Stress relaxation

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

Effect of stress relaxation on distortion in additive manufacturing process modeling

Abstract: A method for modeling the effect of stress relaxation at high temperatures during laser direct energy deposition processes is experimentally validated for Ti-6Al-4V samples subject to different inter-layer dwell times. The predicted mechanical responses are compared to those of Inconel® 625 samples, which experience no allotropic phase transformation, deposited under identical process conditions. The thermal response of workpieces in additive manufacturing is known to be strongly dependent on dwell time. In this work the dwell times used vary from 0 to 40 s. Based on past research on ferretic steels and the additive manufacturing of titanium alloys it is assumed that the effect of transformation strain in Ti-6Al-4V acts to oppose all other strain components, effectively eliminating all residual stress at temperatures above 690 °C. The model predicts that Inconel® 625 exhibits increasing distortion with decreasing dwell times but that Ti-6Al-4V displays the opposite behavior, with distortion dramatically decreasing with lowering dwell time. These predictions are accurate when compared with experimental in situ and post-process measurements.

Stress Relaxation and Self-Adhesion of Rubbers with Exchangeable Links

Abstract: To get intrinsic adhesion or healing properties in a cross-linked rubber, two levers are possible: the inherent relaxation in permanently but lightly cross-linked elastomers or the relaxation due to exchange reactions as recently reported in the case of vitrimers. The former is associated with dangling chain motion and cannot be controlled. Lightly cross-linked rubbers may show interesting adhesion and healing properties, but at the expense of mechanical properties: ultimate properties are limited, and materials are subjected to creep even at low temperature. Conversely, exchange reactions may be triggered by temperature, providing the healable materials with strong elastomeric properties in a wide temperature range. Here a model system based on epoxidized natural rubber is presented which enables to differentiate and quantify the part of each process. A comparative study including stress relaxation, swelling experiments, and adhesion measurements highlights the advantages of the vitrimer chemistry on inh...

Flat GaN epitaxial layers grown on Si(111) by metalorganic vapor phase epitaxy using step-graded AlGaN intermediate layers

Abstract: In this work, we report on the growth by metalorganic vapor phase epitaxy (MOVPE) of GaN layers on AlN/Si(111) templates with step-graded AlGaN intermediate layers. First, we will discuss the optimization of the AlN/Si(111) templates and then we will discuss the incorporation of step-graded AlGaN intermediate layers. It is found that the growth stress in GaN on high-temperature (HT) AlN/Si(111) templates is compressive, although, due to relaxation, the stress we have measured is much lower than the theoretical value. In order to prevent the stress relaxation, step-graded AlGaN layers are introduced and a crack-free GaN epitaxial layer of thickness >1 µm is demonstrated. Under optimized growth conditions, the total layer stack, exceeding 2 µm in total, is kept under compressive stress, and the radius of the convex wafer bowing is as large as 119 m. The crystalline quality of the GaN layers is examined by high-resolution x-ray diffraction (HR-XRD), and the full-width-at-half maximums (FWHMs) of the x-ray rocking curve (0002) ω-scan and (−1015) ω-scan are 790 arc sec and 730 arc sec, respectively. It is found by cross-sectional transmission electron microscopy (TEM) that the step-graded AlGaN layers terminate or bend the dislocations at the interfaces.

Stress Relaxation, Dynamics, and Plasticity of Transient Polymer Networks

Abstract: We propose a theoretical framework for dealing with a transient polymer network undergoing small deformations, based on the rate of breaking and reforming of network cross-links and the evolving elastic reference state. In this framework, the characteristics of the deformed transient network at microscopic and macroscopic scales are naturally unified. Microscopically, the breakage rate of the cross-links is affected by the local force acting on the chain. Macroscopically, we use the classical continuum model for rubber elasticity to describe the structure of the deformation energy, whose reference state is defined dynamically according to when cross-links are broken and formed. With this, the constitutive relation can be obtained. We study three applications of the theory in uniaxial stretching geometry: for the stress relaxation after an instantaneous step strain is imposed, for the stress overshoot and subsequent decay in the plastic regime when a strain ramp is applied, and for the cycle of stretching ...

Strain measurements by convergent-beam electron diffraction: The importance of stress relaxation in lamella preparations

Abstract: Local convergent-beam electron diffraction (CBED) patterns have been acquired on focus ion beam prepared samples in order to determine the strain field generated by a NiSi layer in a n-MOS transistor A broadening of the high order Laue zone lines in the transmitted disk of CBED patterns is observed when approaching the NiSi∕Si interface We show that this broadening is mainly due to the atomic plane bending that occurs as a result of the stress relaxation during the preparation of the thin lamella From the analysis of this relaxation, we are able to determine the initial stress state of the bulk structure The presented CBED procedure appears to be a promising tool to measure the strain and stress in any layer or structure deposited on a crystalline substrate