Stress relaxation

About: Stress relaxation is a research topic. Over the lifetime, 12959 publications have been published within this topic receiving 270815 citations.

Strain-induced crystallization of poly(ethylene terephthalate)

Abstract: The fabrication of poly(ethylene terephthalate), PET, into fibers, films, and containers usually involves molecular orientation caused by molecular strain, which may lead to stress- or strain-induced crystallization (SIC). The SIC of PET was studied by the methods of birefringence, density, thermal analysis, light scattering, and wide-angle X-ray. The development of crystallinity is discussed in relation to the rate of crystallization, the residual degree of orientation, and stress relaxation. The experimental procedure involves stretching samples at temperatures above the glass transition temperature, Tg, to a given extension ratio and at a specific strain rate of an Instron machine. At the end of stretching, the sample is annealed in the stretched state and at the stretching temperature for various periods of time, after which the sample is quickly quenched to room temperature for subsequent measurements. During stretching, the stress strain and the stress relaxation curves are recorded. The results indicate that the SIC of annealed, stretched PET can proceed in three different paths depending on the residual degree of orientation. At a low degree of residual orientation, as indicated by the birefringence value, annealing of stretched PET leads only to molecular relaxation, resulting in a decrease of birefringence. At intermediate orientation levels, annealing causes an initial decrease in birefringence followed by a gradual increase and finally a leveling off of birefringence after a fairly long period of time. At higher orientation levels, annealing causes a rapid increase in birefringence before leveling off. The interpretation of the above results is made using the measurements of light scattering, differential scanning calorimetry, and wide-angle X-ray. The rate of the SIC of PET is also discussed in terms of specific data analysis.

On the influence of stress state, stress level and temperature on γ-channel widening in the single crystal superalloy CMSX-4

Abstract: The present investigation describes the kinetics of γ-channel widening in the single-crystal superalloy CMSX-4 at temperatures above 1000 °C and at stress levels between 50 and 100 MPa. A miniature tensile creep specimen (uniaxial loading in 〈0 0 1〉-direction) and a double shear creep specimen (biaxial loading of the macroscopic crystallographic shear system {0 1 1} 〈0 1 1 〉 ) were used for interrupted creep testing. The volume fractions of the γ- and γ′-phases and the kinetics of γ-channel widening were investigated using high resolution scanning electron microscopy in combination with quantitative metallography. It was found that rafting always occurred perpendicularly to the direction of the maximum principal stress (i.e. perpendicular to the tensile direction in the tensile specimens and in 45° to the shear stress for the shear tests). Over the whole stress range, there was no significant difference between γ-channel widening in 〈0 0 1〉-tension and {0 1 1} 〈0 1 1 〉 -shear creep deformation, when the experiments were performed at the same maximum principal stress. γ-Channel widening is an important process during creep of γ/γ′-microstructures because dislocations propagate in the thin γ-channels. Our present study shows how temperature, stress level and stress state affect this microstructural coarsening process. Superimposed stresses favour the γ-phase, and therefore channel widths increase faster with increasing superimposed stresses.

Influence of strain on semiconductor thin film epitaxy

Abstract: Under typical growth conditions, strain levels greater than or equal to 10−4 are shown to influence thin film surface morphology and strain relaxation pathways. Misfit and threading dislocations in relaxed heterostructures produce long wavelength undulations on the surface and shallow depressions, respectively. Threading dislocation densities greater than ∼105–106 cm−2 in relaxed heterostructures must be due to increased impediments to dislocation motion, which in turn originate from the effect of the misfit dislocations on the surface morphology. Under typical growth conditions, the origin of strain-induced surface features can be identified by recognizing the length scale at which the features occur.

Thickness-dependent magnetism and spin-glass behaviors in compressively strained BiFeO3 thin films

Abstract: Compressively strained BiFeO3 (BFO) films from 19 to 114 nm are epitaxially grown on LaAlO3 substrates, and their thickness-dependent evolutions of structural and magnetic properties are investigated. Across the morphotropic phase boundary, complex strain relaxation behaviors involving low-symmetry intermediate/bridging phases are observed. The fully strained 38 nm BFO film exhibits a saturation magnetization of ∼28 emu/cm3 at 300 K with a coercivity of 130 Oe while all films show a spin-glass behavior. These findings suggest that tailoring film thickness is effective to suppress the cycloidal magnetic modulation in BFO, leading to magnetic properties different from the bulk counterpart.