Growth Of Microvoids Research Articles

Enhanced tensile ductility of an additively manufactured AlSi10Mg alloy by reducing the density of melt pool boundaries

AlSi10Mg components produced by laser powder bed fusion (LPBF) typically exhibit higher strength but lower ductility than those made by conventional mature processing technologies. The effects of melt pool boundaries on the ductility and fracture behavior of the LPBF-produced AlSi10Mg have not been systematically studied. The focus of this work is to investigate the local strain evolution, microvoid growth, and crack formation in the melt pool boundary regions using in situ tensile testing and synchrotron-based X-ray microtomography. Results indicate that decreasing area fractions of melt pool boundaries from 5.48% to 4.48% leads to an increase of tensile ductility from 7.2% to 9.8% in the LPBF AlSi10Mg samples. By influencing the density of melt pool boundaries in the LPBF process, while maintaining part density, we offer a new opportunity to fabricate AlSi10Mg products with an excellent combination of high strength and ductility.

Anisotropic plasticity and fracture of alpha titanium sheets from cryogenic to warm temperatures

Titanium is promising for manufacturing high-performance components in aerospace, marine, energy and healthcare. Whether the forming potential of alpha titanium can be excavated under cryogenic and warm working conditions and how the anisotropic plasticity and fracture evolve over a wide temperature range are the premise and basis for accurate control of inhomogeneous deformation of the material and invention of new forming technologies. In tandem with this, the anisotropic plasticity and fracture of alpha titanium sheets over a wide temperature range of -180 to 200 °C were explored regarding behaviors, mechanisms and modeling. 1) From a series of characterization experiments under different temperatures, compared with room temperature, the tension hardening exponent and fracture elongation of the material are increased by 133% and 48% at -180 °C, and by 7% and 50% at 200 °C. The anisotropy and asymmetry in flow stress are decreased with the temperature increasing from -180 to 200 °C, while the fracture displacements present a significant anisotropy over the given temperature range. 2) Via the macro- and micro-observations regarding dislocation, texture, micro-void and fractography and the viscoplastic self-consistent (VPSC) based slip/twinning analysis, at cryogenic temperature, the synergistic effects of slip and twinning make the dislocation motion ability increase and the grains refined. This causes the large strain hardening ability and inhibits the growth of micro-voids, which results in a relatively large fracture elongation. With increasing temperature, the increased slip systems and slip activities suppress the activation of twinning. At warm temperature, the enhanced dislocation motion ability by thermal activation reduces dislocation pile-up and inhibits void initiation, making the fracture elongation increase. 3) By introducing the temperature/strain related interpolation approach and the linear transformation of stress tensor, an integrated modeling framework was proposed, and the evolution of anisotropic yielding-necking-fracture loci with temperature was established. The results indicate that the necking strain limit is significantly improved at cryogenic temperature and the fracture strain limit is dramatically increased at warm temperature.

Investigation on Strain Hardening and Failure in Notched Tension Specimens of Cold Rolled Ti6Al4V Titanium Alloy.

Uniaxial and notched tension samples are utilized to investigate the damage and failure of titanium alloy Ti6Al4V. The strain fields on the samples are obtained by the digital image correlation (DIC) method. Strain localization occurs before fracturing in all samples, and the width and size of the localized zone are characterized. Slant fractures are observed in uniaxial and notched tension specimen, which indicate that the initiation and propagation of cracks in thin sheet specimens are highly affected by the shear stress. Numerical simulations were performed for identification of hybrid hardening laws, and the results were compared with the experiments. The influence of the stress triaxiality on damage mechanism of Ti6Al4V was analyzed by observation of the specimen fracture surfaces using SEM. The results show that a higher stress triaxiality facilitates the formation and growth of micro-voids, which leads to a decrement of strain at failure.

Simulation of ductile-to-brittle transition combining complete Gurson model and CZM with application to hydrogen embrittlement

A general simulation framework for modelling ductile-to-brittle transition in metals is proposed. The method combines the complete Gurson model and cohesive zone model, which brings ductile and brittle fracture mechanisms into one play. We found that the transition of failure mode is the result of a competition between fracture due to micro-void growth and coalescence and fracture in the cohesive zone. It is found that the fracture mode is dependent on the ratio between the cohesive strength and the yield strength of the material; brittle fracture only occurs when the strength ratio is below a critical value. This generic rule can be used to rationalize various failure scenarios featured by ductile-to-brittle transition, such as low temperature embrittlement and hydrogen embrittlement. As an application of the general framework, hydrogen embrittlement is simulated. It is revealed that a critical hydrogen concentration has to be achieved in order to trigger brittle fracture, which is consistent with many experimental observations.

Evaluation of Fracture Strain for Cold Drawn Thin-Walled Steel Tubes via Small Round-Bar Tensile Test

The evaluation of tube burring formability is a crucial task for finding a suitable material for tube-based automobile parts. The local strain at the ductile fracture site (fracture strain) should be evaluated for this purpose. Moreover, a cold-drawn steel tube has a strong anisotropic shaped microstructure and possibly causes anisotropic fracture strain behavior. Based on this background, the study evaluated the axial and hoop directional fracture strains of cold-drawn steel tubes using the small round-bar tensile specimen. The burnished surface ratio on the pierced surface was also investigated for possibility estimation of in-line formability inspection. As a result, three tubes are presented with inferior, nearly the same, and superior hoop directional fracture strains compared with the axial strains, where exceeding 40% deterioration in the hoop direction occurs by a combination of grain elongation and carbide aggregation. The scanning electron micrographs suggest that the microvoid growth and linkage percolated thorough carbides on the elongated grain boundaries. For the piercing test, the 30% fracture strain deterioration resulted in a 4% decrease in the burnished surface ratio on the pierced surface. This result suggested that the estimation of the pierced surface can detect material defects before the actual tube-burring process.

Dynamic analysis on microvoid growth in viscoelastoplastic materials under thermal shock

The microvoid growth in an infinite medium under thermal shock was analyzed on the basis of the viscoelastoplastic theory. In the mathematical model of the problem, the stage of purely viscoelastic deformation was taken into account. The direct integration method is used to find the analytical expressions of displacement and stress. The moving interface between the viscoelastic and the viscoplastic zones was involved, and its location was determined by the nonlinear ordinary differential equation with initial condition. As an example, the evolution of void with time for the aluminum alloy material was investigated by numerical computation. The effects of the temperature variation, the heating rate, and the viscosity of material on the void growth were examined. The analysis indicated that the relationship between void growth and temperature load is nonlinear due to the appearance of moving interface.

A deep learning method for predicting microvoid growth in heterogeneous polycrystals

In heterogeneous polycrystals, microvoid growth presents inherent randomness and dispersion, which generally follows a statistical law. This statistical characteristic intrinsically arises from randomly distributed grain-orientations around microvoids. It remains a huge challenge to explicitly depict the inherent correlation between microvoid growth and grain-orientation distribution by conventional deterministic damage models. In recent years, deep learning has gained in popularity in materials science and has been demonstrated to exhibit excellent data-mining abilities. To our best knowledge, deep learning has not been applied to investigate the statistical damage-evolution issues hitherto. In this work, a novel microvoid growth model based on deep neural network is creatively designed, incorporating both convolutional and long short-term memory components. The former extracts the spatial grain-orientation information, and the latter captures the causal effect of strain history on the microvoid growth. Moreover, to train and test the deep learning-based model, a microvoid-growth database is generated through a large number of crystal plasticity-based finite element simulations, incorporating randomly-oriented grains and different void locations. All the sample data (i.e., the grain-orientation distributions, microvoid locations and microvoid-growth curves) are processed by specific methods (e.g., the pixel-based method) to be amenable for the training process. Our results show that this novel model well captures the statistical characteristic of the microvoid growth in heterogeneous polycrystals. It is expected that the deep learning-based method can provide a new way to predict the microvoid growth at the grain-level.

The Evaluation of Quenching Temperature Effect on Microstructural and Mechanical Properties of Advanced High Strength Low Carbon Steel After Quenching Partitioning Treatment

The influence of quenching temperature on microstructural and mechanical properties of low alloy steel of the following chemical composition: 0.26 C, 1.70 Mn, 1.42 Si, 1.10 Cr, 1.10 Ni, 0.94 Cu, 0.24 Mo, 0.1 V, Bal. Fe (Wt.%) was investigated after applying a quenching-partitioning (Q-P) treatment. The steel samples were isothermally quenched at 260, 280, and 300 °C, from the austenitizing temperature and then Q-P treated at 340 °C. After the Q-P treatment, the steel showed a multiphase microstructure containing bainite, martensite, and retained austenite. It was determined that the tensile strength and Charpy impact energy increased with a decrease in quenching temperature to 1415 MPa and 43 J, respectively. This effect was attributed to an increase in the volume fraction of austenite/martensite micro blocks that introduces a hard phase mixture strengthening factor and the presence of tempered martensite, which is strengthened by fine particle dispersion and moreover, a decrease in thickness of the bainitic-ferrite subunits that refine the microstructure. The fractographic examination of the Charpy tested specimens showed that the sample quenched at 260 °C contained finer and deeper dimples, which indicates that more energy was spent on the nucleation and growth of ductile fracture microvoids, thus increasing the toughness.

Phase-field modeling of 3D fracture in elasto-plastic solids based on the modified GTN theory

An appropriate failure model of ductile materials needs to consider complex phenomena at the microscopic level, such as the nucleation, growth, and coalescence of microvoids and the final fracture at the macroscopic level. A recent popular continuous phase-field method based on the regularization of sharp crack discontinuities can easily model macroscopic cracks. In this work, we developed a coupled model to predict the crack law when materials undergo ductile fracture. The coupling model combined the ductile phase-field fracture model of the material with the shear-modified GTN model to illustrate the microscopic damage accumulation caused by the growth of voids and the macroscopic crack initiation and propagation. In addition, a modified phase-field degradation function was established, combining phase field, void volume fraction, and shear damage. This paper also considers the crack propagation law when the materials undergo ductile fracture under different stress triaxialities. In this study, we used the subroutines UEL and UMAT in the commercial finite element software Abaqus to give the staggering solution algorithm for ductile phase-field fracture of the materials. The shear-modified GTN model was written by the subroutine UMAT, while the ductile phase-field fracture model was written by the subroutine UEL. The results of the numerical simulations showed good agreement with the experimental damage failure mechanisms and the load–displacement curves.

Experimental study on deformation evolution and fracture behaviors of pure titanium at different stress triaxialities

In this study, the deformation and fracture behaviors of pure titanium sheet specimens under different stress triaxialities are experimentally investigated. Digital image correlation (DIC) technique is used to determine the strain maps on the surface of the specimens during the deformation and failure process. The experimental results show clearly that the strain localization occurs in different specimens which leads to the formation of the crack in the localized region (LR). The characteristics of the LR are quantitatively identified by the strain distribution along axial and transverse directions, the evolution of effective strain is successfully used to obtain the strains of diffuse necking, local necking and strain at fracture. The fracture features of the specimens with different stress triaxialities are studied by the analysis of fracture surface morphology. The nucleation and growth of micro-voids in pure titanium is significantly dependent on the applied stress conditions. The slip mechanism dominates the failure of shear specimens. For tensile loading conditions, the higher stress triaxiality accelerates the growth and coalescence of micro voids which induce the decrement of fracture strain.

Hot damage evolution in a high strength aluminum alloy during hot forming: a study using the Gurson–Tvergaard–Needleman model

The investigation on the formability of 7075-T6 aluminum alloy at elevated temperatures is significant due to its low plasticity at room temperature. Meso-Damage mechanics suggests that the growth and accumulation of micro-voids is an important factor that leads to material damage, fracture, and affects its formability. In this work, the Gurson–Tvergaard–Needleman (GTN) model was used to investigate the damage evolution of the 7075-T6 aluminum alloy during the hot forming process. The effects of different temperatures and strain rates on the formability were investigated through uniaxial hot tensile tests. The parameters of the Gurson–Tvergaard–Needleman (GTN) model were inverse-calibrated using the Central Composite Design (CCD)-Response Surface Method (RSM)-Genetic Algorithm (GA). Control variable method was used to explore the effect of each damage parameter on the fracture initiation and evolution. The results showed that the calibrated Gurson–Tvergaard–Needleman (GTN) model well predicted the high-temperature damage of the 7075-T6 aluminum alloy at 300–450 °C. Predicted necking and fracture volume fractions of voids were less than 3% error from experimental results. Initial void volume fraction (f0) had little effect on damage. Increasing void nucleation volume fraction (fN) significantly advanced the necking and fracture of the material. An increase in the critical void volume fraction (fC) and fracture void volume fraction (fF) resulted in higher peak stresses and fracture strains.

FCG Modelling Considering the Combined Effects of Cyclic Plastic Deformation and Growth of Micro-Voids.

Fatigue is one of the most prevalent mechanisms of failure. Thus, the evaluation of the fatigue crack growth process is fundamental in engineering applications subjected to cyclic loads. The fatigue crack growth rate is usually accessed through the da/dN-ΔK curves, which have some well-known limitations. In this study a numerical model that uses the cyclic plastic strain at the crack tip to predict da/dN was coupled with the Gurson–Tvergaard–Needleman (GTN) damage model. The crack propagation process occurs, by node release, when the cumulative plastic strain reaches a critical value. The GTN model is used to account for the material degradation due to the growth of micro-voids process, which affects fatigue crack growth. Predictions with GTN are compared with the ones obtained without this ductile fracture model. Crack closure was studied in order to justify the lower values of da/dN obtained in the model with GTN, when compared with the results without GTN, for lower ΔK values. Finally, the accuracy of both variants of the numerical model is accessed through the comparison with experimental results.

Study of Influence of Width to Thickness Ratio in Sheet Metals on Bendability under Ambient and Superimposed Hydrostatic Pressure

The effect of the width to thickness ratio on the bendability of sheet metal is investigated using the finite element method (FEM) employing the Gurson–Tvergaard–Needleman (GTN) model. Strain path changes in the sheet with change in the width/thickness ratio. It is shown that bendability and fracture strain increase significantly by decrease in the width/thickness ratio. The stress state is almost uniaxial when the stress ratio (α) is close to zero for narrow sheets. Stress ratio is nothing but the major stress to minor stress ratio. This delays the growth and coalescence of micro-voids as the volumetric strain and stress triaxiality (pressure/effective stress) decrease. On the other hand, ductility decreases with increase in α for wider sheets. Fracture bending strain is calculated and, as expected, it increases with decrease in the width/thickness ratio. Furthermore, a brief study is performed to understand the effect of superimposed hydrostatic pressure on fracture strain for various sheet metals with different width/thickness ratios. It is found that the superimposed hydrostatic pressure increases the ductility, and that the effect of the width/thickness ratio in metals on ductility is as significant as the effect of superimposed hydrostatic pressure. Numerical results are found to be in good agreement with experimental observations.

Numerical Study about the Influence of Superimposed Hydrostatic Pressure on Shear Damage Mechanism in Sheet Metals

It is generally accepted that the superimposed hydrostatic pressure increases fracture strain in sheet metal and mode of fracture changes with applying pressure. Void growth is delayed or completely eliminated under pressure and the shear damage mechanism becomes the dominant mode of fracture. In this study, the effect of superimposed hydrostatic pressure on the ductility of sheet metal under tension is investigated using the finite element (FE) method employing the modified Gurson–Tvergaard–Needleman (GTN) model. The shear damage mechanism is considered as an increment in the total void volume fraction and the model is implemented using the VUMAT subroutine in the ABAQUS/Explicit. It is shown that ductility and fracture strain increase significantly by imposing hydrostatic pressure as it suppresses the damage mechanisms of microvoid growth and shear damage. When hydrostatic pressure is applied, it is observed that although the shear damage mechanism is delayed, the shear damage mechanism is dominant over the growth of microvoids. These numerical findings are consistent with those experimental results published in the previous studies about the effect of superimposed hydrostatic pressure on fracture strain. The numerical results clearly show that the dominant mode of failure changes from microvoid growth to shear damage under pressure. Numerical studies in the literature explain the effect of pressure on fracture strain using the conventional GTN model available in the ABAQUS material behavior library when the mode of fracture does not change. However, in this study, the shear modified GTN model is used to understand the effect of pressure on the shear damage mechanism as one of the individual void volume fraction increments and change in mode of fracture is explained numerically.

Study on the Influence of the Gurson–Tvergaard–Needleman Damage Model on the Fatigue Crack Growth Rate

The fatigue crack growth (FCG) process is usually accessed through the stress intensity factor range, ΔK, which has some limitations. The cumulative plastic strain at the crack tip has provided results in good agreement with the experimental observations. Also, it allows understanding the crack tip phenomena leading to FCG. Plastic deformation inevitably leads to micro-porosity occurrence and damage accumulation, which can be evaluated with a damage model, such as Gurson–Tvergaard–Needleman (GTN). This study aims to access the influence of the GTN parameters, related to growth and nucleation of micro-voids, on the predicted crack growth rate. The results show the connection between the porosity values and the crack closure level. Although the effect of the porosity on the plastic strain, the predicted effect of the initial porosity on the predicted crack growth rate is small. The sensitivity analysis identified the nucleation amplitude and Tvergaard’s loss of strength parameter as the main factors, whose variation leads to larger changes in the crack growth rate.

Revealing tensile behaviors and fracture mechanism of Ti–6Al–4V titanium alloy electron-beam-welded joints using microstructure evolution and in situ tension observation

This study aimed to systematically investigate the microstructure evolution of the nail-shaped weld on electron beam welded joint of Ti–6Al–4V alloy. The microstructure evolution demonstrated that the needle martensitic α′ phases constituted the microstructure in the fusion zone, in which the grain size decreased in the depth direction, and the Widmannstätten structure lamellar α phases covered the base metal. The fusion zone got the strongest tensile strength that reached 980 MPa due to the smallest grain size and the largest number of low angle grain boundary, whereas the tensile strength of the base metal was the lowest in the whole butt joint. The best plastic deformation capacity in the base metal in which the strain approached 13 % was attributed to the largest number of high angle grain boundary in the Widmannstätten structure. Moreover, there were many deformation twins and faults in fusion zone to enhance the resistance to deformation. The tip of the nail-shaped weld was the brittlest location in the entire joint. The experiment also explored the fracture mechanism by in situ observation of tension and fractography, which involved the following three processes: the nucleation and growth of microvoids, the formation of microcracks produced by microvoids, and the occurrence of fracture caused by the growth and propagation of microcracks.

Coarse-grained molecular dynamics simulation of cis-1,4-polyisoprene with silica nanoparticles under extreme uniaxial tension

Coarse-grained (CG) cis-1,4-polyisoprene (PI) models with multiple silica nanoparticles (NPs) are built to study the effect of NPs and crosslinks in the uniaxial tensile simulation. The potential functions of the CG models are obtained mainly via the iterative Boltzmann inversion method. The tensile simulation results show that the grafted silica NPs and the crosslinked structure play reinforcing roles while the smooth silica NPs do the opposite, which have the similar trends with the experiment results. The differences of mechanical properties for these models are studied from different microscopic aspects, such as the network of NPs, the bond lengths, the free molecular chains, the entanglements, the stress and strain distribution and the microvoid evolution. As a result, the main reasons for the weakening of PI models with smooth silica NPs come from the weak interfacial interaction, the inhomogeneity of structural deformation and the reduction of the number of entanglements. However, if there are graft chains, the interfacial interaction can be enhanced by entangling with the matrix molecular chains. The graft chains can make it possible for the aggregated NPs to separate and can hinder the growth of microvoids at the interface. In addition, the inconsistency of the stress and strain distributions at the microscopic level is verified and the nucleation mechanism of microvoids is believed to be caused by the local violent movement of molecular chains.

Effects of Hindered Phenolic Antioxidants on Space Charge and Breakdown Properties of Polypropylene

This paper focuses on the effects of hindered phenolic antioxidants AO300 and AO736 with similar molecular structure on the electrical properties of isotactic polypropylene (iPP). Experiments of conductivity, space charge distribution, and DC breakdown strength after the hetero-polarity DC prestress are performed. The experimental results show that these two antioxidants can increase the conductivity-temperature coefficient, reduce the accumulation of space charge and improve the DC breakdown strength after the hetero-polarity DC prestress of iPP. By calculating the molecular orbitals of these two antioxidants, it is found that the antioxidants introduce local states in the forbidden band of iPP matrix to capture high-energy charges. Besides, the bond dissociation energy (BDE) of the hydrogen-oxygen bond in AO736 is lower than that in AO300, representing a stronger ability of proton transfer, which scavenges the radicals, inhibits the growth of microvoids and prevents the process of breakdown. It is concluded that the improvement of space charge and breakdown properties is attributed to the coupling effects of deep trap sites and radicals scavenging introduced by antioxidant. This research shows that AO736 has potential applications in PP insulation.

Phase characterization of interfacial reactions in the Ni/In/Cu ternary system

This study focuses on the characterization of the Ni/In/Cu microstructure. Ni and Cu are chosen because they are common materials in electronic devices, and In is one of the elements in commercial solders. The benefit of this joining process is the production of a bond at a low temperature. However, knowledge of the behaviors of the Ni/In/Cu ternary system at low temperatures is lacking due to issues with sample preparation. In this study, reactions at low temperatures and in the presence of a thin In layer are discussed. The interfacial reactions of Ni/In/Cu are systematically investigated to provide information about low-temperature reactions that involve the interdiffusion of a thin In layer. The results of this study show that (Cu, Ni)In2 is a stable phase below 100 °C and (Cu, Ni)11In9 is a dominant phase between 120 and 180 °C. In addition, a thin layer of (Ni, Cu)3In7 forms near the Ni side. The sequential microstructural evolution, such as microvoid growth and phase transformations, are discussed in this study.

Nonlinearity in the propagation of acoustic waves: Simulation and experimental validation in a creep damaged material

Abstract A 3-Dimensional mathematical model has been developed to understand the nonlinear characteristics of an acoustic wave propagating in a creep-damaged medium. Nonlinearities were assumed to be originated from the material due to creep damage in terms of change in precipitate size, nucleation, and growth of micro voids. For carrying out mathematical simulation in this research, a nonlinear material model has been used and the dynamic value of second order elastic constants as well as density measured for the specimen after each creep interruption were incorporated into it. For studying acoustic wave propagation inside the model of Inconel 600 alloy, a common excitation and acquiring point (pulse-echo mode) has been selected via the acoustoelastic effect. The signal received has been used for calculating the acoustic nonlinearity parameter (β) which is the indicative parameter for expressing progression of damage in the material. This parameter β can be calculated as the ratio of the amplitude of the transmitted signal to the square of the amplitude of the second harmonic and is dependent on the elastic constants of the material. The simulation results obtained through the model were validated experimentally. A drastic increase in β was observed at the transition region from secondary to tertiary stage of creep in the studied material, which is due to the increase in the volume fraction of precipitates as well as micro void initiation and are the dominating cause of material failure. So, this technique can be used for assessment of localized deformation in any material much prior to failure.