A PHOTOELASTIC STUDY OF CRACK REPAIR

Birefringent coatings have been employed to study the effectiveness of an adhesively bonded repair of a center-cracked tension panel. The re pair was one sided, with photoelastic coatings applied to the opposite side. Photoelastic coatings were also applied over the patch. Analysi s methods are presented to permit the stress intensity factor to be de termined from the isochromatic fringe patterns recorded from both cont inuous and X-and Y-edged coatings. The results showed that the one-sid ed adhesively bonded patch reduced the stress intensity factor; howeve r, the repair did not markedly change the character of stress distribu tions. Fringe loops formed near the crack tips for both the cracked an d repaired tension panels. The primary difference was in the size of t he loops. The reduction in K-I due to repair was smaller than anticipa ted, but even small improvements in Delta K-I markedly enhance the lif e of a repaired panel. The Paris power law is used to show the relatio n between the reduction in Delta K-I and the improvement in the crack growth rate da/dN. Fringe patterns from the birefringent coatings appl ied to the patch provided a means not only to investigate the stresses in the patch but also to detect the initiation of the local debonding of the adhesive in the neighborhood of the crack. The birefringent co ating on the patch is an approach for producing an optically ''smart'' repair.