Cohesive Zone Simulation Research Articles

Prediction of Propagation of a Pre-existing Crack in the Hardfaced Coating Made on Austenitic Stainless Steel Plate Under Quasi-static Loading Using Cohesive Zone Simulation

Prediction of Propagation of a Pre-existing Crack in the Hardfaced Coating Made on Austenitic Stainless Steel Plate Under Quasi-static Loading Using Cohesive Zone Simulation

Investigation of buckling of laminated composites repaired by resin injection using acoustic emission and cohesive zone simulation method

Due to their low weight ratio, excellent damping behavior, and high strength characteristics, composite materials are widely used in different industries, including aerospace. Despite the excellent mechanical behavior, most of these materials suffer from damages such as delamination when undergoing low-energy impacts. In the present study, repairing the composite structures using the resin injection into the damaged zone is analyzed. In some samples, initial delamination is included during the manufacturing process using the Teflon film, while the other samples are left to be intact. Furthermore, the delamination inside the composite sample has been simulated using the cohesive zone method (CZM) using ABAQUS software. The samples with delamination were repaired with the resin injection method. Finally, all samples underwent the buckling test, and the buckling process of the samples was monitored using the acoustic emission method. The amounts of damage mechanisms of delamination growth, fiber breakage, and matrix cracking were calculated using the wavelet method. The results show a good agreement between the experimental results with both the finite element method (FEM) and the acoustic emission results. Besides, the restoration percentages of compressive strength in samples repaired with stacking sequence [0]8, [90]8, and [0/90]8 were 88%, 87%, and 91.7%, respectively.

Detachment Stress Calculation of Projection Stereolithography based on Cohesive Zone Simulation

Detachment Stress Calculation of Projection Stereolithography based on Cohesive Zone Simulation

Understanding interfacial fracture behavior between microinterlocked soft layers using physics-based cohesive zone modeling.

We examine the underlying fracture mechanics of the human skin dermal-epidermal layer's microinterlocks using a physics-based cohesive zone finite-element model. Using microfabrication techniques, we fabricated highly dense arrays of spherical microstructures of radius ≈50μm without and with undercuts, which occur in an open spherical cavity whose centroid lies below the microstructure surface to create microinterlocks in polydimethylsiloxane layers. From experimental peel tests, we find that the maximum density microinterlocks without and with undercuts enable the respective ≈4-fold and ≈5-fold increase in adhesion strength as compared to the plain layers. Critical visualization of the single microinterlock fracture from the cohesive zone model reveals a contact interaction-based phenomena where the primary propagating crack is arrested and the secondary crack is initiated in the microinterlocked area. Strain energy energetics confirmed significantly lower strain energy dissipation for the microinterlock with the undercut as compared to its nonundercut counterpart. These phenomena are completely absent in a plain interface fracture where the fracture propagates catastrophically without any arrests. These events confirm the difference in the experimental results corroborated by the Cook-Gordon mechanism. The findings from the cohesive zone simulation provide deeper insights into soft microinterlock fracture mechanics that could prominently help in the rational designing of sutureless skin grafts and electronic skin.

Variations on R-curves and traction-separation relations in DCB specimens loaded under end opening forces or pure moments

In this work the differences in R-curve response and traction-separation relations due to a finite damage zone or large-scale bridging (LSB) in mode I fracture on double cantilever beams (DCB) were investigated under end opening forces (EOF) and pure moments (PM). To realize PM loading, a novel test-rig was designed, built and used to test adhesive joints with metallic substrates and unidirectional carbon fiber reinforced polymer (CFRP) specimens. In the adhesive joints, no pronounced difference was observed between the R-curves due to the two loading conditions. However, the experiments on the CFRP specimens showed important differences on both R-curve and traction separation relations due to LSB when comparing EOF and PM loading. These differences are attributed to the different curvature of the specimens’ arms in EOF and PM allowing for longer bridging zone in the latter case. The increase in the energy release rates was ~40% for the specimens subjected to PM with about four times higher maximum crack opening displacements at the steady state. Pertinent cohesive zone simulations predict the respone very well.

Cohesive zone simulation of grain size and misorientation effects on hydrogen embrittlement in nickel

Abstract The size and misorientation effects on hydrogen embrittlement of a four grain nickel aggregate are studied with the help of hydrogen informed cohesive zone model. The grain misorientation angle is parameterized by fixing the lower grains while rotating the upper grains about the out-of-plane axis. Brittle failure of the grain aggregate is observed and nominal strength obtained. In the crack-free situation, the grain misorientation exerts an obvious weakening effect on the nominal strength, which is most pronounced at misorientation angles around 20°. Such trend applies to the pre-cracked situation but is much less pronounced. Both misorientation and pre-crack lead to size effect. The nominal strength shows a decreasing trend with the grain size, indicating that grain refinement tends to improve the load bearing capacity, which coincides with the observation in practice. Further, it is shown that the size effect diagram without hydrogen can be divided into three regimes. The conclusions apply to the case with hydrogen, except that the trend of the size effect curve can be affected by large grain sizes due to the longer absolute distance of hydrogen diffusion. These results provide guidelines for grain boundary engineering and for nanomechanical tests aiming at calibrating the intergranular decohesion parameters.

Continuum level simulation of the grain size and misorientation effects on hydrogen embrittlement in nickel

This paper addresses the size and misorientation effects on hydrogen embrittlement of a four grain nickel aggregate. The grain interior is modelled with orthotropic elasticity and the grain boundary with cohesive zone technique. The grain misorientation angle is parameterized by fixing the lower grains and rotating the upper grains about the out-of-plane axis. The hydrogen effect is accounted for via the three-step hydrogen informed cohesive zone simulation. The grain misorientation exerts an obvious weakening effect on the ultimate strength of the nickel aggregate which reaches its peak at misorientation angles around 20◦, but such effect becomes less pronounced in the case with a pre-crack. The misorientation could induce size effect in the otherwise size independent case without a pre-crack. The contribution of misorientation to the size effect is negligible compare to that caused by the existence of a pre-crack. These findings indicate that the misorientation effect in cases with a deep pre-crack is weaker than expected in shallow-pre-crack situations. Most of these conclusions hold for the hydrogen charging situation except that the ultimate strength is lowered in all the sub-cases due to hydrogen embrittlement. Interestingly, it is observed that the size effect becomes less pronounced with hydrogen taken into account, which is caused by the fact that hydrogen takes more time to reach the failure initiation site in larger grains.

Hydrogen embrittlement in nickel, visited by first principles modeling, cohesive zone simulation and nanomechanical testing

Hydrogen (H) can have dramatic consequences on material properties, especially by reducing the fracture toughness. Degradation by H initiates through mechanisms at the nano-scale, and is normally not detectable prior to the final leakage or component fracture. Computational techniques are therefore being developed in order to provide both a wider understanding of the phenomenon and engineering tools for prediction of materials susceptibility toward hydrogen embrittlement. This work presents a preliminary study for the development of a novel computational approach in which density functional theory (DFT), nanoscale experiments and finite element (FE) modeling are combined and interrelated in order to improve the understanding of hydrogen induced intergranular cracking. Two low angle and low coincidence grain boundaries types have been considered: Σ3 and Σ5. Density Functional Theory has been applied to investigate the influence of an increasing number of H atoms on the cohesive strength of these grain boundaries in pure Nickel. This provided relations between the number of H atoms (coverage of the grain boundary) and the cohesive strength, which is further applied in cohesive zone FE modeling of a triangular nanometer sized fracture mechanics cantilever beam. For verification of the model such specimens will be tested experimentally both with and without in-situ electrochemical charging.The simulation results show that the model is suitable for describing the combined effect of grain boundary misorientation and the reduced cohesive energy due to hydrogen on the grain boundary propensity to cleave.

The influence of a brittle Cr interlayer on the deformation behavior of thin Cu films on flexible substrates: Experiment and model

Thin metal films deposited on polymer substrates are used in flexible electronic devices such as flexible displays or printed memories. They are often fabricated as complicated multilayer structures. Understanding the mechanical behavior of the interface between the metal film and the substrate as well as the process of crack formation under global tension is important for producing reliable devices. In the present work, the deformation behavior of copper films (50–200nm thick), bonded to polyimide directly or via a 10nm chromium interlayer, is investigated by experimental analysis and computational simulations. The influence of the various copper film thicknesses and the usage of a brittle interlayer on the crack density as well as on the stress magnitude in the copper after saturation of the cracking process are studied with in situ tensile tests in a synchrotron and under an atomic force microscope. From the computational point of view, the evolution of the crack pattern is modeled as a stochastic process via finite element based cohesive zone simulations. Both, experiments and simulations show that the chromium interlayer dominates the deformation behavior. The interlayer forms cracks that induce a stress concentration in the overlying copper film. This behavior is more pronounced in the 50nm than in the 200nm copper films.

Buckle Driven Delamination in Thin Hard Film Compliant Substrate Systems

Deformation and fracture of thin films on compliant substrates are key factors constraining the performance of emerging flexible substrate devices. [1-3] These systems often contain layers of thin polymer, ceramic and metallic films and stretchable interconnects where differing properties induce high normal and shear stresses. [4] As long as the films remain bonded to the substrates, they may deform far beyond their freestanding form. Once debonded, substrate constraint disappears leading to film failure. [3] Experimentally it is very difficult to measure properties in these systems at sub-micron and nanoscales. Theoretically it is very difficult to determine the contributions from the films, interfaces, and substrates. As a result our understanding of deformation and fracture behavior in compliant substrate systems is limited. This motivated a study of buckle driven delamination of thin hard tungsten films on pure PMMA substrates. The films were sputter deposited to thicknesses of 100 nm, 200 nm, and 400 nm with a residual compressive stress of 1.7 GPa. An aluminum oxide interlayer was added on several samples to alter interfacial composition. Buckles formed spontaneously on the PMMA substrates following film deposition. On films without the aluminum oxide interlayer, an extensive network of small telephone cord buckles formed following deposition, interspersed with regions of larger telephone cord buckles. (Figure 1) On films with an aluminum oxide interlayer, telephone cord buckles formed creating a uniform widely spaced pattern. Through-substrate optical observations revealed matching buckle patterns along the film-substrate interface indicating that delamination occurred for large and small buckles with and without an interlayer. The coexistence of large and small buckles on the same substrate led to two distinct behaviors as shown in Figure 2 where normalized buckle heights are plotted against normalized film stress. The behaviors deviate significantly from behavior predicted by rigid elastic solutions. To address this issue we developed a finite element analysis technique that employed a cohesive zone model to simulate interfacial crack growth. Specifying the traction-separation relationship, cohesive strength, and work of separation along with film thickness, film stress, and film and substrate properties, buckle width and height were determined as a function of interfacial toughness. The simulations indicate that an analysis based on rigid substrate solutions significantly underestimate toughness for prescribed buckle widths: a result consistent with an analysis by Yu and Hutchinson [5] that pieced together a solution based on non-linear plate theory with a solution for the linear film on substrate problem. More importantly, the results defined a lower limiting bound to seemingly disparate buckle deflection data (Figure 2). The variance from linear elastic behavior, especially for the small buckles, indicates more than substrate compliance is controlling behavior. Comparison of the experimental results with cohesive zone simulations suggests that the two buckle behaviors are associated with different levels of substrate yielding. In this presentation we will use the results to show how substrate compliance and deformation affect delamination and buckling of films on compliant substrates and provide a means to predict device performance.

Influence of hydrogen from cathodic protection on the fracture susceptibility of 25%Cr duplex stainless steel – Constant load SENT testing and FE-modelling using hydrogen influenced cohesive zone elements

Laboratory experiments and cohesive zone simulation of Hydrogen Induced Stress Cracking in SENT tests specimens of 25%Cr duplex stainless steel have been performed. A polynomial formulation of the traction separation law and hydrogen dependent critical stress was applied. Best agreement with the experiments was found for an initial critical stress of 2200MPa and a critical separation of 0.005mm. Proposed threshold stress intensity factor and lower bound net section stress is 20MPa√m and 480MPa. High crack growth rates and typical hydrogen influenced fracture topography suggest large influence of the stress and strain in the fracture process zone on the hydrogen diffusion rate.

Cohesive zone simulations of crack growth along a rough interface between two elastic–plastic solids

It has been known for many years that crack propagation along interfaces is influenced by interface topography or roughness profile. This has given rise to a small body of literature in which interface toughening with stochastic surface finishes, produced by grinding, rolling, or grit blasting, has been the primary focus. However, there is very little information currently available on the effect of patterned interfaces that are characterized by a minimal number of geometric parameters. In the present article, roughness-enhanced toughening of a cohesive interface between two identical materials is explored with a pure sinusoidal interface morphology that is characterized by its aspect ratio or ratio of amplitude to wavelength. Sixteen finite element meshes, each with a different aspect ratio, were constructed to study initiation and growth of a semi-infinite interface crack due to remote mode-I loading. The cohesive interface was modeled with a viscosity-modified Xu–Needleman cohesive zone law, and the solids were characterized with continuum elastic and elastic–plastic constitutive models. Predicted relationships between the aspect ratio and the macroscopic toughness point to key differences in the material models. A set of critical parameters which include the aspect ratio, material and cohesive properties is predicted such that catastrophic crack growth is inhibited due to crack blunting. A clear boundary between brittle and ductile fracture behavior is thus identified. The results suggest some guidelines for practical design of failure resistant interfaces through appropriate choice of geometric, material, and cohesive parameters.