Service-Life Induced Failure of Bird Impact Resistant Windshields,
MATERIALS RESEARCH LABS ASCOT VALE (AUSTRALIA)
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Bird-impact resistant transparencies BIRT, which are laminates of modified polymethylmethacrylate, polycarbonate and a rubbery interlayer, have been fitted to all Royal Australian Air Force F111C aircraft. However their service-life has often been severely limited by factors other than bird-impact, and in one instance was as little as 30 flying hours for an alternate design bird-impact resistant transparency ADBIRT windshield. Unserviceability has arisen due to cracks, crazes, delamination, optical distortions or scratches the most critical are cracks and crazes. All failed first generation windshields BIRT examined showed crazing of the inner acrylic layer, while 60 of them had serious crazing of the outer acrylic layer, and in 35 this layer was cracked. With ADBIRT windshields all failures examined were crazed, while some were cracked or showed delamination. The shapes of the crazes seen on these windshields show that the stresses involved in their formation and growth are complex.