Một ước tính trực tiếp, bằng cách sử dụng lý thuyết chung, của sự căng thẳng lưới trong các bản vá trên crack cho thấy một giá trị 7100 microstrain. Tuy nhiên, nếu tải thêm thu hút vào các miếng vá (như là kết quả của hiệu ứng bao gồm) được coi là, sự căng thẳng có thể được như cao như 9500 | Chapter 17. Damage tolerance assessment of bonded composite doubler repairs 487 Wf can be determined experimentally 7 Reference 6 also describes the maximum load Pmax that can be carried by a bond in a symmetrical bonded joint as p nax 2 tWeETỸ12 where Wc is the maximum strain energy density of the adhesive. Thus composite doubler repair design guidelines are that max is greater than the ultimate load for the repaired structure and that Pf is greater than the limit load. Reference 6 also points out that these critical design variables are affected by the loading rate. A conservative estimate for can be obtained by using the value of the maximum von Mises equivalent stress in the adhesive ơe as measured in high strain rate tests. For FM73 the adhesive used in this study ơ pmax 5800 psi and the threshold stress iTth 3600 psi. This analysis approach clearly shows the importance of the adhesive in determining the overall performance of the bonded repair. The approach outlined above can be used to certify that a composite doubler design will satisfy the damage tolerance provisions of the . Federal Aviation Regulations FAR Part 25. The fundamental result from the reference 8 NDI study is that a team of NDI techniques can identify flaws well before they reach critical size. The abilities of nondestructive inspection techniques to meet the DTA flaw detection requirements are presented in Chapter 23. Analysis of composite repairs Numerous efforts have developed refined and advanced the use of methodologies needed to analyze composite doubler installations. Obviously this is a critical element in the repair process since a badly implemented repair is detrimental to fatigue life and may lead to the near-term loss of structural integrity. The difficulties associated with analyzing the stress fields and flaw tolerance of various composite doubler designs and installations are highlighted in references 3 5 9 Doubler design and analysis studies 6 9-17 have led to computer
