Key Engineering Materials Vols. 324-325

Abstract: In the present paper, a micromechanical based damage model and corresponding permeable coefficient tensor are developed for rocks under tensile stress and hydraulic pressure loading based on the concept of the domain of microcrack growth (DMG) . After choosing an appropriate fracture criterion for microcracks, we obtain the equations from which the DMG under a monotonically increasing proportional plane stress and an invariable hydraulic pressure loading is obtained. Then the overall effective stress-strain relations and the permeable coefficient tensor of damaged rocks are calculated. The theory is verified by test.

Abstract: This study gives the problem of a crack in the film oriented perpendicular to the film-substrate interface with the crack tip terminating at the interface. Based on Beuth’s theory, three-dimensional model is simplified to plane strain problems, which obtains fracture mechanisms of a cracked film-substrate medium by applying the boundary element method(BEM). The method aptly resolves the problem involving stress concentration and, further, that this study develops the multi-region boundary element method and applies it to evaluate the cracked film-substrate medium. It shows that the stress intensity factor is affected by the different elastic mismatches and the thickness ratio of the film and the substrate. These results indicate: 1) The stress intensity factor has remarkable increased with the decrease of the thickness ratio of the film and the substrate. The effect of the fracture behavior of film is negligible when the thickness ratio of the film and the substrate is above 10, therefore, it is treated as thin film; 2) The stress intensity factor will decrease with the increase of α ( −1 pα p +1) for β = 0 and β =α / 4 , where α and β are called Dundurs parameters. What’s more, this paper studies the special condition of the film-substrate medium, which is the analysis of the fracture of the absence of any elastic mismatch between the film and the substrate, i.e.α=β=0, and revision of the formula of Xia and Hutchinson is put forward for the stress intensity factor of the deep crack problems by comparing to the former conclusions of Y.Murakami.

Abstract: A ultrahigh carbon steels (UHCS) containing 1.6 wt pct carbon was studied. Through spheroidizing process by divorced-eutectoid transformation (DET), the forged microstructure was spheroidized and the microstructure was fine carbide particles distributed in ferrite matrix. Second-time heat treatment included two kinds of technologies: normalizing and quenching + tempering. Finally, the UHCS obtained ideal mechanical properties. The yield strength and tensile strength of the UHCS were higher than that of 40CrNiMo, moreover plasticity of the UHCS was equal to that of 40CrNiMo. So the UHCS was an excellent structural material.

Abstract: It is very important to detect fiber orientation error in orthotropic composite laminates because the layup of a CFRP (carbon-fiber reinforced plastics) composite laminates affects the properties of the laminate, including stiffness, strength and thermal behavior. In this study, an investigation of shear wave ultrasonic technique was carried out in order to detect stacking orientation error for the orthotropic composite laminates. During testing, the most significant problem is that the couplant conditions do not remain the same because of changing the viscosity of the couplant. Therefore, the design and use of a shear wave transducers would greatly aid in alleviating the couplant problem. A pyramid with an isosceles triangle with two 45o was made of aluminum to generate shear waves using two longitudinal transducers based on ultrasonicpolarized mechanism. Also, the signal splitter was connected to the pulser jack on the pulser/receiver and to the longitudinal transducers which were mounted with mineral oil. The shear transducer was mounted on the bottom as a receiver with burnt honey. It is found that the shear wave was generated at a maximum and a minimum based on the ultrasonic-polarized mechanism. Finally, test results with model data were compared for a fiber orientation of the laminates.

Abstract: The present study shows a new idea for investigating characteristics of stress singularity field around a vertex associated with elasto-plastic properties of materials. FEM formulation for elasto-plastic stress singularity analysis is expressed to investigate an eigenvalue and the intensity of singularity.
The elasto-plastic stress singularity in a flip chip joint was investigated using the FEM formulation. After that, the possibility of delamination on the flip chip joint was discussed.

Abstract: In the present work, the fracture toughness of a NiTi pseudoelastic alloy has been obtained by experiments on CT specimens, which is KIC =39.38MPa·m1/2. Then the stress induced phase transformation behavior in front of the crack tip of the CT specimen is simulated by a micromechanical model considering the different elastic properties between martensite and austenite. The results show that the pre-crack promotes phase transformation at the crack tip. And the phase transformation is localised near the crack tip. It is also shown that phase transformation reduces the Mises stress around the crack tip.

Abstract: In order to study the behavior of fatigue crack propagation under random overloads, a Monte Carlo simulation scheme is proposed. Overloads of Poisson flow with uniform distribution on base-line constant-amplitude cyclic loads are considered. The retardation effect of overload is taken into account using crack closure model and the crack opening stress level is assumed to vary linearly in the yield zone produced by the overload. The fatigue crack growth curve from initial crack size till fatigue failure is simulated step by step. Through the large number of the simulated samples, the average fatigue crack propagation life is calculated. Finally, the influence of overload intensity and magnitude on fatigue crack propagation life is studied.

Abstract: Many military aircraft have reached or exceeded their original design life, and have been subject to significant increase in maintenance and repair cost due to multiple site damage (MSD). In order to assessing the effects of MSD on the structural integrity of aircraft lap joints, the wing lap joint of certain model military aircraft with MSD was analyzed using special code FRANC2D/L. The rivet holes along the top row of the outer skin of lap joint were considered as the independent structural unit for the simulated MSD cracks. The stress intensity factors (SIFs) at each crack tip with different distribution loads at the rivet holes were computed and show that the analysis results have good coherence with the available literature data. It also shows that the SIF at each crack tip
s a function of crack length can be calculated by the crack growth simulation capability of FRANC2D/L. The SIF values are not sensitive to the rivet load distribution manner, which has seriously influence on MSD crack growth direction. Rivet loading can be best molded quadratic load distribution over one half of rivet hole relative to uniform load distribution and point load. As a result of this analysis, it is postulated that for MSD in aircraft lap joints, compliance measurements may provide a useful tool for assessing the structural integrity of the lap joints.

Abstract: Ductile tearing of brittle solid with initial crack is studied numerically. This work is focused on the simulation and analysis of crack path deviation for the respective configuration and the study of relation between crack length and the fracture resistance, and that the configuration of fracture surface is also observed. The simulating tool is a novel numerical code, 3D Realistic Failure Process Analysis code (abbreviated as RFPA3D). The simulating investigation has shown that the fracture resistance decreases with the increase of the angle between the crack ends and horizontal direction. Due to heterogeneity of materials distribution, crack path deviation towards the softer zone or elements is observed. The topography of the non-planar fracture surface has plotted. Concluding the simulation made in the present study, the problem of crack extension in the brittle solid can be handled by RFPA3D. Fracture resistance can be predicted and the crack path deviation be simulated, if the model parameters have been carefully identified and the mesh design is adequate. So this work is beneficial to choose safe load-spots and predicate the direction of the crack path deviation.