Stress relaxation tests at various strain rates and the repeated stress relaxation tests were carried out at room temperature to obtain the internal and effective stresses as well as the Gupta-Li's stress exponent, m*, in NaCl single crystals. It was found that the constant, a, appearing in the time pharenthesis of the integral form of the Gupta-Li equation, decreased with an increase of the strain rate accompanied by the increase of effective stress. With increasing cycle number of repeated relaxation experiment, it was found that the effective stress decreased monotonously, whereas the internal stress increased. The repeated relaxation curves cannot, in general, be superimposed on the initial stress relaxation curve if the re-strain rate is changed. This indicates that the mobile dislocation density (which is considered to be constant during one cycle of testing period) and the effective stress during the repeated stress relaxation test are not altogether equal to those of the initial stress relaxation test. However, if the mobile dislocation density could be chosen to be the same as that at the initial stress relaxation test, it may be possible to superimpose these two relaxation curves by translating along the time axis to the point where each effective stress agrees. Therefore, if the re-strain rate is too small not to produce the mobil dislocation density comparable with that in the initial test, the superposition is not to be expected. This indicates that the increasing rate of mobile dislocation density as a function of strain depends on strain rate.
The probability of failure is treated as a statistical quantity from the viewpoint that the probability can be evaluated only through experimental data. A distribution function for the probability of fatigue failure is derived under the linear damage accumulation hypothesis, and it is pointed out that the allowable probability of failure varies largely with its reliability. A simple but important policy for fatigue-proof design is proposed. This policy requires that a value smaller than a pre-specified allowable value for the probability of failure be correct with a pre-specified reliability.
Several proposed procedures of J integral estimate for center-cracked plates under gross and general yielding conditions were examined based on the finite element analysis. The J values derived through the compliance method, the simple method proposed by Rice et al. and its modified method were compared with those determined by the line integral method. The compliance method was applicable for the case where the energy in load vs. load-point displacement curve changed large with respect to crack length. Rice's method was not applicable for the region of the crack length relative to the specimen width a/W less than 0.4. The best evaluation method that was applicable for a wide range of a/W was found to be the modified Rice's method in which the crack-center opening displacement was used instead of the load-point displacement. The mechanical foundation for the modified method was clarified and the limitations of other methods were discussed. Fatigue crack growth tests in center-cracked specimens of a low-carbon steel under general yielding were conducted to confirm applicability of each estimate method to fatigue crack growth. The relation between the crack growth rate and the J integral range obtained by each estimate procedure was examined experimentally, and it was found that the midified Rice's method yielded the best correlation for a wide range of the crack length relative to the specimen width.
The crack initiation, propagation and branching characteristics of delayed failure under uniaxial and biaxial load were investigated on the Ni-Cr-Mo steel quenched and tempered at 473 or 673K. In the three point (uniaxial) bending tests of smooth rectangular specimens, a straight crack initiated and propagated perpendicular to the longitudinal axis of the specimen without revealing crack branching. In the uniaxial tensile tests of precracked rectangular specimens, the crack propagated to unstable fracture without branching. In the biaxial bending tests of smooth disk specimens, the edge of which was supported and the center of which was compressed by a round punch, a Y-shaped crack initiated from the center of specimen, and the branching occurred at the tip of the Y-shaped crack when the stress of specimen was large. These aspects were almost the same both for the steels tempered at 473 and 673K. For the steel tempered at 473K, the crack initiation time in delayed failure was shorter in the biaxial bending than in the uniaxial bending for the same bending stress.
Sustained load tests were conducted on a high-strength steel SNCM 439 sensitive to hydrogen embrittlement with or without vibratory stresses of small amplitude, superimposed. Furthermore, varying load tests (stress ratio R being near zero) were carried out to compare with those mentioned above. The relation among static SCC under sustained load (R=1.0), cyclic SCC under varying load (R being near zero) and dynamic SCC under high-cycle vibratory stresses superimposed (R being near unity) were discussed, in contrast with those in 7N01 A1 alloy8) sensitive to active path corrosion type SCC. Similarly as those in 7N01 alloy, the threshold stress intensity factor Kmax=KDSCC considerably decreased from KISCC at R=1.0. However, KDSCC at a stress ratio of 0.95 hardly decreased from KISCC. On the other hand, in the Region II, where a subcritical crack growth appears, the crack growth rate in 7N01 alloy increased with a decrease of R. However, in SNCM 439 steel the smaller the stress ratio was, the slower the crack growth rate was. In SNCM 439 steel static SCC was dominant in the Region II just the same as cyclic SCC for the material sensitive to hydrogen embrittlement5).
The effect of applied stress on the SCC susceptibility and the polarization behavior was measured on SUS 316 type stainless steel in 42% MgCl2 solution. In order to study the relation between the SCC susceptibility and the polarization behavior, the potentiostatic SCC test was also performed. Moreover, the effect of applied stress on the crack morphology was observed by a scanning electron microscope. With the applied stress over 100MPa, the rest potential of the specimen and the pitting potential in the polarization curve shifted remarkably toward less noble potentials, and they changed more remarkably with increasing applied stress. The width of the unsteady passive region decreased owing to the shift of the pitting potential mentioned above. From the investigation of the dependence of the SCC susceptibility on the potential, it was clarified that three different SCC susceptible regions existed and these regions were almost coincident with the steady passive region, the unsteady one and the pitting region in the polarization curve, respectively. From these results, it was concluded that the increase of the SCC susceptibility with increasing applied stress was attributable to both the decrease of the width of the unsteady passive region and the shift of the rest potential toward less noble potential due to the applied stress. As to the crack morphology, transgranular cracking occured mainly in the specimen fractured under low applied stress, whereas intergranular cracking appeared mainly under high applied stress.
The erosion behaviors by repeated impacts with a particle (steel ball of 4mm diameter) on the same material surface has been investigated to make clear the mechanism of sand erosion. The effects of impact velocity (V) and angle of attack (α) on erosion damage were discussed. The material used was polymethyl-methacrylate, and the impact velocity of particle was varied from 12 to 24m/s, and the angle of attack was from 40 to 80 deg. From the observation of behaviors of crack growth caused by particle impact, it was found that the new ‘erosion factor’ presented as f=V3.6·sin2.8α·cos0.8α would be very useful in understanding the erosion behavior, and it was concluded that the ‘erosion factor’ was closely related to the energy given to the material by impact. Moreover, the ‘erosion factor’ was found to be applicable for erosion by multiple particle impacts if ‘f’ was generalized as f=Vn·sinn1α·cosn2α (where n=n1+n2). A linear relationship was held between the erosion rate (R) and ‘f’. By using this new generalized ‘erosion factor’ the erosion rate under arbitrary condition could be estimated.