Quantifying spectrum loading effects on fatigue crack growth in 7075-T73 aluminum

The combined effects of four variables on calculated spectrum fatigue crack growth (FCG) in 7075-T73 aluminum were compared using an experimental design approach (NASGRO 3.0 adaptation). The four parameters considered were: (1) load interaction, (2) load sequence (rainflow counted low-to-high, rainflow counted and randomized, and rainflow counted high-to-low), (3) crack tip stress state (plane stress versus plane strain) and (4) spectrum type (FELIX-28 helicopter rotor blade spectrum versus F-16 wing-root-bending moment spectrum), Each of the three load-interaction models available in NASGRO 3.0 was used to compute load-interaction effects. The investigation concluded that the order of cycles and crack tip constraint play relatively minor roles in computed FCG for problems involving repeated application of either of the given spectra, while the effects of the spectrum type itself, inclusion of load-interaction effects and the synergy of spectrum with load interaction are highly significant.