A finite-deformation constitutive model of particle-binder composites incorporating yield-surface-free plasticity

We present a constitutive model for particle-binder composites that accounts for finite-deformation kinematics, nonlinear elasto-plasticity without apparent yield, cyclic hysteresis, and progressive stress-softening before the attainment of stable cyclic response. The model is based on deformation mechanisms experimentally observed during quasi-static monotonic and cyclic compression of mock Plastic-Bonded Explosives (PBX) at large strain. An additive decomposition of strain energy into elastic and inelastic parts is assumed, where the elastic response is modeled using Ogden hyperelasticity while the inelastic response is described using yield-surface-free endochronic plasticity based on the concepts of internal variables and of evolution or rate equations. Stress-softening is modeled using two approaches; a discontinuous isotropic damage model to appropriately describe the softening in the overall loading-unloading response, and a material scale function to describe the progressive cyclic softening until cyclic stabilization. A nonlinear multivariate optimization procedure is developed to estimate the elasto-plastic model parameters from nominal stress-strain experimental compression data. Finally, a correlation between model parameters and the unique stress-strain response of mock PBX specimens with differing concentrations of aluminum is identified, thus establishing a relationship between model parameters and material composition.

Anisotropic stress softening of residually stressed solids

Residual stress in purely elastic solids has been extensively studied in the literature. However, to the best of the author’s knowledge, the influence of residual stresses on anisotropic Mullins materials has not been studied. Hence, the aim of this paper is to propose an anisotropic phenomenological model to describe the Mullins phenomena for residually stressed elastomers; taking note that most materials are not purely elastic and some of them exhibit an anisotropic stress-softening phenomenon widely known as the Mullins effect. The anisotropic model is based on the use of direction-dependent damage parameters and a set of anisotropic spectral invariants presented recently in the literature by the author. The spectral invariants have a clear physical meaning that is useful in aiding the design of a rigorous experiment to construct a specific form of constitutive equation. Since boundary value results for residually stressed Mullins material are not found in the literature, the effect of residual stresses on the Mullins phenomena in simple tension, torsion and equibiaxial deformations is discussed in this paper.

Effect of masterbatch drying methods on the properties of rubber reinforced with renewable hydrophilic filler

Hydrophilic fillers contain functional groups capable of forming hydrogen and/or ionic bonds. Many recent developments with biobased fillers are masterbatch process with rubber latex. The effect of the different process influences the characteristics of filler network and therefore rubber properties. In this study, the rubbers reinforced with hydrophilic filler, soy protein particles, and carbon black processed in two different methods, casting and freeze-drying methods, are investigated using crosslink density, dynamic mechanical properties, stress softening effect, stress relaxation, tensile properties, and thermal degradation. Stress softening effect is analyzed with the Kraus model and shows that the characteristic strains shifted to smaller strains for the rubbers prepared by a casting process. Stress relaxation of the reinforced rubber prepared from the two different processes shows that the rubbers from the casting process have slower relaxation rates because of higher crosslink density and modulus. Overall, the rubber composites prepared by casting method have higher crosslink density, greater softening effect, slower rate of stress relaxation, and higher moduli attributed to greater interactions between hydrophilic components in the reinforced rubber.

Hot Deformation Behavior of a Ni-Based Superalloy with Suppressed Precipitation

Inconel®718 is a well-known nickel-based super-alloy used for high-temperature applications after thermomechanical processes followed by heat treatments. This work describes the evolution of the microstructure and the stresses during hot deformation of a prototype alloy named IN718WP produced by powder metallurgy with similar chemical composition to the matrix of Inconel®718. Compression tests were performed by the thermomechanical simulator Gleeble®3800 in a temperature range from 900 to 1025 °C, and strain rates scaled from 0.001 to 10 s−1. Flow curves of IN718WP showed similar features to those of Inconel®718. The relative stress softening of the IN718WP was comparable to standard alloy Inconel®718 for the highest strain rates. Large stress softening at low strain rates may be related to two phenomena: the fast recrystallization rate, and the coarsening of micropores driven by diffusion. Dynamic recrystallization grade and grain size were quantified using metallography. The recrystallization grade increased as the strain rate decreased, although showed less dependency on the temperature. Dynamic recrystallization occurred after the formation of deformation bands at strain rates above 0.1 s−1 and after the formation of subgrains when deforming at low strain rates. Recrystallized grains had a large number of sigma 3 boundaries, and their percentage increased with strain rate and temperature. The calculated apparent activation energy and strain rate exponent value were similar to those found for Inconel®718 when deforming above the solvus temperature.

A Finite Strain Analytical Solution for Stress-Softening of Hyperelastic Materials Under Cyclic Bending

In this paper, a new approach for stress-softening of an isotropic, incompressible, hyperelastic and rectangular beam that undergoes cyclic bending-unbending deformation, is presented. Employing an exponential softening function, damage response of the hyperelastic beam due to cyclic finite bending is investigated. The stress-softening phenomenon occurs in elastomeric materials when they deform for the first time. Under the same deformation, the stress required in reloading is smaller than the initial loading stage. This is known as the Mullins effect. To verify the accuracy of the proposed solution, finite element analysis of the same problem is carried out. In this study, a principal stretch-based strain energy function i.e., Ogden model and an invariant-based function such as a newly introduced Exp–Exp model are used for all bending, unbending and re-bending procedures. The proposed method needs a much shorter time compared to FEM simulations. Thus, in design and optimization of the structures under bending that requires a large number of analyses, the proposed semi-analytical solution can be considered as an efficient tool for studying the effects of different material and geometrical parameters.