Mechanism Of Void Growth Research Articles

Embrittlement of notched duplex stainless steel: Role of hydrogen-assisted void growth and cleavage

The micro-degradation processes related to the ductile-brittle transition are crucial for revealing hydrogen embrittlement mechanisms. This study investigates the evolution and transition of hydrogen-assisted damages in notched duplex stainless steel using a combination of microstructural characterization, numerical simulations, and mechanistic models. The results show that the brittle area of the fracture surface increases rapidly with decreasing cathodic potential, accompanied by the fractography transition in a sequence of voids, voids mixed with quasi-cleavage, and quasi-cleavage. While hydrogen-assisted dislocation emission from the void surface and stress concentration at the phase boundary promote nanovoid growth, hydrogen-enhanced plasticity facilitates preferential void growth along the ferrite block. At higher hydrogen concentrations, ferrite cleavage cracking may originate from prolate voids and hydrogen-augmented Cottrell dislocation reactions, reducing plastic strain near the crack tip and hence diminishing the formation of vacancies as void nuclei. Previous studies have focused on the statistics of hydrogen's effect on void size. This work demonstrates the dislocation mechanism of hydrogen-assisted void growth and provides new insights into the development of micro-mechanism-based models to predict hydrogen embrittlement.

The role of cavitation in the toughening of elastomer nanocomposites reinforced with graphene nanoplatelets

The toughening of different types of elastomers through the incorporation of graphene nanoplatelets (GNPs) has been investigated. Both the tear strength and tearing energy are increased significantly through a combination of mechanisms such as debonding, pull-out and cavitation. The processes that occur ahead of a tear/crack tip in natural rubber filled with GNPs have been monitored in situ using synchrotron-based nanoscale X-ray computed tomography. The GNP particles are found to debond and form voids that grow under stress leading to considerable energy absorption. The mechanisms of cavitation and void growth are analysed theoretically and it is shown that voids need to be larger than ∼1 μm in size to grow in the triaxial tensile stress field ahead of the tear/crack tip. The high level of cavitation and void growth found for the elastomers filled with micron-sized GNP particles is suggested to be the reason why these nanocomposites have a high tear resistance.

The mechanistic origins of heterogeneous void growth during ductile failure

In 1969, Rice and Tracey proposed that the rate of void growth during ductile failure is a strong function of the hydrostatic stress state. Numerous in-situ x-ray computed tomography (XCT) studies demonstrated that the average rate of void growth is well-predicted by modified versions of the Rice-Tracey equation. However, recent in-situ XCT studies of void growth demonstrated that individual voids grow in highly heterogeneous manners in a way that is not predicted by Rice-Tracey or similar models. Model-based studies using crystal plasticity finite element (CP-FE) suggest that local effects of grain orientation play a strong role during void growth, but we lack experimental data to test this hypothesis. The present study leverages recent advances in laboratory-based diffraction contrast tomography (Lab-DCT) and in-situ XCT to systematically examine the effects of grain orientation on void growth experimentally in an Al-2219 alloy. CP-FE modeling was used to assess the local stress states associated with voids and their influence on void growth rates. These data indicate that void growth is not simply controlled by the local hydrostatic stress or stress triaxiality. Additionally, no clear relationship between void growth rates and grain orientation were observed. Instead, void growth rates were highly stochastic in ways that could not be directly linked to the local stress or strain states. The combination of experimental and modeling data suggests that our current assumptions regarding the mechanisms of void growth are flawed and that additional factors, which may include the local dislocation density, affect the rate of void growth.

Predictive Modeling of Fracture Behavior in Ti6Al4V Alloys Manufactured by SLM Process

This study focuses on ductile fracture behavior prediction for Ti6Al4V alloys fabricated via Selective Laser Melting (SLM). A modified Gurson-Tvergaard-Needleman (GTN) model characterizes void growth and shear mechanisms under uniaxial stress. The research explores the impact of Artificial Neural Network (ANN) architecture, specifically hidden layers and neurons, on predicting fracture parameters. Results reveal that increasing hidden layers substantially enhances accuracy, particularly for fracture displacement. Notably, predicting maximum force requires fewer layers than fracture displacement. Using selected layers and neurons, the system consistently achieved R2-values exceeding 0.99 for both maximum force and fracture displacement. The study identifies the initial void volume fraction (f0) parameter as having the most significant influence on both properties.

Use of effective multiscale cohesive models in the simulation of spall in metal plates

AbstractDuctile fracture of metals is the net result of void nucleation, growth and coalescence mechanisms that operate at the microscale. Optimal scaling analysis provides the analytical form of the effective material law that models the ductile fracture phenomena at the macroscale. The upscaled model of ductile behavior assumes the form of a cohesive relation—surface traction versus displacement—of the power‐law type with well‐defined exponents. In the present work, we demonstrate how the effective cohesive law derived from optimal scaling can be conveniently inserted into macroscale calculations by recourse to cohesive elements. The plastic deformation outside the cohesive elements is of moderate size and can be modeled by means of conventional plasticity models. In particular, the mesh size is dictated by accuracy considerations at the macroscale structural level and is not in any way constrained by microscale features and mechanisms, which happen at the subgrid level and are accounted for by the effective cohesive law. We illustrate the multiscale paradigm by means of spall calculations.

Effects of loading rate on void growth in amorphous glassy polymers

Initiation, growth, and coalescence of microscopic voids are precursors to failure in amorphous glassy polymers. Hence, void growth analysis is important for understanding the failure in these polymers. Under quasi-static conditions, void growth in polymers is well understood; however, a similar analysis under dynamic conditions is not available in the literature. In the present work, we analyse the effect of loading rate on the void growth behaviour using large deformation finite element simulations. An axisymmetric unit cell model of a spherical void is considered for the analysis. The void is subjected to a range of strain rates (10−3s−1 to 105s−1). Remote triaxialities equivalent to uniaxial loading, biaxial loading, and higher triaxiality conditions are considered. Remote mean stresses, which are important from the perspective of void growth, have been characterized as a function of void size, strain rate, and triaxiality. Results show that the critical mean stresses required for unstable void growth increase with increasing triaxiality. Higher loading rates further amplify the critical mean stresses, and the amplification depends upon the material rate sensitivity of polymers. The void growth mechanism does not change because of material rate-sensitivity. During void growth, inertia effects are found to be significant after a strain rate of 103s−1. It is found that inertia increases with increasing triaxiality and increasing void size. It stabilizes the void growth and makes them expand in a more spherical shape, even at low triaxialities.

A shear modified GTN model based on stress degradation method for predicting ductile fracture

The Gurson–Tvergaard–Needleman (GTN) model has provided a powerful description of the nucleation growth and coalescence of micro voids, but it has limitations in simulating shear fracture due to the absence of a description of shear localization behavior. A shear improvement method is proposed for simulating the ductile fracture of materials under different stress states. The modified model not only allows for strain hardening of the matrix material, but also accounts for the stress degradation caused by shear. The strength equation of the material is described by both the shear stress state function and a decay function, making it easier for materials under shear stress state to experience material softening and further inducing shear fracture. The modified GTN model is developed by incorporating the shear stress degradation factor into the yield function, while taking into account both void growth and shear failure mechanisms. By carefully calibrating the model’s parameters, the deformation and fracture processes of tensile, plane strain, notch tensile, and compression specimens in the 7A52 aluminum alloy are simulated. The damage evolution behavior of the material under different stress states is analyzed. The results indicate that the damage include void growth mechanism and void shear mechanism. The proportions of these two mechanisms vary under different levels of stress triaxiality. Upon localizing material deformation, the shear stress state intensifies, and the shear damage mechanism assumes a critical role in fracture. The modified GTN model accurately predicts the load-displacement response and fracture path of the 7A52 aluminum alloy under a wide range of stress states.

An uncoupled ductile fracture model considering void shape change and necking coalescence

In order to accurately describe the fracture behaviour of ductile metals in a wide range of stress states, based on the mechanism of void nucleation, growth, and coalescence during plastic deformation, an uncoupled isotropic ductile fracture model was established, which considered the combination of void shape change and void coalescence through internal necking. Firstly, the effect of the volume and shape changes on the fracture behaviour of the material was summarized and discussed, and the mechanism of interpore dimple shrinkage and consolidation during the void coalescence was studied. Secondly, the parameters of the proposed model are studied in detail to verify the flexibility of the new model. Then, the proposed ductile fracture model was used to construct the 3D fracture locus of AA 2024-T351, AISI 1045, Q460, Mg-Al-Zn-RE, and Ti-6Al-4V materials, and the predicted fracture strain values under corresponding stress states were compared with the experimental results of previous scholars. The effectiveness of the proposed new model is verified. Finally, to demonstrate the superiority of the proposed new model, the predicted results were compared with those of the widely used DF2016, MMC, and Hu models. The prediction results show that the new model has the highest precision compared to the other three. The void shape change and the void coalescence through internal necking play an essential role in the ductile metal fracture process, which should be fully considered.

Dynamic fracture characteristics of nanocrystalline Al containing He bubbles

This Letter reports the individual and combined effects of He bubbles and grain boundaries on the spallation of aluminum at different strain rates. Molecular dynamics simulations show that both He bubbles and grain boundaries reduce the spall strength, with the effect of He bubbles being reduced by about 50% when a grain boundary is present. Furthermore, it is found that He bubbles inhibit void nucleation and growth, while grain boundaries determine the location and mechanism of void or He bubble nucleation and growth, leading to a more rapid reduction in the spall strength of polycrystals than single crystals as the strain rate decreases. Additionally, a linear relationship is found between the spall strength of polycrystalline Al and the fraction of He atoms when He concentration is below 0.4 at.%.

The void formation behaviors in working solid-state Li metal batteries.

The fundamental understanding of the elusive evolution behavior of the buried solid-solid interfaces is the major barrier to exploring solid-state electrochemical devices. Here, we uncover the interfacial void evolution principles in solid-state batteries, build a solid-state void nucleation and growth model, and make an analogy with the bubble formation in liquid phases. In solid-state lithium metal batteries, the lithium stripping-induced interfacial void formation determines the morphological instabilities that result in battery failure. The void-induced contact loss processes are quantified in a phase diagram under wide current densities ranging from 1.0 to 10.0 milliamperes per square centimeter by rational electrochemistry calculations. The in situ-visualized morphological evolutions reveal the microscopic features of void defects under different stripping circumstances. The electrochemical-morphological relationship helps to elucidate the current density- and areal capacity-dependent void nucleation and growth mechanisms, which affords fresh insights on understanding and designing solid-solid interfaces for advanced solid-state batteries.

Multiple strengthening via high-entropy alloy particle addition in titanium matrix composites fabricated by spark plasma sintering

The strong interface bonding of particle reinforced metal matrix composites has been a longstanding puzzle. Unlike the existing titanium matrix composites (TMC) processed, which are mainly reinforced by ceramic particles, this work investigates the in-situ triggered alloying via high entropy alloy (HEA) addition, which promotes multiple strengthening to the metal matrix. The effects of HEA content on the microstructure evolution, mechanical properties, void growth mechanism, sintering model and underlying strengthening mechanism were investigated. The results show that the enhanced diffusion effect in the diffusion region leads to the formation of a multiphase structure composed of an FCC solid solution and a σ phase. In addition, with the addition of HEAs content, the yield strength and hardness values of all composites were higher than those of unreinforced Ti6Al4V (TC4). The obtained composite reinforced with 4.5 wt% HEA achieves an unprecedented tensile elongation to failure of 8.8% and a high ultimate tensile strength of 1150 MPa. The enhancement of the strength is attributed to the combined effects of grain refinement strengthening, load transfer strengthening and dislocation strengthening.

On the mix-mode fracture of carbon fibre/epoxy composites interleaved with various thermoplastic veils

This work studied the mix mode-I/II fracture behaviour of an aerospace-grade carbon fibre/epoxy composite that was interlayer-toughened by Polyamide-12 (PA), Polyphenylene-sulphide (PPS), Polyimide (PI), Polyethersulfone (PES) and Polyethylenimine (PEI) fibres. During the laminate curing process, the PA fibres melted, the PPS and PI fibres kept in their original form and the PES and PEI fibres dissolved in the epoxy matrix. This resulted in different toughening mechanisms of the veils for the mix mode-I/II fracture of the laminates, which was studied using a cracked lap-shear test. The main toughening mechanisms were observed to be plastic deformation and failure of the thermoplastic resin for the meltable PA veils, thermoplastic fibre debonding and bridging for the intact PPS and PI veils, and thermoplastic particle debonding and plastic void growth for the dissolvable PES and PEI veils. The experimental results revealed that the fibre debonding and bridging mechanism was superior for toughness enhancement, followed by the thermoplastic particle debonding and plastic void growth mechanism. For instance, interleaving the PPS and PEI veils increased the mix-mode fracture propagation energy of the laminates by 345% and 171%, respectively. However, the toughening performance of the PA and PI veils was poor, since the crack mainly propagated at the vicinity around the interlayer/laminate interface.

Secondary void interactions under shear in thin porous sheets

A compressive stress state decreases the mechanism of void growth in metals. However, in reality, void sheets are evidenced on the fracture surface of metallic shear-compression specimens. The prevailing stress state in the gauge is in contrast to the features observed on the fracture surface. This work examines the mechanical fields in the surroundings of a void under shear as a realization of the mechanical behavior of a ligament between coalescing voids. It is shown that the presence of secondary voids in the surroundings of one primary void reduces the average triaxiality in the cell and homogenizes its distribution; thus, it can be surmised to be mechanically favored. This paper examines this point from a micromechanical perspective utilizing the periodic unit cell method with a single void in its center as a reference case to be compared with other configurations of voids distributed in the cell. It is hypothesized that nucleated voids grow and promote nucleation and growth of voids around them – a mechanism to describe void sheet formation as a positive feedback loop process. The effect of extra secondary voids on the mechanical fields is examined and correlated to their size and location with respect to the existing primary void.

Theoretical models of void nucleation and growth for ductile metals under dynamic loading: A review

Void nucleation and growth under dynamic loading are essential for damage initiation and evolution in ductile metals. In the past few decades, the development of experimental techniques and simulation methods has helped to reveal a wealth of information about the nucleation and growth process from its microscopic aspects to macroscopic ones. Powerful and effective theoretical approaches have been developed based on this information and have helped in the analysis of the damage states of structures, thereby making an important contribution to the design of damage-resistant materials. This Review presents a brief overview of theoretical models related to the mechanisms of void nucleation and growth under dynamic loading. Classical work and recent research progress are summarized, together with discussion of some aspects deserving further study.

The effects of main toughening mechanisms and functionally interphase properties on fracture energy and fatigue characteristics of nanocomposites containing various fillers

Nowadays, novel technologies like nanofillers-based nanocomposites constitute one of the most active research fields in the extremely high performance materials, caused by the fact that this nanomaterial enhances the properties of different polymers in a significant way. Using these materials in polymer structure could enhance the mechanical properties such as fracture energy and fatigue behavior, which caused by variable or cyclic loadings. In the present study, by using the hierarchical multi-scale modeling procedure the fracture toughness (resistance to cracking), fracture energy and fatigue behavior of nanofillers/epoxy nanocomposites with consideration the contributions of the toughening mechanisms of interfacial debonding and plastic void growth in dissipation energy were investigated and discussed. Afterwards, the effect of CNT/matrix interphase on the fracture toughness, fracture energy and fatigue behavior was evaluated by varying the parameters of Young’s modulus and thickness of interphase. Furthermore, analysis of multi-scale modeling strategy was utilized in order to assess the influences of interphase properties and interfacial fracture energy on major parameters such as the critical stress debonding stress and radial component of the stress concentration tensor. In addition, different micromechanical models such as Halpin-Tsai and Maxwell and models were utilized to determine the Young’s modulus of nanocomposite as functionally graded (FG) interphase properties. Fracture energy and fatigue behavior measurements indicated that the properties of interphase region have a crucial importance on the fracture toughness of CNTs/epoxy nanocomposites. In order to verify the model, comparisons were carried out between the results of model with those predicted by experiments. The consistency between results for most fillers show the good performance of the procedure which has been utilized in this study.

Ductile fracture prediction of additive manufactured Ti6Al4V alloy based on an extended GTN damage model

Selective Laser Melting (SLM) is one of the revolutionary additive manufacturing (AM) technologies to produce metallic mechanical parts. To thoroughly understand the damage and fracture behavior of SLM-fabricated Ti6Al4V alloy under complex stress states, a number of tensile, compressive and torsional experiments covering various stress states were conducted with different types of the specimens. A modified GTN (Gurson-Tvergaard-Needleman) model with dual damage variables characterizing the void growth mechanism and void shear mechanism was adopted to describe and predict the ductile fracture behavior of the SLM-fabricated Ti6Al4V alloy under complex stress states. A new stress state-dependent function was introduced to extend the application of the model to negative stress triaxiality state. A finite element (FE) based inverse analysis strategy was proposed to calibrate the model parameters. The simulation results showed that the damage and fracture behavior of SLM-fabricated Ti6Al4V alloy can be well described by the modified GTN model. The morphologies of the fracture surfaces were successfully predicted by using the developed damage constitutive model, in which the variable of void fraction dominates fibrous zone failure based on the void growth mechanism, and the shear damage variable facilitates shear fracture based on the void shear mechanism.

A strain gradient plasticity model of porous single crystal ductile fracture

A strain gradient void-driven ductile fracture model of single crystals is proposed and applied to simulate crack propagation in single and oligo-crystal specimens. The model is based on a thermodynamical framework for homogenized porous solids unifying and generalizing existing thermodynamical formulations. This porous single crystal ductile fracture model relies on a multi-surface representation of porous crystal plasticity in which the standard Schmid law is enhanced to account for porosity, including void growth and void coalescence mechanisms. A new criterion to detect the onset of void coalescence in porous single crystals is proposed and validated by comparison to porous single crystal unit-cell simulations. This criterion can either be used as an additional yield surface or it can be used to follow the well established Gurson–Tvergaard–Needleman approach based on an effective porosity to model void coalescence. The strain gradient formulation relies on a Lagrange multiplier based relaxation of strain gradient plasticity. Material points simulations are performed in order to depict the elementary features of the porous single crystal ductile fracture model without strain gradient effects. The model is then applied to the simulation of plane strain single crystal specimen loaded in tension up to failure. The regularization ability and convergence with mesh refinement are demonstrated. Finally two- and three-dimensional simulations of ductile fracture of single and oligo-crystal specimens are presented. The significant influence of plastic anisotropy on the crack path, ductility and fracture toughness is highlighted.

An assessment of a mechanism for void growth in Li anodes

The formation of dendrites within the solid-state electrolyte of a Lithium (Li) ion battery is exacerbated by the presence of voids at the interface between the electrolyte and the Li anode. It is assumed that voids initiate and grow by the focussing of Li flux at the periphery of pre-existing small imperfections along the interface between the solid electrolyte and Li anode. Void growth in the Li anode, driven by stripping of the Li+ ions from the anode, is accompanied by creep within the anode. Consequently, the initiation and growth of these voids involve electrochemical stripping of Li+ from the anode, creep deformation of the anode and flux of Li+ through the adjacent solid electrolyte. Here we present a numerical analysis of this problem. We consider a single-ion conductor electrolyte, with Butler–Volmer kinetics governing the interfacial flux and the Li anode modelled as a power-law creeping solid. The study reveals that void growth can only initiate from relatively large pre-existing interfacial imperfections of size > 1200 μm along the interface of the solid electrolyte (LLZO) and the Li anode. In contrast, experimental observations suggest that voids as small as 1μm can initiate along the LLZO/Li interface and thus the simple picture described above involving power-law creep of the Li anode coupled with Butler–Volmer kinetics, even with interfacial diffusion accounted for, is insufficient to explain these observations. Our calculations reveal that the degree of flux focussing on the periphery of small imperfections must exceed that predicted by Butler–Volmer kinetics in order for interfacial voids to initiate and grow.

Void growth via dislocation pileup impingement on grain boundary

The effect of edge dislocations, impinging on grain boundary in fcc Fe near a preexisting void on the volume of this void, was studied using MD simulation. It was found that virtually unlimited growth of the void via this mechanism is possible under certain conditions. To the best of our knowledge this is the first atomistic simulation study, demonstrating feasibility of this void growth mechanism.

A novel uncoupled ductile fracture criterion for prediction of failure in sheet metal forming

The ductile fracture behavior of anisotropic materials was modeled by the uncoupled ductile fracture criterion. The ductile fracture model was concerned with the micro mechanisms of void nucleation, void growth, and evolution of void coalescence. The advance plastic model of Hill’48 non-associated flow rule is considered in proposed ductile fracture criterion. A series of basic fracture testing covering a wide range of stress state for aluminum alloy 6016-AC200 are carried out. Strain field on the surface of specimens is captured by the non-contact measurement DIC system. The fracture locus constructed using the proposed criterion is close to the experimental data points over a wide stress triaxiality range from negative to intermediate and high stress triaxiality. Then, a square cup drawing test is conducted in order to verify efficiency of the proposed fracture criterion in predicting fracture behavior of metal sheet. The results indicate that the proposed ductile fracture criterion can be utilized for predicting initial fracture in sheet metal forming.