Rate mechanisms of plasticity in semi-crystalline polyethylene

Based on our experiments on polyethylene where we have observed a constant level of plastic resistance, independent of lamella thickness exceeding 40 nm, we have fundamentally re-considered the rate controlling mechanisms of crystal plasticity in semi-crystalline polymers. In this we have not only re-examined and made modifications to the widely accepted mechanism of Young (Young RJ. Mater Forum 1988;11:210.) of monolithic nucleation of screw dislocations from edges of crystalline lamellae predicting an increase in plastic resistance with increasing lamella thickness, but we are proposing here two new modes of nucleation of both edge and screw dislocation half loops from lamella faces that are independent of lamella thickness. These two new modes of dislocation nucleation explain well the observed transition from a plastic resistance increasing with lamella thickness to one of constant resistance above a lamella thickness of ca. 35 nm in polyethylene. They also provide a more satisfactory framework to explain the temperature and strain rate dependence of the plastic resistance of polyethylene and predict the observed levels of activation volumes determined by us.     

Piecewise regression model construction with sample efficient regression tree (SERT) and applications to semiconductor yield analysis

Forward stepwise regression analysis selects critical attributes all the way with the same set of data. Regression analysis is, however, not capable of splitting data to construct piecewise regression models. Regression trees have been known to be an effective data mining tool for constructing piecewise models by iteratively splitting data set and selecting attributes into a hierarchical tree model. However, the sample size reduces sharply after few levels of data splitting causing unreliable attribute selection. In this research, we propose a method to effectively construct a piecewise regression model by extending the sample-efficient regression tree (SERT) approach that combines the forward selection in regression analysis and the regression tree methodologies. The proposed method attempts to maximize the usage of the dataset's degree of freedom and to attain unbiased model estimates at the same time. Hypothetical and actual semiconductor yield-analysis cases are used to illustrate the method and its effective search for critical factors to be included in the dataset's underlying model.     

Argon: Systematic Review on Neuro- and Organoprotective Properties of an “Inert” Gas

Argon belongs to the group of noble gases, which are regarded as chemically inert. Astonishingly some of these gases exert biological properties and during the last decades more and more reports demonstrated neuroprotective and organoprotective effects. Recent studies predominately use in vivo or in vitro models for ischemic pathologies to investigate the effect of argon treatment. Promising data has been published concerning pathologies like cerebral ischemia, traumatic brain injury and hypoxic ischemic encephalopathy. However, models applied and administration of the therapeutic gas vary. Here we provide a systematic review to summarize the available data on argon’s neuro- and organoprotective effects and discuss its possible mechanism of action. We aim to provide a summary to allow further studies with a more homogeneous setting to investigate possible clinical applications of argon.     

Note: Micromechanical testing of toughness in nylon 6/PDMS block copolymers and blends

Small specimens of nylon 6/PDMS compositions were screened for relative levels of toughness using a micromechanical testing device. Measurements were made of the total crack length emanating from pre-cut notches in a specially designed specimen. At room temperature, cracks propagated readily in nylon 6 homopolymer and somewhat more slowly in a composition containing 4 wt% PDMS; at 10 and 28 wt% PDMS the cracks were blunted in a diffuse whitened zone and did not propagate through the specimens. At −40°C, the 10 wt% blend appeared to be as tough as the 4 wt% blend at room temperature.     

Role of controlled debonding along fiber/matrix interfaces in the strength and toughness of metal matrix composites

In metal matrix composites toughness is derived primarily from the plastic work of rupture of ductile matrix ligaments between the fractured fibers and from the plastic work of simple shear separation along steps connecting major fracture terraces. In the optimization of tensile strength in the longitudinal and transverse directions together with the respective works of fracture the most important factor is the control of the extent of debonding along interfaces between the fibers and the matrix, which develops locally in the course of deformation in a continuously changing mix of modes. In Al alloy matrix composites reinforced with Al₂O₃ fibers an effective means of controlling the key interface fracture toughness is through coarsening of Al₂Cu intermetallic interface precipitates which prescribe a ductile fracture separation layer. A combined experimental approach and micromechanical modeling, utilizing a specially tailored novel tension/shear: traction/separation law provides the means for further optimization of overall behavior. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Steady-state creep of single-phase crystalline matter at high temperature

Over the past 15 years important advances have been made in the experimental study of the microstructural changes occurring during the non-linear steady-state creep of single phase crystalline matter at elevated temperatures. Curiously, although the results of these painstaking studies have gone a long way toward elucidating the mechanism of this phenomenon, they have been largely ignored in favour of some simple dislocation mechanisms that are not only inconsistent with these observations, but are also unable to describe correctly the known phenomenology. This review concentrates primarily on the recent experiments on microstructural alterations occurring during creep; however, it also surveys the many mechanistic models that attempt to describe this phenomenon, and finds them all deficient.On leave from The Institute for Solid State Physics, The University of Tokyo, during 1974–1975.     

Interfaces with controlled toughness as mechanical fuses to isolate fibres from damage

A source of inadequate performance of metal matrix composites has been the loss of strength due to the reaction layers between the fibre and the surrounding metal matrix. Here, we propose that the traditional diffusion barrier coatings on the fibre can be utilized to serve as mechanical fuses to isolate the impinging reaction zone cracks by interface delamination. Requirements on the interface strength and toughness for the specific tailoring of the fibre-coating interface are given. Special problems associated with the graphite-aluminium system are identified. A double cantilever beam experiment has been developed to measure the work of separation of thin coatings (0.1 to 0.3 μm) from bulk substrates. This test has been successfully applied to measure the work of fracture of the interface between a planar pyrolytic graphite substrate with the same chemistry and closely related microstructure as that of the 10 μm Pitch-55 graphite fibre and SiC coatings on them. A value of 60 J m⁻² was obtained for the critical energy release rate for the PG-SiC interface. Additional measurements of energy release rates in thin layers of glue used to model the aluminium matrix and in PG itself, have given values which account for the high toughness of the main interfaces through the accompanying inelastic deformation work in the glue and the PG while the crack travels along the interface.     

Plasticity by martensite transformations in cobalt base metallic glasses?

The possibility of mechanical coupling to applied stresses of martensitic transformations in Co₈₀Nb₁₄B₆ and C0₈₄Nb₁₀B₆ metallic glasses reported by O'Handley and co-workers in the temperature range 295 to 473 and 513 K, respectively, was investigated. Since martensitic transformations strongly couple with externally applied shear stresses, internal friction scans and plastic resistance measurements in both tension and plane-strain compression were carried out in thermal cycles in the same temperature range, complemented with differential scanning calorimetry observations. No transformation plasticity was detected in these glasses as internal friction showed no peaks, DSC showed no heat evolution or absorption, and the temperature dependence of flow stress exhibited a monotonically decreasing behaviour.Visiting Associate Professor in the Department of Mechanical Engineering (1984–6), on leave from Middle East Technical University, Ankara, Turkey.     

Morphological mechanisms and kinetics of large-strain plastic deformation and evolution of texture in semicrystalline polymers

The development of deformation-induced texture in high-density polyethylene is described from the point of view of a micromechanically based composite computer model of crystalline lamellae associated with attached amorphous layers. The predictions of the model are compared and contrasted with detailed recent experimental observations of this process by WAXS, SAXS, TEM, atomic force microscopy, light microscopy and a whole host of conventional and unconventional means of mechanical probing. A separate set of experiments probing the rate controlling process of chain slip in quasi-single-crystalline nylon-6 are also presented together with implications related to the kinetics of long-range dislocation motion in such highly textured solids.