Fatigue crack acceleration effects during tensile underloads in 7010 and 8090 aluminium alloys

This study investigates the combined action of small amplitude high R ratio cycles and large amplitude low R ratio tensile underloads on fatigue crack growth in aluminium alloys 8090 T852 and 7010 T76351. Compact tension samples were subjected to repeated two level load spectra in which near threshold load cycles at R = 0.9 were interrupted with underloads to zero load. Kop was measured during underload cycles using a crack mouth gauge. Under these loading conditions crack growth rates are accelerated with respect to the rates calculated from constant amplitude data using linear summation. In 7010, the acceleration was about 30%, in 8090 it ranged from 30% to over 1200%, depending on the number of small cycles between underloads, as well as other loading parameters. Fastest growth rates occurred under loading conditions producing a flat fracture surface and Kop levels reduced from the constant amplitude case. It is concluded that there are two mechanisms causing acceleration, both arising from increases in ΔKeff of the underloads in the two level tests. Changes to plasticity based closure produces moderate accelerations of 30% in both alloys; changes to microstructure based closure in 8090 produce additional large accelerations.