Detailed Observation of Change in the Strain near Fatigue Crack Tip

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In this study, for the purpose of proper understanding of the fatigue crack propagation mechanism, detaild observations of change in strain near the fatigue crack tip were carrind out. And successful measurement of the hysteresis curve of load (P) vs. strain (epsilon') with high accuracy was made.First, from introducing the curve of compliance (depsilon'/dp) and the change rate compliance (d^2epsilon'/dp^2) during one cycle, the following observations have been obtained concernig the fatigue crack opening/closure phenomenon.1) Unloading part of the hysteresis curve is found to be "S" shape, and the point of inflection of the unloading part is considered to be the crak closure point. So if the accuracy of measurement is high enough to be able to obtain a smooth curve of compliance, the crack closure point can be measured automatically as the maximum point of the unloading part of this curve.2) Loading part of the curve of compliance is found to be "S" shape, and the point of inflection … More of the loading part is considered to be RPG (Re-tensile Plastic zone' Generated) point. So if the accuracy of measurement is high enough to be able to obtain a amooth curve of the change rate of compliance, the RPG point can be measured automatically as the minimum point of the loading part of this curve.Secondly, from dividing the loading and unloading processes into several characteristic areas on the basis of the tendency which the compliance and the change rate of compliance show, the following observations have been obtained concerning the fatigue crack propagation parameters.3) In unloading process, it seems to be impossible to isolate the area corresponding to the creation of the re-compressive plastic zone and there is also doubt about using the stress intensity factor because of the influence of the crack closing. So it seems to be rational to adopt the area in loading process, corresponding to the creation of the re-tensile plastic zone, for the fatigue crack propagation parameter.4) According to the experimental results, it is found to be clear that the re-tensile plastic zone's generated point disagrees with the crack opening point.5) From the facts described above 3) & 4), it may be concluded that DELTAK_<RP>, , not DELTAK_<OP> nor DELTAK_<C1>, is considered the most suitable parameter which controls the fatigue crack propagation. Less