A. S. Argon

Plastic deformation in metallic glasses

A theory is presented for the plastic deformation of metallic glasses below their glass transition temperature. The theory is based on two modes of thermally activated shear transformations initiated around free volume regions under an applied shear stress. The regions are typically conceived to be about 5 atom diameters across. At high temperatures (0.6 Tg ≲ T ≲ Tg) the transformation is a diffuse rearrangement producing a relatively small local shear strain in a roughly spherical region. At low temperatures (0 < T ≲ 0.6 Tg) the transformation is in a narrow disk shaped region and resembles closely the nucleation of a dislocation loop. The theory is in good accord with experimental observations. Based on the theory, possible levels of flow dilatation have been computed from which rates of shear localization can be obtained. At low temperatures, very rapid shear localization is predicted which is in good accord with the observations reported in the literature and with recent cinematographic observations.

Large inelastic deformation of glassy polymers. part I: rate dependent constitutive model

Abstract Glassy polymers constitute a large class of engineering solids. In order to successfully analyze the warm (near the glass transition temperature) mechanical processes by which many glassy polymeric products are manufactured, as well as to ascertain the response of the resulting part to service life loading conditions, a constitutive law that properly accounts for the large, inelastic deformation behavior of these materials is required. Such behavior is known to exhibit strain rate, temperature, and pressure dependent yield, as well as true strain softening and hardening after yield. This paper develops a three-dimensional constitutive model based on the macromolecular structure of these materials and the micromechanism of plastic flow which encompasses these above dependencies. The experiments necessary to determine the material properties used in the model are also identified. The model predictions for the true stress-strain behavior of PMMA are then compared with experimental data reported in the literature.

Cavity formation from inclusions in ductile fracture

The previously proposed conditions for cavity formation from equiaxed inclusions in ductile fracture have been examined. Critical local elastic energy conditions are found to be necessary but not sufficient for cavity formation. The interfacial strength must also be reached on part of the boundary. For inclusions larger than about 100Å the energy condition is always satisfied when the interfacial strength is reached and cavities form by a critical interfacial stress condition. For smaller cavities the stored elastic energy is insufficient to open up interfacial cavities spontaneously. Approximate continuum analyses for extreme idealizations of matrix behavior furnish relatively close limits for the interfacial stress concentration for strain hardening matrices flowing around rigid non-yielding equiaxed inclusions. Such analyses give that in pure shear loading the maximum interfacial stress is very nearly equal to the equivalent flow stress in tension for the given state of plastic strain. Previously proposed models based on a local dissipation of deformation incompatibilities by the punching of dislocation loops lead to rather similar results for interfacial stress concentration when local plastic relaxation is allowed inside the loops. At very small volume fractions of second phase the inclusions do not interact for very substantial amounts of plastic strain. In this regime the interfacial stress is independent of inclusion size. At larger volume fractions of second phase, inclusions begin to interact after moderate amounts of plastic strain, and the interfacial stress concentration becomes dependent on second phase volume fraction. Some of the many reported instances of inclusion size effect in cavity formation can thus be satisfactorily explained by variations of volume fraction of second phase from point to point.

A theory for the low-temperature plastic deformation of glassy polymers

Abstract A theory of yielding of glassy polymers by thermally-activated production of local molecular kinks is described. It is possible to obtain the activation free enthalpy of this process by modeling the intermolecular energy barrier as resulting from the stress fields of two equal and opposite closely spaced wedge disclination loops extending over the molecular cross section at the points of rotation of the molecular kinks. The theory predicts the yield stress at absolute zero to be dependent only on the shear modulus and the Poissons ratio, and is capable of describing the temperature, pressure, and strain rate dependences of the flow stress from absolute zero to near the glass transition temperature. Comparison of the theory with the available experimental, results on polystyrene, polyethylene-terephthalate, polycarbonate of bisphenol A, and poly-methyl-methacrylate shows excellent agreement in nearly all respects.

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.

Mechanical Behavior of Materials

Mechanical behavior of materials , Mechanical behavior of materials , کتابخانه دیجیتالی دانشگاه علوم پزشکی و خدمات درمانی شهید بهشتی

Separation of second phase particles in spheroidized 1045 steel, Cu-0.6pct Cr alloy, and maraging steel in plastic straining

Experiments were performed on spheroidized 1045 steel, Cu-0.6 pct Cr alloy, and maraging steel containing respectively Fe3C, Cu-Cr, and TiC particles of nearly equiaxed shape. The local interfacial stresses for separation of these particles during plastic deformation were evaluated by the methods described in the two preceding papers. The results show that the interfacial strengths for these particles in their respective matrices are 242, 144, and 264 ksi. In the spheroidized steel the average diam of the separated particles is distinctly larger than the average diam of the whole population. This is quantitatively explained by the enhanced interfacial stresses developed in regions of above average volume fraction of second phase which frequently occur in very dense populations of particles. No such effect was observed in the other two systems which is consistent with their much lower volume fraction of second phase. Some tension experiments have also been performed with the spheroidized 1045 steel at elevated temperature, giving results qualitatively similar to those at room temperature.

Toughenability of polymers

Abstract We demonstrate that all solid polymers are intrinsically brittle and will undergo a ductile to brittle fracture transition based on the nature of their bonding alone. The most effective way of avoiding a ductile to brittle transition is to reduce the plastic resistance to delay reaching the brittle strength which in unoriented polymers is governed by intrinsic cavitation. While a number of possibilities for this exist, the most widely used techniques involve incorporation of rubbery particles that can cavitate or rigid particles that can debond prior to plastic flow. In both approaches the continuous homo-polymer is transformed into a quasi-regular cellular solid that is much more capable of undergoing large local plastic flow by ligament stretching between cavitated particles and is less susceptible to the propagation of brittle cracks under the usual conditions of tensile straining. Under impact conditions, however, in a notched sample which concentrates the strain rate at the notch root, the plastic resistance of the stretching ligaments rises sharply due to two separate but related effects. First, by an increase in the shear modulus due to the high frequency nature of the Izod impact test to fracture, viewed as a quarter cycle oscillation, which directly elevates the flow resistance and second, by the further effect of increase due to the much increased plastic strain rate. At the notch root then, the plastically stretching and strain hardening ligaments are left with a much reduced capacity to strain further before the cavitation stress is reached. While rubber particle-modified polymers can still exhibit considerable toughening, rigid-particle-modified polymers suffer severely from clustering of rigid particles into super critical flaws that trigger brittle response, much like what is encountered in structural steels. Based on their known mechanical response in neat form six, semi-crystalline polymers have been analyzed in detail to evaluate their potential for toughening under impact conditions. The results correlate very well with the experimental findings.

Plastic flow in a disordered bubble raft (an analog of a metallic glass)

Abstract As Bragg and his coworkers have demonstrated in their classical investigation soap bubbles of diameter about 1.2 mm floating on the surface of water interact according to a potential closely resembling that existing in close-packed metals. A raft made of a randomized mixture of two different sizes of bubbles in roughly equal numbers has many structural characteristics of a metallic glass and can serve as a quantitatively accurate analog for it. The two-dimensional pair distribution function of such a raft resembles those in glassy metals. In addition this structure permits a simple definition of free area as a two-dimensional analog of free volume and the accurate determination of its distribution. When sheared the rafts are observed to change shape by a collection of very local shear transformations. These transformations are in the nature of either relatively equiaxed regions of about 5 bubble diameters undergoing complex internal rearrangements or are in the form of two-dimensional slip patches involving sharp shear translations of two adjacent nearly close packed bubble rows that are about 5 bubbles in length. These observations lead by analogy to the postulate that in metallic glasses the corresponding three-dimensional processes are also shear transformations in correspondingly small volume elements. These are either in the form of equiaxed regions undergoing some redundant internal rearrangements of atoms imposed by the specific disordered structure or are in the form of very narrow pennyshaped shear transformations resembling dislocation loops.

Toughening mechanism of rubber-modified polyamides

Abstract Rubber-modified polyamides were probed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), small-angle X-ray scattering and various mechanical tests. TEM studies showed that in tough samples the crystalline orientation in the interparticle region is of a distinctly different character. The lamellae are organized perpendicular to the rubber-matrix interface, while the hydrogen-bonded planes of low slip resistance are aligned parallel to these interfaces. Based on this observation a model is proposed to elucidate the deformation and toughening mechanisms of these materials. Further SEM studies in the stress-whitened regions of both the tensile bars and Izod specimens revealed the evolution of a cavitation process in the rubber particles. The shape and the size of the cavities in tough samples is related to the initial morphology of the matrix.