Tham khảo tài liệu 'metal fatigue part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 224 Chapter 12 e N 106 Influence of Si-Phase on Fatigue Properties of Aluminium Alloys 225 Figure Fracture surface near fracture origin which is at Si phase extrusion 20A11 a 147 MPa Nf X to7 rotating bending. Fig. shows the fatigue life percentage for crack initiation and propagation. It can be seen that shear-type cracks grow very quickly. Fatigue Behaviour of Specimens Containing an Artificial Hole In order to clarify the difference in the fatigue mechanisms for the continuously cast material and for the extruded material fatigue tests were carried out on specimens containing an artificial hole. Fig. shows the S-N curves obtained. Fig. shows the crack growth behaviour for a specimen which survived for N 109 cycles. S-N curve 3 in Fig. shows a fatigue limit with a clear knee point. Fig. implies that a plasticity induced crack closure mechanism caused non-propagation or at least tendency towards non-propagation of cracks emanating from the artificial hole. Comparing the S-N curves 1 2 and 3 in Fig. we can interpret the S-N curve for unnotched specimens to be a combination of curves 1 and 2 namely curve 1 which is for failure from Si phase and has a hypothetical clear fatigue limit and curve 2 which is for failure by shear-type cracking but has no clear fatigue limit even at N 108. The 226 Chapter .
