| 1. |
- Ahlqvist, Max, et al.
(author)
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Accounting for defect position in ultrasonic fatigue test specimen with heterogeneous stress distribution
- 2024
-
In: Engineering Fracture Mechanics. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0013-7944 .- 1873-7315. ; 308
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Journal article (peer-reviewed)abstract
- To account for or neglect the defect position, i.e. the fatigue initiating defect location, both radially and axially, is evaluated for hourglass-shaped ultrasonic fatigue specimen. The commonly used analytical equations to calculate the stress is compared against a finite element (FE) based approach, which is able to fully considering the stress state at the defect position. Notably, the effects on several common fatigue analyses are evaluated: the fatigue strength distribution, the stress-life and the stress-defect relationships. Fracture mechanical assessment is also performed, for a comprehensive VHCF characterization of the EN-GJS-500-7 ductile cast iron used in the study. The VHCF properties are characterized up to 3 & sdot; 108 8 cycles, using the Step-Stress fatigue testing method under fully reversed loading. The FE-model and Weibull distribution as the choice of fatigue strength distribution, enables size effect evaluation by Weakest-link effective volume with the highly stressed volume method as benchmark. The work shows that it is imperative to use the local stress state at the defect position, as the distribution of failures can diverge largely from the center of the specimen, and that neglecting this causes systematic error and flawed potentially results.
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| 2. |
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| 3. |
- Boåsen, Magnus, et al.
(author)
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Analysis of thermal embrittlement of a low alloy steel weldment using fracture toughness and microstructural investigations
- 2022
-
In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 262
-
Journal article (peer-reviewed)abstract
- A thermally aged low alloy steel weld metal is investigated in terms of its fracture toughness and microstructural evolution and compared to a reference. The main purpose of the study is to investigate the effects of embrittlement due to thermal ageing on the brittle fracture toughness, and its effects on the influence of loss of crack tip constraint. The comparison of the investigated materials has been made at temperatures that give the same median fracture toughness of the high constraint specimens, ensuring comparability of the low constraint specimens. Ageing appears to enable brittle fracture initiation from grain boundaries besides initiation from second phase particles, making the fracture toughness distribution bimodal. Consequently, this appears to reduce the facture toughness of the low constraint specimens of the aged material as compared to the reference material. The microstructure is investigated at the nano scale using atom probe tomography where nanometer sized Ni-Mn-rich clusters, precipitated during ageing, are found primarily situated on dislocation lines.
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| 4. |
- Cao, Ri-hong, et al.
(author)
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Failure and Mechanical Behavior of Transversely Isotropic Rock under Compression-Shear Tests : Laboratory Testing and Numerical Simulation
- 2021
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 241
-
Journal article (peer-reviewed)abstract
- The failure and mechanical behavior of transversely isotropic rock are significantly affected by the original bedding planes. Until now, few studies have been performed to investigate the influence of the geometrical and mechanical parameters of the bedding planes on the fracture characteristics of transversely isotropic rocks under planar shear fracture loading conditions. For this purpose, experimental and numerical compression-shear tests on double-notched specimens are conducted to investigate the fracturing characteristics of transversely isotropic rock under planar shear fracture loading. The experimental study that focuses on the influence of bedding plane inclination on fracture load, fracture pattern and AE evolution, and six inclination angles is conducted in this study. Based on the flat joint contact model (for the rock matrix) and smooth joint contact model (for the original bedding plane) in PFC2D (particle flow code), the microscale fracturing process of transversely isotropic rock with different inclinations is simulated and analyzed. The results show that the inclination has an important influence on the fracture load and fracture pattern, and the maximum and minimum fracture loads are obtained for specimens with inclination angles of 30° and 60°, respectively. Moreover, the strength and spacing of the original bedding planes also play an important role in fracture loads. Higher bedding plane strength and wider bedding plane spacing result in higher fracture loads. In addition, with a moderate inclination angle, transversely isotropic rock with higher bedding plane strength tends to form cracks that cut through the rock matrix. However, with the decrease in the bedding plane strength, more fractures form along the bedding planes.
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| 5. |
- Carlsson, Jenny, 1984-, et al.
(author)
-
Simulating fracture in a wood microstructure using a high-resolution dynamic phase field model
- 2020
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 232
-
Journal article (peer-reviewed)abstract
- As a simplified model, wood can be considered as consisting of hollow cells held together by a weaker middle lamella. We use a high-resolution finite element model, endowed with a phase field model for brittle fracture to model such a structure. The micro-structured discretised materials, with and without middle lamella, differ significantly from a homogenous continuum reference in terms of damage patterns. There is also significant difference between the models with and without middle lamella, since damage localises in the weaker middle lamella, which underlines the importance to include the middle lamella in mechanical analyses. At increased loading velocity, the damage becomes scattered over a larger region ahead of the crack tip, indicating that dynamic effects affect the crack behaviour and that dynamics need to be taken into consideration when loading rates are high or when crack propagation is unstable.
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| 6. |
- Chen, Bin, et al.
(author)
-
Element-removal global digital image correlation for accurate discontinuous deformation field measurement in fracture mechanics
- 2023
-
In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 290
-
Journal article (peer-reviewed)abstract
- We propose an element-removal (ER) global digital image correlation (DIC) method to improve the measurement accuracy of discontinuous deformation fields, such as crack propagation. The occurrence of cracks in materials or structures inevitably deteriorates the tracking accuracy, and, consequently, the strain field accuracy obtained by regular subset and global DIC. The proposed ER-global-DIC algorithm iteratively identifies and removes all the elements covering the crack, during the updating of displacement fields. In the remaining elements, the continuous shape function is applicable for accurate deformation measurement. In principle, although elements that contain the cracks are removed, the algorithm preserves the same number of nodes since the nodes are retained by the remaining elements. Synthetically deformed images based on analytical discontinuous displacement fields validate the effectiveness and accuracy of the proposed method. The ER-global-DIC is further applied to measure the discontinuous displacement fields containing a crack deflection, generated from a finite element model with a cohesive zone model. The results demonstrate the potential of the proposed method for discontinuous deformation measurement on advanced materials, e.g., fiber-reinforced composites.
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| 7. |
- Chen, Shaohui, et al.
(author)
-
Numerical analysis of crack path stability in brittle porous materials
- 2022
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 275
-
Journal article (peer-reviewed)abstract
- Porous materials are widely used in engineering applications because of their high stiffness, strength, and low density. Those advantages are mainly due to their open microstructures, which also makes it challenging to obtain a thorough understanding of their fracture mechanisms and to predict trustworthy crack paths. In this study, we analyse numerically fracture trajectories in brittle porous solids with varying porosity (or relative density) subject to different mixed-mode loading conditions using a phase-field theory for brittle fracture. The results reveal that the crack paths in porous solids with high porosity (low relative density) are very different from those in porous solids with low porosity (high relative density). The latter resemble stable crack paths in homogeneous solids, whereas the former seems somewhat arbitrary, more stochastic. In high porosity materials, the crack paths are governed by the local microstructure rather than by the global remote loading. A key observation is that there is a transition of the fracture behaviour in porous materials at relative densities of around 50%. At relative densities above 50%, the ma-terial behaves nearly as a classical continuum, and the crack paths are reasonably well captured by traditional fracture mechanics theories. The stochastic nature of the porous microstructure is also examined.
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| 8. |
- Elkhodbia, Mohamed, et al.
(author)
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Machine Learning Augmentation of the Failure Assessment Diagram Methodology for Enhanced Tubular Structures Integrity Evaluation
- 2024
-
In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 307
-
Journal article (peer-reviewed)abstract
- Failure assessment diagrams (FADs) are essential engineering tools for evaluating the structural integrity of components. However, their widespread application can be limited by complexity and computational expense. This study presents a novel machine learning-based approach to streamline FAD analysis, offering accuracy and efficiency while overcoming these limitations. The approach integrates numerical contour integral-based FADs with artificial neural networks (ANNs). To ensure reliable material modeling for the Finite Element Analysis (FEA) used to generate J-integral based FADs that train the ANNs, careful experimental and numerical procedures were employed. This involved uniaxial tensile tests, an iterative method for obtaining precise true stress–strain curves, and a Ramberg–Osgood material model for accurate material behavior representation. The ANNs themselves not only analyze large datasets to generate precise FAD envelopes but also predict limit loads and the Φ parameter, incorporating the effect of residual stress on the FAD methodology. To verify and test the proposed method, hypothetical fitness-for-service assessment cases were conducted, incorporating experimental residual stress measurements from split-ring tests on P110 and L80 pipes. These assessments were compared to both traditional FAD methods and computationally intensive FEA-based FADs. Results demonstrate a closer agreement with FEA-based calculations than traditional methods provided in engineering standards. Ultimately, this work provides a rather innovative and adaptable approach for structural integrity evaluations and critical engineering assessments through the proposal of an ANN enhanced FAD approach, simplifying these calculations while maintaining high fidelity.
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| 9. |
- Frómeta, D., et al.
(author)
-
Identification of fracture toughness parameters to understand the fracture resistance of advanced high strength sheet steels
- 2020
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 229
-
Journal article (peer-reviewed)abstract
- The fracture toughness of four advanced high strength steel (AHSS) thin sheets is evaluated through different characterization methodologies, with the aim of identifying the most relevant toughness parameters to describe their fracture resistance. The investigated steels are: a Complex Phase steel, a Dual Phase steel, a Trip-Aided Bainitic Ferritic steel and a Quenching and Partitioning steel. Their crack initiation and propagation resistance is assessed by means of J-integral measurements, essential work of fracture tests and Kahn-type tear tests. The results obtained from the different methodologies are compared and discussed, and the influence of different parameters such as specimen geometry or notch radius is investigated. Crack initiation resistance parameters are shown to be independent of the specimen geometry and the testing method. However, significant differences are found in the crack propagation resistance values. The results show that, when there is a significant energetic contribution from necking during crack propagation, the specific essential work of fracture (we) better describes the overall fracture resistance of thin AHSS sheets than JC. In contrast, energy values obtained from tear tests overestimate the crack propagation resistance and provide a poor estimation of AHSS fracture performance. we is concluded to be the most suitable parameter to describe the global fracture behaviour of AHSS sheets and it is presented as a key property for new material design and optimization.
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| 10. |
- Halilovic, Armin, et al.
(author)
-
An experimental fracture mechanics study of the combined effect of hydrogen embrittlement and loss of constraint
- 2023
-
In: Engineering Fracture Mechanics. - : Elsevier Ltd. - 0013-7944 .- 1873-7315. ; 289
-
Journal article (peer-reviewed)abstract
- This work presents a systematic investigation of the combined effect of hydrogen embrittlement and loss of constraint. The fracture mechanics experiments are performed on an advanced martensitic high strength steel using a single-edge-notch bend specimen, with different crack over height ratio, subjected to electrochemical in-situ hydrogen charging at various loading rates. It is found that the environmentally driven ductile-to-brittle transition region in fracture toughness is obtained for both the high and low constraint specimen configurations. This region is characterized by a change from transgranular dimple rupture to an intergranular mode of fracture. The transition region for the low constraint specimen is shifted towards longer hydrogen exposure times, which is an effect of the reduced hydrostatic stress ahead of the crack front compared to the high constraint specimen. The low constraint specimen exhibits significant plastic straining, which is reflected in a significant decrease in the fracture toughness due to hydrogen assisted transgranular dimple rupture.
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| 11. |
- Halilovic, Armin, et al.
(author)
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An experimental-numerical screening method for assessing environmentally assisted degradation in high strength steels
- 2021
-
In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 245
-
Journal article (peer-reviewed)abstract
- In this work, an experimental-numerical screening method for studying the elastic-plastic properties in high strength steel subjected to environmentally assisted degradation due to hydrogen is proposed. The experiments were performed on single-edge-notch bend specimens loaded with a monotonic constant displacement rate, and the specimens were electrochemically hydrogen pre-charged and/or in-situ. A systematic investigation was conducted of the influence of current density, pre-charging time and loading rate on the fracture mechanical properties. It was found that the loading rate had the greatest effect on the J-R curves, and that the environmental ductile-to-brittle transition region was obtained in a less than a day of experimental time. In this transition region it was found from the fractography that the dominating mode of failure changed from dimple to dominating intergranular fracture.
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| 12. |
- Hultgren, Gustav, et al.
(author)
-
Fracture toughness assessment of surface cracks in slender ultra-high-strength steel plates
- 2023
-
In: Engineering Fracture Mechanics. - : Elsevier Ltd. - 0013-7944 .- 1873-7315. ; 289
-
Journal article (peer-reviewed)abstract
- Safe design against unstable fractures in load-bearing structures is crucial at sub-zero temperatures where the brittle fracture toughness can be unfavourable, especially for high-stress designs incorporating ultra-high-strength steels. The brittle fracture toughness of surface cracks in structural steel with a minimum yield strength of 1300 MPa is, for this reason, tested in the present study at sub-zero temperatures. The realistic flaws are compared with single-edge notched specimens (SEN(B)) from thicker plates with the same chemical composition, using a representative fracture toughness for a three-dimensional crack front according to the Master Curve method. A novel approach determines the latter without considering the local temperature and constraint variation through empirical relations. The experimental result shows a difference in the reference temperature between the two specimen types, which likely is the natural variation of the manufactured materials and/or a loss of constraint due to the difference in the scaled specimen deformation level.
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| 13. |
- Jungstedt, Erik, et al.
(author)
-
On the high fracture toughness of wood and polymer-filled wood composites – Crack deflection analysis for materials design
- 2024
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 300
-
Journal article (peer-reviewed)abstract
- Cracks oriented in the toughest direction across the grain of wood (0°) tend to deflect at 90° to the precrack rather than extending in 0° direction. Fracture toughness data across the grain are therefore difficult to interpret. Crack growth mechanisms and effects from replacing wood pore space with a polymer are investigated. Crack growth is analyzed in four-point bending fracture mechanics specimens of birch and two different polymer-filled birch composites using strain-field measurements and finite element analysis (FEA). Calibrated cohesive zone models in both precrack and 90°-directions describe fracture process zone properties in orthotropic FEA-models. Conditions for 0° crack penetration versus 90° crack deflection are analyzed based on cohesive zone properties. Stable, subcritical crack deflection takes place at low load, reduces crack tip stress concentration, and contributes to high structural toughness, provided the 90° toughness is not too low. Polymer-filled neat birch composites have the best structural toughness properties in the present investigation, since 90° toughness is not compromised by any chemical treatment.
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| 14. |
- Klein, Daniela V., et al.
(author)
-
Influence of heterogeneity due to toughness variations on weakest-link modeling for brittle failure
- 2023
-
In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 292
-
Journal article (peer-reviewed)abstract
- The effect of heterogeneous microstructures on the macroscopic probability of failure is studied by use of weakest-link modeling. Heterogeneity is here associated with a local variation of toughness, where a size scale characteristic of this variation defines a length parameter. The ratio between this length parameter and the size of the active fracture process zone, defined as the heterogeneity ratio, is key to evaluating the impact of a heterogeneous microstructure. Two extremes are identified; small-scale heterogeneity (SSH) and large-scale heterogeneity (LSH). For these cases, it is possible to formulate analytical expressions based on the weakest-link concept, and references are made to existing models in the literature. Typically, heterogeneity along the crack front, where gradients of the mechanical fields are small, falls under the category of SSH. On the other hand, the effect of heterogeneity in a plane perpendicular to the crack front depends strongly on the heterogeneity ratio. Cases that can neither be identified with SSH nor LSH must be addressed with care. How this can be done is discussed, and examples are given for four different microstructure configurations of interest. The investigation is carried out by numerical analysis of a modified boundary layer model. The cumulative probability of failure by cleavage fracture is evaluated in a post-processing step, where two different statistical models are examined; the Beremin model and the Kroon–Faleskog model. Both models render the same conclusion about the alteration of the overall failure probability distributions caused by heterogeneity.
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| 15. |
- Lin, Meichao, et al.
(author)
-
Simulation of ductile-to-brittle transition combining complete Gurson model and CZM with application to hydrogen embrittlement
- 2022
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 268
-
Journal article (peer-reviewed)abstract
- A general simulation framework for modelling ductile-to-brittle transition in metals is proposed. The method combines the complete Gurson model and cohesive zone model, which brings ductile and brittle fracture mechanisms into one play. We found that the transition of failure mode is the result of a competition between fracture due to micro-void growth and coalescence and fracture in the cohesive zone. It is found that the fracture mode is dependent on the ratio between the cohesive strength and the yield strength of the material; brittle fracture only occurs when the strength ratio is below a critical value. This generic rule can be used to rationalize various failure scenarios featured by ductile-to-brittle transition, such as low temperature embrittlement and hydrogen embrittlement. As an application of the general framework, hydrogen embrittlement is simulated. It is revealed that a critical hydrogen concentration has to be achieved in order to trigger brittle fracture, which is consistent with many experimental observations.
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| 16. |
- Molavitabrizi, Danial, et al.
(author)
-
Hydrogen embrittlement in micro-architectured materials
- 2022
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 274
-
Journal article (peer-reviewed)abstract
- Hydrogen embrittlement is a classical problem in bulk materials while it is rather untouched for advanced materials such as micro-architectured materials. This can be a barrier to industrial adoption of these materials where hydrogen is present as a popular source of energy. In this study, we developed a numerical scheme to assess the hydrogen degradation in metallic micro-architectured materials. The numerical scheme is built on the concept of elastoplastic homoge-nization and two hydrogen embrittlement theories, i.e. hydrogen enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP). The use of homogenization allows for explicit definition of a unit-cell, drastically improving the computation time. The hydrogen degradation loci of two specific micro-architectured materials, that is cubic (with 10%, 20% and 30% relative densities) and body-center cubic (with 20% relative density), are numerically characterized. Additionally, the influence of unit-cell topology, relative density, and trap hydrogen on the degradation of homogenized macroscopic material is determined. Finally, a unique failure locus is provided for generic cubic unit-cell with arbitrary relative densities. This degradation law is in-dependent of the relative density and can be interpreted as a material property, contributing to the material design charts.
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| 17. |
- Negi, Alok, et al.
(author)
-
A gradient-enhanced damage model for anisotropic brittle fracture with interfacial damage in polycrystalline materials
- 2023
-
In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944 .- 1873-7315. ; 280
-
Journal article (peer-reviewed)abstract
- This article presents a nonlocal gradient-enhanced damage model that uses direction-dependent damage evolution and interfacial damage to predict transgranular and intergranular cracks in polycrystalline materials at the microstructural level. The distinct grains within the polycrystalline morphology are modeled as anisotropic linear elastic domains with random spatial orientation and cubic symmetries. Transgranular micro-cracks are assumed to occur along specific preferential cleavage planes within each randomly oriented crystal and are described using a bulk damage variable. For intergranular fracture, a smeared description of interface decohesion is incorporated through an interface damage variable which depends on the modified interface kinematics based on a cohesive law that uses a smoothed displacement jump approximation. The coupled system of equations in the proposed computational framework is decoupled using an operator-split methodology to ensure a robust and straightforward computational implementation. Several numerical examples are presented, and simulations are performed on single crystal, bicrystals, and polycrystalline domains to demonstrate the capabilities and validation of the proposed model.
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| 18. |
- Suchorzewski, Jan, et al.
(author)
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Size effect at aggregate level in microCT scans and DEM simulation – Splitting tensile test of concrete
- 2022
-
In: Engineering Fracture Mechanics. - : Elsevier Ltd. - 0013-7944 .- 1873-7315. ; 264
-
Journal article (peer-reviewed)abstract
- The paper describes an experimental and numerical study of size effect on concrete cylindrical specimens in splitting tensile test. Own experimental campaign was performed on specimens with 5 various diameters from D = 74, 105, 150, 192 and 250 mm with hardboard loading strips (distributed load according to standard methods) scaled proportionally to the specimen diameter. The crack opening-control system was applied to obtain the post-peak behaviour of all tested specimens including catastrophic behaviour (snap-back). The tested specimens at a certain point were unloaded and scanned with novel high-resolution micro tomography to analyse the macro cracks and phenomena like aggregate breakage, crack branching etc. at the aggregate level. Based on realistic mesostructure the discrete element method (DEM) 2D model of 3 specimens with diameters of D = 74, 150 and 250 mm were constructed and tested. The fracture was analysed at macro and micro-level in DEM and directly compared with microCT scans. DEM simulations revealed additional information related to the loss of material strength and ductility with increasing specimen size (size effect). The simulation and experimental results were in good agreement. © 2022 The Authors
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| 19. |
- Tarhouni, I., et al.
(author)
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Assessing the effect of the experimental parameters in the evaluation of the essential work of fracture in high-strength thin sheets
- 2022
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 270
-
Journal article (peer-reviewed)abstract
- The essential work of fracture methodology (EWF) has been successfully adopted to evaluate the fracture toughness of various metals and polymers. However, some aspects of the methodology are still far less understood, such as the influence of the experimental parameters on EWF measurement in thin metal sheets. In the present paper, the ligament range criterion of the EWF approach was revised for several advanced high-strength steels (AHSS). The validity of the upper and lower ligament length limits given by the ESIS protocol is redefined and rationalized according to the necking capability and the plasticity behaviour of the different AHSS grades. The work provides a new criterion to define the minimum ligament length to be tested, based on the minimum distance required by the crack to fully develop the necking capability of the material. The width constraint is too restrictive and has no effect on the deviation from linearity in the upper range. On the other hand, the maximum ligament length is proven to be controlled by the size of the plastic zone as proposed by the ESIS protocol.
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| 20. |
- Tavares da Costa, Marcus Vinicius, 1989-, et al.
(author)
-
Energy-release rates and opening of cracks in thin barrier coatings on polymer substrates subjected to tensile loading
- 2020
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 235
-
Journal article (other academic/artistic)abstract
- Thin barrier coatings on polymer films are used increasingly in various applications. In coatings development, it is of interest to characterise the fracture toughness. The tensile fragmentation test potentially provides data to determine the critical energy-release rate for channelling cracks in the coatings during crack accumulation with increasing strain. Also, the crack-opening of displacement is of importance to predict barrier properties. In this work, a method based on finite-element simulations is presented to predict the energy-release rate and crack-opening displacement. The results are compared with experimental findings for TiO2 and mixed TiO2-Al2O3 coatings, with a thickness down to 4-20 nm, on polymer films. It was found that the non-linear stress-strain behaviour of the polymer substrate material needs to be accounted for in the numerical model, in particular for the high strains close to crack saturation observed in these types of materials. Considering the small scales for the cracks and coating thickness, the predicted crack openings compare favourably with those measured experimentally using high-resolution scanning-electron microscopy.
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| 21. |
- Xu, Zijie, et al.
(author)
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The mode I fracture toughness alternation and crack propagation behavior evolution due to long-term Sc-CO2 saturation
- 2022
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 263
-
Journal article (peer-reviewed)abstract
- The mode I fracture toughness is a critical parameter which defines the rock's resistance to crack propagation, especially in hydraulic fracturing. Recently, Supercritical carbon dioxide (Sc-CO2) has been proposed as a fracturing fluid candidate in hydraulic fracturing stimulations of shale reservoirs. However, its effects on fracture toughness transition and crack propagation behaviors have not been understood well. In this study, we performed a series of semicircular bend speci-mens (SCB) before the Longmaxi shale specimens' saturation. Three-point bending tests along divider orientation showed that after Sc-CO2 saturation, mode I fracture toughness (KIC), elastic modulus (E) and absorbed energy (U-e) of shale were decreased by 22.1%, 24.5% and 44.3%, respectively. High speed camera images indicated that after Sc-CO2 saturation, mode I crack directly propagated straight along artificial pre-crack direction, decreasing the degree of crack deviation as in KIC. The results of Cronos high-precision 3D scanning system and scanning elec-tron microscopy (SEM) revealed complicated fracture mechanisms (transgranular, intergranular and mutual coupling crack mechanisms) of shale after Sc-CO2 saturation, which reduced the roughness and area of fracture crack surface. The generation of pore and cracks was the main reason for the decrease of shale resistance to fracture. Furthermore, Sc-CO2 saturated shale specimens only needed to absorb less energy to more rapidly cause the initiation and propagation of mode I cracks with the main fracture mode being transgranular cracks.
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| 22. |
- Yu, Haiyang, et al.
(author)
-
Is it possible to measure the tensile strength and fracture toughness simultaneously using flattened Brazilian disk?
- 2021
-
In: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 247
-
Journal article (peer-reviewed)abstract
- The Flattened Brazilian Disk test was designed for measuring both the tensile strength and fracture toughness of rocks. Using cohesive zone modelling, we found that the method is practically not suitable for extracting the fracture toughness, while it works well in calibrating the tensile strength. The flattened Brazilian disk provides a very narrow window for the variation of load during failure, <15%, insufficient to reflect the change in fracture toughness among different materials. The crack propagation throughout failure is highly unstable and dynamic, where linear elastic fracture mechanics is not applicable. These findings were supported by experimental data. In contrast, the Semi-Circular Bend test, with a pre-crack, can properly measure the fracture toughness but cannot capture the change in the tensile strength. In general, un-cracked disk specimens are suitable for measuring the tensile strength and pre-cracked specimens for the fracture toughness. It is very challenging, if not impossible, to measure both with a single test. It is further implied that the first peak on the loading curve in such kind of tests is trustworthy for extracting mechanical properties, while the following second turning point (if existing) is unreliable as it may contain dynamic effect. These results provide useful reference for the selection and design of test methods using Brazilian disk related geometries.
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| 23. |
- Daniel, Pierre M., et al.
(author)
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A method for modelling arbitrarily shaped delamination fronts with large and distorted elements
- 2024
-
In: Engineering Fracture Mechanics. - 0013-7944. ; 306
-
Journal article (peer-reviewed)abstract
- The simulation of delamination in layered composites is currently limited for large structures. Typically, the use of Cohesive Zone Modelling leads to a requirement of keeping the elements smaller than 1.0 mm. As an alternative, this article presents an Energy Release Rate-based cohesive method enabling the use of elements significantly larger (up to 5 mm). A novel algorithm is presented to use the Virtual Crack Closure Technique with distorted elements not aligned with the delamination front. When the propagation criterion is met, a cohesive law is introduced to model the progressive crack growth along the newly created crack surface, ensuring to dissipate the correct amount of energy. The method is validated for different propagation tests. Notably, Double Cantilever Beam and End-Notched Flexure tests are accurately modelled with large and distorted elements. Finally, a partially reinforced DCB test demonstrates the ability of the method in representing an evolving delamination front.
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| 24. |
- Främby, Johannes, 1983, et al.
(author)
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An adaptive shell element for explicit dynamic analysis of failure in laminated composites Part 1: Adaptive kinematics and numerical implementation
- 2020
-
In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944. ; 240
-
Journal article (peer-reviewed)abstract
- To introduce more fibre-reinforced polymers in cars, the automotive industry is strongly dependent on efficient modelling tools to predict the correct energy absorption in crash simulations. In this context, an adaptive modelling technique shows great potential. However, as the critical energy absorption in a crash occurs over a very short period of time, and since the deformation behaviour is very complex, car crash simulations are usually performed using explicit dynamic finite element solvers. Therefore, any practical adaptive technique must be adapted to an explicit setting in a software available to the automotive companies. In this paper, we propose an adaptive method for explicit finite element analysis and describe its implementation in the commercial finite element solver LS-DYNA. The method allows for both so-called weak discontinuities (discontinuities in strain), which are crucial for accurate stress and intralaminar damage predictions, and strong discontinuities (discontinuities in displacements), needed for a proper representation of growing delamination cracks. In particular, we detail the implementation of the proposed method into LS-DYNA and also how we propose to remedy the non-physical oscillations arising from the implementation of the adaptive scheme in a explicit dynamic setting. The paper is concluded with numerical examples where we demonstrate the potential for the adaptive approach and also perform a detailed study on its accuracy and stability.
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| 25. |
- Främby, Johannes, 1983, et al.
(author)
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An adaptive shell element for explicit dynamic analysis of failure in laminated composites Part 2: Progressive failure and model validation
- 2021
-
In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944. ; 244
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Journal article (peer-reviewed)abstract
- To enable modelling of the progressive failure of large, laminated composite components under crash or impact loading, it is key to have a numerical methodology that is both efficient and numerically robust. A potential way is to adopt an adaptive method where the structure is initially represented by an equivalent single-layer shell model, which during the analysis is adaptively transformed to a high-resolution layer-wise model in areas where higher accuracy is required. Such a method was recently developed and implemented in the commercial finite element solver LS-DYNA, aiming at explicit crash analysis (Främby, Fagerström and Karlsson: An adaptive shell element for explicit dynamic analysis of failure in laminated composites - Part 1: Adaptive kinematics and numerical implementation, 2020). In the current work, the method is extended to the case of interacting inter- and intralaminar damage evolution. As a key part, we demonstrate the importance of properly regularising the intralaminar failure described by a smeared-crack model, and show that neglecting to account for the crack-versus-mesh orientation may lead to significant errors in the predicted energy dissipation. We also validate the adaptive approach against a four-point beam bending test with matrix-induced delamination growth, and simultaneously show the capability of the proposed method to – at lower computational expense – replicate the results from a refined, non-adaptive model.
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| 26. |
- Salahi Nezhad, Mohammad, 1993, et al.
(author)
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Numerical predictions of crack growth direction in a railhead under contact, bending and thermal loads
- 2022
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In: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944. ; 261
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Journal article (peer-reviewed)abstract
- The effect of different operational loading scenarios on predicted crack growth direction for a propagating inclined railhead crack is assessed by 2D finite element simulations. Studied load scenarios include a moving Hertzian contact load, a temperature drop, rail bending due to a passing wheelset, and combinations thereof. The direction of the unbiased crack propagation is predicted using an accumulative vector crack tip displacement criterion. The numerical model is validated for the individual load scenarios. Restraints due to crack face locking are imposed by a threshold parameter, whose influence is also assessed. For combinations of thermal and contact loads, the predicted crack path is found to diverge gradually from transverse growth, corresponding to pure thermal loading, to shallow growth, corresponding to a pure contact load. For combined bending and contact loading, there is a discrete jump in the predicted crack direction as the contact load increased while the bending load is kept constant. These results are well aligned with empirical experience.
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