Key Engineering Materials Vols. 324-325

Abstract: In this work, we investigated the influences of residual stress on the load shifts, irreversible work from load vs. depth curves. It is found that there are linear relationships between the level of surface residual stress, the load shifts, and the variation of irreversible work for Ni, Ti, TiFe and A316L studied in this work. From these effects of residual stresses on load versus depth curves, depth-sensing indentation can be expected as a non-destructive method for measuring the residual stresses. Using the simulation results, the stresses estimated from the relationship of the level of surface residual stress and the load shifts, agree well with the applied stresses during simulations.

Abstract: The surface cracks are easy to occur in the solid missile motor grain. The solid missile motor grain with cracks is very dangerous when it is lunched, because the surface cracks might be propagation unstable when the internal pressure enters the surface cracks cavity. In order to analyze the surface crack of solid motor grain, with accelerated aging test, the variation law of mechanics properties of propellant in the storage period is obtained. Under internal pressure and axial acceleration loading, the three-dimension finite element models of the motor grain are established, three-dimension singular crack elements at the tips of surface cracks are established to simulate the cracks propagation. Along with the surface crack propagation, the stress intensity factors of the crack tips in different storage period are calculated to prejudge its stability respectively. The method and conclusions are available for using solid missile motor grain with surface cracks.

Abstract: Splitting force is the key to the process of fracture splitting connecting rod. In this paper, numerical simulation is made in the process of fracture splitting Jetta car connecting rod by using software MSC.MARC. Through analysis, the curve between J integral and splitting force is established. According to the experiential formula of impact toughness and fracture toughness, critical J integral ( JIC ) can be achieved. After getting JIC , the splitting force can be determined by linear interpolation method and verified by tensile experiment and the error is below 1%. It is proved that this method can also used to analyze other kinds of connecting rods with different materials.

Abstract: By the theory of complex functions, dislocation distribution function concerning mode
dynamic crack propagation problem under the conditions of unit-step loads and moving increasing loads was studied respectively. Analytical solution representations are attained by the methods of self-similar functions. The problems investigated can be transformed into Riemann-Hilbert problems and their closed solutions are obtained rather simple by this approach.

Abstract: The interaction behavior of two non-aligned through-wall cracks in flat plates is investigated by the finite element method (FEM) under extensive creep condition. The time-dependent fracture parameter C*-integral along the crack tips are calculated and compared to the results of a single crack of the same size. For comparison purpose, the interaction of stress intensity factors (SIFs) is also examined in the study. The results indicated that interaction of multiple cracks is different between the time- dependent fracture characterized by C*-integral and linear elastic fracture noted by SIF. The magnifying factors of time-dependent fracture are obviously larger than that of the linear elastic fracture cases. Therefore, the current re-characterization rule for multiple cracks developed from linear elastic fracture analysis may lead to a non-conservative result and should be modified when it is used in the assessment of time dependent failure.

Abstract: A damage detection method reflecting initial damage of the elastic structure is presented. It is based on the idea that the damage will decrease the structure stiffness. From eigenvalue issue, regard Poison ratio and elastic modulus as a constant and a random variable, respectively, and in terms of known data and a combination of the FEM, then equations solving random modulus of the undamaged and the damaged structure are obtained, respectively. Based on the element damage indicator, initial damage of the structure can be detected. At last, a five-storey shearing structure is simulated, and the results indicate that initial damage situation can be accurately calculated, and initial damage locations can be estimated based on the proposed method, which is simple and effective, and contributes to the application in Engineering.

Abstract: The debonding of a rigid particle embedded in an infinite non-linear viscoelastic material is investigated in this paper. Under sphere-symmetric deformation, a non-linear equilibrium equation expressed by velocity of a particle in the viscoelastic matrix material is derived. The strain rate is obtained by solving non-linear equation in terms of iterative method. According to the energy criterion, the critical instant of the interfacial debongding is calculated. Numerical results show that the influence of non-linear viscosity on the interfacial debonding is significant.

Abstract: More and more engineering practice indicates rock mass is prone to lose its stability through crack coalescence under dynamic loading, such as blasting and earthquake. However, the crack coalescence pattern of rock specimens containing two or more flaws has not been studied comprehensively under dynamic loading. In this paper, the mechanism of the crack coalescence and peak strength of sandstone-like materials containing two parallel flaws are studied under uniaxial static and dynamic loading with strain rates 1.7×10-5 s-1 and 1.7×10-1 s-1. Through the comparisons of the propagation length, coalescence pattern of the cracks and strength increase of the pre-cracked specimens under static and dynamic loading, the dynamic response of the crack coalescence is found different from static loading under different geometric setting of the flaws. The inertia effect of the crack propagation is revealed under dynamic loading, that is to say, the growth of the secondary cracks tends to the original propagation direction, and the direct and immediate coalescence is taken place easily between two pre-existing flaws, which is different from the kinking coalescence under static loading. So, the inertia effect of the crack propagation is regarded as the main cause of the strength increase of the brittle material under dynamic loading for medium strain rates. In virtue of the explanation, another cause of the mode II shear fracture occurred under earthquake is opened out.

Abstract: In the framework of percolation theory, a simple void-coalescence model combined with the constitutive relations for describing the stress relaxation and material softening during the void-coalescence process, name as the percolation-relaxation (P-R) model, is proposed to describe the dynamic tensile spallation of ductile metals. A critical damage is introduced and coupled into the model to identify the onset of the void coalescence. Mesoscopically, the critical damage corresponds to the critical intervoid ligament distance (ILD), indicating the start of transition from the void-growth to the void-coalescence.

Abstract: The influence of surface roughness in crack face on the behavior of crack growth is studied. Numerical results derived from two kinds of mechanical models representing the macroscopic and microscopic asperity supposed to exist in the face of an initial crack are compared. In macro-asperity model, the influence of location and size of single-triangular asperity on the stress intensity factor is evaluated. In this model, it is found that the mode-I stress singularity appears in certain circumstances even under overall compression. In micro-asperity model, influence of friction coefficient on the trajectory of crack path is discussed. There exists a tendency that a path of wing crack growth becomes more straight and parallel to axis of compression as the friction coefficient increases.