Liguo Zhao

On the uniaxial mechanical behaviour of an advanced nickel base superalloy at high temperature

The uniaxial mechanical behaviour of an advanced nickel base superalloy has been studied under strain-controlled cyclic loading at two elevated temperatures, 300 and 650 °C. High-temperature related features, such as a transition from initial cyclic hardening to softening and stress relaxation during load-holding periods, were observed. The materials low cycle fatigue life experienced heavy reduction with increase in temperature and strain range. Using constitutive models for cyclic plasticity/viscoplasticity, attempts were made to simulate the stabilized cyclic loops for different strain ranges at both temperatures. The model parameters were determined simultaneously. Comparison of the simulated and experimental results is made for the stabilized cyclic loops and the stress relaxation behaviour.

Finite element simulation of creep-crack growth in a nickel base superalloy

Abstract Creep-crack growth in a standard compact-tension specimen has been simulated for an elastic/power-law creeping material, Waspaloy, at 650°C using the finite element code ABAQUS. Two cases of crack growth rates, relatively fast (3.25×10−2 mm/h) and slow (3.25×10−5 mm/h), were considered. For both cases, the ratio of the load-line deflection rate due to creep ( V c ) to the total deflection rate ( V ) was found to be well below unity, i.e., V c / V ≪1 , which suggests the creep-brittle characteristics. The development of the creep zone was determined and the creep-zone expansion rate along the crack growth direction was compared to the crack growth rate. Attempts were made to describe the crack tip stress fields by the Hui–Riedel, Hutchinson–Rice–Rosengren and K types at different stages of crack growth.

Stress intensity factor K and the elastic T-stress for corner cracks

The stress intensity factor K and the elastic T-stress for corner cracks have been determined using domain integral and interaction integral techniques. Both quarter-circular and tunnelled corner cracks have been considered. The results show that the stress intensity factor K maintains a minimum value at the mid-plane where the T-stress reaches its maximum, though negative, value in all cases. For quarter-circular corner cracks, the K solution agrees very well with Pickards (1986) solution. Rapid loss of crack-front constraint near the free surfaces seems to be more evident as the crack grows deeper, although variation of the T-stress at the mid-plane remains small. Both K and T solutions are very sensitive to the crack front shape and crack tunnelling can substantially modify the K and T solutions. Values of the stress intensity factor K are raised along the crack front due to crack tunnelling, particularly for deep cracks. On the other hand, the difference in the T-stress near the free surfaces and at the mid-plane increases significantly with the increase of crack tunnelling. These results seem to be able to explain the well-observed experimental phenomena, such as the discrepancies of fatigue crack growth rate between CN (corner notch) and CT (compact tension) test pieces, and crack tunnelling in CN specimens under predominantly sustained load.

On the contribution of subinterface microcracks near the tip of an interface macrocrack to the J-integral in bimaterial solids

Abstract The J-integral analysis is performed for the plane problems of multiple subinterface microcracks near the tip of an interface macrocrack in bimaterial solids. The analysis starts from a general solution based on the “pseudo-traction” method which has been addressed thoroughly in homogeneous cases. The contribution to the J-integral induced from the subinterface microcracks is shown in a consistent relation with those induced from an interface macrocrack tip and the remote stress field. A new technique is developed to evaluate the second component (being expressed by J 2 ∗ in this paper) of the well-known Jk-vector of a subinterface crack for considering a contour enclosing the whole crack, which is necessary to evaluate the contribution to the J-integral arising from the subinterface microcracks. The consistency of the J-integral for numerical examples is proved, where two kinds of material combinations presented by Hutchinson et al. [ASME J. Appl. Mech., 1987, 54, 828–832] are considered. Some discussion and conclusions are then given which seem very useful in the investigation of microcrack shielding problems in bimaterial cases.

Crack channelling and spalling in a plate due to thermal shock loading

The propagation of a pre-existing edge crack across a finite plate subjected to cold shock has been studied. The plate, initially at uniform temperature, is exposed to a cold shock on one surface whilst three different types of heat transfer boundary condition are separately considered for the opposing face: cold shock, thermal insulation and fixed temperature. For all three boundary conditions, the plate experiences tensile stress near the cold-shocked surface and compressive stressing near the mid-plane. Consequently, a Mode I edge crack extending into the compressive region may grow in one of three different modes: continued extension in plane strain, channelling and spalling. The thermal shock conditions governing each failure mode are quantified, with a focus on crack channelling and spalling. The dislocation method is employed to calculate the energy release rates for plane strain cracking and steady-state channelling. For steady-state spalling, the energy release rate is obtained by an energy analysis of elastic beams far ahead and far behind the crack tip. Analytical solutions are also obtained in the short crack limit in which the problem is reduced to an edge crack extending in a half space; and the parameter range over which the short crack solution is valid for a finite plate is determined. Failure maps for the various cracking patterns are constructed in terms of the critical temperature jump and Biot number, and merit indices are identified for materials selection against failure by thermal shock.

Effects of material, coating, design and plaque composition on stent deployment inside a stenotic artery: finite element simulation

Finite-element simulations have been carried out to study the effects of material choice, drug eluting coating and cell design on the mechanical behaviour of stents during deployment inside a stenotic artery. Metallic stents made of materials with lower yield stress and weaker strain hardening tend to experience higher deformation and stronger dogboning and recoiling, but less residual stresses. Drug eluting coatings have limited effect on stent expansion, recoiling, dogboning and residual stresses. Stent expansion is mainly controlled by the radial stiffness of the stent which is closely associated with the stent design. In particular, open-cell design tends to have easier expansion and higher recoiling than closed-cell design. Dogboning is stronger for slotted tube design and open-cell sinusoidal design, but reduced significantly for designs strengthened with longitudinal connective struts. After deployment, the maximum von Mises stress appears to locate at the U-bends of stent cell struts, with varying magnitude that depends on the materials and severity of plastic deformation. For the artery-plaque system, the stresses, especially in the plaque which is in direct contact with the stent, appear to be distinctly different for different stent designs and materials in terms of both distribution and magnitude. The plaque composition also strongly affects the expansion behaviour of the stent-artery system and modifies the stresses on the plaque.

Interaction between an interface crack and a parallel subinterface crack

In this paper, the pseudo-traction method addressed thoroughly in homogeneous cases is combined with the edge dislocation method to solve the interaction problem of an interface crack with a parallel subinterface crack. After deriving the fundamental solutions for a typical interface crack loaded by the normal and tangential concentrated tractions on both crack surfaces and the fundamental solutions for an edge dislocation beneath the interface, the interaction problem is reduced to a system of singular integral equations which can be solved numerically with the aid of the Chebyshev polynomial technique. Numerical results for the stress intensity factors are shown in the figures in which six kinds of material combinations presented by Hutchinson et al. [1] are considered.

A computational study of stent performance by considering vessel anisotropy and residual stresses

Finite element simulations of stent deployment were carried out by considering the intrinsic anisotropic behaviour, described by a Holzapfel-Gasser-Ogden (HGO) hyperelastic anisotropic model, of individual artery layers. The model parameters were calibrated against the experimental stress-stretch responses in both circumferential and longitudinal directions. The results showed that stent expansion, system recoiling and stresses in the artery layers were greatly affected by vessel anisotropy. Following deployment, deformation of the stent was also modelled by applying relevant biomechanical forces, i.e. in-plane bending and radial compression, to the stent-artery system, for which the residual stresses generated during deployment were particularly accounted for. Residual stresses were found to have a significant influence on the deformation of the system, resulting in a re-distribution of stresses and a change of the system flexibility. The results were also utilised to interpret the mechanical performance of stent after deployment.

Effect of the T-Stress in Microcrack Shielding Problems

The effect of the T-stress in microcrack shielding problems is studied by solving the interaction problem of a macrocrack with near tip microcracks applying a discrete model. The T-stress has no effect on the results for the parallel microcrack cases; however, it plays an important role for the oriented microcrack cases, especially for large values of the T-stress and large distances of the microcrack center from the macrocrack tip. In determining the shielding or amplification effect of the oriented microcracks, it is necessary to consider the effect of the T-stress. The effect of the T-stress on microcrack shielding or amplification is substantially dependent on the sign and magnitude of the T-stress as well as on the geometry of the microcrack arrangement. The contribution to the J-integral induced from the microcracks is reexamined by considering the T-stress and shown to be still in a consistent relation with those induced from the macrocrack tip and the remote stress field.

Modeling of Oxygen Diffusion Along Grain Boundaries in a Nickel-Based Superalloy

Finite element analyses of oxygen diffusion at the grain level have been carried out for a polycrystalline nickel-based superalloy, aiming to quantify the oxidation damage under surface oxidation conditions at high temperature. Grain microstructures were considered explicitly in the finite element model where the grain boundary was taken as the primary path for oxygen diffusion. The model has been used to simulate natural diffusion of oxygen at temperatures between 650C and 800C, which are controlled by the parabolic oxidation rate and oxygen diffusivity. To study the effects of mechanical stress on oxygen diffusion, a sequentially coupled deformation-diffusion analysis was carried out for a generic specimen geometry under creep loading condition using a submodeling technique. The material constitutive behavior was described by a crystal plasticity model at the grain level and a unified viscoplasticity model at the global level, respectively. The stress-assisted oxygen diffusion was driven by the gradient of hydrostatic stress in terms of pressure factor. Heterogeneous deformation presented at the grain level imposes a great influence on oxygen diffusion at 750C and above, leading to further penetration of oxygen into the bulk material. Increased load level and temperature enhance oxygen concentration and penetration within the material. At 700C and below, mechanical loading seems to have negligible influence on the oxygen penetration because of the extremely low values of oxygen diffusivity and pressure factor. In the case of an existing surface microcrack, oxygen tends to accumulate around the crack tip due to the high stress level presented near the crack tip, leading to localized material embrittlement and promotion of rapid crack propagation.