Stress relaxation

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

Yielding and flow of sheared colloidal glasses

Abstract: We have studied some of the rheological properties of suspensions of hard-sphere colloids with particular reference to behaviour near the concentration of the glass transition. First we monitored the strain on the samples during and after a transient step stress. We find that, at all values of applied step stress, colloidal glasses show a rapid, apparently elastic, recovery of strain after the stress is removed. This recovery is found even in samples which have flowed significantly during stressing. We attribute this behaviour to 'cage elasticity', the recovery of the stress-induced distorted environment of any particle to a more isotropic state when the stress is removed. Second, we monitored the stress as the strain rate of flowing samples was slowly decreased. Suspensions which are glassy at rest show a stress which becomes independent of as . This limiting stress can be interpreted as the yield stress of the glass and agrees well both with the yield stress deduced from the step stress and recovery measurements and that predicted by a recent mode coupling theory of sheared suspensions. Thus, the behaviours under steady shearing and transient step stress both support the idea that colloidal glasses have a finite yield stress. We note however that the samples do exhibit a slow accumulation of strain due to creep at stresses below the yield stress.

Stress relaxation and thermal evolution of film properties in amorphous carbon

Abstract: A model for the stress relaxation of amorphous carbon films containing high concentrations of fourfold coordinated carbon is presented. The onset of stress relaxation in these materials occurs following thermal annealing at temperatures as low as 100°C, and near full stress relaxation occurs after annealing at 600°C. The stress relaxation is modeled by a series of first order chemical reactions which lead to a conversion of some fourfold coordinated carbon atoms into threefold coordinated carbon atoms. The distribution of activation energies for this process is derived from the experimental measurements of stress relaxation and is found to range from 1 eV to over 3 eV. Permanent increases in the electrical conductivity of the carbon films are also found following thermal annealing. The electrical conductivity is found to be exponentially proportional to the number of additional threefold atoms which are created upon annealing, with the increase in threefold atom concentration being deduced from the stress relaxation model. This indicates that the increase in electrical conductivity and the stress relaxation originate from the same fourfold to threefold conversion process and that electrical transport through these films is dominated by a hopping conduction process.

SiGe Coherent Islanding and Stress Relaxation in the High Mobility Regime

Abstract: Real-time stress measurements during Si{sub 8 }Ge{sub 2}/Si(001) heteroepitaxy, combined with {ital ex situ} microscopy, are used to examine islanding dynamics under conditions of relatively low strain and high adatom mobility, where morphological evolution bypasses dislocation formation. We show that growth in this regime proceeds similarly to growth of Ge/Si(001) (i.e., at high strain, low temperature), but with the length scales expanded by the reduced strain. This greatly facilitates measurement of the coupled kinetics of morphological evolution and stress relaxation. {copyright} {ital 1997} {ital The American Physical Society}

Inhomogeneous cartilage properties enhance superficial interstitial fluid support and frictional properties, but do not provide a homogeneous state of stress

Abstract: It has been well established that articular cartilage is compositionally and mechanically inhomogenous through its depth. To what extent this structural inhomogeneity is a prerequisite for appropriate cartilage function and integrity is not well understood. The first hypothesis to be tested in this study was that the depth-dependent inhomogeneity of the cartilage acts to maximize the interstitial fluid load support at the articular surface, to provide efficient frictional and wear properties. The second hypothesis was that the inhomogeneity produces a more homogeneous state of elastic stress in the matrix than would be achieved with uniform properties. We have, for the first time, simultaneously determined depth-dependent tensile and compressive properties of human patellofemoral cartilage from unconfined compression stress relaxation tests. The results show that the tensile modulus increases significantly from 4.1 +/- 1.9 MPa in the deep zone to 8.3 +/- 3.7 MPa at the superficial zone, while the compressive modulus decreases from 0.73 +/- 0.26 MPa to 0.28 +/- 0.16 MPa. The experimental measurements were then implemented with the finite-element method to compute the response of an inhomogeneous and homogeneous cartilage layer to loading. The finite-element models demonstrate that structural inhomogeneity acts to increase the interstitial fluid load support at the articular surface. However, the state of stress, strain, or strain energy density in the solid matrix remained inhomogeneous through the depth of the articular layer, whether or not inhomogeneous material properties were employed. We suggest that increased fluid load support at the articular surface enhances the frictional and wear properties of articular cartilage, but that the tissue is not functionally adapted to produce homogeneous stress, strain, or strain energy density distributions. Interstitial fluid pressurization, but not a homogeneous elastic stress distribution, appears thus to be a prerequisite for the functional and morphological integrity of the cartilage.