Hong Zuo

A universal crack extension criterion based on the equivalent stress gradient: I. theory and numerical verification

In this contribution, the microscopic fracture mechanism and extension criterion for mixed type crack in ductile material under plane mixed mode loading are investigated in details. A universal extension criterion for the mixed type crack, i.e. the crack propagates along the direction of the maximum gradient of equivalent stress, is suggested. This new criterion is used to predict the propagation direction of mixed type crack, showing a good agreement with other theories for different types of mode mixity. Moreover, the numerical verification is also carried out for the case of an edge crack with different mixed mode loadings. Finally, a potential application to three-dimensional fracture in the ductile material induced by holes is also discussed.

Finite element simulation and optimization of radial resistive force for shape memory alloy vertebral body stent

Vertebral body stent of shape memory alloy is an innovative device which helps recover the compression fractural vertebral at normal height due to its excellent superelasticity and the shape memory effect. A finite element modeling is carried out to optimize the mechanical behavior of the vertebral body stent of shape memory alloy accounting for the stress-induced phase transformation of the shape memory alloy. The radial resistive force of stent is paid more attention in order to meet the functional and surgical requirement. First, experimental procedure and finite element modeling computations are constructed to calculate the compression resistance force of an original design of vertebral body stent of shape memory alloy. It is found that numerical results are consistent with those of experimental observations so as to validate the accuracy of the finite element modeling model. Second, a series of numerical simulations are performed to optimize the topological structures of vertebral body stent of shape memory alloy by response surface methodology. The surgical condition of the lumen structure of fracture vertebral body and the size constrain condition of puncture instrument of minimally invasive surgery are introduced during finite element modeling simulation and response surface methodology optimization. Finally, an innovative design optimization is proposed by the series–parallel connection of four S-type representative stents. The proposed new structure can obtain the maximum radial resistive force of 831 N which is able to meet the functional requirement. In conclusion, it is expected that the finite element modeling simulations and optimizations can help the researchers to design and optimize the topological structures of vertebral body stent of shape memory alloy systems.

A New Method for M-Integral Experimental Evaluation

In this article, the experimental measurement method of M-integral is investigated. Through the detailed analysis to the nondestructive evaluation method of J- and M-integrals suggested by King and Herrmann, it is found that the specimen geometry which they selected and the corresponding clamping mode in their test exists a conflict with the stress distribution assumption on the integral contour. The formulas they proposed cannot represent the selected specimen geometry and the related integral contours. To avoid this conflict, a new experimental measurement method and a simper specimen style is proposed in this study. According to the method, the M-integral is nondestructive evaluated experimentally through the new specimen and the new clamping mode.

The peritubular reinforcement effect of porous dentine microstructure

In the current study, we evaluate the equivalent stiffness of peritubular reinforcement effect (PRE) of porous dentine optimized by the thickness of peritubular dentine (PTD). Few studies to date have evaluated or quantitated the effect of PRE on composite dentine. The miscrostructure of porous dentine is captured by scanning electron microscope images, and then finite element modeling is used to quantitate the deformation and stiffness of the porous dentine structure. By optimizing the radius of PTD and dentine tubule (DT), the proposed FE model is able to demonstrate the effect of peritubular reinforcement on porous dentine stiffness. It is concluded that the dentinal equivalent stiffness is reduced and degraded with the increase of the radius of DT (i.e., porosity) in the certain ratio value of Ep/Ei and certain radius of PTD, where Ep is the PTD modulus and Ei is the intertubular dentine modulus. So in order to ensure the whole dentinal equivalent stiffness is not loss, the porosity should get some value while the Ep/Ei is certain. Thus, PTD prevents the stress concentration around DTs and reduces the risk of DTs failure. Mechanically, the overall role of PTD appears to enhance the stiffness of the dentine composite structure. These results provide some new and significant insights into the biological evolution of the optimal design for the porous dentine microstructure. These findings on the biological microstructure design of dentine materials are applicable to other engineering structural designs aimed at increasing the overall structural strength.

Crack-Tip Fields of a Crack Impinging upon the Yielding/Debonding Slippage in Anisotropic Body

This paper presents a fundamental solution for a crack impinging normally upon the slippage in anisotropic materials under tensile loading. The slippage could occur in the form of yielding (e.g., a well-bonded ductile layer with plastic yielding) or debonding (e.g., a weak, sliding-free one). A superposition method is employed to explicitly solve the problem which combines the solution of a crack in an elastic homogeneous medium, the solution of a continuous distribution of dislocations which represent slippage, and an appendix solution which offsets the stress on the crack faces induced by the dislocations. This procedure reduces the problem to a singular integral equation which can be numerically solved by using Chebyshev polynomials. Numerical implementations are performed to analyze the influence of slippage on cracking and stress redistribution near the crack tip in anisotropic bodies. It is found that yielding or debonding slippage redistributes the stress ahead of the crack tip. The presence of yielding or debonding lowers the high stress concentration in the tensile stresses ahead of the crack tip. It is also concluded that debonding appears to be more effective in lowering the stress concentration than yielding.

Different Failure Styles for Aluminum-Alloy Strip with Two Holes under Cyclic Tensile Loading

This paper reports seven failure styles found in our experimental investigation for the failure between two neighbor holes located at different angle in an Aluminum-Alloy strip under cycle tensile loadings. In some cases, unlike expected, the initial fatigue crack is found not to nucleate at the center region between the two holes, where the stress concentration is larger than other region. In fact, the initial fatigue crack always nucleates at the outside rim of two holes far apart from the center region. The coalescence between the two holes always occurs far after the initial fatigue crack nucleation.

The Damage and Texture Features of High-Speed Rail(THS) Wheel Tread Based on EBSD

In this paper, the microscopic features of damage and texture of a wheel tread in high-speed rail were investigated by EBSD. The microscopic sample was cut from the wheel tread zone of a used high-speed rail wheel. It was found from the microscopic investigation that the density of austenite distribution is increased near the wheel tread, and the damage voids distribute far away from the wheel tread zone. The texture analysis with two magnify zone indicated that there exists large amount of twin-crystal within the ferrite in the dimension of 20 micrometer, and the dimension of austenite and martensite decreases in the edge zone of the wheel tread.