Increase In Internal Stress Research Articles

Thermal contraction and isometric heating of oriented polyethylene obtained by shear deformation

A study was made of morphological and structural changes taking place during thermal contraction and isometric heating of oriented, low density polyethylene samples. It was shown that in the initial stages of thermal contraction first slight interfibrillar and later intralamellar shear takes place. Subsequent increase in temperature and particularly, transition to the region of partial melting markedly increase contraction and result in a change in the shape of the sample. During the entire process of heating the polymer up to melting point no mutual disorientation of crystalline lamellae was observed in spite of the fact that before melting the order of these centres deteriorated considerably resulting in the disappearance of individual small angle X-ray scattering. Structural regions were formed which begin to melt mainly on increasing temperature. It was shown that three processes taking place in oriented low density polyethylene (extension of the long period, marked contraction and inhibition, followed by the cessation of internal stress increase in the sample) are observed in the same temperature interval, which may be due to one cause—diffusion mobility of macromolecules in polymer crystallites.

単純せん断による鉄単結晶の塑性変形

Observations were made on the plastic deformation of iron single crystals subjected to the simple shear on the single and multiple slip systems to examine the work hardening behavior. When the applied shear stress was matched to the single slip systems, the easy glide in stage I was clearly observed in the stressstrain curves. On the other hand, when the applied shear stress coincided with the multiple glide system, the linear hardening in stage II was clearly recongnized from the early stage of deformation.To examine the work hardening mechanisms, the strain rate-cycling tests were performed on crystals in easy glide and linear hardening regions and the average effective and internal stresses were estimated as a function of strain in these regions.In the easy glide in stage I. the effective stress is essentially independent of the strain whilst the internal stress increases with strain. The work hardening in stage I resulted from the increased long range internal stress field due to dislocation tangles and cell walls. On the other hand, the work hardening in stage II is caused by the increase in both internal and effective stresses. The ratio of the internal and effective stresses is approximately 2.6. This result suggests that the work hardening in stage II is associated with the long range internal stress field as well as the hardening in stage I.

Plastic deformation and Lüders band propagation in α-brass

Three methods are used to study the dislocation dynamics in α-brass. These are stress relaxation, double strain rate cycling and Lüders band propagation. All three methods yield similar values, 4 ± 1, for the dislocation velocity-stress exponent. In the four alloys studied, the internal stress accounts for 90–95% of the applied stress. The effective stress does not vary appreciably with strain suggesting that work hardening is mainly due to the increase of internal stress. For a given grain size and strain, the internal stress increases rapidly near 15% Zn, consistent with a change of dislocation arrangement at that composition.

Work hardening and dislocation structure in Ta and Ta-base alloys

Abstract The work-hardening characteristics of Ta and Ta-base alloy single crystals were investigated as a function of temperature, orientation and interstitial concentration; associated dislocation structures were characterized parallel and perpendicular to the primary slip vector. The presence or absence of stage II hardening can be related to the magnitude of the effective stress τ∗. When τ∗≃0, stage II hardening occurs and the work-hardening slope θII is a maximum. Asτ∗ increases, θII decreases, and in the limit only stage I is observed. In comparison to f.c.c. metals, there is a large degree of overshoot (∼10°), and it appears that secondary slip is a necessary condition for the existence of stage II hardening. An explanation for the dependency of stage II and the magnitude of θII on the level of τ∗ is advanced, based on two factors: (1) the probability of the stable reaction α/2[111] + α/2[111] = α[100] occurring increases with decreasing τ∗; (2) the increase in internal stress required for secondar...

Investigation of the Effect of O2, N2, and H2O on Nickel Thin Films

The role of oxygen, nitrogen, and water vapor adsorbed during the evaporation process was investigated for nickel films grown on room-temperature glass substrates. A change in pressure from 10−10 to 10−7 Torr oxygen caused a marked shift in the resonance field and the perpendicular anisotropy (i.e., the anisotropy resulting from the rotation of the magnetization out of the plane of the film). This shift could be accounted for by an increase in internal stress from 5×109 dyn/cm2 at 10−10 Torr to 2×1010 dyn/cm2 at 10−7 Torr. The wall-motion coercive force also increased with increasing pressure, ranging from 50 to greater than 200 Oe. Samples made in pressures as high as 1×10−5 Torr nitrogen appeared identical (as determined by the measurements) to films made in residual vacuum of 10−10 Torr. The presence of water vapor caused a slight increase in internal stress and Hc when the pressure changed from 10−10 to 10−5 Torr. The effect appears to be related to an internal oxidation phenomenon. These experiments clearly demonstrate the role of impurity atoms in determining the stress-sensitive magnetic properties of films and emphasises the need for controlling the partial pressures of gases during evaporation, and not just the total pressures.