On the USAF 'risk of failure' approach and its applicability to composite repairs to metal airframes

The USAF report on the risk analysis of aging aircraft fleets notes that the operational life of individual airframes is seldom equal to the design life of the fleet and that the life of an aircraft fleet tends to be determined more by its inherent operational capability and maintenance costs rather than by the number of flight hours specified at the design stage. As such this paper focuses on whether the USAF approach to risk assessment can be used for airframes repaired with a composite patch/doubler. To this end the present paper describes a test program designed to study the effect of adhesively-bonded composite repairs to fatigue cracks that, prior to repair, have grown from small naturally-occurring materials discontinuities. This study reveals that crack growth in composite repairs conforms to the exponential growth equation used in the USAF approach to assessing the risk of failure. Furthermore, the exponent, ω, in the exponential growth law can be determined from the crack growth history associated with the unrepaired specimens and the simple reduction in the stress due to the application of the composite patch/doubler, using the 'cubic rule' that was previously used to assess crack growth in the RAAF F/A-18 (Hornet) fleet.