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Tham khảo tài liệu 'mechanics.of.materials.saouma episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 14.1 Maximum Circumferential Tensile Stress. 5 12. If the pair of SIF is inside the fracture loci then that crack cannot propagate without sufficient increase in stress intensity factors. If outside then the crack is locally unstable and will continue to propagate in either of the following ways a With an increase in the SIF and the energy release rate G thus resulting in a global instability failure of the structure crack reaching a free surface will occur. b With a decrease in the SIF and the energy release rate G due to a stress redistribution the SIF pair will return to within the locus. Victor Saouma Mechanics of Materials II 6 Dian MIXED MODE CRACK PROPAGATION Victor Saouma Mechanics of Materials II Draft Chapter 15 FATIGUE CRACK PROPAGATION 1 When a subcritical crack a crack whose stress intensity factor is below the critical value is subjected to either repeated or fatigue load or is subjected to a corrosive environment crack propagation will occur. 2 As in many structures one has to assume the presence of minute flaws as large as the smallest one which can be detected . The application of repeated loading will cause crack growth. The loading is usually caused by vibrations. 3 Thus an important question that arises is how long would it be before this subcritical crack grows to reach a critical size that would trigger failure To predict the minimum fatigue life of metallic structures and to establish safe inspection intervals an understanding of the rate of fatigue crack propagation is required. Historically fatigue life prediction was based on S N curves Fig. 15.1 or Goodman s Diagram Endurance Limit Figure 15.1 S-N Curve and Endurance Limit using a Strength of Material Approach which did NOT assume the presence of a crack. 15.1 Experimental Observation 4 If we start with a plate that has no crack and subject it to a series of repeated loading Fig. 15.2 between ơmin and ơmax we would observe three distinct stages Fig. 15.3 1. Stage 1 Micro coalescence of
