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- Stenström, Christer
(författare)
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Modelling of fracture toughness using peridynamics : A Study of J-integral, essential work and homogenisation
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fracture toughness is one of the most important properties of a material. Being able toaccurately estimate the energy that goes into forming new crack surfaces is essential for the development of new materials, quality assurance, structural monitoring and failure analysis. Fracture toughness parameters are routinely determined by mechanical testing and are often used in numerical tools. Furthermore, fracture toughness is a common property in material specification. Numerical simulation of fracture toughness can reduce the need of mechanical testing and is sometimes the only viable alternative when mechanical testing is not an option, for example in component optimisation and in the assessment of operational structural components. However, complex fracture is a challenge in material modelling, which comes from that a material body is assumed to remain continuous in classical continuum mechanics. Classical continuum mechanics is formulated assuming a continuous body and that spatial derivatives are defined. However, this is not the case at cracks and other dis continuities. Complementing continuum mechanics with supplementary procedures for modelling discontinues can also add further challenges. Besides, the assumption of locality, that each material point only interacts with is immediate neighbouring points, becomes invalid for nanoscale geometries. Thus, fracture cannot easily be modelled. An alternative is therefore of interest. Peridynamics is a nonlocal extension of continuum mechanics with the constitutive model formulated as an integro-differential equation. The advantages of using an integral expression are foremost that long-range forces can be handled and that the theory is valid even in the presence of discontinuities, such as cracks, allowing unguided modelling of fracture. Since damage is introduced to the constitutive model of peridynamics, there is no requirement of supplementary procedures that can add further complications. Due to its nonlocal formulation, the method is also capable of capturing nano-effects. However, the use and reporting of fracture toughness parameters in peridynamics is a routine in its infancy as the method is under development.In this thesis, two fracture toughness methods, the classical J-integral and the essential work of fracture (EWF), are studied with peridynamics. Also, as the nonlocality of peri dynamics give rise to certain boundary effects, e.g. on crack faces, homogenisation is a part of the study. The thesis consists of two parts; an introductory summary with discussion and conclu sions, followed by a series of appended papers. The first paper concerns application of Rice's J-integral on displacement derivatives formulation in peridynamics with comparison to an exact analytical stress-strain-displacement specimen solution. The next two papers concerns homogenisation of a peridynamic bar, to remove the end effects, arisen from the nonlocality of peridynamics, to obtain an elastic behaviour exact to a classical continuum mechanics bar. The fourth paper is an implementation of the J-area integral into peridynamics, with study of various discretisation methods. Thereafter, in the last paper, Rice's J-integral and the nonlocal peridynamic J-integral are compared on various specimens, followed by an extension of the research to study EWF with peridynamics for the first time. The study includes a novel automated calibration at the interparticle bond level to simulate nonlinear elastic behaviour, which subsequently is complemented with softening and used for EWF modelling. As a part of introducing the peridynamic J-integral, the study also includes a proof of path independence.Major findings of the study includes:• The classical J-integral on a displacement derivative formulation gives accurateestimations of fracture toughness in peridynamics.• The peridynamic lD bar can be homogenised to obtain a linear elastic behaviour identical to that of an corresponding continuum mechanics body.• The bond calibration method gives a nonlinear elastic peridynamic model that can accurately recover an experimentally obtained stress-strain response. Up to the start of material softening, the nonlinear elastic model recovered the experimentally obtained stress-strain response of two very different materials; a lower-ductility martensitic-bainitic steel and a higher-ductility bainitic steel.• The nonlinear elastic model were able to match very well the experimentally measured EWF for the higher-ductility bainitic steel.• The J-integral value obtained from the peridynamic model, matched the experimentally obtained EWF value for the higher-ductility bainitic steel.
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| 4. |
- Gustafsson, David, et al.
(författare)
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Effect of cutting clearance and sandblasting on fatigue of thick CP800 steel sheets for heavy-duty vehicles
- 2022
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Ingår i: Hot Sheet Metal Forming of High-Performance Steel: proceedings. - : Wissenschaftliche Scripten. ; , s. 315-322
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Konferensbidrag (refereegranskat)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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| 5. |
- Hammarberg, Samuel, 1988-, et al.
(författare)
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Calibration of orthotropic plasticity- and damage models for micro-sandwich materials
- 2022
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Ingår i: SN Applied Sciences. - : Springer Nature. - 2523-3963 .- 2523-3971. ; 4:6
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Jonsson, Simon, Doktorand, 1987-, et al.
(författare)
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Impact crash tests of high-strength steels using 3D high-speed digital image correlation and finite element analysis
- 2022
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Ingår i: 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
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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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| 9. |
- Larsson, Fredrik, et al.
(författare)
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A Testing Methodology for Hot Rolled High Strength Steels Under Warm Forming Conditions
- 2022
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Ingår i: Hot Sheet Metal Forming of High-Performance Steel: proceedings. - : Wissenschaftliche Scripten. ; , s. 411-418
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Konferensbidrag (refereegranskat)abstract
- For the reduction of the environmental footprint of Heavy-Duty vehicles (HDV), lighter chassiscomponents can be considered. A lighter HDV chassis gives the opportunity of lower fuel consumption, increased payloads, and savings of material resources. One way of achieving this, is to reduce thicknesses of components in combination of using higher strength steels. For the aim of forming UHSS into complex geometries the need to characterize thick sheet metal at elevated temperatures arises. This work aims at expanding earlier research of characterization of thinner sheet metal and create a testing methodology for tensile tests of 7 mm thick steel sheets at elevated temperatures. An experimental methodology for evaluating high strength steel under warm conditions have been developed and demonstrated. A Digital Image Correlation system is used to extract strain fields for all three testing temperatures. This together with an automatized induction system pre-defined temperature cycles are applied. When the desired Hollomon-Jaffe constant is obtained the tensile test is executed. The methodology shows promising results with good repeatability of stress-strain curves. The methodology shows good stability and are promising for future development and investigations of high strength steels under warm forming conditions.
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