Fatigue Crack Propagation in Mirage IIIO Wing Main Spar Specimens and the Effects of Spectrum Truncation on Life

摘要

As part of an investigation into the life extension and safe operation of the wings of the Mirage IIIO aircraft, a fatigue testing program and extensive fractographic examination was undertaken on specimens representing the critical section of the spar to assess the effects of truncating the maximum positive loads of the spectrum and provide information relating to fatigue crack propagation rates. Under the fighter-type load spectrum adopted, truncation of the maximum load from +7.5g to +6.5g or to +5g did not result in an increase in fatigue life, presumably because of the loss of the crack retardation potential of this rarely occurring high positive load. A good linear relationship was found between the standard deviations of log. life to crack 'initiation', for propagation or to final failure, nor in the corresponding standard deviations of arithmetic lives. In considering the more general question of whether the greater contribution to variability in total life comes from that in 'initiation' life or that in 'propagation' life, cognizance must be taken of the analytical basis of the assessment-whether arithmetic crack of specific depth decreases with increasing crack depth, whereas on arithmetic basis it increases with crack depth, that at a depth of 5 mm being about seven times that at a depth of 0.5 mm. However the standard deviation of log. crack propagation rate is substantially constant irrespective of crack depth.