In the present study the correlation between the crack opening and crack propagation behavior under selected loading histories was systematically investigated on center cracked Al 7475-T7351 specimens. The development of the crack opening stress intensity Kop as the lower limit of the effective stress intensity factor range ΔKeff, has been determined using the near threshold crack growth (NTCG) method as proposed by Döker and Bachmann. Significant crack closure (i.e., ΔKeff < ΔK) was found for all sequences, and also for those at very high R-ratios (R > 0.83). The crack growth behavior was determined by the DC-potential drop method and by striation measurements and showed significant retardation. The Kop values correlated with the da/dN measurements.
The experimentally determined Kop and da/dN behavior was compared to predictions as performed by J. C. Newman, Jr. applying his advanced FASTRAN II model. The predictions of the development of the Kop and of the da/dN values compare quite well except in one case, where the model predicted complete crack arrest. The affected cycle numbers for the retardation periods were also satisfactorily predicted (except in that one case). From the investigations performed in the present study it is concluded that exact measurements of the trends in Kop as caused by load sequence effects form an important basis for further improvements of crack propagation calculations.