Micro-mechanical damage accumulation in airframe materials and structural components

A simple and flexible ACPD probe design methodology incorporating current focusing technique was developed for on-line as well as off-line measurements in laboratory and industrial scale test environments. The effectiveness of the current focusing technique and the resulting three-dimensional control of the current density distribution was demonstrated. The ACPD technique was successfully used to obtain strain calibration and crack calibration curves in 7075 Al alloys. This also included quantifying the effect of residual stresses on ACPD signal. The smallest crack depth detected was 140 mum at 120 kHz, 5 amp. and 3000 gain. A quadratic relation was found to correlate crack depth with potential change. The ACPD sensors were installed on a F-18 aircraft undergoing full scale test at Canadair. The sensors were able to measure the response of the structure to spectrum loading. A crack was detected by the ACPD sensors only after 3400 SFH and it was established that ACPD probes were able to detect the crack long before (∼2500 SFH) any other established techniques utilized by DND/Canadair was able to detect the crack.; The technique was successfully used to characterize short crack growth behaviour of naturally initiated fatigue cracks in Al alloys from 40 mum crack depth onwards. The cracks were semi-elliptical in shape and crack growth along the notch root was faster than crack growth into the specimen. The effect of various grain orientations on short crack growth behaviour was studied. The crack tip interactions with grain boundaries were shown to control the SCG behaviour of Al alloys even at notch root peak stresses close to yield stress of the material. This effect was less prominent at notch root peak stresses considerably higher than yield stresses. The fatigue crack growth rates were correlated with the SIF (K) calculated using Newman's model for thumb nail cracks. This correlation resulted in a very large scatter in the data. The fatigue crack growth rate data were better correlated using modified parameters, DeltaK2 /sigma and DeltaK/C.