| 1. |
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| 2. |
- Gustafsson, David, et al.
(author)
-
Effect of cutting clearance and sandblasting on fatigue of thick CP800 steel sheets for heavy-duty vehicles
- 2022
-
In: Hot Sheet Metal Forming of High-Performance Steel: proceedings. - : Wissenschaftliche Scripten. ; , s. 315-322
-
Conference paper (peer-reviewed)abstract
- Effect from manufacturing processes on fatigue properties of high-strength thick steel sheets have gained increased attention the recent years, due to new demands on the heavy-duty vehicle (HDV) industry to reduce the carbon footprint. The aim of this study is to add knowledge of the effect of shear cutting clearance on the fatigue behaviour of complex phase CP800 thick steel sheets. In addition, sandblasting and its effect on the fatigue properties are studied. Service loads are fluctuating loads acting on chassis component making fatigue an important failure mode. The fatigue strength usually follows the yield strength of the material and hence weight could in theory be saved by using steels of higher strength. However, in the presence of edge defects this relation does not necessarily hold, this leads to large safety factors of the design and under-utilization of the high-strength material. Thus, an increased knowledge about the effect from manufacturing processes on fatigue properties is important for the quest to achieve weight reduction. This is particularly true for thick sheets which, to the best of our knowledge, are less investigated than their thinner counterparts, but of high importance for the HDV development. In this paper, empirical results from fatigue testing of complex phase steel CP800, subjected to punching and trimming, are presented. Results for different cutting clearances are compared as well as the effect of sandblasting. A fast fatigue testing method called Rapid fatigue test based on stiffness evolution is utilized. The results show the improvement obtained by using sandblasting as well as illustrating the effect of different cutting clearances. These results can be used as a guidance for design and production of HDV components, where cutting clearance is set. Furthermore, the results can be used as an input for discussions whether the extra costs associated with sandblasting is motivated or not for components made from high strength, thick steel sheets.
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| 3. |
- Gustafsson, David
(author)
-
Effect of shear cutting on metal fatigue
- 2024
-
Licentiate thesis (other academic/artistic)abstract
- Lightweighting of automotive and heavy-duty vehicle components is an important task that does not need any further motivation or background. It can be read in a large part of the technical papers in the field. A common approach for finding lighter solutions is to increase the material grade while decreasing the material thickness. Often in combination with design changes. For perfectly smooth components this is not an issue, but when cut edges from manufacturing processes are present the situation changes. One topic to address is that increased material grade often means increased notch and surface damage sensitivity. This has implications both on forming and fatigue. The reason for selecting a higher strength material is to allow for higher stresses in design. It has however been shown that for a given stress level the fatigue performance of a higher strength material could be worse than for a lower strength counterpart if punched holes or trimmed edges are present. This means that in the search of lower weight there is a risk of increasing stresses, and at the same time selecting a material that is less suited to handle this increase. Hence, engineers and developers are put in a position where these effects must be quantified to find the most efficient solution. This quantification is a cumbersome and expensive task, often including a considerable amount of testing. Important sources of fatigue life reduction in this context are the residual stresses in the loading direction and the surface roughness in the cut edge. This thesis aims to present an overview of metal fatigue in the context of shear cut components. Necessary knowledge regarding the shear cutting process is provided along with a description of numerical methods and considerations for process simulations. These findings are then applied to the presented papers where the first introduces a simplified approach for numerical simulation of shear cutting to obtain residual stresses. In this approach the simplification mainly lies in the failure model calibration. The second paper studies the possibility of using the obtained residual stresses together with measured values of surface roughness to quantify fatigue life reduction of shear cut specimens.
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| 4. |
- Hammarberg, Samuel, 1988-
(author)
-
A Study on Sandwich Structures : Development, Mechanical Characterization and Numerical Modeling
- 2021
-
Doctoral thesis (other academic/artistic)abstract
- Legislative demands force the automotive industry to reduce greenhouse gas (GHG) emissions. At the same time, crashworthiness must not be compromised. A ve-hicle’s GHG emissions, such as carbon dioxide, is dependent on its fuel consump-tion. Lowering the vehicle weight, reducing fuel consumption, will therefor reduce emissions. Thus, high performance lightweight materials and structures are on demand. Several methods for achieving high-performance lightweight components are available. One of the most successful approaches has been replacing mild steels with press-hardened steels, e.g. ultra high strength steels (UHSS). In the press-hardening process, a low-alloyed boron steel blank is austenitized followed by simultaneously forming and cooling. By controlling cooling rates, a martensitic microstructure can be obtained, resulting in components with superior properties compared to mild steels. Other methods of achieving lightweight components in-clude the usage of sandwich structures where stiff skins are bonded to a low-density core. In the present thesis, several types of sandwich structures are studied both numerically and experimentally. A UHSS sandwich with a bidirectionlly corru-gated core, intended for stiffness application, is manufactured and evaluated in three-point bending. Finite element models are utilized to recreate the three-point bend test. A large amount of finite elements are required for precise discretization of the core. The number of finite elements are reduced by replacing the sandwich with an homogeneous, equivalent model with input data obtained from analyzing representative volume elements (RVEs) of the core, subjected to periodic and ho-mogeneous boundary conditions. Good agreement is found between experiments and finite element models. A UHSS sandwich with a partly perforated core is evaluated numerically for energy absorption applications. Several hole configu-rations for the core are evaluated with respect to specific energy absorption. A fracture criterion is utilized for the sandwich skins. Computational time is re-duced by homogenization of the core using a stress-resultant based constitutive model. It is found that the sandwich concept allows for an increase in specific energy absorption and that the computational time can be reduced while still be-ing able to predict energy absorption. An experimental methodology is developed for mechanical characterization of micro-sandwich materials. Tools are developed for loading the micro-sandwich in out-of-plane tension and shear, where digital image correlation is used for measuring displacements fields and fracture of the micro-sandwich core. Statistical methods are adopted for analyzing the variation in the mechanical properties of the micro-sandwich from which statistical means may be obtained. The experimental data is used as input for constitutive models, simulating the micro-sandwich material subjected to peeling, using a T-peel test. The numerical models are validated against experiments, found to agree within one standard deviation, suggesting that the experimental methodology produces robust data.The present work has thus presented methods, further increasing the usability of UHSS with regard to lightweighting, and explored how such components may be simulated numerically with adequate accuracy and reasonable computation time. Furthermore, the present thesis contributes by presenting methods for character-izing micro-sandwich materials, including statistical methods for analyzing scatter in mechanical properties, and how such sandwich materials may be modeled, tak-ing elasto-plasticity and damage into account. These results opens up possibilities for further development and optimization of lightweight constructions.
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| 5. |
- Hammarberg, Samuel, 1988-, et al.
(author)
-
Calibration of orthotropic plasticity- and damage models for micro-sandwich materials
- 2022
-
In: SN Applied Sciences. - : Springer Nature. - 2523-3963 .- 2523-3971. ; 4:6
-
Journal article (peer-reviewed)abstract
- Sandwich structures are commonly used to increase bending-stiffness without significantly increasing weight. In particular, micro-sandwich materials have been developed with the automotive industry in mind, being thin and formable. In the present work, it is investigated if micro-sandwich materials may be modeled using commercially available material models, accounting for both elasto-plasticity and fracture. A methodology for calibration of both the constitutive- and the damage model of micro-sandwich materials is presented. To validate the models, an experimental T-peel test is performed on the micro-sandwich material and compared with the numerical models. The models are found to be in agreement with the experimental data, being able to recreate the force response as well as the fracture of the micro-sandwich core.
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| 6. |
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| 7. |
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| 8. |
- Hammarberg, Samuel, 1988-, et al.
(author)
-
Modeling of Ultra High Strength Steel Sandwiches with Lightweight Cores
- 2019
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In: Hot sheet metal forming of high-performance steel. - : Wissenschaftliche Scripten. ; , s. 313-320
-
Conference paper (peer-reviewed)abstract
- Legislation, du to greenhouse gas emissions, is forcing the automotive industry to reduce emissions and energy consumption. High-performance lightweight materials and structures are essential for meeting these demands. In this work, two types of lightweight sandwich materials are investigated and developed; one intended for crash applications (Type I) and another for stiffness applications (Type II). In order to predict the final properties of the sandwich materials, numerical modeling strategies are established. To achieve reasonable computational time, homogenization is adopted to overcome the complex core geometries of the sandwich materials. Type I, based on press-hardened boron steel, consists of a perforated core between two face plates. Evaluation of energy absorption during crash is conducted by utilizing numerical deformation models of a hat-profile geometry. The intention is to compare the energy absorption of the hat-profile based on the Type I sandwich to a hat-profile based on solid steel with equivalent weight. Type II, based on press-hardened boron steel, consists of a bidirectionally corrugated core between two face plates. The geometry of the bidirectional core requires a large amount of finite elements for precise discretization, causing impractical simulation times. This is adressed by suggesting an equivalent material formulation, to reduce the computational time. The results from Type I indicate an increased specific energy absorption capacity of 20 % as compared to solid steel. From the equivalent material procedure of Type II, it is found that the computational cost is reduced by 95 % with a maintained accuracy for structural stiffness. Validation is carried out by subjecting the sandwich to three-point bending. Good agreement is found between numerical and experimental data. Thus, this work shows that sandwich materials are an interesting and promising approach for reducing weight of vehicle components while maintaining performance, in terms of stiffness and crashworthiness.
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| 9. |
- Hammarberg, Samuel, 1988-, et al.
(author)
-
Novel Methodology for Experimental Characterization of Micro-Sandwich Materials
- 2021
-
In: Materials. - : MDPI. - 1996-1944. ; 14:16
-
Journal article (peer-reviewed)abstract
- Lightweight components are in demand from the automotive industry, due to legislation regulating greenhouse gas emissions, e.g., CO2. Traditionally, lightweighting has been done by replacing mild steels with ultra-high strength steel. The development of micro-sandwich materials has received increasing attention due to their formability and potential for replacing steel sheets in automotive bodies. A fundamental requirement for micro-sandwich materials to gain significant market share within the automotive industry is the possibility to simulate manufacturing of components, e.g., cold forming. Thus, reliable methods for characterizing the mechanical properties of the micro-sandwich materials, and in particular their cores, are necessary. In the present work, a novel method for obtaining the out-of-plane properties of micro-sandwich cores is presented. In particular, the out-of-plane properties, i.e., transverse tension/compression and out-of-plane shear are characterized. Test tools are designed and developed for subjecting micro-sandwich specimens to the desired loading conditions and digital image correlation is used to qualitatively analyze displacement fields and fracture of the core. A variation of the response from the material tests is observed, analyzed using statistical methods, i.e., the Weibull distribution. It is found that the suggested method produces reliable and repeatable results, providing a better understanding of micro-sandwich materials. The results produced in the present work may be used as input data for constitutive models, but also for validation of numerical models.
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| 10. |
- Hammarberg, Samuel, 1988-, et al.
(author)
-
Numerical evaluation of lightweight ultra high strength steel sandwich for energy absorption
- 2020
-
In: SN Applied Sciences. - : Springer. - 2523-3963 .- 2523-3971. ; 2:11
-
Journal article (peer-reviewed)abstract
- Legislation regarding greenhouse gas emissions forces automotive manufacturers to bring forth new and innovative materials and structures for weight reduction of the body-in-white. The present work evaluates a lightweight ultra high strength steel sandwich concept, with perforated cores, for energy absorption applications. Hat-profile geometries, subjected to crushing, are studied numerically to evaluate specific energy absorption for the sandwich concept and solid hat-profiles of equivalent weight. Precise discretization of the perforated core requires large computational power. In the present work, this is addressed by homogenization, replacing the perforated core with a homogeneous material with equivalent mechanical properties. Input data for the equivalent material is obtained by analyzing a representative volume element, subjected to in-plane loading and out-of-plane bending/twisting using periodic boundary conditions. The homogenized sandwich reduces the number of finite elements and thereby computational time with approximately 95%, while maintaining accuracy with respect to force–displacement response and energy absorption. It is found that specific energy absorption is increased with 8–17%, when comparing solid and sandwich hat profiles of equivalent weight, and that a weight saving of at least 6% is possible for equivalent performance.
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| 11. |
- Hammarberg, Samuel, 1988-, et al.
(author)
-
Ultra high strength steel sandwich for lightweight applications
- 2020
-
In: SN Applied Sciences. - : Springer Nature. - 2523-3963 .- 2523-3971. ; 2:6
-
Journal article (peer-reviewed)abstract
- Methods for reducing weight of structural elements are important for a sustainable society. Over the recent years ultra high strength steel (UHSS) has been a successful material for designing light and strong components. Sandwich panels are interesting structural components to further explore areas where the benefits of UHSS can be utilized. The specific properties of sandwich panels make them suitable for stiffness applications and various cores have been studied extensively. In the present work, bidirectionally corrugated UHSS cores are studied experimentally and numerically. A UHSS core is manufactured by cold rolling and bonded to the skins by welding. Stiffness is evaluated experimentally in three-point bending. The tests are virtually reproduced using the finite element method. Precise discretization of the core requires large amounts of computational power, prolonging lead times for sandwich component development, which in the present work is addressed by homogenization, using an equivalent material formulation. Input data for the equivalent models is obtained by characterizing representative volume elements of the periodic cores under periodic boundary conditions. The homogenized panel reduces the number of finite elements and thus the computational time while maintaining accuracy. Numerical results are validated and agree well with experimental testing. Important findings from experimental and simulation results show that the suggested panels provide superior specific bending stiffness as compared to solid panels. This work shows that lightweight UHSS sandwiches with excellent stiffness properties can be manufactured and modeled efficiently. The concept of manufacturing a UHSS sandwich panel expands the usability of UHSS to new areas.
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| 12. |
- Jonsson, Simon, Doktorand, 1987-, et al.
(author)
-
Evaluation of Crashworthiness Using High-Speed Imaging, 3D Digital Image Correlation, and Finite Element Analysis
- 2023
-
In: Metals. - : MDPI. - 2075-4701. ; 13:11
-
Journal article (peer-reviewed)abstract
- To promote the use of newhigh-strengthmaterials in the automotive industry, the evaluation of crashworthiness is essential, both in terms of finite element (FE) analysis aswell as validation experiments. Thiswork proposes an approach to address the crash performance through high-speed imaging combined with 3D digital image correlation (3D-DIC). By tracking the deformation of the component continuously, cracks can be identified and coupled to the load and intrusion history of the experiment. The so-called crash index (CI) and its decreasing rate (CIDR) can then be estimated using only one single (or a few) component, instead of a set of components with different levels of intrusion and crushing. Crash boxes were axially and dynamically compressed to evaluate the crashworthiness of TRIP-aided bainite ferrite steel and press-hardenable steel. Acalibrated rate-dependent constitutivemodel, and a phenomenological damage model were used to simulate the crash box testing. The absorbed energy, the plastic deformation, and the CIDR were evaluated and compared to the experimentally counterparts. When applying the proposed method to evaluate the CIDR, a good agreement was found when using CI:s reported by other authors using large sets of crash boxes. The FE analyses showed a fairly good agreement with some underestimation in terms of energy absorptions. The crack formation was overestimated resulting in too high a predicted CIDR. It is concluded that the proposed method to evaluate the crashworthiness is promising. To improve the modelling accuracy, better prediction of the crack formation is needed and the introduction of the intrinsic material property, fracture toughness, is suggested for future investigations and model improvements.
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| 13. |
- Jonsson, Simon, Doktorand, 1987-, et al.
(author)
-
Impact crash tests of high-strength steels using 3D high-speed digital image correlation and finite element analysis
- 2022
-
In: 8th International Conference Hot Sheet Metal Forming of High-Performance Steel CHS2: May 30th - June 2nd, 2022, Barcelona, Spain. - Auerbach : Verlag Wissenschaftliche Scripten. - 9783957351500 ; , s. 119-126
-
Conference paper (other academic/artistic)abstract
- The automotive industry is currently adapting to a new reality, where anthropogenic emissions need to decrease significantly. To meet present and future demands of vehicle design, press harden-ing techniques to produce complex geometries with high strength and ductility as well as good precision are of great interest. New generations of hot forming steels enable both further weight reductions by using thinner sheets as well as better crash performance due to its ability to improve the structural integrity of the body-in-white. To promote the use of these new steel grades, it is important to study their performance using well-instrumented lab scale test of full-scale compo-nents. Since these tests are often time consuming and expensive, calibrating constitutive models with tensile specimens and using finite element analysis is a more cost-effective alternative. How-ever, these calibrated models should be validated against full-scale experiments to verify their effectiveness in predicting the material behaviour in complex crash environments. In this paper, a high-speed 3D digital image correlation experiment is performed on a crash box under axial com-pression. The material is a hot forming steel grade with a specified tensile strength of 1000 MPa. The axial crash tests are modelled based on a visco-plastic model calibrated by high-speed tensile tests. The computed results in terms of force response and obtained deformations agree well with the corresponding measurements.
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| 14. |
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| 15. |
- Jonsson, Simon, 1987-, et al.
(author)
-
Studying the rate-dependence of Essential Work of Fracture in press hardening steels
- 2024
-
Conference paper (other academic/artistic)abstract
- The automotive industry is currently in a paradigm shift transferring the fleet over from internal combustion vehicles to battery electric vehicles (BEV). This introduces new challenges when designing the Body-In-White (BIW) since it is essential to protect the heavy and energy-dense battery in a crash scenario. Press hardening steels (PHS) have emerged as an excellent choice when designing crash safety parts due to their high-strength and forming flexibility. It is however crucial to evaluate the crash performance of the selected materials before producing parts. Component testing is cumbersome, and it is difficult to separate material behaviour from other influences such as spot welds. Fracture toughness measured in the frame of fracture mechanics using the Essential Work of Fracture (EWF) has emerged as an interesting tool to rationalise crash performance of steel sheet designs. The EWF has previously been shown to increase, sometimes substantially, with increasing strain rate, but the cause of this is still largely unknown. For high strain rates, adiabatic heating can have a significant effect on the mechanical and fracture properties, especially for steel grades with metastable austenite. However, the rate dependence is still evident for press hardened martensitic steel grades. In this work, quasi-static and high-speed EWF experiments are performed on two PHS grades designed for anti-intrusion and energy absorption, respectively. The thermals are monitored using infrared thermography to study the adiabatic heating and thus the thermal environment in the FPZ during EWF at high-loading rates. The results are compared and discussed.
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| 16. |
- Jonsson, Simon, Doktorand, 1987-
(author)
-
Towards energy-based fracture modelling for crashworthiness applications
- 2024
-
Licentiate thesis (other academic/artistic)abstract
- The automotive industry is currently adapting to progessively more stringent emission and safety regulations imposed by governmental agencies. This introduces significant design difficulties due to the conflicting nature of passenger safety in automotive manufacturing, namely that increased crashworthiness generally leads to heavier vehicles, which in turn leads to more severe crashes. Significant industry effort to introduce lightweight materials into automotive Body-in-White (BIW) design has thus been introduced in recent years to reduce curb weight while improving crashworthiness. Third generation Advanced High Strength Steels (3rd-gen AHSS) and new generations of press hardening steels (PHS) has emerged as cost-effective and natural substitutes in the safety critical crush zones of the vehicle. The limited ductility of these higher strength materials can however make them more prone to cracking, which in turn make reliable deformation behaviour difficult in a crash event. Thus, predicting cracks in the material and its resistance to further propagate them are essential in evaluating crash performance of a design. Fracture toughness measured within the frame of fracture mechanics using the Essential Work of Fracture (EWF) has shown to correlate well with AHSS crashworthiness for steel sheets, making it an interesting parameter for further study in this area. EWF is however strain rate dependent, and most available EWF testing for AHSS is still performed using quasi-static loading rates, conditions completely different from common high-speed crash scenarios. Furthermore, since full-scale testing is a costly endeavor, numerical modelling is used in Computer Aided Engineering (CAE) to test designs before proceeding with a physical prototype. To promote the use of new high strength steel grades in the industry, reliable and properly characterised material models are thus necessary. These models then need to be validated with component experiments to ensure that the models are accurate enough. This is usually done using crash box components in an axial compression or three-point bending setup because of their similarity to real structural components used in crash zones. In this work, EWF at the higher loading rates common in crash scenarios is further investigated to contribute additional data regarding strain rate dependence of fracture toughness measured within the frame of fracture mechanics for AHSS sheets. Furthermore, the crashworthiness of dynamically loaded axially compressed AHSS and PHS crash boxes are evaluated both experimentally using full-field measurements and numerically using a commercially available damage model. The high-speed photography allow for a more efficient component crashworthiness evaluation with fewer components due to the possibility to track crack initiations and their propagation during the deformation. The results from the commercial damage model show that although the prediction of the first cracks is decent, the damage evolution is not captured accurately. These results show the need for further development of economically feasible (shell) damage models that take propagation energy into account in crash simulations. This would also help promote the use of fracture toughness in the automotive industry.
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| 17. |
- Kajberg, Jörgen, et al.
(author)
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Characterisation of materials subjected to large strains by inverse modelling based on in-plane displacement fields
- 2004
-
In: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 41:13, s. 3439-3459
-
Journal article (peer-reviewed)abstract
- A method for characterisation of materials subjected to large strains beyond the levels when plastic instability occurs in standard tension tests is presented. Thin sheets of two types of hot-rolled steel are subjected to tension loading until fracture occurs. The deformation process is captured with a digital camera and by digital speckle photography (DSP) in-plane pointwise displacement fields are obtained. By numerical differentiation and assuming plastic incompressibility the equivalent plastic strain is determined. The characterisation performed in this paper consists of estimating material parameters in two constitutive models. These models are a piecewise linear plasticity model and a parabolic hardening model. By using inverse modelling including finite element analyses (FEA) of the tension tests the material parameters are adjusted to achieve a minimum in a so-called objective function. The objective function is basically a least-square functional based on the difference between the experimental and FE-calculated displacement and strain fields. Due to the large deformations an adaptive meshing technique is used in order to avoid highly distorted elements. The DSP- technique provided measurements, where the uncertainty of the equivalent plastic strain varied between 0.0015 and 0.0056. The maximum obtained strain was approximately 0.8. The true stress-strain curves based on the estimated parameters are validated in the low strain region by comparison with curves from standard tension tests
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| 18. |
- Kajberg, Jörgen
(author)
-
Displacement field measurement using digital speckle photography for characterisation of materials subjected to large deformations and high strain rates
- 2003
-
Doctoral thesis (other academic/artistic)abstract
- In many technical processes, material is deformed under conditions involving large deformations (strains) and/or high strain rates. Examples of such processes are collisions, impact, penetration, metal forming, powder compaction and crack propagation. For description of these kinds of situations a variety of constitutive models, based on both physical foundations and empirical considerations, is available. Common for all models is that they contain material parameters, which have to be estimated by utilising experimental methods. For material characterisation under quasi-static conditions standardised tension tests of uniaxially loaded specimens are commonly used. With these tests stress-strain relations are obtained up to moderate strain values, whereupon the onset of strain localisation, so-called necking, restricts their validities. Correction methods have been developed to compensate for the onset of necking (e.g. Bridgman's correction method for round bars). The Taylor impact test and the split Hopkinson bar arrangement are frequently used methods for the investigation of incompressible (volume conserving) materials in the high strain rate regime. Typically, the specimens are short and stubby cylinders, which ideally facilitate a homogeneous state of loading necessary for a simple interpretation of the experimental results. In this thesis a methodology is suggested for characterisation of materials subjected to large deformations and high strain rates, where neither homogeneously loaded specimens nor incompressible behaviour are necessary. Experimental methods similar to standardised tension tests and split Hopkinson bar arrangement are complemented with an optical method, digital speckle photography (DSP), for in-plane point-wise displacement and strain measurements. By using a common digital camera in the former tests and a high-speed camera with a CCD-unit (Charged Coupled Device) in the latter tests digitised images are obtained for the method of DSP. An inverse method is used to estimate the material parameters in constitutive models. Three-dimensional numerical simulations of the specimens are performed by the finite element method (FEM). By adjusting the parameters to give a best fit between experimental and numerical results (displacements and strains) in least-square sense optimal values are obtained. In the quasi-static tension tests true strain values up to 0.8 were obtained for a hot-rolled steel. The mild steel specimens in the high strain rate tests were subjected to strain rates of magnitudes 10^2-10^3 1/s.
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| 19. |
- Kajberg, Jörgen
(author)
-
High strain-rate experiments using high-speed photography
- 2002
-
Licentiate thesis (other academic/artistic)abstract
- In many technical processes, material is deformed under conditions that contain high strain rates. Examples of such processes are collisions, impact, penetration, metal forming and crack propagation. Constitutive models including viscoplasticity have been proposed for these kinds of situations. Common for all models is that they contain material parameters, which are not well known. Experimental techniques like Taylor impact test and split Hopkinson pressure bar have been designed for the investigation of materials subjected to high strain rates. In these methods high strain rates are achieved by subjecting short specimens to rapid loading. For a simple interpretation of experimental results, a homogeneous state of stress and strain is desirable. Short specimens imply high strain rate but if the length and width are similar a nonhomogeneous state of stress and strain will result and the reliability in the evaluated quantities decreases. With the testing techniques mentioned above, it is difficult to use specimens, which are short but slender. In this thesis an experimental method to study material behaviour at high strain rates is developed. In contrast to the classical techniques, this method does not require a homogeneous state of stress and strain. A very small specimen (sub mm) and several larger specimens (up to 5 mm) have been used in the experiments. They are subjected to rapid tensional loading in devices similar to the Hopkinson bar arrangement. For the larger specimens a complete split Hopkinson bar is used, while one of the bars, namely the incident bar, has been omitted when the shorter specimen is tested. The deformation of the specimens is captured with a high-speed camera of image converter type. For the small specimen of sub mm size, the extension of its entire length is evaluated. The estimated strain rate reached well over 10^4 1/s. The larger specimens are evaluated using digital speckle photography (DSP) to give in-plane strain fields. Strains in the domain, 0.01-0.25, are evaluated and strain rates up to 3000 1/s are achieved.
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| 20. |
- Kajberg, Jörgen, et al.
(author)
-
High strain-rate tensile testing and viscoplastic parameter identification using microscopic high-speed photography
- 2004
-
In: International journal of plasticity. - 0749-6419 .- 1879-2154. ; 20:4-5, s. 561-575
-
Journal article (peer-reviewed)abstract
- A combined experimental/numerical method for determination of constitutive parameters in high strain-rate material models is presented. Impact loading, using moderate projectile velocities in combination with small specimens (sub mm) facilitate tensional strain rates in the order of 104-105 s-1. Loading force is measured from one-dimensional wave propagation in a rod using strain gauges and deformation is monitored with a high-speed camera equipped with a microscope lens. A sequence of digital photographs is taken during the impact loading and the plastic deformation history of the specimen is quantified from the photographic record. Estimation of material parameters is performed through so called inverse modelling in which results from repeated FE-simulations are compared with experimental results and a best choice of constitutive parameters is extracted through an iterative optimisation procedure using the simplex method. Results are presented from a preliminary tension test of a mild steel (A533B) at a strain rate well over 104 s-1. The sensitivity of the evaluated material parameters to errors in measured quantities is studied. The method, especially the optical technique for measurement of deformation will be further developed.
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| 21. |
- Kajberg, Jörgen, et al.
(author)
-
High-temperature split-hopkinson pressure bar with a momentum trap for obtaining flow stress behaviour and dynamic recrystallisation
- 2014
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In: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 50:6, s. 547-554
-
Journal article (peer-reviewed)abstract
- In hot forming processes at elevated temperatures like wire rolling, microstructural changes such as repeated dynamic recrystallisation and grain growth occur. An experimental method to obtain the flow stress behaviour and to capture the recrystallised microstructure for materials subjected to large deformations, high temperatures between 900 and 1200°C and high strain rates around 5000s-1 is presented. The method is based on the split-Hopkinson pressure bar arrangement complemented with an inductive heat source. Furthermore, a momentum trap is added to ensure that the specimen is loaded only once. By quenching the specimen directly after the single loading, the dynamically recrystallised microstructure is preserved. The quenching is performed within 0.1s of loading by dropping the specimen into a water bath. By applying the momentum trap technique, the compressive loading of the specimen could be interrupted at a strain level slightly above the strain level corresponding to the peak stress, which is a good estimation for the onset of dynamic recrystallisation. © 2014 Wiley Publishing Ltd.
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| 22. |
- Kajberg, Jörgen, et al.
(author)
-
Material characterisation using high-temperature Split Hopkinson pressure bar
- 2013
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In: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 213:4, s. 522-531
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Journal article (peer-reviewed)abstract
- In order to characterise the mechanical response of materials in manufacturing processes, such as wire and bar rolling involving very high strain rates, temperatures and level of straining, an experimental device is presented. The device is suitable for testing at strain rates up to approximately 4000 s-1, temperatures up to 1200 °C (≈1500 K) and strains around 0.5. It is based on the classical Split Hopkinson pressure bar and is complemented with an inductive heating source for achieving requested temperatures. By keeping the specimen separated from the Hopkinson bars just until an instant before impact (50 ms) considerable cooling and temperature gradients in the specimen are avoided. Three steel grades, two stainless steels and a high-speed steel, were tested. Four different material models whose parameters were fitted to the obtained experimental data were used for mechanical characterisation: two empirically based and two physically based. Overall, one of the physically based models showed the best agreement between experimental results and the predicted flow stresses.
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| 23. |
- Kajberg, Jörgen, et al.
(author)
-
Optical method to study material behaviour at high strain rates
- 2000
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In: IUTAM Symposium on Field Analysis for Determination of Material Parameters - Experimental and Numerical Aspects. - Dordrecht : Encyclopedia of Global Archaeology/Springer Verlag. - 1402012837 ; , s. 37-49
-
Conference paper (peer-reviewed)
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| 24. |
- Kajberg, Jörgen, et al.
(author)
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Viscoplastic parameter estimation by high strain-rate experiments and inverse modelling : speckle measurements and high-speed photography
- 2007
-
In: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 44:1, s. 145-164
-
Journal article (peer-reviewed)abstract
- A methodology based on inverse modelling for estimating viscoplastic material parameters at high strain-rate conditions is presented. The methodology is demonstrated for a mild steel exposed for compression loading in a split Hopkinson pressure bar arrangement. By using dog-bone shaped specimens nonhomogeneous states of deformation are obtained throughout the entire deformation process. The resulting nonhomogeneous deformation of the specimens is evaluated using digital speckle photography (DSP) to give in-plane point-wise displacement and strain fields. The photographs are captured with a high-speed camera of image converter type, which acquire time resolved images during the impact loading. The experiments are simulated using finite element analysis (FEA), where the material model suggested by Johnson-Cook for high-strain rate conditions are utilised. Experimental and FE-calculated field information are compared in order to estimate the viscoplastic parameter in the Johnson-Cook material model. The estimation is performed by minimising least-square functions that contain the differences in displacements and strains, respectively. The quality of the estimated parameters is studied from statistical point of view.
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| 25. |
- Lindroos, Matti, et al.
(author)
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Micromechanical and multi-scale modeling of manganese containing slag comminution in the design of energy efficient secondary raw material beneficiation processes
- 2021
-
In: Minerals Engineering. - : Elsevier. - 0892-6875 .- 1872-9444. ; 170
-
Journal article (peer-reviewed)abstract
- Efficient separation of valuable metals from various slags is of great interest for the industry to effectively utilize valuable raw materials. Present work focuses on modeling the deformation and damage behavior of manganese containing slag materials at the microstructural level, which dictates the macroscopic material behavior and allows one to investigate possibilities to perform metal separation after comminution of the slags. The model includes finite element micromechanical description of the material behavior and slag microstructure. Computational micromodels are constructed based on direct input characterization data and statistically representative synthetic models. The damage model treats brittleness and ductility of the material together with phase specific material behavior, all relevant to comminution of the slag. Finally, a simplified jaw crusher simulation accounts for freeing materials, assisting the evaluation of empirical random breakage, all together with a microstructural particle study which is analyzed against micromechanical modeling. Crystal plasticity level simulations of surface deformation and hardening in jaw crusher are presented to couple macroscale crushing events with microscale deformation of wear parts. The work overall presents a workflow and proposes a methodology how digitalization and multi-scale material modeling can contribute to the development of efficient comminution means for hard to process secondary raw materials.
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| 26. |
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| 27. |
- Lundholm, Erik
(author)
-
On the relationship between microstructure, process parameters and mechanical properties of boron steels
- 2023
-
Licentiate thesis (other academic/artistic)abstract
- The continuous development of the press hardening technology has led to stronger, lighter and more environmentally friendly components. Utilising the varying properties of boron steel at different temperatures enables great design freedom, while also attaining high strength in the final component. This is achieved by heating the initial material to an austenitic state, where it has good formability, followed by forming and quenching using pressing tools. However, in order to simulate this thermo-mechanical process the microstructure evolution must be understood. Research has been performed using various initial material states, evaluating possible effects on the final mechanical properties. Studies have also been performed to evaluate the grain growth during austenitisation. The influence of the initial material and the evolution of the austenite morphology during austenitisation has previously been less researched compared to other parts of the process.In this work, samples from commercially available materials have been heat treated to create test specimens, which subsequently have been used for mechanical testing and microstructure analysis. Digital image correlation was used to determine local fracture strains and anisotropic properties during plastic deformation. Samples were also heat treated using varying process parameters in order to study the grain growth during austenitisation. It was found that if hot rolled, cold rolled and soft annealed cold rolled samples were compared after hardening, their mechanical properties only exhibited minor variations. However, all samples displayed anisotropic properties during plastic deformation. There is therefore some microstructural trace from the production which is unaffected by soft annealing, austenitisation and subsequent quenching. The grain growth observed during the austenitisation was consistent within a temperature range not exceeding 930 ◦C. Using data retrieved from isothermal experiments a model could be fitted which described the growth using the temperature and current grain size. At 960 ◦C the microstructure was irregular, with large single grains and considerable variations in the average grain size within the same sample. The bending performance was not affected in a major way by the austenitisation temperature.The lack of variation of the mechanical properties due to the initial microstructure or parent austenite grain size is a testament to the robustness of the process. It should be noted however, that all samples were rapidly quenched. If the microstructure is formed through diffusion dependent phase transformations, the final mechanical properties could be more sensitive to process parameters. Further research is needed to fully understand the microstructural evolution and thus the mechanical properties where a more general thermal cycle can be used.
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| 28. |
- Marth, Stefan
(author)
-
An Approach on Material Model Calibration for Modelling of Sheet Metal Deformation and Failure
- 2021
-
Doctoral thesis (other academic/artistic)abstract
- Sheet metals are often used in automotive and aerospace applications for safety-relevant components. Weight reduction is one possibility to reduce fuel consumption or increase the payload capacity and therewith reduce the carbondioxide emission of these trans-portation vehicles. The weight reduction can be achieved by using new sheet metal alloys and thereby reducing the sheet metals thickness. Advanced material process-ing technologies like for example the press hardening process to manufacture ultra high strength steels (UHSS) are an important contribution to weight reduction. Furthermore, the usage of many different sheet metal materials and grades, like the new generation of advanced high strength steels (AHSS) and aluminium alloys will replace further low strength steel components.To challenge the balance between safety and weight reduction, while maintaining safety, reliable and efficient engineering tools are needed. Finite Element (FE) simulations are commonly used to prove a maintained safety for parts with a decreased sheet thickness and weight. This leads to a high demand on the simulation precision of sheet metals, where an accurate prediction of the failure behaviour and the post-necking hardening of materials is needed. Therefore, an approach on material model calibration for modelling of sheet metal deformation and failure is developed. The ability for companies to predict the performance envelop of all these new sheet metal alloys and components is of great importance for the metal manufacturer as well as for the automotive industry.In this thesis work a method to characterize the elasto plastic post necking behaviour of sheet metal materials, the Stepwise Modelling Method (SMM), is presented. The method uses full field measurements of the deformation field on the surface of tensile specimen. The hardening relation is modelled as a piecewise linear relation in a step by step procedure. The linear hardening parameter is adapted to reduce the residual between experimental and calculated tensile forces. The SMM is used to characterize the post necking behaviour of a ferritic boron steel and the results are compared with the commonly used inverse modelling method. It is shown that the stepwise modelling method characterizes the true stress, true plastic strain relation in an effective and com-putational efficient way. Furthermore, the SMM is used to characterize the stress state evolution during tensile testing, which is an important factor for failure and fracture mod-elling. This method is shown in an aerospace application for the nickel based super alloy Alloy 718. A study on simulating the whole comments lifespan from blank to fractured component is presented by producing a laboratory scale UHSS-component and testing it until fracture. The component performance simulation is based on results obtained by SMM for paint baked fully hardened boron steel. To enable the post necking characterization of anisotropic sheet metals like aluminium alloys an updated SMM version based on an anisotropic plasticity model is presented and evaluated for the aluminium alloys AA6016 and AA5754. Finally, the fracture behaviour of an automotive 6000 series alu-minium alloy in different directions is presented. In this study a GISSMO failure model is calibrated based on full field measurements under different stress states and evaluated on a multi triaxiality tensile specimen.The results shown in this thesis are that the presented Stepwise Modelling Method is an effective and efficient alternative method to characterize the deformation and failure of sheet metals. Based on the results of this method plasticity and fracture models can be calibrated and used for advanced forming and component performance simulations. This can lead to reduce time and costs during the development processes of new materials and products.
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| 29. |
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| 30. |
- Marth, Stefan, et al.
(author)
-
Stepwise modelling method for post necking characterisation of anisotropic sheet metal
- 2021
-
In: Modelling and Simulation in Materials Science and Engineering. - : Institute of Physics (IOP). - 0965-0393 .- 1361-651X. ; 29:8
-
Journal article (peer-reviewed)abstract
- Modelling and simulation are important tools during design and development processes. For accurate predictions of, e.g. manufacturing processes or final product performance, reliable material data is needed. Usually, the applied material models are calibrated by utilising direct methods such as conventional uniaxial tensile/compression tests but also inverse methods are occasionally applied. Recently, an effective inverse method, the stepwise modelling method (SMM), was presented. By using SMM, the flow stress from initial yielding, beyond necking to final fracture, can be determined. However, the method is developed for sheet materials having isotropic von Mises hardening. In this paper the SMM is extended for post necking characterisation of anisotropic sheet metals using the Barlat yield 2000 criterion. The novel method was applied to analyse the post necking plasticity of the widely used aluminium alloy AA6016 in T4 condition and the aluminium alloy AA5754 in H111 condition. The latter alloy has reported to show serrated yielding, also known as the Portevin–Le Chatelier effect. The obtained flow stress curves agree well with the curves form conventional uniaxial tensile tests up to the point of necking and show credible post necking predictions to final fracture. Furthermore, SMM showed that it could handle the effect of serrated yielding for AA5754-H111. Hence, the novel approach can be used to characterise the post necking hardening of a variety of anisotropic sheet metals and thereby contributes to efficient and reliable material model calibration.
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| 31. |
- Moretti, Marie Anna, 1995-
(author)
-
Microstructure and property models of alloy 718 applicable for simulation of manufacturing processes
- 2022
-
Licentiate thesis (other academic/artistic)abstract
- This thesis focuses on experimental characterization, understanding and modelling of nickel-based alloy 718, for a large range of loading conditions. Alloy 718 is the most widely used nickel-based superalloy, due to its high strength, high corrosion resistance and excellent mechanical properties at high temperatures. In this work, the mechanical behavior and microstructure evolution of this alloy during high strain rate deformation is investigated. Compression tests using a Split-Hopkinson pressure bar (SHPB) device were performed and the microstructure of the deformed sample was observed using optical microscope (OM) and scanning-electron microscope (SEM) coupled with electron back-scattered diffraction (EBSD) technique. The microstructural evolution according to the deformation conditions was characterized. For high deformation temperatures (1000 C and above), recrystallisation is identifed as the main deformation mechanism. A physics-based model was employed to simulate the deformation behavior of alloy 718. This type of models accounts for the microstructural mechanisms taking place during deformation. Knowledge about the deformation mechanisms of alloy 718, acquired experimentally and from literature, enables to formulate mathematically the microstructural phenomena governing the deformation behavior of the alloy. The proposed model includes the effects of strain hardening, grain boundary strengthening (Hall-Petch), solid solution strengthening, phonon and electron drag and recovery by dislocation glide and cross-slip. It is calibrated and validated using data obtained from mechanical tests, as well as values captured by the microstructural analysis.
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| 32. |
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| 33. |
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| 34. |
- Sandin, Olle, 1989-
(author)
-
Predicting Sheared Edge Characteristics of High Strength Steels
- 2022
-
Licentiate thesis (other academic/artistic)abstract
- An efficient way of reducing CO2 emissions from the transportation sector is to reduce the vehicle weights, i.e. lightweighting. A common strategy for lightweighting of vehicles is to replace the steels used to build structural parts of the vehicle, usually manufactured by metallic sheets, with stronger, advanced high strength steel (AHSS) grades. Using stronger steel grades enables down-gauging while the structural integrity of the parts remain unchanged. However, the increase in strength of AHSS typically comes with a loss of ductility, affecting their forming properties. A common AHSS manufacturing defect is edge cracking occurring when a sheared edge (damaged by the shearing operation) is bent or stretched. It is known in the sheet metal forming industry that the shear cutting process introduces damage, in terms of micro-cracks and notches, to sheared edges from which edge cracks can grow. Conventional forming analyses do not include the effects from sheared edge damage and therefore can not predict edge cracking during forming. Numerical modelling of the shear cutting process can aid the understanding of sheared edge damage and how it affects the AHSS edge cracking phenomena.This thesis presents experimental and numerical methods for calibration of acommercial damage- and failure model, intended for shear cutting simulations. Crack initiation and propagation govern the shear cutting process of AHSS sheets. Therefore, a commercial numerical damage- and failure model was studied regarding its ability to predict shear edge damage. The investigation shows that the damage and failure model has limitations concerning prediction of crack initiation, thus concluding that modelling of processes including formation of cracks using the said damage- and failure model risks to generate erroneous results. This phenomena was also seen in modelling of shear cutting, where the crack-driven fracture process following burnish formation was delayed. Through sensitivity analysis of uncalibrated areas on the failure locus could accurate correlation between numerical and experimental cut edge morphology be obtained. Such results show that additional calibration experiments are necessary, but also the need for development of stress-state dependent failure modelling of AHSS that includes the effect from cracks.
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| 35. |
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| 36. |
- Sjöberg, Ted, 1986-, et al.
(author)
-
Calibration and Validation of Three Fracture Criteria for Alloy 718 Subjected to High Strain Rates and Elevated Temperatures
- 2018
-
In: European journal of mechanics. A, Solids. - : Elsevier. - 0997-7538 .- 1873-7285. ; 71, s. 34-50
-
Journal article (peer-reviewed)abstract
- The aerospace industry has an important role to play in lowering greenhouse gas emissions and thereby reducing its ecological footprint. Aerospace manufacturers do therefore push components and materials closer to their limits in order to design for minimal weight. An example of component is the metallic turbine case of the aircraft engine for protection of the surrounding structure in situations such as blade-off events. This implies design, where knowledge regarding mechanical material response from onset of yield to fracture, at both high strain rates and elevated temperatures, are needed. Alloy 718 is a nickel-based superalloy commonly used in the containment structures of the hot parts of the aircraft engines. Three established criteria, Modified-Mohr-Coulomb, Maximum Shear Stress and Magnitude of Stress Vector, have been applied to characterise the fracture behaviour of Alloy 718 supplied to conditions present at a blade-off event. The calibrations of the criteria were based on high-speed tensile tests of thin sheet specimens with different geometries for varying different stress states. The fracture strains were determined using high-speed photography combined with digital image correlation. The temperature and strain rate were varied from 20 to 650 °C and 1 to 1000 s−1 respectively. The calibrated criteria were validated through finite element analyses and reverse impact testing at room temperature up to 650 °C. Discs were fired against a instrumented rod with different shapes of their tips for obtaining varying stress states. It was found that the Modified-Mohr-Coulomb locus showed the best agreement with measured fracture strains in the calibration experiments. This criterion did also predict the fractures of discs in the reverse impact tests fairly well. The possibility to accurately predict fracture facilitates the use of modern numerical software for containment design as a complement to time-consuming and expensive full-scale tests.
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| 37. |
- Sjöberg, Ted, et al.
(author)
-
Experimental characterisation of the evolution of triaxiality stress state for sheet metal materials
- 2017
-
In: European journal of mechanics. A, Solids. - : Elsevier. - 0997-7538 .- 1873-7285. ; 66, s. 279-286
-
Journal article (peer-reviewed)abstract
- Sheet metals are often used as safety structures in automotive applications where the fracture behaviour is a key design parameter. Theoretical and experimental observations have shown that the fracture behaviour of many metals depends on the stress state. Modelling the stress state dependency of fracture in Finite Element (FE) simulations has led to the development of advanced stress state dependent fracture criteria. The calibration of advanced fracture models is currently limited by the characterisation methods, which have not developed much during the last decades. Experimental characterisation methods that can determine the stress state accurately are necessary to ensure reliable calibrations of advanced fracture models. In this article, an experimental method to obtain the stress state and its evolution during deformation is presented. The stress state evolution is determined using measured local displacement field data, which were obtained by digital image correlation, coupled with a stepwise modelling method. This article shows that the stepwise modelling method can capture the stress state evolution for three different specimen geometries subjected to tensile loading. The resulting experimentally determined stress state evolutions are compared with the results of FE simulations, and both results are in good agreement. The accurate stress state evolutions characterised directly from experiments using the proposed method enables calibration of advanced fracture models rapidly and reliably
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| 38. |
- Sjöberg, Ted, et al.
(author)
-
Fracture behaviour of Alloy 718 at high strain rates, elevated temperatures, and various stress triaxialities
- 2017
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 178, s. 231-242
-
Journal article (peer-reviewed)abstract
- A methodology for fracture characterisation at strain rates up to 1000 s−1, temperatures up to 650 °C, and various stress triaxialities is presented. High-speed photography combined with digital image correlation is used to evaluate the strain at fracture. The methodology was successfully demonstrated on aged nickel based Alloy 718, commonly used in the containment structure of aircraft engines. Tensile specimens with four different geometries were loaded to get a wide range of positive stress triaxialities. All specimens originated from one single heat and batch to ensure consistent mechanical properties. The results showed evident stress state dependency on the failure strain, where lower failure strains were observed at higher stress triaxialities for all combinations of temperatures and strain rates. A coupled relationship between the temperature and the stress triaxiality controlling the fracture strain was found. However, any clear dependency on strain rate was hard to detect.
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| 39. |
- Sjöberg, Ted, 1986-
(author)
-
Method Development for Characterisation of Superalloy used in Containment Design
- 2017
-
Doctoral thesis (other academic/artistic)abstract
- Due to the trend of increasing environmental demands put on civil aviation, manufacturersof commercial aircraft engines meet increased pressure to reduce weight. Modernturbofan engines represent up to almost one tenth of an aircraft's total weight, meaning areduction of engine component weight of just 30 kg is estimated to reduce CO2 emissionsby 400 tonnes over the lifetime of a medium sized commercial aircraft. At the sametime turbine casings are required to fully prevent debris to escape in the event of bladefailure, to prevent further damage to critical systems. For new designs to be approvedthe Federal Aviation Regulations (FAR) states that the containment capability of a suggesteddesign solution must be experimentally established, a process associated with highcosts and long lead times. The industry therefore more frequently relies on numericalsimulations as part of all stages in the design process. For simulations to replace theexpensive experiments in nding the nal optimum design regarding weight and safety,the accuracy of the used models have to be improved.This thesis aims to provide increased accuracy in the numerical predictions by developingexperimental procedures to test material close to the operational conditions of thecontainment structure. This is realised by performing experiments at high-strain ratesand elevated temperatures in a high-velocity tensile testing machine combined with aninduction heater. Sheet specimens of varying geometries are loaded in tension to achievedierent stress states for covering dierent failure modes. Furthermore, high-speed photographyand Digital Image Correlation are utilised to track in-plane deformations. Theresulting local deformations are then used to derive the stress-strain hardening relationand the evolution of the stress state from initial loading up to fracture. The obtaineddata are nally used to calibrate strain rate and thermal dependent plasticity and fracturemodels. To validate the calibrated models so-called reverse impact testing was used,where the resulting force of a material sample impacting an instrumented target wasquantied. The experiment was straightforward to model numerically since the specimenies freely without constraints, thereby avoiding complex boundary conditions.The characterisation method was developed and performed on nickel based Alloy 718.This material is known for its high strength and good corrosion resistance at high temperaturesand is therefore commonly used in hot parts of aircraft engines, such as thecontainment structures of the low-pressure part of the engine turbine. All material fortesting and validation was supplied from one single heat and batch, aged using the sameheat treatment conditions, to ensure consistent mechanical properties. The results fromthe characterisation procedure showed that the plastic ow of Alloy 718 is moderatelystrain rate and temperature dependent while the fracture is clearly stress state dependent.
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| 40. |
- Sjöberg, Ted, et al.
(author)
-
Reverse ballistic experiment resembling the conditions in turbine blade off event for containment structures
- 2016
-
In: Thin-walled structures. - : Elsevier BV. - 0263-8231 .- 1879-3223. ; 107, s. 671-677
-
Journal article (peer-reviewed)abstract
- An experimental technique has been developed which allows loading of heated sheet material under impact conditions with simultaneous measurement of the impact force history. The combined characteristics of impact loading at elevated temperature makes the experiment ideal for validation of models used to simulate the containment structure surrounding aircraft engines. In this paper experimental results for Alloy 718 are presented, a nickel based super alloy commonly used in hot parts of the containment structure. The experimental results are then compared to simulations in order to validate previously calibrated material parameters. The basic principle of the validation experiment is based on reverse ballistics, in which a thin circular specimen with free boundaries impacts the end of an instrumented rod. Using induction heating the specimen is heated to temperatures up to 650 °C and a gun driven by compressed air accelerates the specimens to desired velocity. In the reported work velocities are kept low enough to avoid cracking and thus the study is limited to plastic conditions, even though the technique is applicable also for fracture studies. The free boundaries of the experiment makes numerical modelling and simulation straightforward, making it valuable as a validation tool. All numerical simulations are performed using the commercial finite element code LS-Dyna and plastic behaviour of the material was modelled with the Johnson-Cook material model
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| 41. |
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| 42. |
- Suarez, Laura, 1991-
(author)
-
Mechanical Characterization of Heterogeneous Brittle Materials
- 2021
-
Licentiate thesis (other academic/artistic)abstract
- Comminution is one of the highest energy-consuming processes in the mining and mineral processing industry by consuming around 2% of the global generated energy with an overall efficiency of 1-3%. Different approaches to the optimization of processes have been developed, but there is still room for improvement. The macro events where energy is mostly spent require numerical methods, so an overall optimization of the system is performed by the analysis and optimization of individual subsystems, such as machines and material to be crushed. The challenge when applying numerical analysis lies on the calibration of the models with mechanical parameters inherent to the constitutive laws and physics of the system. It has been seen that mineral material is exposed to a great variety of time dependent forces within the process. A baseline to understanding the interaction of the material with the machines is the analysis of fracture processes under different loading conditions. This thesis focuses on the mechanical characterization of manganese slag core material for the development, calibration and validationof constitutive models via direct and indirect measurements of the strength and fracture behavior. Diametrial and axial compressive tests under quasi-static and dynamic conditions were used by the hand of optical techniques to obtain information about the evolution of damage. Digital image correlation in 2D and 3D was implemented, considering that it is a method virtually independent ofthe geometry, size, material and deformation rate. Quasi-static tests on both Brazilian disc and unconfined axial compression configurations exposed a mechanical behavior of composite-like material where random failure of the components caused high variability of the elastic parameters. Irreversible damage was perceived globally as non-linearities in load-strain curves, while cyclic loading revealed a degradation of the material affecting the elastic modulus where a weakening of the matrix and dominant behavior of the inclusions on the mechanical response is perceived. Dynamic tests were performed in an in-house built Split Hopkinson Pressure Bar which follows the wave propagation theory in the material generated by the impact of a pressure driven projectile. 2D high speed imaging was performed in order to obtain informationabout the crack initiation and fracture process so that a sampling frequency of 380,000 fps and 663,200 fps was obtained for axially and diametrically loaded samples, respectively. Full-field deformations showed a staggered fracture process were on set failure points vary due to the internal events happening in the material. Localized frictional occurrences and inertial effects acting in the pre-cracked matrix have a strong effect on the global mechanical response and, therefore, a great variability of ultimate compressive and tensile strengths was found. The overall strain/loading rate dependency of the material was perceived as a general increase of the UCT and maximum load compared to quasi-static values. In general, the objective of this work was to study the effect of different loading conditions on the mechanical behavior and material parameters of unprocessed slag for the future development of numerical models of large-scale comminution processes.
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| 43. |
- Suarez, Laura, 1991-, et al.
(author)
-
Mechanical Characterization of Highly Heterogeneous Brittle Materials by Optical Techniques
- 2022
-
In: Minerals Engineering. - : Elsevier. - 0892-6875 .- 1872-9444. ; 185
-
Journal article (peer-reviewed)abstract
- Fragmentation processes like crushing and grinding are complex and extensively energy-consuming activities in the mining and mineral processing industry. Numerical analysis of different materials and loading conditions will gain more knowledge and support in the improvement of the efficiency of the fragmentation process. Although, a challenge is the lack of experimental data both for calibration and validation hampering the development of constitutive models. As a case of study, a mechanical characterization of pre-processed MnSiFe-slags was performed. Diametral and axial compressive tests under quasi-static conditions were used to load the mineral material and obtain a strain field (ε) during increasing and cyclic loading until failure accounting for progressive damage. The evolution of the strain captured by digital image correlation (DIC) techniques exposed a mechanical behavior of composite-like material where random failure of the components caused high variability of the elastic parameters. These were found to be load dependent and they are strongly related to the ability of the material to internal rearrangement during loading. Irreversible damage affects the structure of the material and is perceived as non-linearities in the load-strain curves. It was found a degradation of the material under repetitive loading decreasing of the elastic modulus perceived as a weakening of the matrix and dominant behavior of the inclusions on the mechanical response of the material.
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| 44. |
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| 45. |
- Suarez, Laura, 1991-, et al.
(author)
-
Valorization of Air-Cooled EAF Manganese Slag in Comminution Processes: an Investigation into the Breakage Characterization
- 2023
-
In: Mining, Metallurgy & Exploration. - : Springer Nature. - 2524-3462 .- 2524-3470. ; 40:6, s. 2449-2462
-
Journal article (peer-reviewed)abstract
- In recent years, slag, a residue from pyrometallurgical processes, has become more attractive in circular economy frameworks to increase the efficient use of resources throughout the life cycle of steel products and help in the reduction of carbon emissions. Its applicability is strongly dependent on the particle size, and therefore, the optimization of breaking processes should be approached by increasing the knowledge of the dynamics of slag to promote fracture. Increasing the knowledge on the mechanical response of manganese slag opens up the potential for the development of cost-effective numerical models, e.g., constitutive models based on inverse engineering calibration frameworks or digital twins. In this study, rate-dependent tests of manganese slag have been performed using a split Hopkinson pressure bar device for testing its dynamic mechanical response. In order to obtain information about the crack initiation and fracture process, 2D ultra-high speed imaging was implemented with a sampling frequency of 663,200 fps for diametrically loaded specimens. Full-field deformation measurements using digital image correlation (DIC) techniques showed a staggered fracture process where failure points on mechanical response curves vary due to the internal events happening in the material. Localized frictional occurrences and inertial effects acting inside the pre-cracked matrix have a strong effect on the global mechanical response, and therefore, a great variability of strengths was obtained.
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| 46. |
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| 47. |
- Wessling, Albin, 1993-, et al.
(author)
-
A Statistical Bonded Discrete Element Model for heterogeneous brittle materials
- 2021
-
Conference paper (other academic/artistic)abstract
- Numerical modelling of the fracture of heterogeneous brittle materials is of interest for several industries,such as rock excavation and comminution applications. A numerical model of brittle materials needs tobe able to capture the unpredictable results, e.g. with regards to measured strength and fracture pattern,as observed experimentally. In this study a new approach, based on the Parallel Bond Model (PBM) [1] and theWeibull distribution, for modelling brittle heterogeneous materials in 3D is proposed and appliedto the Brazilian Disc Test (BDT) [2]. The PBM is used to generate irregular grains with varying bondstrengths and stiffnesses. For the grain generation, a parent particle is chosen at random in the rockbody and a randomized ellipsoid is generated around the particle. The mean grain bond stiffnesses andstrengths are associated with the grain and all particles within the ellipsoid surface are bonded togetherwithin +/- 10 % of these mean values. Further, the bond parameters of the cement between a grain andits neighbours is scaled based on the mean grain properties. An example of a generated sample is shownin Figure 1 a). In order to evaluate the model, a series of simulations of the BDT were conducted.The effects of the Weibull heterogeneity index and cement strengths on the predicted tensile strengthand crack pattern were evaluated. Specifically, the initiation, propagation, coalescence and branching ofcracks were examined in detail. Apart from demonstrating challenges with the BDT, the results also showthat the proposed model is able to capture key phenomena related to brittle heterogeneous materials, suchas unpredictable fracture pattern and a large variation in tensile strength, see Figure 1 b-c).
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| 48. |
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| 49. |
- Wessling, Albin, 1993-, et al.
(author)
-
A statistical bonded particle model study on the effects of rock heterogeneity and cement strength on dynamic rock fracture
- 2024
-
In: Computational Particle Mechanics. - : Springer Nature. - 2196-4378 .- 2196-4386. ; 11:3, s. 1313-1327
-
Journal article (peer-reviewed)abstract
- Numerical modelling and simulation can be used to gain insight about rock excavation processes such as rock drilling. Since rock materials are heterogeneous by nature due to varying mechanical and geometrical properties of constituent minerals, laboratory observations exhibit a certain degree of unpredictability, e.g. with regard to measured strength and crack propagation. In this work, a recently published heterogeneous bonded particle model is further developed and used to investigate dynamic rock fracture in a Brazilian disc test. The rock heterogeneities are introduced in two steps—a geometrical heterogeneity due to statistically distributed grain sizes and shapes, and a mechanical heterogeneity by distributing mechanical properties using three Weibull distributions. The first distribution is used for assigning average bond properties of the grains, the second one for the intragranular bond properties and the third one for the bond properties of the intergranular cementing. The model is calibrated for Kuru black diorite using previously published experimental data from high-deformation rate tests of Brazilian discs in a split-Hopkinson pressure bar device, where high-speed imaging was used to detect initiations of cracks and their growth. A parametric study is conducted on the Weibull heterogeneity index of the average bond properties and the grain cement strength and evaluated in terms of crack initiation and propagation, indirect tensile stress, strain and strain rate. The results show that this modelling approach is able to reproduce key phenomena of the dynamic rock fracture, such as stochastic crack initiation and propagation, as well as the magnitude and variations of measured quantities. Furthermore, the cement strength is found to be a key parameter for crack propagation path and time, overloading magnitudes and indirect tensile strain rate.
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| 50. |
- Wessling, Albin, 1993-, et al.
(author)
-
A statistical DEM approach for modelling heterogeneous brittle materials
- 2022
-
In: Computational Particle Mechanics. - : Springer Nature. - 2196-4378 .- 2196-4386. ; 9:4, s. 615-631
-
Journal article (peer-reviewed)abstract
- By utilizing numerical models and simulation, insights about the fracture process of brittle heterogeneous materials can be gained without the need for expensive, difficult, or even impossible, experiments. Brittle and heterogeneous materials like rocks usually exhibit a large spread of experimental data and there is a need for a stochastic model that can mimic this behaviour. In this work, a new numerical approach, based on the Bonded Discrete Element Method, for modelling of heterogeneous brittle materials is proposed and evaluated. The material properties are introduced into the model via two main inputs. Firstly, the grains are constructed as ellipsoidal subsets of spherical discrete elements. The sizes and shapes of these ellipsoidal subsets are randomized, which introduces a grain shape heterogeneity Secondly, the micromechanical parameters of the constituent particles of the grains are given by the Weibull distribution. The model was applied to the Brazilian Disc Test, where the crack initiation, propagation, coalescence and branching could be investigated for different sets of grain cement strengths and sample heterogeneities. The crack initiation and propagation was found to be highly dependent on the level of heterogeneity and cement strength. Specifically, the amount of cracks initiating from the loading contact was found to be more prevalent for cases with higher cement strength and lower heterogeneity, while a more severe zigzag shaped crack pattern was found for the cases with lower cement strength and higher heterogeneity. Generally, the proposed model was found to be able to capture typical phenomena associated with brittle heterogeneous materials, e.g. the unpredictability of the strength in tension and crack properties.
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