EFFECTS OF OVERLOAD AND SUPPLYING OIL ON CRACK GROWTH BEHAVIOR IN MG ALLOYS

A single overload was applied during the crack growth process under constant stress amplitude, and retardation of crack growth was observed in the case of magnesium alloys as well as carbon steel, aluminum alloys, etc. The retardation of crack growth was related to crack closure, the fracture surface roughness, and crack tip deformation. In addition, the effects of supplying oil into the crack on crack growth behavior of an overloaded specimen were investigated in this study. The crack growth rate in the case of supplying oil became lower than in the case without supplying oil. In the case of the magnesium alloy AZ31, powder of oxide magnesium appeared from the crack after overloading. It is one of the typical behaviors of AZ31. In the case of AZ31 and AZX912, the crack growth behavior after overloading was slightly different due to the deformation of the crack tip.

Effects of a Single Overload on Fatigue Crack Growth of Type 316 Stainless Steel

The effect of a single overload on the fatigue crack growth rate was investigated for Type 316 stainless steel. Fatigue crack growth tests were conducted by controlling strain and load. Tensile and compressive overloads were applied during constant amplitude cycling. The overload ratio, which was defined as the ratio of overload size to baseline constant amplitude, was also changed. The constant amplitude tests were conducted at the strain or the stress ratio of −1.0 which was defined as the ratio of the minimum value to the maximum value. The crack opening point was obtained by the unloading elastic compliance method. The crack growth rate increased after the single compressive overload. The accelerating rate increased with the overload ratio. In contrast, not only the acceleration but also the retardation of the crack growth rate was observed for some tensile overload cases. The crack growth rate increased for relatively small tensile overload cases and decreased for relatively large tensile overload cases. The change in the crack opening level was examined. The crack growth rates after tensile and compressive single overloads correlated with the effective strain and stress intensity factor ranges both for load and strain controlling modes.

Fatigue Life Prediction for Variable Strain at a Mixing Tee by Use of Effective Strain Amplitude

Mixing flow causes fluid temperature fluctuations near the pipe walls and may result in fatigue crack initiation. The authors have previously reported the loading sequence effect on thermal fatigue in a mixing tee. The fatigue damage around the hot spot, which was heated by the hot jet flow from the branch pipe, obtained by Miner’s rule was less than 1.0. Since the strain around the hot spot had waveforms with periodic overload, the loading sequence with periodic overload caused reduction of the fatigue life around the hot spot. In this study, the effect of a single overload on the fatigue crack growth rate was investigated in order to clarify the reduction of the fatigue life at the mixing tee due to strain with periodic overload. In addition, the prediction method of the fatigue life for the variable thermal strain at the mixing tee was discussed. It was shown the crack growth rate increased after an overload for both cases of tensile and compressive overloads. The effective strain amplitude increased after the application of a single overload. The fatigue life curve was modified by considering the increment of the effective strain range. The fatigue damage recalculated using the modified fatigue life curve was larger than 1.0 except in a few cases. The fatigue life could be assessed conservatively for variable strain at the mixing tee using the developed fatigue curve and Miner’s rule.

Estimation of the residual strength resource of machines

For objective reasons, machines experience short-term overload, which can lead to damage or destruction of the structure. Under these conditions, it is important to estimate the residual resource of machine fatigue characteristics. For this purpose in engineering practice the method of single overloads (from a high level to low) is widely used. The method allows simulating a mode of adverse irregular loading. Single overload at a level exceeding the initial endurance limit for a certain number of cycles reduces the endurance limit and the residual resource of the structure durability. The empirical dependences proposed in the literature for estimating the relative decrease in endurance limits from the overload coefficient and the cyclic ratio give a complete decrease in the secondary endurance limit at a cyclic ratio equal to one. This is inconsistent with experimental results indicating the existence of a marginal reduction in secondary endurance limits. The formula allowing to correct slightly these dependences and to describe marginal decrease of secondary endurance limits, also under some conditions gives full decrease of secondary endurance limits or loses physical sense. Although the dependence proposed for titanium alloys gives a marginal reduction in the secondary endurance limits other than zero, it determines the anomalous nature of the asymptotic curve which does not correspond to numerous experimental data and mathematical models of damage. Earlier, the author has developed a mathematical model of single overloads and on its basis he has performed an estimation of the residual resource of machine durability based on the results of statistical tests of laboratory samples. In this paper, this model is used to estimate the residual structural strength of machines. The calculated dependences are proposed that satisfactorily describe the experiment and are free from the above shortcomings. These relations can be recommended for implementation in the practice of engineering calculations.