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Advances in the Bonded Composite Repair o f Metallic Aircraft Structure phần 9

Đối với bảng điều khiển vá, vá lý thuyết cho thấy rằng K, khoảng 53MPam1 / *. Mặc dù Ko0 khá gần với Grit, trước đây là một ước tính trên giới hạn của cường độ căng thẳng do đó, nó là dự kiến kết luận rằng các vết nứt trong kim loại không phải là nguyên nhân của thất bại. | Chapter 15. Graphitelepoxy patching efficiency studies 439 Fig. 15.18. Different damage configurations of equivalent width. that the combined contribution of AT J and Alni to the effective stress intensity factor was less than 8 for the configurations studied. Fatigue testing confirmed that various forms of damage could be repaired effectively with single patches. For example the fatigue lives of panels containing 40 mm diameter holes and either single cracks or four 45 kinked cracks were improved by factors of 5.1 and 15 respectively by single-sided repairs with a 1 mm thick patch 80 mm X 80mm The measured fatigue crack growth rates were Table 15.5 Comparison of stress intensity factor ranges for four 45 kinked cracks and two diametrically opposite cracks with tips at X 40 mm and hole radius 20mm 0.1 crmax 65 MPa . Crack configuration Patch thickness mm AKP AK6 MiP kKu 4 X 45 kinked 2.0 6.0 22.3 0.30 cracks 1.0 8.8 22.3 0.39 2 X diam. opposite 2.0 11.3 27.7 0.41 cracks 1.0 13.7 27.7 0.49 440 Advances in the bonded composite repair of metallic aircraft structure in good general agreement with theoretical predictions. For example the mean crack growth rate of 9 X 10 9m cycle measured for four kinked cracks at a half crack length of 35 mm during R 0.1 ơmax 41.25 MPa loading was in good agreement with predicted values of KP 5.7 MPam1 2 and fmin fmax 0.51 and da dN-AK data for 2024-T351 aluminium alloy sheet. Furthermore the observed crack paths indicated little effect due to Mode II loading. In the case of a 40 mm hole and two cracks double sided patching resulted in crack arrest in agreement with theoretical predictions. Work is in progress to determine the effectiveness of patch repairs for other damage configurations. 15.10. Future work Although adhesively bonded gr ep patch repair of cracked metallic structures has been studied extensively and service experience with repairs has been good it appears that further work is required to address some remaining problems