Issue34

A. Tajiri et alii, Frattura ed Integrità Strutturale, 34 (2015) 347-354; DOI: 10.3221/IGF-ESIS.34.38 349 should be noted that onion ring patterns are more strongly formed under high strain rate FSP condition (1000 rpm). Figure 2 : Macroscopic structure observed on the cross section of stir zone; (a) L specimen, (b) H specimen. Fatigue properties The tensile strengths of the as-received, L and H specimens are 218, 199 and 215 MPa, respectively. L specimen has slightly lower ultimate strength, but the tensile strength is insensitive to the tool rotational speed. The S-N diagram is shown in Fig. 3, in which high cycle fatigue strengths were highly improved by FSP compared with the as-received material. It should be noted that the FSP under lower rotational speed had resulted in the higher fatigue strength than that under higher speed. Fig. 4 reveals the fracture surface near crack initiation site in the as-received material. Large casting defect is recognized at the crack initiation site, indicating the typical fatigue crack initiation mechanism of A356 cast Al alloy. On the contrary, casting defects were not found in L and H specimens as indicated in Fig.5, showing fracture surfaces near crack initiation sites. It could be concluded that casting defects were successfully eliminated by both FSPs under low and high tool rotational speeds. Consequently, the increase of fatigue strengths in the FSPed specimens could be mainly attributed to the transition of crack initiation mechanism from defect-dominated to cyclic-slip-dominated crack initiation. Figure 3 : S-N diagram. Small fatigue crack growth behavior was monitored by a plastic replication technique. A fatigue test was periodically terminated, and the shape of fatigue crack was copied to the plastic replica film. Consequently, fatigue crack growth rates, d a /d N , was measured from the crack length as a function of number of cycles. Fig. 6 shows the relationship between d a /d N and maximum stress intensity factor, K max . In this case, K max was calculated assuming the aspect ratio of surface SZ R-side A-side 1mm (b) SZ A-side R-side 1mm (a) 1 10 4 10 5 10 6 10 7 10 8 50 100 150 200 Number of cycles to failure N f Stress amplitude  a (MPa) A356 Plane bending R = −1 as−received L H