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Additive manufacturing (AM) enables fabricating intricate objects with complex geometries previously unattainable through conventional methods. This process encompasses various techniques, including powder bed fusion (PBF), such as selective laser sintering (SLS) and multi-jet fusion (MJF). These techniques involve selectively melting powdered polymer material, predominantly utilizing engineering thermoplastics layer by layer to create solid components. Although their mechanical properties have been extensively characterised, very few works have addressed the influence of additive manufacturing on fracture behaviour. In this context, we present our work demonstrating the presence of anisotropy in fracture behaviour due to the build orientation as well as the PBF methods. To evaluate this anisotropy, the fracture behaviour of polyamide 12 polymer manufactured by SLS and MJF were investigated with experiments and numerical modelling of Mode I compact tension (CT) specimens. Experiments were monitored by digital image correlation (DIC) and infra-red thermography (IRT). Additionally, the fractured surfaces are analysed using scanning electron microscopy. Comparative analyses between SLS and MJF technologies unveiled dissimilar trends in mechanical strength, build-orientation effects, and fracture properties. [ABSTRACT FROM AUTHOR]

This study focuses on the prediction of the fracture mechanics behaviour of a highly flexible adhesive (with a tensile elongation of 90%), since this type of adhesive is becoming widely used in automotive structures due to their high elongation at break and damping capacity. Despite their extensive applications, the understanding of their fracture mechanics behaviour under varying loading rates and temperatures remains limited in the literature. In addition, current prediction models are also unable to accurately predict their behaviour due to the complex failure mechanism that such bonded joints have. This study aims to determine whether a simple triangular cohesive zone model (CZM), which predefines the crack path, can reproduce the load–displacement curves of adhesives under various temperatures and strain rates. To achieve this, a calibrated CZM is used, adapting the model for reference joints and then validating it with independent test results conducted in a wide range of loading and environmental conditions. The tests were performed at speeds between 0.2 and 6000 mm/min and at three different temperatures ranging from −30 °C to 60 °C. Mode I fracture toughness was measured using the DCB (double cantilever beam) specimens. Using a simple triangular CZM may not be optimal for predicting the mechanical response of highly flexible adhesives with complex failure mechanisms and multiple crack paths. However, by correctly adjusting the cohesive zone properties for a limited set of reference conditions, it is possible to accurately predict the mechanical response of these joints across various test speeds and temperatures, significantly reducing costs and effort. [ABSTRACT FROM AUTHOR]

Novel engine fan blades are made of 3D woven composite materials and incorporate a protective metallic layer at the leading edge. The end-of-life of such structures involves complicated disassembly and recycling processes. In this paper, laser-shock is being investigated as an environmentally friendly disassembly method. In this context, symmetric laser-shock experiments that have been conducted in a previous work using a time delay between the shots have been proven successful for debonding initiation and propagation as they manage to effectively focus the tensile stress field developed by the shock waves at the bondline. In this paper, a numerical model simulating the symmetric laser-shock disassembly of titanium/CFRP specimens has been developed using the LS-Dyna explicit FE code. The aim is to give a deeper insight into the physical mechanisms and to optimize the experimental process. To simulate the behavior of the titanium part, the Johnson-Cook material model in combination with Grüneisen equation of state has been employed, while for the composite part a progressive damage material model incorporating high-strain rate effects has been used. The bondline between the two parts has been modeled using cohesive zone elements. To obtain tensile and shear elastic and strength properties at high strain rates for the composite damage model, tension and punch shear Split-Hopkinson Bar tests have been conducted. The numerical results correlate well with back-face velocity profiles obtained from single sided laser shock experiments and with debonding patterns in the adhesive, observed in electron microscopy images, induced by symmetrical laser shock experiments. After the validation, the numerical model has been used successfully to simulate a larger disassembly process composed of 16 consecutive shots. [ABSTRACT FROM AUTHOR]

The escalating importance of lightweight design in engineering demands innovative strategies to tackle this challenge. Traditionally, the joining of these materials involves rivets, bolts, or adhesives. However, contemporary manufacturing techniques, such as 3D printing, present the potential to fabricate joints without the necessity for additional binding mechanisms. This paper delves into a promising initiative concerning the joining of multimaterial systems, specifically composites and metals. The fabrication of the metal component of the joint through additive manufacturing (AM) enables the manipulation of surface geometry by incorporating patterned anchors. This, in turn, facilitates the direct co-curing of the composite onto the modified metallic surface. The primary objective is to enhance mechanical interlocking without relying on traditional fastening elements or adhesives. The study evaluates various anchor geometries to assess their efficacy in increasing the overall joint strength. This assessment employs the cohesive zone modeling (CZM) method to simulate joint specimens, followed by comparative analyses to quantify the strengths of the joints

The double cantilever beam (DCB) specimen is widely used to characterise the mode I fracture of adhesive joints. This paper analyses some particular characteristics of adhesively bonded composite DCB specimens which could affect test results. Three-dimensional (3D) and two-dimensional (2D) finite element analyses (FEA) were conducted in order to evaluate the effects bondline constraint and adherend through-thickness flexibility on the specimen response. Since beam theory-based data reduction schemes are widespread, beam models were also employed to analyze the effects of adherend through-thickness flexibility and fracture process zone relative size. It is shown that, although composite adherends are usually thinner and have much lower transverse moduli than metal adherends, the level of bondline constraint is similarly high. This may: limit the level of adhesive plastic deformations in the fracture process zone; generate high bondline tractions that increase the likelihood of interface failure and interlaminar damage in the composite adherends. The present analyses also show relevant effects of adherend through-thickness flexibility in the adhesive elastic loading stage. Finally, smaller fracture process zones relative to metal adherend DCB specimens were predicted by a beam cohesive zone model. This may explain lower fracture energy values reported with composite adherends in some studies. [ABSTRACT FROM AUTHOR]

This article discusses the use of numerical cohesive zone modeling (CZM) in analyzing self-anchoring additive manufacturing (AM) metal-CFRP joints. The study explores various anchor geometries and their effects on joint strength through numerical analysis using the CZM method. The article also mentions other research in the field of joint configurations, including investigations into mechanical properties, joining processes, fracture toughness, and adhesive characteristics. The authors emphasize the importance of lightweight structures in engineering applications and the need for innovative approaches to achieve durable joint configurations. The document discusses the use of the cohesive zone model (CZM) in analyzing the performance of adhesively bonded joints. The study validates the CZM method through simulations of single lap joints (SLJ) using different adhesives. The results show a correlation between the CZM simulations and experimental parameters, demonstrating the reliability of the methodology. The simulations also reveal that certain anchor geometries and adhesives result in higher failure loads, indicating a more robust joint design. The contact status of the joints is analyzed, and it is observed that complete separation occurs after the propagation of separation passes the second row of anchors. Overall, the CZM proves to be an effective tool for analyzing the performance of adhesively bonded joints. The document discusses the impact of different anchor geometries on joint strength. The study focuses on the assumption that joint failure occurs in cohesive or adhesive mode and aims to identify the optimal geometry for potential cohesive failure. The research evaluates the effectiveness of [Extracted from the article]

Additive manufacturing (AM) enables fabricating intricate objects with complex geometries previously unattainable through conventional methods. This process encompasses various techniques, including powder bed fusion (PBF), such as selective laser sintering (SLS) and multi-jet fusion (MJF). These techniques involve selectively melting powdered polymer material, predominantly utilizing engineering thermoplastics layer by layer to create solid components. Although their mechanical properties have been extensively characterised, very few works have addressed the influence of additive manufacturing on fracture behaviour. In this context, we present our work demonstrating the presence of anisotropy in fracture behaviour due to the build orientation as well as the PBF methods. To evaluate this anisotropy, the fracture behaviour of polyamide 12 polymer manufactured by SLS and MJF were investigated with experiments and numerical modelling of Mode I compact tension (CT) specimens. Experiments were monitored by digital image correlation (DIC) and infra-red thermography (IRT). Additionally, the fractured surfaces are analysed using scanning electron microscopy. Comparative analyses between SLS and MJF technologies unveiled dissimilar trends in mechanical strength, build-orientation effects, and fracture properties.

The augmented finite element method (AFEM) embeds cracks within solid elements. These cracks are modelled without additional degrees of freedom thanks to a dedicated static condensation process. However, static condensation can induce a lack of constraint problem, resulting in singular stiffness matrices. To address this issue, we propose a new method called the stabilised augmented finite element method (SAFEM), which produces non-singular stiffness matrices. We conducted 2D experiments involving stationary traction-free cracks and propagating cohesive discontinuities to compare the performance of the SAFEM with the AFEM. The SAFEM outperforms the AFEM in modelling traction-free cracks.

In the present work, single- and double-dowel joints following different geometric configurations are experimentally and numerically investigated to derive the splitting behaviour of beech wood (Fagus sylvatica L.), one of the most widespread hardwood species in Europe for structural purposes. The influence of the spacing between dowels, their distance to the supports, and the slenderness of the beams is analysed. The correlation of the experimental failure loads with those predicted numerically by cohesive zone finite element-based models using the fracture properties of the species is discussed. The experimental results are also compared with those obtained from the normative expression included in Eurocode 5 and two other design models reported in the literature. The splitting failure loads predicted by both the analytical and numerical models were found to be conservative, the latter being closer to the experimental values. [ABSTRACT FROM AUTHOR]

This study investigates the effects of ground tire rubber (GTR) on the cracking resistance of asphalt mortar from different analysis scales. Furthermore, the effectiveness of different fracture test methods was compared for GTR-modified asphalt mortar, including the indirect tensile cracking test (IDEAL-CT), disk-shaped compact tension test (DCT) and Illinois flexibility index test (I-FIT). To achieve this, a mesoscopic fracture test based on the SEM-Servo Plus device was proposed firstly to obtain the micro-parameters of constitutive models in discrete-element modelling (DEM). A unified fracture modelling procedure that can avoid any interference factors was carried out for comparative fracture analysis. Results from the tests and simulations show that rubber filler plays a positive role in enhancing the fracture properties of the asphalt mortar. The fracture energy measured by IDEAL-CT is almost 5–6 times of the DC(T) test and 1.5–2 times of the I-FIT test. It is not an inherent attribute of material and can only be regarded as a relative index for fracture resistance evaluation. The FI and CTindex obtained from the I-FIT and IDEAL-CT test show a weak correlation of fracture resistance, especially when comparatively evaluating the GTR-modified asphalt mortar with minor difference in filler content. [ABSTRACT FROM AUTHOR]

The augmented finite element method (AFEM) embeds cracks within solid elements. These cracks are modelled without additional degrees of freedom thanks to a dedicated static condensation process. However, static condensation can induce a lack of constraint problem, resulting in singular stiffness matrices. To address this issue, we propose a new method called the stabilised augmented finite element method (SAFEM), which produces non-singular stiffness matrices. We conducted 2D experiments involving stationary traction-free cracks and propagating cohesive discontinuities to compare the performance of the SAFEM with the AFEM. The SAFEM outperforms the AFEM in modelling traction-free cracks. [ABSTRACT FROM AUTHOR]

Epoxies are the most common structural adhesive type in bonded joints. They are viscoelastic materials that show time and temperature dependent behavior under creep. Cohesive zone models (CZM) are often used to model the adhesive bonds, but rarely to model the creep behavior of adhesive bonds. The application of creep load to end notched flexure (ENF) samples leads to a change in the shear fracture energy, which depends on both creep time and creep load. Accordingly, in order to model the creep behavior in ENF joints, fracture energies must be accessed for each time increment along the simulation. For this purpose, the proposition of functions must be addressed, which must contain two variables including creep load level and creep time. The current research presents two different functions to determine the shear fracture energies according to the different creep times and creep loads. The function parameters are obtained by linear and nonlinear regressions. The models were validated and then used in a numerical analysis as part of the adhesive constitutive behavior, using CZM. Very good agreement between the numerical and experimental creep curves was observed. [ABSTRACT FROM AUTHOR]

The integrity of delaminated composite structures can be restored by introducing a thermally-based healing effect on continuous fiber-reinforced thermoplastic composites (CFRTPC). The phenomenon of thermoplastics retaining their properties after melting and consolidation has been applied by heating the delaminated composite plates above their glass transition temperature under pressure. In the current investigation, the composite is comprised of Methyl methacrylate (MMA)-based infusible lamination resin combined with benzoyl peroxide initiator, which polymerizes into a Polymethyl methacrylate (PMMA) matrix. For the reinforcement, unidirectional 220 gr/m2 glass filament fabric was used. Delamination damage is artificially induced during the fabrication of laminate plates. The distributed delamination region before and after thermally activated healing was determined by using non-destructive testing with active thermography. An experimental approach is employed to characterize the thermal healing effect on mechanical properties. Experimentally determined technological parameters for thermal healing have been successfully applied to repair delamination defects on composite plates. Based on the compression-after-impact (CAI) test methodology, the intact, damaged, and healed composite laminates were loaded cyclically to evaluate the healing effect on stiffness and strength. During the CAI test, the 3D digital image correlation (DIC) technique was used to measure the displacement and deformation fields. Experimental results reveal the difference between the behavior of healed and damaged specimens. Additionally, the numerical models of intact, damaged, and healed composite laminates were developed using the finite element code LS-Dyna. Numerical models with calibrated material properties and tie-break contact constants provide good correlation with experimental results and allow for the prediction of the mechanical behavior of intact, damaged, and healed laminated plates. The comparison analysis based on CAI test results and modal characteristics obtained by the 3D Laser Doppler Vibrometer (Polytec GmbH, Karlsbad, Germany) proved that thermal healing partially restores the mechanical properties of damaged laminate plates. In contrast, active thermography does not necessarily indicate a healing effect. [ABSTRACT FROM AUTHOR]

In this work, the fracture behaviour of repaired honeycomb/carbon–epoxy sandwich panels under edgewise compression and three-point bending loading was analysed. Assuming the occurrence of damage resulting from a complete perforation leading to an open hole, the followed repair strategy consists of plug filling the core hole and considering two scarf patches with an angle of 10° in order to repair the damaged skins. Experimental tests were performed on undamaged and repaired situations in order to address the alteration in the failure modes and assess the repair efficiency. It was observed that repair recovers a large part of the mechanical properties of the corresponding undamaged case. Additionally, a three-dimensional finite element analysis incorporating a mixed-mode I + II + III cohesive zone model was performed for the repaired cases. Cohesive elements were considered in the several critical regions prone to damage development. The failure modes and the resultant load–displacement curves obtained numerically were compared with the experimental ones. It was concluded that the numerical model is suitable for estimating the fracture behaviour of sandwich panel repairs. [ABSTRACT FROM AUTHOR]

Cohesive elements are routinely used to simulate delamination between plies in composites. However, the allowable maximum size of these elements is limited, owing to the presence of a very small process zone ahead of the cohesive crack-tip, which needs to be discretised with multiple elements for obtaining an accurate failure response. This restricts their applicability beyond analysing delamination in coupon scale problems, due to the need for a very fine mesh and associated large problem size. In this paper, a novel hp cohesive element is developed to address this problem. This element is capable of simultaneous mesh refinement and elevation of interpolation order, locally and adaptively around the propagating cohesive crack tip. Additional degrees of freedom associated with the hp refinement are handled by extra nodes already supplied in the initial mesh file, that are selectively activated by the program as required. Several example problems are presented showing the ability of this method to improve the prediction of delamination onset and growth in extremely coarse meshes with a reduced computational time. [ABSTRACT FROM AUTHOR]

Bonding steel plate has been used as a strengthening approach to repair disrupted segmental lining of operational tunnels. This paper introduces numerical investigation into the composite behaviour of the initially deformed segmental tunnel linings strengthened by bonding steel plates using finite element modelling. Cohesive zone modelling was used to simulate the interface bonding behaviour between the segmental linings and steel plates. The full history of the tunnel behaviour before and after strengthening were simulated, where the segmental tunnel lining is initially loaded to create some deformation, then after bonding steel plate, the strengthened tunnel is reloaded until failure occurs. By comparing the results with experimental data from the literature, the proposed model was proved to be capable of simulating the strengthened lining behaviour and able to capture the strengthening failure process in terms of the interface debonding. Subsequently, the segmental lining response and interface shear stress distribution and propagation were analysed to interpret the interaction and failure mechanism of the steel plate strengthened segmental linings. The influence of the initial deformation and the steel plate thickness were investigated and discussed in terms of the strengthened stiffness and capacity. It has been found that the interface shear stress concentration occurred at the positions of the segment joints, where bond damage first initiated. The ultimate failure of the steel plate strengthening happened suddenly once a local debonding zone close to the segmental joint was formed. In addition, the predicted results indicate that a delay in strengthening would result in an increase in strengthened capacity but a decrease in strengthened stiffness. By using thicker steel plates, the strengthened stiffness was improved, while the strengthened capacity could be improved only if the thickness was relatively thin.

The advantages of Carbon Fibre Reinforced Polymers (CFRPs) are well established, but repairing CFRP components remains difficult and costly, posing challenges for industries like aerospace. This paper explores the design, modelling, inspection, and testing of a Variable Length Stepped Scarf (VLSS) repair scheme for highly loaded composite structures. A fully nonlinear 2D Finite Element Model (FEM) is used to design the VLSS repair, predict failure loads and modes, and model adhesive cohesion and delamination. The model incorporates a validated progressive damage model, general contact, and both force and geometric nonlinearities. Two manufacturing techniques involving hard repair patches and glass beads to maintain a constant bond line are employed. A 3D FEM validated against repaired composite coupons under uniaxial tension shows excellent agreement with experimental data. The static strength repair efficiency is approximately 80 % of a pristine sample, with failure displacements at 87 %, and Hooke's stiffness at 102 % of pristine laminates. Cohesive failure at adhesive overlap edges is identified as the cause of stiffness degradation, confirming experimental observations. This study contributes to both composite repair modelling and repair design optimisation. [ABSTRACT FROM AUTHOR]

The interest in the design and numerical modelling of adhesively-bonded components and structures for industrial application is increasing as a research topic. Although research on joint failure under pure mode is widespread, applied bonded joints are often subjected to a mixed mode loading at the crack tip, which is more complex than the pure mode and affects joint strength. Failure of these joints under loading is the objective of predictions through mathematical and numerical models, the latter based on the Finite Element Method (FEM), using Cohesive Zone Modelling (CZM). The Single leg bending (bending) testing is among those employed to study mixed mode loading. This work aims to validate the application of FEM-CZM to SLB joints. Thus, the geometries used for experimental testing were reproduced numerically and experimentally obtained properties were employed in these models. Upon the validation of the numerical technique, a parametric study involving the cohesive laws' parameters is performed, identifying the parameters with the most influence on the joint behaviour. As a result, it was possible to numerically model SLB tests of adhesive joints and estimate the mixed-mode behaviour of different adhesives, which enables mixed-mode modelling and design of adhesive structures. [ABSTRACT FROM AUTHOR]

The objective of this work is to determine the fracture energy of a honeycomb/carbon-epoxy sandwich panel under mode II loading using the Asymmetric End-Notched Flexure (AENF) test. Experimental tests and numerical analyses aiming to validate all the procedures were performed. An equivalent crack length data reduction scheme was developed in order to simplify the experimental determination of the Resistance-curves (R -curves) using exclusively the load-displacement (P - δ) data. This strategy makes unnecessary the crack length monitoring during its propagation, which is hard to perform in mode II fracture tests. A mixed-mode I+II trapezoidal cohesive zone model was used for validating the proposed data reduction method. It was concluded that this methodology constitutes a suitable procedure for mode II fracture characterisation of debonding failure in composite sandwich panels. [ABSTRACT FROM AUTHOR]

The asymmetric double cantilever beam (ADCB) test was used to measure the fracture energy of a honeycomb/carbon-epoxy sandwich panel under mode I loading. A data reduction scheme based on equivalent crack length theory was developed for this case. The experimental Resistance-curves were obtained using exclusively data ensuing from the load-displacement curves avoiding the usual and non-rigorous crack length monitoring during the test. Furthermore, a mode partitioning methodology lying on cohesive zone modelling was adopted, aiming to estimate the fracture energy under mode I loading from the total fracture energy under mixed-mode I+II ensuing from the ADCB test. Numerical simulations of the ADCB test considering cohesive zone modelling were performed for the sake of validation of the followed procedure. [ABSTRACT FROM AUTHOR]

In this work, a grain boundary formulation for intergranular and transgranular micro-cracking in three-dimensional polycrystalline aggregates is presented. The formulation is based on the displacement and stress boundary integral equations of solid mechanics and it has the advantage of expressing the polycrystalline problem in terms of grain boundary variables only. The individual grains within the polycrystalline morphology are modelled as generally anisotropic linear elastic domains with random spatial orientation. Transgranular micro-cracking is assumed to occur along specific cleavage planes, whose orientation in space within the grains depend upon the crystallographic lattice. Both intergranular and transgranular micro-cracking are modelled using suitably defined cohesive laws, whose parameters characterise the behaviour of the two mechanisms. The algorithm developed to track the inter/transgranular micro-cracking history is presented and discussed. Several numerical tests involving pseudo-3D and fully 3D morphologies are performed and analysed. The presented numerical results show that the developed formulation is capable of tracking the initiation and evolution of both intergranular and transgranular cracking as well as their competition, thus providing a useful tool for the study of damage micro-mechanics.

• A novel semi-discrete continuum damage model for matrix cracking in composites is presented. • The damage model uses a kinematically enriched two-scale constitutive law. • A geometric algorithm preserves discrete nature of cracks and enables mesh-independent crack growth. • A known pathological problem of traditional continuum damage models under large deformation is eliminated. • Accurate modelling of complex and interactive progressive damage in benchmark simulations is presented. Continuum Damage Mechanics models are widely used for simulation of ply-level damage such as matrix cracking in composites. Although their ease of implementation within the finite element framework is appealing, several limitations exist such as a smeared representation of sharp cracks, mesh orientation bias of crack growth, inaccurate energy dissipation due to approximate definition of characteristic length etc. In this work, a new 'semi-discrete' continuum damage model is proposed for matrix cracking, that includes sharp crack kinematics. The mesh orientation bias is eliminated by a fibre-directed growth driving criterion. The crack characteristic length is exactly computed due to an accurate geometric representation of the crack surface. The model also addresses a well-known problem of spurious stress transfer across crack faces when deformation becomes large. Through several challenging benchmark problems, the model is shown to perform nearly as good as discrete crack models while being simpler and more robust. [ABSTRACT FROM AUTHOR]

Delaminations are a concern for the structural integrity of fibre composite structures. The decisive material parameter for delaminations is fracture toughness. Structures usually have a multidirectional ply stacking. However, interface orientation dependant fracture toughness values and R-curve effects are neglected in numerical delamination analysis. This work investigates interface orientation specific fracture toughness values and considers R-curve effects in mode I to improve simulation accuracy. Therefore, mode I, mode II and mixed mode fracture toughness values of different interface ply orientations are determined experimentally and verified using numerical analysis of the characterisation specimens with cohesive zone modelling. In this way, an engineering methodology is provided for the experimental characterisation and comprehensive numerical modelling of delaminations in mesoscale progressive damage analysis of multidirectional composite structures. In addition, a full parameter set for the simulation of four different interfaces of M21-T700GC prepreg material is given. It can be shown that the use of standard 0°//0°-values leads to very conservative results. The use of interface specific values increases the accuracy. Ultrasonic scans of the DCB specimens are used to compare the crack front shapes for validation. Not only the load displacement curves of the characterisation specimens are well captured, but also the crack front shapes. This demonstrates that by smearing the microscale effects, the material behaviour can be captured phenomenologically correct by mesoscale modelling suitable for industrial use.

Mixed-mode delamination is one of the common failures of composites which has not been studied under low-impact loading. This paper studies the influence of displacement rate on mixed-mode I/II delamination of unidirectional carbon/epoxy composites. Single leg bending test is performed at displacement rates of 1, 10, 100, and 500 mm/min. Experimental results reveal that the mixed-mode I/II fracture toughness is invariant with the displacement rate. In addition, scanning electron micrographs shows that shear cusps are more obvious at 1, 10, and 100 mm/min. At 500 mm/min, significant matrix debris is noticed. Furthermore, the proposed three-dimensional rate-dependent fracture criterion is found to well predict the fracture toughness. Numerical simulation using cohesive zone model suggests that the lower numerical peak load is due to lower damage dissipated energy. In addition, the theoretical and numerical traction-separation responses show significant differences, which is also reflected in the numerical phase angle. This implies that the local mixed-mode ratio is not constant throughout the simulation process.

Fracture arising from cracks nucleating and propagating along twin boundaries is commonly observed in metals that exhibit twinning as a plastic deformation mechanism. This phenomenon affects the failure of macroscopic mechanical components, but it is not fully understood. We present simulations in which a continuum model for discrete twins and a cohesive zone model are coupled to aid the understanding of fracture at twin boundaries. The interaction between different twin systems is modelled using a local term that depends on the continuum twin variables. Simulations reveal that the resolved shear stress necessary for an incident twin to propagate through a barrier twin can be up to eight times the resolved shear stress for twin nucleation. Interface elements are used at the interfaces between all bulk elements to simulate arbitrary intragranular cracks. An algorithm to detect twin interfaces is developed and their strength has been calibrated to give good agreement with the experimentally observed fracture path. The elasto-plastic deformation induced by discrete twins is modelled using the crystal plasticity finite element method and the stress induced by twin tips is captured. The tensile stress caused by the tip of an incident twin on a barrier twin is sufficient to nucleate a crack. A typical staircase fracture path, with cracks propagating along the twin interfaces, is reproduced only if the strength of the twin interfaces is decreased to about one-third of the strength of the bulk material. This model can be used to help understand fracture caused by the activation of multiple twin systems in different materials. [ABSTRACT FROM AUTHOR]

The present work aims to assess the joint behaviour of friction stir spot welded sandwich sheets by changing the quality of the epoxy core layer. Lab scale experiments and numerical simulations are conducted for the purpose. The core property is altered by varying hardener to resin (h/r) ratio within a suitable range. Joint mechanical performance and joint characterization are evaluated from experiments. Cohesive zone modelling has been incorporated in Abaqus to monitor the hook formation and interface delamination, which is not performed until now in literature. The core quality influences the behaviour of sandwich sheet and hook geometry significantly. Desired h/r ratio is also evaluated from such results, and FSSW is found advantageous at some h/r ratios. The failure mode is independent of h/r ratio and loading conditions. The final joint shape and hook geometry obtained from FE simulations agree well with that of from experiments. Cohesive zone modelling helped in accurate prediction of delamination, and without it, the hook geometry and plastic energy dissipation predictions are approximated. It has been suggested to incorporate cohesive zone model during FE simulations to have a realistic prediction of sandwich joint performance. [ABSTRACT FROM AUTHOR]

Bonded composite scarf joints with bonding flaws were tested to study their tensile behaviors. Based on the failure modes obtained by various observation methods, an improved numerical methodology with appropriate model width was developed, considering the marginal low stiffness regions in ± 45° plies. The results show that the modelling approach provides accurate predictions on the strength, stiffness, and the failure modes considering variations in scarf angle, flaw size, and flaw location. Marginal low stiffness regions in ± 45° plies influence the stress distributions in the adhesive layer and the failure mode. Adhesive layer failure is the main cause of the final fracture of the pristine and the defective scarf joints, and damages within composite adherend especially interlaminar delamination, may accelerate the growth of the bondline stress at an early stage. The traditional damage tolerance design approach for bonded composite joints needs to be improved to avoid confusing and adventurous results. [ABSTRACT FROM AUTHOR]

Highlights: Dynamic ductile fracture propagation of pipeline steels was studied using 3D CZM.JIS STPG370 steel was examined.CZM calibrated for a small‐scale specimen (DWTT) transferred to large‐scale pipes.Calibrated CZMs can be used to reproduce experimental data with good agreements.The fracture velocities and the CTOAs are in good agreements. [ABSTRACT FROM AUTHOR]

• Honeycomb core sandwich sheets made by FSSW with disc insert and adhesive bonding. • Modeling attempted using homogenized core and equivalent cohesive layer. • Cohesive zone modeling attempted using actual core and adhesive layer. • Hybrid joint (FSSW with disc insert + adhesive bonding) performed better in peel test. • Peel test numerical predictions agreed well with experimental values. The present work examines the fabrication and numerical modelling of honeycomb core sandwich sheets utilizing Friction Stir Spot Welding (FSSW) techniques as prospective substitutes for adhesive-bonded structures. Here, the sandwich sheet contains AA5052-H32 skins and AA3003 honeycomb core. Two strategies, FSSW with disc insert (FSSW_D) and FSSW with disc insert and adhesive bonding (FSSW_D_AB), were evaluated against traditional adhesive-bonded sandwiches using peel test performance and finite element (FE) simulation with cohesive zone modelling (CZM). In the CZM, a homogenized core with an equivalent cohesive layer was substituted for the honeycomb core and cohesive layer. The utilization of FSSW_D and FSSW_D_AB methods resulted in substantial improvements in maximum load capacity, with enhancements in the hybrid sandwich specimen ranging from 254 % to 399 % compared to adhesive-bonded connections. The maximum load of hybrid joints (FSSW_D_AB) was 15 %–23 % higher than that of ordinary spot-welded joints (FSSW_D). An approach to model the hybrid sandwich specimen using both the homogenized core and equivalent cohesive zone model accurately was proposed. FE analysis, including peel tests of adhesive-bonded, spot-welded, and hybrid joints, was performed, and the numerical predictions agreed satisfactorily with the experimental values for all the joint types. [ABSTRACT FROM AUTHOR]

Composite stepped repairs can achieve high strength recovery without the addition of bolts or fasteners to the structure. They are therefore a major issue in the field of aerospace composite structure damage repair. However, there is no standardized method to design this type of repairs. Many analytical, semi-analytical and finite element models were proposed throughout the years to predict the strength of stepped repairs, using various hypotheses and simplifications. The aim of this paper is to investigate the influence of modelling hypotheses on stress distribution and strength prediction of composite stepped repairs. Five simplified stepped joint models using macro-element (ME) modelling and finite element method (FE) are compared to a full 3D FE model of a stepped repaired panel. The influence of step length and adhesive fracture toughness was investigated to determine the field of validity of each model. Among FE models, it was shown that modelling the equivalent joint under 2D generalized plain strain gives a very close strength prediction to the 3D stepped repair model while saving computation time. Simplified macro-element models under bar or beam hypotheses are fairly close to the results of FE modelling, but the deviation between those and FE is sensitive to step length and adhesive fracture toughness. [ABSTRACT FROM AUTHOR]

This data article provides useful information often required for numerical modeling of the so-called microbond tests. It includes the experimental and simulation data of the microbond testing using Fibre Bragg Grating (FBG) fibres for optical strains. Microbond testing was performed on five different droplets of varying embedded length and diameter to collect the data. Finite element simulation was carried out and modelling was validated, by using two variables force and strain, to collect the data. The output data of the fitted models is given and is also visualized via graphs of force-strain derivative curves. The data of the simulations is provided for different finite element mesh densities. Here, to clarify the type and form of the data for the use by readers, the energy distribution curves describing various functionalities of the droplet, fibre and interface are presented. For further reading, the interpretation and analysis of this data can be found in a research article titled “3D interfacial debonding during microbond testing: Advantages of local strain recording” (R. Dsouza et al., 2020) [1].

The interplay between twinning and fracture in metals under deformation is an open question. The plastic strain concentration created by twin bands can induce large stresses on the grain boundaries. We present simulations in which a continuum model describing discrete twins is coupled with a crystal plasticity finite element model and a cohesive zone model for intergranular fracture. The discrete twin model can predict twin nucleation, propagation, growth and the correct twin thickness. Therefore, the plastic strain concentration in the twin band can be modelled. The cohesive zone model is based on a bilinear traction-separation law in which the damage is caused by the normal stress on the grain boundary. An algorithm is developed to generate interface elements at the grain boundaries that satisfy the traction-separation law. The model is calibrated by comparing polycrystal simulations with the experimentally observed strain to failure and maximum stress. The dynamics of twin and crack nucleation have been investigated. First, twins nucleate and propagate in a grain, then, microcracks form near the intersection between twin tips and grain boundaries. Microcracks appear at multiple locations before merging. A propagating crack can nucleate additional twins starting from the grain boundary, a few micrometres away from the original crack nucleation site. This model can be used to understand which type of texture is more resistant against crack nucleation and propagation in cast metals in which twinning is a deformation mechanism. The code is available online at https://github.com/TarletonGroup/CrystalPlasticity. [ABSTRACT FROM AUTHOR]

Understanding the fatigue response of adhesive joints under cyclic shear stress is important to avoid fatigue failure of the bonded structures. Recently, a cohesive zone modelling (CZM) technique was developed where the cohesive properties of the elements were degraded by a proposed empirical relation. However, based on this degradation approach, the fatigue life of the joints should be known before running the analysis. The aim of the current study is to improve the numerical method in which the fatigue life of the joints will be automatically estimated during the analysis. To achieve this, the concepts of fracture mechanics are considered by using the Paris' law combined with the degradation model. A user material subroutine is developed to conduct the numerical calculations based on the concepts of CZM. End notched flexure (ENF) tests were carried out to evaluate the numerical data. Based on the results, it was found that the modified approach, in which the total fatigue life is obtained automatically, can be employed for fatigue life assessment of adhesive joints subjected to cyclic shear stresses. [ABSTRACT FROM AUTHOR]

In modern lightweight structures, the use of fasteners is preferred to other joining techniques. An approach to increase the bearing strength is the local metal hybridisation, where carbon fibre-reinforced plastics layers are substituted locally by metal foils of the same thickness. The local replacement leads to a transition zone between the hybrid region and the pure carbon fibre-reinforced plastics region. The present work deals with the investigation of different transition zone patterns of carbon fibre-reinforced plastics-steel hybrid specimens in combined tension–bending tests and accompanying non-linear static simulation. The simulation includes delamination and intralaminar damage with the use of a cohesive zone model and Cuntze's failure mode concept. Furthermore, residual thermal stresses are considered. A satisfying agreement of test and simulation is achieved, which allows the identification of beneficial transition zone configurations and also validates the numerical model for further parametric studies. [ABSTRACT FROM AUTHOR]

This work addresses fracture characterisation of composite bonded joints under mixed-mode I+II loading. A proper strategy, including only two different loading conditions applied to five tests, is performed to obtain the fracture law describing the mixed-mode fracture behaviour. The peel test loading condition is employed to characterise fracture under pure mode I loading considering a symmetrical double-cantilever beam specimen and under mixed-mode I+II with mode I supremacy with an asymmetrical double-cantilever beam specimen. Three-point bending loading condition was also employed. The end-notched flexure test was used to characterise fracture under pure mode II, while the asymmetric single-leg bending tests were performed to cover mixed-mode I+II loading with mode II predominance up to a mode-mixity with moderate mode I prevalence. A methodology using cohesive zone modelling was applied to perform loading mode partition, which is crucial for an appropriate identification of the criterion describing the fracture behaviour of these joints. [ABSTRACT FROM AUTHOR]

• Interface properties of second phase particles in AA7075 matrix are calculated using molecular dynamics (MD) under various loading conditions. A methodology to estimate quasistatic cohesive properties is introduced. • The cohesive properties, extracted from MD simulations, are implemented in single particle and two particle unit cell finite element (FE) models along with idealized particle field (IPF) FE models to assess the effect of morphology and interparticle alignment. Insights from MD simulations, unit cell FE models and IPF FE models are used to interpret decohesion in a real microstructure using in-situ SEM uniaxial tensile tests and corresponding real particle field FE models. • Larger particles or those with smaller radial dimensions along the loading direction result in increased normal stresses at the interface, aiding decohesion. Interparticle alignment of 0 degrees promotes elevated normal stresses, while interparticle alignment of 45 degrees is most conducive for development of elevated shear stresses. As a result, particles aligned at 0–45 degrees are likelier to undergo decohesion. • Void created by a decohered particle facilitates local matrix flow, leading to elevated shear stress at interfaces of neighboring particles. As a result, neighboring particles can also undergo decohesion as interfaces of all second phase particles are weaker under shear loading. • Due to its larger size and more irregular morphology, θ precipitates are 10 times more likely to undergo decohesion than η precipitates. However, when the effect of size and morphology is removed, θ precipitates undergoing decohesion is only 1.7 times more than η precipitates, thereby underlying the significance of size and morphology. This research investigates stress evolution and plastic deformation characteristics influencing particle–matrix interface decohesion in AA7075-O aluminum sheets. Employing a combined molecular dynamics (MD) - finite element (FE) approach, three interfaces (Al-η, Al-θ, Al-Fe-rich intermetallic) are studied under various loading conditions. Cohesive properties, represented by traction-separation (T-S) curves, are derived using a novel methodology from MD simulations to estimate critical peak traction (t) initiating decohesion and the work of separation (G) at quasistatic strain rates. It is shown that all three interfaces are weaker in shear than in normal loading. The cohesive property estimates from MD simulations are utilized as input in FE-based models with both real and simplified microstructures of AA7075-O sheet material. These models are subjected to large plastic strains, and the decohesion behavior of each particle type are analyzed. It is shown that particle decohesion is a function of inherent cohesive properties, local inter-particle alignment with respect to loading direction, particle morphology and particle size. Interparticle alignment between 0 and 45 degree promote particle decohesion. Larger sized particles with smaller radial dimensions along loading direction aid early decohesion of particles. Decohesion of a particle can also facilitate debonding of neighbouring particles under continued loading. Fe-rich particles have higher likelihood for decohesion due to their weaker interface. The θ precipitates, despite having comparable interface strength as η particles, manifest a tenfold increase in susceptibility to decohesion due to the effect of particle morphology and size. [ABSTRACT FROM AUTHOR]

Injection-moulded short-fibre composites are lightweight materials suitable for high-volume applications; however, current simulation methods (based on failure initiation criteria) to design components using these materials cannot yet accurately predict failure. This work presents a methodology to predict failure of injection-moulded short-glass-fibre reinforced thermoplastic (IM-SFRP) composite subcomponents, based on experimentally measured properties. The material's fracture toughness was characterised by Compact Tension tests for different fibre orientations and environmental conditions. These fracture toughnesses were used as the input for cohesive zone modelling in Finite Element simulations of subcomponents representative of automotive applications, coupled with fibre orientation fields predicted by an injection-moulding process simulation. These coupled simulations presented excellent agreement with the experimental results for subcomponents both in terms of (i) the peak load (highlighting the importance of accounting for the finite fracture toughness of the material to accurately predict the ultimate failure of the subcomponents), and (ii) the pre- and post-peak sequence of failure events (verified using fractographic analyses). This work also verified the applicability of temperature-moisture equivalence, not only for material characterisation using coupons including the material's fracture toughness, but also for the mechanical response of subcomponents until final failure. The methodology demonstrated in this paper contributes to designing safer and more efficient damage-tolerant IM-SFRP components. [Display omitted] • Toughness was measured for several fibre orientations, temperatures & moisture levels. • Subcomponents representative of automotive applications were tested and simulated. • Coupled FEA using cohesive zone modelling predicted failure load within a 2.6% error. • FE predicted a sequence of failure events consistent with fractographic results. • Temperature-moisture equivalence was verified at coupon and subcomponent levels. [ABSTRACT FROM AUTHOR]

Cohesive zone modelling is a common approach to capture delamination in composite laminate structures. Recent experimental advancements now enable the direct measurement of Mode I traction-separation responses (TSRs) from a single specimen using the composite rigid double cantilever beam (cRDCB), overcoming a major obstacle in using cohesive zone modelling to model delamination. However, TSRs measured experimentally with the cRDCB specimen capture damage response as well as the stiffness contribution of the adjacent laminae, which can introduce significant artificial compliance into numerical models when modelling delamination separately from intralaminar behaviour. A two-stage analysis procedure utilizing a crack tip compensation function is presented to enhance the TSRs measured with the cRDCB specimen to accurately model Mode I delamination. The analysis procedure is demonstrated to improve the accuracy of delamination prediction within the statistical variation of published experimental data. Furthermore, the transferability of TSRs measured with cRDCB specimens is explored using available experimental DCB data. It is shown that the onset of damage and early damage behaviours measured with the cRDCB specimen appear to be transferable between geometries, whilst large-scale damage mechanics remain geometry dependent. [ABSTRACT FROM AUTHOR]

We show here, based on VAlN, TiAlN and the related oxynitrides, that the (brittle) fracture and elastic properties may be consistently modelled from quantum- to continuum mechanics using micromechanical testing to link both scales. The measured elastic moduli match closely with those predicted by density functional theory calculations. Good agreement was also observed between the micro-cantilever bending experiments and cohesive-zone-finite element modelling. These scale-bridging data serve as a baseline for future improvements of the fracture toughness of these coating systems based on microstructure and coating architecture optimization.

Adhesives are increasingly being used in the aerospace and automotive industries. They allow for light weight vehicles, fuel savings and reduced emissions. However, the environmental degradation of adhesive joints is a major setback in its wider implementation. Moisture degradation of adhesive joints includes plasticization, attacking of the interface, swelling of the adhesive and consequent creation of residual stresses. The main factors affecting the strength of adhesive joints under high and low temperatures are the degradation of the adhesive mechanical properties and the creation of residual stresses. To model the long term mechanical behaviour of adhesive joints, the temperature and moisture dependent properties of the adhesives must be known. However, few studies focus on the combined moisture and temperature degradation, which difficults the prediction of the long term mechanical behaviour of these joints. In this study the prediction of moisture and temperature dependent cohesive properties of a structural adhesive is analysed.

In the experimental studies of mode III delamination using the edge crack torsion test, the crack initiation and propagation measurement are always difficult. This information could be obtained through numerical modelling. The objective of this study is to propose a guideline to model mode III delamination behaviour using cohesive elements. Finite element models of an edge crack torsion specimen were developed based on the data from the literature. The delamination behaviour of the specimen along the pre-crack, which was located at the mid-thickness location, was modelled using cohesive elements. Through parametric studies, it was found that for reliable numerical modelling, a mesh size of 0.5 mm was suggested, which provided three elements in the cohesive zone. As for the interface strength, it was recommended to choose 80 MPa. In addition, a viscosity parameter of 110-3 was found to be a good choice for reasonable computational time and converged numerical results. Besides, the interface stiffness was suggested to be 4106 MPa/mm. Furthermore, the fracture process zone contour revealed that the delamination was started at a normalised location of approximately 0.7. Not only that, the fracture energy and strain distribution plots have shown the delamination was mode III dominated within the normalised distance of 0.34-0.86. The results from this study suggested that cohesive zone modelling is a useful method for the detailed analysis of the mode III delamination of an ECT specimen. The numerical modelling approach suggested from this study could be applied to ECT specimens at various different initial crack lengths. It also has the potential to be used to simulate the mode III delamination of other various types of laminated composites.