Đang chuẩn bị liên kết để tải về tài liệu:
Advances in the Bonded Composite Repair o f Metallic Aircraft Structure phần 6

Như là kết quả của chương này, chúng tôi thấy rằng những cân nhắc thiết kế chính, tức: cường độ căng thẳng tối đa yếu tố, cho phép bản chất nhớt, nhựa chất kết dính, nên càng thấp càng tốt và tốt nhất là dưới giá trị quan trọng thứ k cho vết nứt mệt mỏi tăng trưởng trong vật liệu. | 262 1 R Fig. 9.34. Composite repairs to cracked holes. 9.12. Findings relevant to thick section repair As a result of this chapter we find that the major design considerations are viz 1. The maximum stress intensity factor allowing for the visco-plastic nature of the adhesive should be as low as possible and preferably below the critical value Kth for fatigue crack growth in the material. For cracks at holes or notches or repairs to corrosion damage load bi-axiality should be accounted for in the design process. 2. The maximum adhesive stresses energy should be below the value at which fatigue damage accumulates in the adhesive see 8 16 21 . For FM73 this NORMALIZED STRESS INTENSITY FACTOR KJK- Fig. 9.35. Crack-tip stress-intensity factors for short and long cracks from 29 SEMI-INFINITE BONDED JOINT HART-SMITH S SOLUTION TRANSITION FROM SHORTTO LONG CRACK Chapter 9. Numerical analysis and design 263 value is 25 MPa. However to minimise errors in measuring and computing the adhesive stresses and allowables see 20 it is best to compute and measure the energy in the adhesive w l 2ơụ j Ị ơijd ij. These measurements are best performed using the ASTM thick adherend test ASTM D 1002 see 16 3. The composite patch must not experience failure by interply delamination. This can be checked by ensuring that the polynomial failure criteria is not greater than one. The commonly used failure criteria are Tsai-Hill Hoffman and Tsai-Wu. These failure criteria are generally written in the form Tsai-Hill criterion Failure is assumed to occur when -2 _ _ -2 -2 d 9.72 Here the material is assumed to have equal strengths in tension and compression i.e. X xc X and Yi Yc Y Hoffman criterion Failure is assumed to occur when 1 1 1 1 . ơ . ơ . ơ 2 Ơ1Ơ2 x XJ1 TJ 2 T V S1 X X C Tsai-Wu criterion Failure is assumed to occur when 9-73 9.74 The coefficient F 2 is experimentally determined from test specimens under biaxial loading and F 2 must satisfy a stability criterion of the form 9.75 creates