Anindito Purnowidodo

Deceleration and Acceleration of Crack Propagation after an Overload under Negative Baseline Stress Ratio

In a previously reported study, it was found that the fatigue crack growth rate actually accelerated after a tensile overload under a negative value of baseline stress ratio R. This type of crack propagation behavior is related to the change in the compression residual stress to tensile residual stress distributed in the vicinity of the crack tip upon unloading to minimum load after an overload. In the present investigation, focused on the negative stress ratio, it was found that the deceleration and acceleration of crack propagation were associated with crack opening displacement at the overload point and the conditions of applied baseline cyclic stress. The fatigue crack growth rate decelerated after being overloaded at a positive R value. However, that rate accelerated rather than decelerated in some stress conditions at a negative R value. Therefore, the residual fatigue life after overloading became longer or shorter, depending on the overload level and baseline stress conditions.

Crack Growth Behavior in Overloaded Specimens with Sharp Notch in Low Carbon Steel

The purpose of the present study is to evaluate fatigue crack propagation after tensile overloading. A 6-mm long u-shaped notch was cut in the center section of the specimen. Notch root radii of 0.1, 1, and 2 mm were chosen for ρ. These were regarded as center-notched specimens. The crack propagation behavior of these specimens was compared with a center-cracked specimen. Push-pull fatigue tests were carried out under stress ratios R=0, −1, and −1.5. A special investigation was made into the results for ρ=1 mm. After overload, retardation of crack growth was observed under R=0 and −1 for the notched specimens and center-cracked specimens. However, where R=−1.5, crack growth acceleration was observed after overload in both types of specimens. Even though crack growth was accelerated or decelerated, the rate was able to be evaluated by the effective stress intensity factor range after 0.2 mm of crack growth from the notch root.

Detecting Overload from Strain Information during Fatigue Crack Propagation under Negative Stress Ratio

In the present study, a detection method of an overload application during stress cycles under constant amplitude was investigated. Also, the effect of the tensile overload was shown at three stress ratios: R = 0, -1, and -1.5, to understand the effects of R on crack propagation after an overload. At the baseline of R = 0, after the overload, retardation in the crack propagation was observed, and the crack growth rate decreased. However, in the case of R = -1.5, the fatigue crack growth rate actually accelerated after the tensile overload. The detection of that crack propagation behavior was attempted through the information of the strain waveform h; h = ey + 1.2λx, where ex and ey are the local strains at the specimen axis, and λ is the strain range ratio Δey/Δex. The waveform shape of h was changed after the overloading. Also, the application of the overload could be detected by the variation of the strain range ratio λ. Especially, the present method is useful for cases of the crack propagation stage under negative R conditions.

Effect of Negative Stress Ratio on Fatigue Crack Propagation Behavior after Single Tensile Overload

It is well known that a single tensile overload applied during constant amplitude cycling at a positive stress ratio causes the crack growth retardation. In a case of a negative stress ratio R, however, the fatigue crack growth rate actually accelerated after a tensile overload. This crack propagation behavior is related to the local bulging of specimen thickness, the blunting behavior of crack tips and the residual stress distributions. When the crack tips were blunted during tensile overload and the succeeding compression stress with a high value was applied after the overload, the crack propagation rate got higher level than that before the overload application. In this case, a tensile residual stress was created in front of the crack. To investigate the effect of the negative stress ratio on the fatigue crack propagation, the fatigue tests under R=0, -1 and -1.5 applying the single tensile overload were performed using annealed 0.15% carbon steels. In the present study, a residual life after an overloading was specially investigated, and it was found that the transition phenomenon of the residual life was appeared in the case of the negative stress ratio.