Triaxial Creep Behavior of Red Sandstone in Freeze-Thaw Environments

To investigate the time-dependent mechanical properties of rock masses in cold regions under the effects of freeze-thaw cycling and long-term loading, triaxial multilevel loading and unloading creep tests were performed on saturated red sandstone samples subjected to different numbers of freeze-thaw cycles. The effects of freeze-thaw cycles and confining pressure on the creep properties, long-term strength, and creep failure mode of the rock were analyzed. The effect of freeze-thaw cycles on the microstructure of the rock was analyzed using scanning electron microscopy. The results showed that as the number of freeze-thaw cycles increased, the rock particle boundaries became more distinct, and more pores formed. The effect of freeze-thaw cycles on the creep deformation of red sandstone was related to the loading stress level. At low stress levels, the rock viscoelastic strain increased gradually and almost linearly with an increasing number of freeze-thaw cycles; in contrast, at high stress levels, the rock viscoelastic strain increased nonlinearly. The viscoplastic strain increased almost linearly with increasing freeze-thaw cycles. The fourth loading stress level (70% σ c ) corresponded to the transition of the creep deformation of the red sandstone. When the confining pressure was low, a higher stress level caused the confining pressure to have a more significant effect on the creep strain. However, as the confining pressure continued to increase, the effect of the confining pressure on the creep strain eventually disappeared. The long-term strength of the red sandstone decreased approximately linearly with an increase in the number of freeze-thaw cycles. When the number of freeze-thaw cycles and the confining pressure were high, the rock samples formed a transverse shear plane and were more fragmented than those without a transverse shear plane. These results provide a reference for construction in rock mass engineering and long-term stability analysis in cold regions.

Nonlinear Material Model for Quasi-Unidirectional Woven Composite Accounting for Viscoelastic, Viscous Deformation, and Stiffness Reduction

To clarify the individual contribution of viscoelastic and viscous deformation to the global nonlinear response of composites, multilevel cyclic loading-unloading recovery tensile tests were carried out. The experimental results show that there is a linear relationship between the viscous strain and viscoelastic strain of composites, regardless of the off-axis angle or loading stress level. On the basis of experimental results, a coupled damage-plasticity constitutive model was proposed. In this model, the plasticity theory was adopted to assess the evolution of viscous strains. The viscoelastic strain was represented as a linear function of viscous strains. Moreover, the Weibull function of the effective stress was introduced to evaluate the damage variables in terms of stiffness reduction. The tensile stress-strain curves, predicted by the proposed model, showed a good agreement with experimental results.

Effect of degree of cure on viscoplastic shear strain development in layers of [45/−45]s glass fibre/ epoxy resin composites

Effect of degree of cure on irreversible (viscoplastic) shear strain development in layers of glass fibre/ epoxy resin (LY5052 epoxy resin) [+45 °/−45 °]s laminate is studied performing a sequence of constant stress creep and viscoelastic strain recovery tests. For fixed values of degree of cure in range from 79.7% to 100%, the viscoplastic strains were measured as dependent on time and stress and Zapa's integral representation was used to characterize the observed behaviour. It is shown that at all degrees of cure the viscoplastic behaviour can be described by Zapa's model with parameters dependent on degree of cure. It is shown that for degree of cure lower than 80% the viscoplastic strains grow much faster and are much more sensitive to the increase of the applied shear stress. These irreversible strains developing in the final phase of the curing can significantly alter the residual stress state in the composite structure.

Microdamage, Viscoelasticity and Viscoplasticity as Main Phenomena in Thermal Stress Relaxation in Laminated Composites

Microdamage, viscoplastic and viscoelastic strain development in 90-layers of cross-ply laminates subjected to tensile loading is studied on unsymmetrical GF/EP laminates measuring the thermal curvature change. All three phenomena partially compensate for the effect of the thermal mismatch reducing the residual stress (specimen curvature). The viscoplastic strain contribution to curvature change is the largest whereas the effect of transient viscoelasticity is the smallest. Damage is included in the analysis through its effect on the effective transverse modulus of the 90-layer.

Viscoelastic Strain Response (ViSR) Ultrasound in Pre-Clinical and Clinical Application

Viscoelastic Strain Response (ViSR) ultrasound is a novel acoustic radiation force (ARF)-based imaging method that noninvasively interrogates the viscoelastic properties of tissue by measuring the relaxation time constant for constant stress in the Voigt biomechanical model. The time constant is defined as the ratio of coefficient of viscosity to elastic modulus, so ViSR differentiates tissue with disparate viscosities and elasticities. ViSR ultrasound is performed by delivering two successive ARF impulses to a single region of exciation (ROE) and tracking the micrometer-scale displacements induced by the propagating longitudinal waves. ViSR does not rely on transverse wave propagation, which can be disrupted and difficult to track in heterogeneous and/or geometrically complex media. Another advantage to ViSR ultrasound is a large axial range relative to conventional ARF Impulse (ARFI) ultrasound. In this overview, ViSR methods are discussed and demonstrated in calibrated viscoelastic tissue mimicking materials. ViSR ultrasound is then applied to differentiating fatty and fibrous deposition in muscle in a golden retriever muscular dystrophy (GRMD) dog model and in boys with Duchenne muscular dystrophy (DMD) with correlation to standard physical testing. ViSR is also applied to delineating the structure and composition of atherosclerotic plaques in a hypercholesterolemic pig model with histochemical validation. ViSR’s key advantages and disadvantages are discussed in regard to its general clinical utility.

Viscoelastic Strain Hardening Model for Gasket Creep Relaxation

This paper proposes a novel strain hardening model for investigating gasket creep relaxation under compressive step-loading at room temperature. A closed form solution is developed for predicting the steady-state gasket pressure. Step-loading of the gasket may be directly achieved and controlled, or indirectly estimated through the bolt tightening and retightening torque. The effect of gasket material, time duration at each stress level, as well as the geometric parameters of the gasket are investigated. An experimental procedure and test setup are used to validate the proposed gasket model.