Issue 35

Takamasa Abe et alii, Frattura ed Integrità Strutturale, 35 (2016) 196-205; DOI: 10.3221/IGF-ESIS.35.23 202 Figs. 14 and 15), but the overall patterns do not evolve, and the leading edge of the crack maintains its complex shape. These observations indicate that multiple cracks initiated at multiple origins coalesce as they propagate, eventually causing complete fracture. The three-dimensional crack observation method cannot provide a continuous picture of the fatigue crack propagation behavior. Therefore, we investigate the leading edge from several crack initiation origins using the beach mark method. In the beach mark test, the load amplitude was F a=6kN, and the results are shown in Fig. 16. The beach mark was repeated at regular intervals, and the fracture accompanying the crack leading edge was clarified. Fig. 16 also presents a schematic of the crack leading edge emanating from the fracture, and the number of cycles to failure. The N f of this test piece was 3.5×10 6 cycles. This continuous observation of the crack leading edge by beach marks revealed multiple crack initiation origins, and coalescence of the multiple cracks as the number of cycles increased. From the three- dimensional and beach mark observation results, we confirm that the combination of fatigue cracks plays an important role in fatigue damage, and should therefore be considered in the fatigue mechanics. Fig. 17 is a three-dimensional picture compiled at a load amplitude of 5kN, where the test piece survives to N =10 7 cycles. Even at this low force level, there are fatigue crack initiations and crack propagations. Figure 14 : 3-dimentional observation of fatigue crack propagation at the root region ( F a =6 kN). Figure 15 : 3-dimentional observation of fatigue crack propagation at the root region ( F a =9 kN).