Key Engineering Materials Vols. 348-349

Abstract: A constitutive model for isotropic damage of geomaterial is put forward, and its numerical computation procedure is given also. The calculated results for a simple example could highlight our understanding about the mechanism of geomaterial damage, and several useful conclusions can be drawn: The series assumption can not be used to synthesis mode of stress and strain for complexus theory of geomaterials damage; strictly speaking, the parallel assumption can not be used to that of geomaterials damage also; the complexus stress-strain behavior like that of ideal natural geomaterial under small stress, and like that of reconstituted soil under large stress.

Abstract: Crack propagation tests were conducted with CCT specimens made by 2024-T3 Aluminum alloy in benign and corrosive environments to investigate the stochastic characteristics of crack growth. A random variable probability crack growth model, base on the modified form of Paris law, was used to characterize the stochastic performance. Distribution significance test has been done, which shows that the random variable X can be considered reasonably to follow log-normal distribution in all the 4 kind of environments. Variance analysis indicates that the scatter in wet air is greater than that in laboratory air significantly. Expressions for the probability distribution of crack size at any given loading cycle and the probability distribution of the random fatigue life at which a given crack size is reached were discussed. The corresponding calculations for the crack length distribution and P-a-N curves with certain reliability were carried out. The comparisons with experimental data indicate the validity of the proposed method.

Abstract: In this paper, the method of Green’s function is used to investigate the problem of dynamic stress concentration of circular lining and interior linear crack impacted by incident SH-wave. The train of thought for this problem is that: Firstly, a Green’s function is constructed for the problem, which is a fundamental solution of displacement field for an elastic space possessing a circular lining while bearing out-of-plane harmonic line source force at any point in the lining. In terms of the solution of SH-wave’s scattering by an elastic space with a circular lining, anti-plane stresses which are the same in quantity but opposite in direction to those mentioned before, are loaded at the region where the crack existent actually, we called this process “crack-division”. Finally, the expressions of the displacement and stress are given when the lining and the crack exist at the same time. Then, by using the expressions, some example is provided to show the effect of crack on the dynamic stress concentration around circular lining.

Abstract: Based on Dankert’s et al. [1] initial model for the elastic-plastic evaluation of fatigue crack growth in sheets providing elliptical notches, a generalized procedure enabling an improved evaluation of the effective ranges of the crack driving force (i.e. the J-Integral) as well as the application to arbitrary notched components has been developed [2]. The present paper presents the basic topics of the calculation model as well as its verification using experimental results from notched specimens with various notch shapes subjected to cyclic loading with various load ratios.

Abstract: This paper attempts to critically review some numerical methods for fatigue strength assessment of welded joints by means of stress analysis tools (usually FE models). In particular, it focuses on the significance of geometrical model, by distinguishing beams from shells and solids, more than distinguishing among nominal, structural and notch stress type. In addition, basing on continuous mechanics theories, a distinction between “local” and “non-local” stress approaches is proposed. Finally the advantages of notch stress approaches are showed by commenting also problems connected to multiaxiality and three-dimensionality of stress states.

Abstract: In this paper, RFPA－dynamic numerical simulation system was adopted to simulate the entire process of the three–point bending specimen with off–center edge-crack propagation to the specimen macroscopic crack which under the dynamic load function. And showed the whole process of the cement mortar specimen crack propagation which under the dynamic load function, as well as the whole field stress distribution picture, and has carried on analysis the specimen destruction process, has pointed out the influence of the no dimension quantity a/L change to crack expansion path and the destruction form.

Abstract: Rotating bending fatigue tests up to 108 cycles were carried out to investigate the effects of shot peening on the fatigue strength and the fracture mechanism in an 18 % Ni maraging steel by using shot particles of various sizes or hardness. Fatigue strength was increased markedly by shot peening in the wide region of fatigue life. The S-N curves showed duplex S-N properties because of the transition of fracture origin from the specimen surface in the short life region to the subsurface in the long life one. Double shot peening by using super-hard fine particles was effective to improve the fatigue strength for surface fracture, though the fatigue strength for an internal fracture was hardly influenced. These results were discussed from the points of view of effects of surface roughness, residual stress and work hardening on the fatigue strength.

Abstract: Rotating bending fatigue tests were carried out to investigate the influence of grain size on the resistance to crack growth of Ni-base super alloy, Inconel 718, using the materials with grain sizes of about 20, 50,100 and 200 たm, at room temperature. The larger grain size, the smaller crack growth rate, though the static strength was decreased with increase in grain size. The growth rate of a small crack was uniquely determined by the term ja na (Small crack growth law), where ja and a are the stress amplitude and the crack length, respectively, and n is a constant. The resistance to crack growth among materials was evaluated based on the law and showed a good correlation with tensile strength jB, meaning that fatigue life can be predicted by using only j B BB.

Abstract: Temper embrittlement and fracture control method based on both non-equilibrium grain-boundary segregation (NGS) theory and grain refinement technique are studied in this paper. Grain refinement technique by deformation induced phase transformation in low-alloy steels, 12Gr1MoV and 2.25Gr1MoNb, is investigated. A single-pass hot rolling process by using a Gleeble-1500 system is performed. Experimental results show that steel strength and toughness may be controlled and improved by grain refinement, and that the grain sizes were affected by the deforming temperature, strain reduction, and strain rate. According to the NGS theory, a control method of brittle fracture along grain-boundary is proposed so that some catastrophically brittle fracture failure may be averted. Grain refinement may decrease both the concentration of phosphorus at grain boundaries and the critical time. With the grain refinement technique, the temper embrittlement of steel may be improved, and the critical time may be shortened. The cost of heat treatment for fracture control will therefore be reduced.

Abstract: When failure occurs in material, it is often occurs by fracture along some grain boundaries and often by the micro-segregation of embrittling impurity to the grain boundaries. In the present work, the non-equilibrium grain-boundary segregation (NGS) kinetics of phosphorus and the temper embrittlement at the same solution treatment and different isothermal holding temperature in steel 2.25Cr1Mo are studied. The NGS kinetics curves of phosphorus at the same solution temperature （1050 oC ）and different isothermal holding temperature (540 oC and 600 oC) are given. Experimental results provide a direct evidence of NGS kinetic model and show that the grain boundary segregation concentrations of phosphorus for specimen isothermal holding at 540 oC are higher than those at 600 oC. The peak values of AES patterns of solute atoms for specimen isothermal holding at 540 oC are also higher than those at 600 oC. It is therefore concluded that the lower the isothermal holding temperature, the higher the segregation concentration of phosphorus at the grain-boundaries, and also the higher the degree of embrittlement.