Void interaction and coalescence in polymeric materials

Beyond pressure-sensitivity, plastic deformation of glassy polymers exhibits intrinsic softening followed by progressive rehardening at large strains. This highly nonlinear stress–strain behavior is captured by a constitutive model introduced in this work. In the first part of the paper, we focus on void growth and coalescence in an axisymmetric representative material volume consisting of a single large void and a population of discrete microvoids. Our study shows that microvoid cavitation, enhanced by strain softening, accelerates the process of void coalescence resulting in brittle-like failure at lowered stresses and strains. Pressure-sensitivity also reduces stress-carrying capacity as well as influences the strain for void coalescence; plastic dilatancy effects are relatively milder. In the second part of the paper, we introduce a population of discrete spherical voids within a three-dimensional computational model to study void growth and damage ahead of a crack front. Our studies reveal a distinctive change in the deformed void shape from oblate to prolate when strain softening is followed by high rehardening at large plastic strains. By contrast, an extended strain softening regime promotes oblacity and facilitates multiple void interaction and their cooperative growth over large distances ahead of the crack front. This multi-void failure mechanism is exacerbated by pressure-sensitivity.     

Void collapse/growth in solid materials under overall shear loading

The present work aims at studying numerically the influence of void concentration, number of voids and absence/presence of inclusion on void collapse/growth and coalescence in materials submitted to shear loading. Starting from the experimental observation that voiding mostly forms within bands of localised deformation in the form of void sheets, the geometrical configuration retained to that purpose is a layer of periodic cells with 1–5, empty or particle-containing voids, subject to simple shearing.     

Void growth and interaction experiments: Implications to the optimal straining rate in superplastic forming

In this work, a set of parametric experiments was conducted on a superplastic material (eutectic tin–lead alloy) with one or more pre-drilled holes in each specimen. The small-sized holes were for simulating microvoids that occur and grow during superplastic forming. All holes were axially aligned with the tensile axis. The results revealed an increase in ductility with the number of holes up to 10 holes and a decrease thereafter. The ductility enhancement was explained based on the m-curve as due to a rise in the strain rate sensitivity locally around the holes. The decrease was explained due to strong void interaction that resulted in shear failure. This was further verified by a separate set of experiments of only two interacting voids with various interspacing. Finally, the void size versus applied strain was fully characterized and the results supported the ductility observations.     

Void growth and coalescence in single crystals

Void growth and coalescence in single crystals are investigated using crystal plasticity based 3D finite element calculations. A unit cell involving a single spherical void and fully periodic boundary conditions is deformed under constant macroscopic stress triaxiality. Simulations are performed for different values of the stress triaxiality, for different crystal orientations, and for low and high work-hardening capacity. Under low stress triaxiality, the void shape evolution, void growth, and strain at the onset of coalescence are strongly dependent on the crystal orientation, while under high stress triaxiality, only the void growth rate is affected by the crystal orientation. These effects lead to significant variations in the ductility defined as the strain at the onset of coalescence. An attempt is made to predict the onset of coalescence using two different versions of the Thomason void coalescence criterion, initially developed in the framework of isotropic perfect plasticity. The first version is based on a mean effective yield stress of the matrix and involves a fitting parameter to properly take into account material strain hardening. The second version of the Thomason criterion is based on a local value of the effective yield stress in the ligament between the voids, with no fitting parameter. The first version is accurate to within 20% relative error for most cases, and often more accurate. The second version provides the same level of accuracy except for one crystal orientation. Such a predictive coalescence criterion constitutes an important ingredient towards the development of a full constitutive model for porous single crystals.     

Void growth and coalescence in anisotropic plastic solids

Large strain finite element calculations of unit cells subjected to triaxial axisymmetric loadings are presented for plastically orthotropic materials containing a periodic distribution of aligned spheroidal voids. The spatial distribution of voids and the plastic flow properties of the matrix are assumed to respect transverse isotropy about the axis of symmetry of the imposed loading so that a two-dimensional axisymmetric analysis is adequate. The parameters varied pertain to load triaxiality, matrix anisotropy, initial porosity and initial void shape so as to include the limiting case of penny-shaped cracks. Attention is focussed on comparing the individual and coupled effects of void shape and material anisotropy on the effective stress–strain response and on the evolution of microstructural variables. In addition, the effect of matrix anisotropy on the mode of plastic flow localization is discussed. From the results, two distinct regimes of behavior are identified: (i) at high triaxialities, the effect of material anisotropy is found to be persistent, unlike that of initial void shape and (ii) at moderate triaxialities the influence of void shape is found to depend strongly on matrix anisotropy. The findings are interpreted in light of recent, microscopically informed models of porous metal plasticity. Conversely, observations are made in relation to the relevance of these results in the development and calibration of a broader set of continuum damage mechanics models.     

Void growth and cavitation in nonlinear viscoelastic solids

This paper discusses the growth of a pre-existing void in a nonlinear viscoelastic material subjected to remote hydrostatic tensions with different loading rates. The constitutive relation of this viscoelastic material is the one recently proposed by the present authors, which may be considered as a generalization of the non-Gaussian statistical theory in rubber elasticity. As the first order approx-imation, the above constitutive relation can be reduced to the “neo-Hookean” type viscoelastic one.Investigations of the influences of the material viscosity and the loading rate on the void growth, or on the cavitation are carried out. It is found that: (1) for generalized “inverse Langevin approximation” nonlinear viscoelastic materials, the cavitation limit does not exist, but there is a certain (remote) stress level at which the void will grow rapidly; (2) for generalized “Gaussian statistics” (neo-Hookean type) viscoelastic materials, the cavitation limit exists, and is an increasing function of the loading rate.The present discussions may be of importance in understanding the material failure process under high triaxial stress.     

Void growth to coalescence in a non-local material

The size-effect in metals containing distributed spherical voids is analyzed numerically using a finite strain generalization of a length scale dependent plasticity theory. Results are obtained for stress-triaxialities relevant in front of a crack tip in an elastic-plastic metal. The influence of different material length parameters in a multi-parameter theory is studied, and it is shown that the important length parameter is the same as under purely hydrostatic loading. It is quantified how micron scale voids grow less rapidly than larger voids, and the implications of this in the overall strength of the material is emphasized. The size effect on the onset of coalescence is studied, and results for the void volume fraction and the strain at the onset of coalescence are presented. It is concluded that for cracked specimens not only the void volume fraction, but also the typical void size is of importance to the fracture strength of ductile materials.     

Auxetic mechanics of crystalline materials

In the present paper, we analyze uniaxial deformation of crystals of different systems with negative Poisson’s ratios, known as auxetics. The behavior of auxetic crystals is studied on the basis of extensive knowledge on the experimental values of elastic constants of different crystals, gathered in the well-known Landolt-Börnstein tables. The competition between the anisotropy of crystal structures and the orientation of deformable samples results in the dependence of the elastic characteristics of deformation, such as Young’s modulus and Poisson’s ratio, on the orientation angles. In the special case of a single angle, a large number of auxetics were found among crystals of cubic, hexagonal, rhombohedral, tetragonal, and orthorhombic systems and the character of variations in their response due to changes in orientation was determined.     

Defects in crystalline materials and their relation to mechanical properties

This paper is divided into two major sections. The first of these describes the types of defects which may arise in crystalline materials. These may be classified as point, line, sheet and volume defects, examples of the four groups being vacancies, dislocations, grain boundaries and precipitates. The basic defects of crystal structures are first described, particular attention being paid to dislocation lines and the way in which a perfect dislocation may dissociate into two partial dislocations and a ribbon of stacking fault. The motion of defects, including glide, climb and cross-slip is discussed. This section ends with a summary of the ways in which basic defects may interact and combine, and be used to describe such microstructural features as deformation twins and precipitate boundaries. These defects form the basis of the mechanisms, given in the second part of the paper, which have been used to explain the various phenomena of hardening and fracture. No attempt has been made to give detailed theories of these mechanical properties. The treatment is intended for the nonspecialist, interested in obtaining an understanding of how a knowledge of the microstructure of materials may be applied to specific problems.     

压力敏感材料中的空洞长大研究及其在页岩及高分子材料中的应用

主要研究压力敏感材料中含内压的空洞长大,如页岩或者高分子材料。采用数值方法研究含内压空洞的对称和非对称球形和柱形胞元的宏观力学行为。结果表明,压力敏感性及其空洞内压将极大影响空洞的形核与长大。在球形胞元情形中未出现柱形胞元的单轴拉伸现象。将胞元有限变形的数值计算结果与基于近期提出的考虑压力敏感材料中空洞长大的塑形力学模型的分析结果进行了对比。     

Thermal effects in the superelasticity of crystalline shape-memory materials

This paper develops a three-dimensional theory for the superelastic response of single-crystal shape-memory materials. Since energetic considerations play a major role in the phase transformations associated with the superelastic response, we have developed the theory within a framework that accounts for the laws of thermodynamics. We have implemented a special set of constitutive equations resulting from the general theory in a finite-element computer program, and using this program have simulated the superelastic response of a single crystal Ti-Ni shape-memory alloy under both isothermal and thermo-mechanically coupled situations. Both manifestations of superelasticity—stress-strain response at fixed temperature and strain-temperature response at fixed stress—are explored. The single-crystal constitutive-model is also used to discuss the superelastic response of a polycrystalline aggregate with a random initial crystallographic texture. The overall features of the results from the numerical simulations are found to be qualitatively similar to existing experimental results on Ti-Ni.     

Dislocations and disclinations: continuously distributed defects in elasto-plastic crystalline materials

The paper deals with elasto-plastic models for crystalline materials with defects, dislocations coupled with disclinations. The behaviour of the material is described with respect to an anholonomic configuration, endowed with a non-Riemannian geometric structure. The geometry of the material structure is generated by the plastic distortion, which is an incompatible second-order tensor, and by the so-called plastic connection which is metric compatible, with respect to the metric tensor associated with the plastic distortion. The free energy function is dependent on the second-order elastic deformation and on the state of defects. The tensorial measure of the defects is considered to be the Cartan torsion of the plastic connection and the disclination tensor. When we restrict to small elastic and plastic distortions, the measures of the incompatibility as well as the dislocation densities reduced to the classical ones in the linear elasticity. The constitutive equations for macroforces and the evolution equations for the plastic distortion and disclination tensor are provided to be compatible with the free energy imbalance principle.     

Void growth in power-law creeping solids: Effect of surface diffusion and surface energy

This paper addresses the growth of a void in a nonlinearly creeping material in the presence of the void-surface energy effect and void-surface diffusion driven by surface curvature gradients. Large strain finite element analysis of the coupled problem indicates that microstructural variables (porosity and void aspect ratio), as well as macroscopic deformation rates are strongly affected by the relative strength of the void-surface energy effect and the void-surface diffusion process vis-a-vis the rate of creep deformation in the bulk of the solid. The phenomenon is characterized by two-dimensionless groups, one measuring the strength of the surface diffusion process with respect to the nonlinear creep deformation in the interior of the solid, and the other the magnitude of the surface energy of the void in relation to the applied load and the size of the void. The computations reveal a rich variety of solutions that reflect a wide range of external load, material, and geometric parameters. Classical void growth studies that ignore both surface diffusion and surface energy effects are shown to recover only one case of this family of solutions. The computations also serve to quantitatively evaluate recent constitutive theories for porous nonlinear materials that account for continuously evolving microstructure, but do not include surface diffusion or surface energy effects.     

Rate-independent crystalline and polycrystalline plasticity,application to FCC materials

This paper deals with the simulation of the mechanical response and texture evolution of cubic crystals and polycrystals for a rate-independent elastic–plastic constitutive law. No viscous effects are considered. An algorithm is introduced to treat the difficult case of multi-surface plasticity. This algorithm allows the computation of the mechanical response of a single crystal. The corresponding yield surface is made of the intersection of several hyper-planes in the stress space. The problem of the multiplicity of the slip systems is solved thanks to a pseudo-inversion method. Self and latent hardening are taken into account. In order to compute the response of a polycrystal, a Taylor homogenization scheme is used. The stress–strain response of single crystals and polycrystals is computed for various loading cases. The texture evolution predicted for compression, plane strain compression and simple shear are compared with the results given by a visco-plastic polycrystalline model.     

Deformation and fracture of crystalline materials under complex loading-program conditions

In predicting the deformation and conditions of fracture of materials under complex loading program conditions one must consider the entire duration of the deformation process.The problem becomes complicated in cases of a simultaneous operation of various mechanisms of deformation and fracture, e. g., when plastic deformation is superposed (once or repeatedly) on creep.A promising phenomenological approach to this problem may be based on concepts of the mechanical equation of state of materials. A hypothesis of the existence of the equation of state depending on a finite number of structural parameters was formulated by Kröner [1] for the case of the three-dimensional law of plasticity and by Rabotnov [2] for the case of creep and fracture under uniaxial stress state conditions.This article is concerned with the application of the hypothesis of the mechanical equation of state to the problem of deformation and fracture of materials (in the uniaxial case) under complex loading program conditions.     

Void growth and coalescence in ductile solids with stage III and stage IV strain hardening

State of the art ductile fracture models often rely on simple power laws to describe the strain hardening of the matrix material. Power laws do not distinguish between the two main stages of hardening observed in polycrystals, referred to as stage III and stage IV hardening, and which emerge from the evolution of the dislocation substructure. The aim of this study is to couple a physics based strain hardening law including these two stages to a micromechanics based ductile damage model. One of the main motivations is that, the stage IV constant hardening rate stage, occurring only at large strain, will be attained in most ductile failure problems if not at the overall level of deformation, at least locally around the growing voids. Furthermore, proper modelling of the stage III involving dislocation storage and recovery terms and the transition to stage IV provides a link with the underlying physical mechanisms of deformation and with the microstructure. First, in order to evaluate the effects of the stage III and stage IV hardening on void growth and coalescence, an extensive parametric study is performed on two-dimensional (2D) axisymmetric finite element (FE) unit cell calculations, using a Kocks-Mecking type hardening law. The cell calculations demonstrate that accounting for the stage IV hardening can have a profound effect on delaying void coalescence and increasing the ductility. The magnitude of the recovery term during stage III has also a significant effect on the void growth rate. Then, the Kocks-Mecking law is incorporated into the Gologanu-Leblond-Devaux (GLD) porous plasticity model supplemented by two different versions of the Thomason void coalescence criterion. The predictions of the damage model are in good agreement with the results of the FE calculations in terms of the stress-strain curves, the evolution of void shape and porosity, as well as the strain value at the onset of void coalescence.     

Shock wave interaction with cellular materials

The equations governing the head-on collision of a planar shock wave with a cellular material and a numerical scheme for solving the set of the governing equations were outlined. In addition, the condition for the transmitted compression waves to transform into a shock wave, inside the cellular material was introduced. It was proved analytically that a cellular material cannot be used as a means of reducing the pressure load acting on the end-wall of the shock tube. In subsequent papers, the interaction of planar shock waves with specific cellular materials, e.g., foams and honeycombs will be described in detail.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.