Stress relaxation

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

Influence of AlN nucleation layers on growth mode and strain relief of GaN grown on 6H–SiC(0001)

Abstract: We study the growth mode and strain state of GaN layers grown either directly on 6H–SiC(0001) or on thin (5 nm), coherently strained AlN nucleation layers. Using a combination of structural, optical, and vibrational characterization methods, we show that the 3.4% compressive lattice mismatch strain is fully relieved in the former case, whereas in the latter case a significant amount (0.3%) remains even after 1 μm of growth. This finding is clarified by in situ reflection high-energy electron diffraction and transmission electron microscopy. We demonstrate that the strain state of the GaN layer is determined by its growth mode, which in turn is governed by the degree of wetting of the underlayer rather than by lattice mismatch.

Measurement of biaxial and uniaxial extensional flow behavior of polymer melts at constant strain rates

Abstract: Biaxial extensional flow behavior of polystyrene and polypropylene melts was measured utilizing the lubricated squeezing flow method under the condition of constant strain rate. The uniaxial extensional flow behavior of these samples was also measured using a Meissner‐type rheometer for comparison with the biaxial behavior. The biaxial stress growth coefficient η B + grows more slowly than the uniaxial stress growth coefficient η E + at long times. Deviation of η B + from the low strain rate asymptote is not so prominent as that of η E +. The deviation of η B + and the upturn of η E + from the low strain rate asymptotes are more apparent in broad distribution samples than in a narrow distribution sample. It is found that η B + decreases with strain rate at long times near steady state. The upturn behavior of η E + at high strain rates in a narrow distribution sample is discussed based on the Doi–Edwards theory and stress relaxation data. The upturn is clearly observed when the strain rate exceeds the contraction (retraction) rate τeq −1, where the contraction time τeq is evaluated from the relaxation data. Consideration on chain stretching in various multi‐axial extensional flows gives a prediction for deviation of stress growth coefficients from the low strain rate asymptotes in uniaxial, biaxial, planar, and ellipsoidal extensional flows.

Competition Between Stress Generation and Relaxation During Oxidation of an Fe± Cr± Al± Y Alloy

Abstract: The residual biaxial stress inAl2O3 scales formed duringoxidation of Fe-22.0%Cr-4.8%Al-0.3%Y in the temperaturerange 1000-1300°C have been measured usingCr3+ luminescence piezospectroscopy. Thestress measurements together with substantialdimensional changes of the specimens show that oxidationproduces high compressive growth stresses in theAl2O3 scale in addition tothermal-mismatch stresses that arise during cooling from the oxidationtemperature. The magnitude of the growth stress isdetermined by two opposing processes: stress generation,associated with lateral growth of the scale, and stress relaxation in both the metal and oxide. Thescale lateralgrowth strain continuously increasesconcurrently with the scale thickening. Creep in theoxide is a major relaxation process when the scale isthin, while metal deformation becomes significantduring the later stages of oxidation at highertemperatures.