The nature of yielding and plastic deformation in rubber-modified polystyrene

Obliquely grooved sheet samples of high-impact polystyrene were used to establish conditions that lead to the early stage of plastic deformation under combined-stress loading conditions. By applying the plasticity theory and the method proposed by Hill to the thermoplastic, it was demonstrated that the basis for relating incremental strain rate with the corresponding deviatoric stress could be established. Yielding under the combined-stress loading condition was also shown to be strongly dependent on the sign of stress. Some insight into the asymmetric yielding behaviour was gained by determining the density and orientation dependence of crazes around the rubber-modified particles. It was shown that the process of craze initiation depended on the prevailing stress state and did not follow the stress or strain criterion. Based on the testing method used, a simple procedure of predicting sheet drawability is outlined.     

On the nature of plastic deformation generated by hydrostatic pressure in silicon single crystals

Macroscopic plastic deformation of silicon single crystals, caused by annealing at hydrostatic pressure and high temperature, was studied by X-ray topography and transmission electron microscopy. The analysis is given of elastic and thermal properties of material around surface cracks and scratches from which deformation process is propagated. The idea of elastic misfit between damaged self-strained material at cracks and scratches and defect-free silicon matrix, is introduced. On the basis of theoretical and experimental data it is concluded that the plastic deformation of silicon at high pressure consists of two processes. The first is a loss of coherency of cracks and scratches by the emission of dislocations at misfitting second phase precipitates present in silicon. The second is the macroscopic yielding from incoherent cracks and scratches at lower elastic strain energies.The presented mechanism explains also the deformation behaviour of silicon crystals subjected to tensile stress at high temperatures; the generation and propagation of dislocations at oxide precipitates before the macroscopic yielding [3].     

Thermo-electro-elastic fields accompanying plastic deformation

The thermo-electro-elastic coupled fields excited by an arbitrary dynamic dislocation ensemble in a piezoelectric medium are studied in a general statement on the basis of the developed 5D formalism. The found general relations are specified for the case when coupled fields are produced by a single straight dislocation moving at a constant speed v. The limiting behavior of derived expressions is analyzed at v → 0. In particular, the static electro-elastic field of a straight dislocation in a piezoelectric is expressed in a compact form similar to the known Barnett-Swanger formula describing dislocation distortions in a purely elastic medium. It is shown that the thermal component of dislocation fields may be essential only for fast dislocations with the speed v of order of the sound velocity. The temperature distribution around the moving dislocation is also analytically studied in details for the case of non-piezoelectric elastic materials. It is shown that metals and ionic crystals manifest a qualitative difference in described thermal effects. The results of the developed theory are compared with the existing experimental data. It is shown that there is a quantitative agreement between them.     

Dilatant plastic deformation of granular materials

A geometrical interpretation is given to the modified associated flow rule derived in the previous paper[l]. According to it, the dilatancy must be regarded as an internal constraint of deformation. The modified associated flow rule then gives equations of plastic deformation which exhibits the specified dilatancy. Hardening and elastic strains can also be incorporated. It is shown that the deformation is non-coaxial in general.     

Structure and plastic deformation of polyethylene

A review is given of the structure of semi-crystalline polymers and the mechanisms of plastic deformation in them. High-density polyethylene (HDPE) is taken as the specific example because of the large number of detailed studies performed on this material. The early findings are also compared and contrasted with very recent detailed large-strain deformation studies and computer simulations of deformation-induced texture development in HDPE.     

Damage evolution under severe plastic deformation

The development and recovery of damage in continuously cast aluminium alloy 6061 due to plastic deformation is investigated for different stress histories. The processes of Equal Channel Angular Extrusion and Equal Channel Angular Drawing are used to introduce damage into the specimen for a specified stress history. The amount of plastic deformation is determined by the angle between the two intersecting channels, while the stress history is varied by applying different back-pressures. The damage is related to the density, measured using Archimedes' principle.The development of damage was observed to increase proportionally with the extent of accumulated plastic shear strain. The influence of stress history, characterised by a stress index, was found to be twofold. First, the stress index defines the intensity of the porosity development, which increases with the stress index as it changes from negative to positive values. Second, the stress index, when in the negative value region, governs the recovery process. A superimposition of high compressive stresses on the plastic shear deformation leads to a recovery of damage and an associated density increase. The kinematic equation for damage evolution is proposed and its coefficients are defined.     

A physical model for plastic deformation

A general theoretical model for plastic deformation is presented, which is based on considerations of the variation of internal stored energy during deformation. It is proposed that the deformation rate will always be such that the rate of energy dissipation in the deforming material is minimal. The physical justification of this principle is discussed. The model is applied to dislocation deformation in metals and the result is then compared with experimental observations in aluminium.     

The plastic deformation of oriented polypropylene and polyethylene: deformation mechanisms

Polypropylene and high-density polyethylene, oriented by hot drawing, have been tensile testedin situ in a low angle X-ray camera. Two orientations of polypropylene, ₀=31° and ₀=60°, and one orientation of polyethylene, ₀=30°, were examined. ( ₀ is the initial angle between the tensile axis and the molecular axis.) Low-angle and wide-angle X-ray patterns were taken at successive stages of increasing strain up to approximately 100%. The rotations of the molecular axis and lamellar normal for both materials oriented near ₀=30° were quantitatively consistent with predominantly intermolecular shear, occurring within the lamellae. In the case of polypropylene, it is proposed that small amounts of interlamellar and interfibrillar shear were also present.At ₀=60°, the polypropylene was shown to deform by void opening or fibril separation, followed by intermolecular shear. The behaviour of polypropylene was consistent with the yield criterion based on a fibre reinforced composite model which was presented in a previous paper [1].     

Steady in-plane deformation of noncoaxial plastic soil

Presented in this paper is the theory of the steady in-plane deformation, obeying the Coulomb yield criterion, of plastic soils whose strain rate and stress principal directions are noncoaxial. The constitutive equations including an unknown noncoaxial angle are derived by use of the geometry of the Mohr circle and the theory of characteristic lines. A boundary value problem is solved by assigning to the non coaxial angle a set of such values that enable us to accommodate the presupposed type of flow satisfying the given boundary conditions in a given domain. The plastic material regulated by the Coulomb yield criterion in in-plane deformation is, therefore, a singular material whose constitutive equations are not constant with material but are variable with flow conditions.     

The size effect on micro deformation behaviour in micro-scale plastic deformation

When the geometry of metal deformed part is scaled down to micro-scale, the understanding and prediction of micro deformation behaviour becomes difficult. This is because the conventional material deformation models are no longer valid in micro-scale due to the size effect, which affects the deformation behaviour in micro plastic deformation, and thus leveraging the traditional knowledge of plastic deformation from macro-scale to micro-scale is not meaningful. In this paper, the size effect on micro-scale plastic deformation and frictional phenomenon are investigated via micro-cylindrical compression test, micro-ring compression test and Finite Element (FE) simulation. The experimental results show the occurrence of various size-effect related deformation phenomena, including the decrease of flow stress and the increases of: (a) irrational local deformation, (b) the amount of springback, and (c) the interfacial friction stress with the decreasing specimen size. The research further verifies that the established surface layer models, with the identified surface grain, the internal grain properties and the measured friction coefficients, are able to predict micro deformation behaviour. The research thus provides an in-depth understanding of size effect on deformation and frictional behaviours in micro-scale plastic deformation.     

Finite plastic deformation for metals under torsion

Abstract

Making use of the concept of plastic spin, endochronic theory is applied to describe the torsional deformation in the finite strain range. Deformation of tubular specimens of 70:30 brass, Ni‐200 and Al‐1100 are discussed. Special attention is given to torsion of tubular specimens with constrained and unconstrained ends. In the finite strain range, axial stress or elongation may be observed in addition to the shear stress and strain, depending on the end condition of the tube.     

Severe plastic deformation processes for thin samples

Among the known severe plastic deformation (SPD) techniques, one particular group can be defined as SPD processing of thin samples. Their distinctive feature is that one of the sample dimensions, namely the thickness, is much smaller than the other two dimensions. Examples include High Pressure Torsion and two recently developed techniques: the Cone–Cone Method and the High Pressure Tube Twisting. The mentioned group of SPD processes involve frictional forces acting on the large surfaces and a high hydrostatic pressure within the deformation zone. These techniques are particularly suited for microforming of metals. In this article, we outline the commonalities between these three techniques. The microstructure of copper samples deformed by all the three processes is presented and compared with those obtained by equal-channel angular pressing as a reference bulk forming SPD technique.