Development of a proof test for through-life monitoring of bond integrity in adhesively bonded repairs to aircraft structure

This paper describes the development of a proof test to evaluate the through-life integrity of structural adhesive bonds. The test is focussed on adhesively bonded patch repairs for aircraft structure; especially those where flight safety depends on the integrity of the repair patch. The test could be used either as an alternative or as an addition to structural health monitoring of bonded repairs to increase confidence to the extent that they could be certified for application to flight-critical structure. The implementation of the test is as follows: thin coupons of the patch material are bonded to the surface of the parent structure simultaneously with, and therefore under very similar conditions to the repair patch. These coupons are proof tested periodically in shear using a torque wrench. Failure of the coupon below a predetermined proof load provides an indication that the adhesive bond to the patch or (possibly) the patch itself has degraded and should be replaced. It was concluded that this test is a very promising cost-effective approach for detecting defective or deteriorated adhesive bonds; however, to raise the technology readiness level to the extent that it could be considered for aircraft applications more work is required to improve the test database and increase practicality.