Evolution of crystalline orientation and texture during solid phase die‐drawing of PP‐Talc composites

The objective of the present work was to examine the development of crystalline orientation and texture in the polypropylene matrix of talc‐filled i‐PP and in unfilled i‐PP with increasing draw ratio during solid‐phase die‐drawing at high strain rates (～1 s^?1) and a die temperature of 145 °C. After drawing, the entire billet was cooled rapidly “under tension” to room temperature before releasing the billet and cutting specimens from different axial locations for analysis. Orientation distributions of the three crystal axes for increasing axial strains have been presented as pole figures in the MD‐TD plane with the direction of draw (MD) as the reference direction. While disruption of spherulites was noticed within the die for neat PP at a draw ratio of 1.5, transcrystalline domains within the composite persisted even at a draw ratio of 3.5 in the free draw region outside the die. The transformation to fibrillar crystal morphology was complete in both materials at a draw ratio of 4.5 but the texture continued to develop beyond this stage. While the (110)[001] texture component was found to be dominant at all draw ratios for neat PP, the (010)[001] texture component was dominant at the higher draw ratios in the drawn composite. This may be attributed to the (010)[001] slip system being more active as the transverse spacing between elongated voids encasing the particles was decreased. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1528–1538     

A unified criterion for the growth and coalescence of microvoids

A yield criterion is developed which unifies void growth and void coalescence theories. Standard void growth theory assumes that plastic flow is diffuse, if not prevalent everywhere within the matrix of the elementary cell considered. On the other hand, void coalescence theory assumes states of post-localized plasticity whereby plastic flow is restricted to intervoid ligaments. The new theory accommodates both scenarios through some appropriate choice of microscopic velocity fields. An important implication for actual evolution problems is a seamless transition from void growth to void coalescence. This is in contrast with previous hybrid approaches whereby abrupt transitions are associated with the presence of unavoidable corners in the effective yield surface. More generally, the new criterion is applicable to describe yielding in porous metal plasticity for both low and high void volume fractions.     

On the Deformation of Functionally Graded Porous Elastic Cylinders

This paper is concerned with the linear theory of inhomogeneous and orthotropic elastic materials with voids. We study the problem of extension and bending of right cylinders when the constitutive coefficients are independent of the axial coordinate. First, the plane strain problem for inhomogeneous and orthotropic elastic materials with voids is investigated. Then, the solution of the problem of extension and bending is expressed in terms of solutions of three plane strain problems. The results are used to study the extension of a circular cylinder with a special kind of inhomogeneity. The influence of the material inhomogeneity on the axial strain is established.      

Mechanical modelling of ductile fracture

Mechanical models of material failure by void growth to coalescence are described to give a brief overview of methods applied in the analysis of ductile fracture. Approximate constitutive relations for porous ductile materials are discussed, modelling both the nucleation and growth of voids. The application of the material models is illustrated by numerical analyses for a tensile test specimen and for dynamic, ductile crack growth. Unstable void growth is a relevant mechanism in ductile materials subject to a high level of triaxial tension. The analysis of such cavitation instabilities in elastic-perfectly plastic materials is discussed for axisymmetric stress states, and the relevance to metal/ceramic components is emphasized.General Lecture presented at the 10th Italian National Congress of Theoretical and Applied Mechanics; AIMETA, Pisa, October 1990.     

A discrete-fibers model for bridged cracks and reinforced elliptical voids

A model of crack bridging and reinforced elliptical voids is proposed, in which the fibers joining the surfaces of the void or crack are modelled as discrete, linear elastic bars. We show that a theory recently developed by us to analyze structural interfaces permits analytical attack and solution of multiple important previously unsolved problems of stress concentration and fracture. In particular, an analytical solution is provided for a reinforced elliptical void, which, by superposition, allows treatment of arbitrary fiber distributions, which can be even randomly distributed and oriented. In the special case of small or null ratio between a void's axes, new stress intensity factor expressions are obtained, which account for fibers’ inclination and geometry.     

考虑损伤时带微孔粘弹性体的应力分布

本文根据粘弹性理论和带空沿材料的线性理论，利用Laplace变换及其逆变换，给出了粘弹性损伤材料的一种本构模型。应用这种本构关系。讨论了带损伤的圆环形板或圆筒体在内外压力作用下的平面问题，得到了圆孔边缘附近的应力场和损伤增量场的分布情况，同时根据Laplace变换的终值定理，得到圆环或圆筒的终态应力和损伤增量的分布，并分析了工程中的一个典型的例子。得到了最终损伤的一种近似分布。可为工程设计提供一定的参考。     

Poromechanics of freezing materials

When subjected to a uniform cooling below the freezing point a water-infiltrated porous material undergoes a cryo-deformation resulting from various combined actions: (i) the difference of density between the liquid water and the ice crystal, which results in the initial build-up of an in-pore pressure at the onset of crystallization; (ii) the interfacial effects arising between the different constituents, which eventually govern the crystallization process in connection with the pore access radius distribution; (iii) the drainage of the liquid water expelled from the freezing sites towards the air voids; (iv) the cryo-suction process, which drives liquid water towards the already frozen pores as the temperature further decreases; (v) the thermomechanical coupling between the solid matrix, the liquid water and the ice crystal. We work out a comprehensive theory able to encompass this whole set of actions. A macroscopic approach first provides the constitutive equations of freezing poroelastic materials, including the interfacial energy effects. This approach reveals the existence of a thermodynamic state function—namely the liquid saturation degree as a function of the temperature only. The macroscopic ice-dependent poroelastic properties are then upscaled from the knowledge of the elastic properties of the solid matrix, of the pore access radius distribution, and of the capillary curve. The theory is finally illustrated by analysing quantitatively the effects of the cooling rate and of the pore radius distribution upon the cryo-deformation of water-infiltrated porous materials. The theory succeeds in accounting for the experimentally observed shrinkage of embedded air voids, while predicting the partial melting of the ice already formed when the cooling suddenly stops.     

多孔Mooney-Rivlin材料矩形板的单向拉伸

本文利用不同可压超弹性材料大变形的Mooney-Rivlin应变能函数研究了含有多个微孔的矩形板在单向拉伸作用下的有限变形和受力分析。首先利用不可压条件得到了文中所给的含有某种对称性分布的多个微孔的矩形板的变形模式函数，其中所含的一个参数可由远离微孔的无穷远处的变形状态确定，另一个参可用最小势能原理导出变分近似解。文中详细分析了板中微孔（一个，三个和五个）随载荷作用的增长情况和微孔边缘应力的分布情况，并进行了比较。讨论了微孔的个数和排列方式，微孔的孔间距离等因素对微孔增长和应力分布的影响。     

微孔线弹性材料的守恒律和完备性定理

本文用无穷小变换群使作用量不变的思想证明了广义Noether定理,且得到一类守恒律,对线性均匀微孔弹性材料阐明了尺度变换下守恒律的可能性,且给出了完备性定理的证明。