Computing and Testing for Mixed Mode Thresholds on Bending Specimens
Tension plates with inclined cracks are frequently used for mixed mode threshold tests. However, when this kind of specimen configuration is adopted to measure the mixed mode thresholds, initially some larger plates are machined and are precracked in fatigue under I mode loading condition so that a sharp crack can be obtained along each initial notch machined by electric spark technique, these plates are then remachined with cracks at various inclined angles. Evidently, it is unconvenient to carry out two cuts for each specimen. In this paper, three point bending specimens in which the straight cracks take different deviations (S1 values) from the centerline of the span, as shown in Fig.1, are adopted. For these specimens, precracks along initial notches can be attained easily in the manner of I mode crack (namely S1=0) without second cut, and meanwhile I mode thresholds can be measured. Therefore, it is convenient to use these bending specimens to measure the mixed mode thresholds(mixed mode means I+II mode in this paper) compared with tension plate specimens with inclined cracks. But the question of how to compute KI and KII of these bending specimens has to be solved. The empirical relations of KI and KII for the bending specimens are developed based on numerical results of the finite element method in this paper so that mixed mode threshold tests on these bending specimens with different crack deviations can be made without the trouble of computing stress intensity factors.
KeywordsStress Intensity Factor Stress Ratio Mixed Mode Mode Crack Incline Crack
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