Evaluation of fatigue life and crack growth rates in 7050-T7451 aluminum plate for T-L and L-S oriented failure under truncated spectra loading

Abstract

Spectrum crack growth testing of 7050-T7451 plate was performed for T-L and L-S specimen orientations to determine the influence of spectrum truncation on fatigue crack growth rates and overall life. This testing was performed on center-hole (double crack), flat, rectangular tensile specimens machined from material blanks cut from 10.16 cm plate. Additional comparison T-L tests were also accomplished using blanks from available 7050-T7451 1.27 cm plate. Behavior of through-thickness long (>5 mm) cracks is examined. Spectrum loading used for the testing was based on a nominal fighter load spectra and truncation levels of testing were comprised of baseline (5%), plus 20% and 30% ranges. The target, ordered tensile-compressive, stress levels were based on a combination of legacy and predicted use of US fighter aircraft. Results produced representative trends in life-to-failure increases for truncation levels relative to 5% truncated levels for the L-S and T-L orientation specimens. Crack growth rate data was recorded and comparisons are exhibited for varied orientation and crack length. Findings show that although relatively small changes in life are experienced due to orientation, L-S oriented plate material life is affected by the greatest extension of up to 17%. Crack growth rates for shorter cracks in the L-S material exceed growth rates in the T-L oriented material by more than two times the rate. Characteristics of splitting, crack wandering, branching, and shielding were observed in the L-S specimen tests. These potentially undesirable characteristics resulted in temporary crack arrest or retardation in the direction perpendicular to loading and resulted in overall specimen rupture well beyond the life predicted critical crack lengths.