Hui-Ji Shi

Effects of expanded and non-expanded hole on the delay of arresting crack propagation for aluminum alloys

The purpose of this paper is to study the effects of inserting an expanded hole or a non-expanded hole at the tip of a fatigue crack on the subsequent delay of fatigue crack propagation. Single edge pre-cracked specimens of two aluminum alloys were used to perform the crack arresting tests under cyclic stress loading. An obvious increase in the delay of crack propagation is obtained for the specimens with expanded hole. The compressive residual stresses produced by the hole expansion process play an important role in increasing the fatigue resistance of the material. The observation of micro-structure at the fatigue failure section reveals that the hole expansion process modifies the condition and properties of the surface layer of materials and affects, significantly the length of the nucleation stage of the fatigue process.

Branched crack growth behavior of mixed-mode fatigue for an austenitic 304L steel

Experiments on mixed-mode fatigue crack initiation and propagation in an austenitic stainless steel 304L were carried out using a circular ring specimen containing a V-notch on the internal radius. The branched crack behavior was experimentally investigated with respect to the direction of the inclined loading and the stress distribution near the notch root or crack tip, especially, for the second and third branched cracks. The behavior of the crack initiating from the notch root under the mixed-mode condition is identified well by the maximum tangential stress criterion. The energy release rate criterion can be used to simulate the path of the branched cracks initiating from the parent crack. Optical microscope observation was performed to examine the microstructure of the material for the tested specimens and all crack growth was typically transgranular mode

Thermomechanical fatigue of a 316L austenitic steel at two different temperature intervals

Many investigations of the fatigue and fracture behavior of 316L austenitic steel at high temperatures have been carried out, because of wide applications of this material in high temperature components in the energy, transport and nuclear industries. However, most of these studies deal with problems under isothermal test conditions, and less attention has been paid to thermomechanical fatigue (TMF) studies in comparison with its great importance in engineering applications. In the scientific literature, a few studies are found concerning the TMF behavior of 316L stainless steel at different temperature ranges. In this paper, total strain-controlled TMF tests were performed in in-phase and out-of-phase forms on an AISI 316L austenitic stainless steel. The mechanics of deformation and damage for TMF is more complex than high temperature low cycle isothermal fatigue, for in thermomechanical fatigue the stress-strain behavior is associated with the mechanical cycles as well as the thermal ones and it is difficult to determine the effect of many variables which influence this behavior. In their study, the influences of temperature interval and the stress-strain response on the crack propagation and microstructural behaviors were investigated.

Numerical simulation on the interface debonding in solid propellant under large deformation by a cohesive zone model

A new algorithm based on the artificial interaction force between neighbouring particles are introduced to build microstructure models of solid propellant, and the numerical simulation on the non-linear properties of solid propellants under large deformation are performed by using the remeshing technique based on the finite element code ANSYS. The binder/particle interface debonding is modelled through cohesive zone models. The results show that the interface debonding of large particles precedes that of small particles. The overall strains at which interface debonding takes place and the predicted effective stresses of propellants decrease with the decreasing of interface strength. At large strains, the load is mainly carried by the binder network, which is formed due to severe interface debonding. The simulated microscopic deformation modes of solid propellants are well consistent with those of experiments.

Numerical simulation on the impact resistance of functionally graded materials

The stress wave propagation and damage evolution within the sandwich structure of ceramic faceplate/Functionally Graded Materials (FGMs) interlayer/metal backplate are numerically studied in this paper. The microstructure models of FGM interlayer are constructed by introducing a new algorithm based on Voronoi discretisation. Four typical microstructure models of FGM interlayer with different composition gradient are constructed to analyse the effect of composition gradient of FGM interlayer on the stress wave propagation and damage evolution in the sandwich structures under impulse loading. The results demonstrate that the composition gradients of FGM interlayer significantly affect the stress propagation and damage evolution in the sandwich structures. The unidirectivity of stress wave in these models varies with the variation of composition gradient, and a slow variation of composition gradient is helpful for reducing the stress on the surface of backplate.

Mixed mode fatigue behavior under inclined loading conditions for an austenitic 304L steel

tsing hua univ, dept engn mech, beijing 100084, peoples r china. acad sinica, inst met res, state key lab fatigue & fracture mat, shenyang 110015, peoples r china.;shi, hj (reprint author), tsing hua univ, dept engn mech, beijing 100084, peoples r china