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 desi gn approach (NASGRO 3.0 adaptation). The four parameters considered were: ( 1) load interaction, (2) load sequence (rainflow counted low-to-high, rainf low counted and randomized, and rainflow counted high-to-low), (3) crack ti p stress state (plane stress versus plane strain) and (4) spectrum type (FE LIX-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 conclude d that the order of cycles and crack tip constraint play relatively minor r oles in computed FCG for problems involving repeated application of either of the given spectra, while the effects of the spectrum type itself, inclus ion of load-interaction effects and the synergy of spectrum with load inter action are highly significant.