Advances in the Bonded Composite Repair o f Metallic Aircraft Structure phần 8

Kể từ khi, từ 13,12 hình, omax khoảng 160 MPa và mm là 33, Kcdt ước tính là khoảng 56 MPam '/ 2. Kết quả tương tự cho đã được thu được từ một số tấm khác chưa được vá. Những giá trị này cho & được trong thỏa thuận hợp lý với công bố giá trị cho các tấm 2024T3 độ dày này. | 390 Advances in the bonded composite repair of metallic aircraft structure assumed to apply for this specimen configuration A rit 1 1 Since from Figure ffmax is around 160 MPa and ứ is 33 mm A crit is estimated to be about 56MPam1 2. Similar results for A7crit were obtained from several other unpatched panels. These values for A cril are in reasonable agreement with published values for 2024T3 panels of this thickness. For the patched panel patching theory suggests that Koo is approximately 53MPam1 2. Although A oo is fairly close to Acrit the former is an upper-bound estimate of stress intensity so it is tentatively concluded that crack propagation in the metal was not the cause of the failure. Strain capacity analysis A direct estimate using joint theory of net strain in the patch over the crack indicates a value of 7100 microstrain. However if the extra load attracted to the patch as a result of the inclusion effect is considered the strain could be as high as 9500 microstrain. Since strain capacity of the boron epoxy is measured to be about 7300 microstrain the conclusion is that failure was probably a result of initial failure of the patch. Furthermore as discussed in reference 1 for the patch configuration employed the ratio inner-surface strain outer-surface strain in the patch is significantly greater than unity. In this case it is estimated to be about . On this basis the inner strain could have exceeded 12000 microstrain however the strain elevation would be very localised. The conclusion is thus reached that failure in the patched panels resulted from initial failure of the patch possibly associated with the strain concentration at its inner surface. This failure mode may change where significant disbond growth occurs during fatigue cycling for two reasons Stress intensity Koo may exceed A crit allowing the crack to grow catastrophically under the patch. The strain concentration in the patch over the crack will be reduced if even minor .

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