PSI - Issue 19

Shinji Hashimura et al. / Procedia Structural Integrity 19 (2019) 204–213 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The square symbols shows the results for Q m =6 kN. The circle symbol shows the results for Q m =8 kN. The colours of symbols were the same as Fig. 4. It can be seen in Fig. 5 that the fatigue strengths of A6056 bolts were significantly higher than those of A5056. Although the fatigue strengths of A6056 bolts decrease with an increase in the mean load Q m if the mean load Q m was 6 kN or less, the fatigue strengths have not decreased with an increase in the mean load if the mean load Q m was higher than 6 kN. This result suggest that the shakedown behaviour have occurred at the root of the first thread when the mean load Q m was higher than 6 kN. The fracture points of the bolt which broke by the stress amplitude just above the fatigue limits were almost the root of the first thread for all mean stress. It can be seen that the fatigue lives depend on the mead load Q m . 4.3. Haigh diagram of test bolts Figure 6 (a) shows Haigh diagram for A5055 bolts and A6056 bolts. In Fig. 6 (a), the ordinate is the fatigue strength  w and the abscissa is the mean stress  m . The square symbols show the results for A5056 bolts and the circle symbols show the results for A6056 bolts. For each symbol, the black colour shows that the bolts broke at the root of the first thread and the grey colour shows the bolts broke at the incomplete thread root under bolt head. Pictures of the fracture surface show in the right side in Fig. 6 (b) and (c). It can be seen in Fig. 6 (a) that the fatigue strength  w of A5056 bolts decrease with an increase in the mean stress  m although the fatigue fracture point has changed. If the stress amplitude was comparatively high, the bolts broke at the root of the first thread. One of the reasons is considered that work hardening occurred at the root of the first thread at tightening process because the stress concentration was very high. In the region that the stress amplitude was comparatively low, it is considered that the bolt broke at the root of the incomplete thread which the stress concentration was the second highest because the surface hardness at the root of the first thread has already been high. We can also see that the fatigue strength  w of A6056 bolts decrease with an increase in the mean stress  m until Q m =6KN and then the fatigue strength  w is constant similar to Haigh diagram in Fig. 1. FE analysis was conducted in order to verify the cause of kink for A6056 bolt in Fig. 6 (a).

Fig. 6 Haigh diagram of aluminium alloy A5056 bolts and A6056 bolts, and their fracture surfaces.