On inelasticity of damaged quasi rate independent anisotropic materials

This paper deals with a body that has a random 3D-distribution of two phase inclusions: spheroidal mutually parallel voids, and differently oriented reinforcing parallel stiff spheroidal short fibers. By the effective field approach the effective stiffness fourth order tensor is formulated and found numerically. Simultaneous and sequential embeddings of inclusions are compared. Damage evolution is described by a modified Vakulenko approach to the endochronic thermodynamics. A brief account of the problem of effective elastic symmetry is considered. The results of the theory are applied to the damage-elasto-viscoplastic strain of a reactor stainless steel AISI 316H.

Viscoelasticplastic–Fracture Modeling of Asphalt Mixtures Under Monotonic and Repeated Loads

Rutting and cracking occur simultaneously in asphalt mixtures as observed in the field and in the laboratory. Existing mechanical models have not properly addressed viscoelastic and viscoplastic deformation together with cracking attributable to model deficiencies, parameter calibration, and numerical inefficiency. This study developed viscoelasticplastic–fracture (VEPF) models for the characterization of viscoelasticity by Prony model and viscoplasticity by Perzyna’s flow rule with a generalized Drucker–Prager yield surface and a nonassociated plastic potential. Viscofracture damage was modeled by a viscoelastic Griffith criterion and a pseudo J-integral Paris’s law for crack initiation and propagation, respectively. The VEPF models were implemented in a finite element program by using a weak form partial differential equation modeling technique without the need for programming user-defined material subroutines. Model parameters were derived from fundamental material properties by using dynamic modulus, strength, and repeated load tests. Simulations indicated that the viscoelastic–viscoplastic–viscofracture characteristics were effectively modeled by the VEPF models for asphalt mixtures at different confinements and temperatures. An asphalt mixture under monotonic compressive loads exhibited a sequenced process including a pure viscoelastic deformation stage, a coupled viscoelastic–viscoplastic deformation stage, a viscoelastic–viscoplastic deformation coupled with a viscofracture initiation and a propagation stage, and then a viscoelastic–viscofracture rupture stage with saturated viscoplastic deformation. The asphalt mixture under repeated loads yielded an increasing viscoplastic strain at an increasing rate during the first half of the haversine load, while the increment of the viscoplastic strain (per load cycle) decreased with load cycles. The finite element program, which is based on a partial differential equation, effectively modeled the coupled viscoelastic–viscoplastic–viscofracture behaviors of the asphalt mixtures.

Curvature of unsymmetric cross-ply laminates: Combined effect of thermal stresses, microcracking, viscoplastic and viscoelastic strains

Curvature of unsymmetrical [0/90] specimens caused by thermal stresses changes if the specimen is subjected to large axial strains introducing intralaminar cracks in the 90-layer. It is shown that the large curvature reduction can not be explained by cracking related stress release only. The large irreversible viscoplastic strains introduced during the axial tensile loading (with 5 min holding at high strain for crack counting) give the main contribution to the curvature change. The effect of transient viscoelasticity (VE) was found to be of minor significance. Simple approach based on effective damaged layer stiffness and constant irreversible strain is used in the framework of laminate theory to extract the viscoplastic and VE strains from experimental curvature data. The obtained fitting expressions for viscoplastic- and VE-strain development are successfully used to describe curvature change in [0/902] laminate subjected to the same test procedure. It is suggested that the used curved beam tests could be efficient to characterize the viscoplastic strain development in the thin 90-layers.

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–Viscoplastic Cyclic Deformation of Polycarbonate Polymer: Experiment and Constitutive Model

A series of uniaxial tests (including multilevel loading–unloading recovery, creep-recovery, and cyclic tension–compression/tension ones) were performed to investigate the monotonic and cyclic viscoelastic–viscoplastic deformations of polycarbonate (PC) polymer at room temperature. The results show that the PC exhibits strong nonlinearity and rate-dependence, and obvious ratchetting occurs during the stress-controlled cyclic tension–compression/tension tests with nonzero mean stress, which comes from both the viscoelasticity and viscoplasticity of the PC. Based on the experimental observation, a nonlinear viscoelastic–viscoplastic cyclic constitutive model is then constructed. The viscoelastic part of the proposed model is constructed by extending the Schapery's nonlinear viscoelastic model, and the viscoplastic one is established by adopting the Ohno–Abdel-Karim's nonlinear kinematic hardening rule to describe the accumulation of irrecoverable viscoplastic strain produced during cyclic loading. Furthermore, the dependence of elastic compliance of the PC on the accumulated viscoplastic strain is considered. Finally, the capability of the proposed model is verified by comparing the predicted results with the corresponding experimental ones of the PC. It is shown that the proposed model provides reasonable predictions to the various deformation characteristics of the PC presented in the multilevel loading–unloading recovery, creep-recovery, and cyclic tension–compression/tension tests.