Evolution of the stiffness tetrad in fiber-reinforced materials under large plastic strain

The main aim of this work is to track the evolution of the stiffness tetrad during large plastic strain. Therefore, the framework of a general finite plasticity theory is developed. Some special cases are examined, and the case of a material plasticity theory is considered more closely. Its main feature is that the elasticity law changes during plastic deformations, for which we develop an approach. As sample materials, we use three types of fiber-reinforced composites. For numerical experiments and verification of the model’s predictions, finite element simulations of representative volume elements for uni-, bi- and tri-directional reinforced materials with periodic boundary conditions are used. From these, we extract the stiffness tetrads before and after large deformations of the material. We quantify the change of the stiffness tetrads due to the fiber reorientation. Finally, we propose an analytical evolution with three parameters that account reasonably well for the evolution of the stiffness tetrad.

Strain-Based CTOD and J-Integral Estimations for Pipelines With a Surface Crack Under Large Plastic Strain and Internal Pressure

Engineering solutions for crack-tip opening displacement (CTOD) and J-integral estimations for pipelines with a surface crack are proposed based on parametric finite element (FE) analyses for various geometries, material properties, and internal pressure conditions. Two kinds of CTOD definitions are considered in relation to strain-based estimation solutions for dealing with confusion regarding the definition of CTOD and to extend the applicability of tensile strain capacity (TSC) assessment. Moreover, influence functions of internal pressure are also suggested to take account of the effect of internal pressure on TSC. Using the proposed solutions, TSCs for cracked X65 and X70 pipes were assessed based on initiation and ductile instability. Curved wide plate tests were performed to obtain experimental TSCs, which were compared with those from the proposed solutions. Moreover, TSCs from the proposed solutions were also compared with those from other TSC-predicted models in order to assess their validity.

Normal and dwell fatigue behavior of a near-alpha titanium alloy - IMI 834

Dwell sensitivity of titanium alloys at ambient temperature (~250 C) is a well-known phenomenon, although the question about the exact micromechanical reasons responsible for this still remains open. In this work, the normal and dwell fatigue response of a near-alpha titanium alloy, IMI 834, is studied. Samples with three different microstructures, namely, fully lamellar, fully equiaxed and bimodal, are evaluated for their dwell fatigue behaviors. A reduction in fatigue life by at least an order of magnitude is seen in all the three microstructures. Large plastic strain accumulation (almost equal to the monotonic ductility) was observed during the dwell fatigue loading condition and this is held responsible for this large debit in fatigue life. The normal fatigue lives decreased in the order, bimodal > fully equiaxed > fully lamellar, while the dwell fatigue lives decreased in the order, fully equiaxed > fully lamellar > bimodal. Bimodal microstructure showed a dwell fatigue debit of 17, while fully lamellar and fully equiaxed showed a debit of 9 and 10, respectively.

Finite thermoelastoplasticity and creep under small elastic strains

A mathematical model for an elastoplastic continuum subject to large strains is presented. The inelastic response is modelled within the frame of rate-dependent gradient plasticity for non-simple materials. Heat diffuses through the continuum by the Fourier law in the actual deformed configuration. Inertia makes the nonlinear problem hyperbolic. The modelling assumption of small elastic Green–Lagrange strains is combined in a thermodynamically consistent way with the possibly large displacements and large plastic strain. The model is amenable to a rigorous mathematical analysis. The existence of suitably defined weak solutions and a convergence result for Galerkin approximations is proved.