Issue 35

S. Barter et alii, Frattura ed Integrità Strutturale, 35 (2016) 132-141; DOI: 10.3221/IGF-ESIS.35.16 139 P ATH EFFECTS ON CRACK GROWTH RATE he apparent retardation effect that builds up for CA loading, as discussed for Fig. 6, was examined by taking crack growth measurements of short sub-blocks of CA embedded between VA loads. The coupons were tested with CA sub-blocks of: 500, 1000, 2000, 5000 or 10000 cycles; an example of one of these spectra is shown in Fig. 7. In Fig. 7A, a single 10000 cycle sub-block is shown, and it appears to roughen as CA cycles progress; apparently due to a multitude of alternate crack paths being sampled locally by the crack front. The inclusion of VA sub-blocks allowed the measurement of the CA bands due to the notable change in the crack path and morphology of the crack surface between them and the CA sub-blocks, i.e. Fig. 7B and Fig. 8A. Generally, it was found that the CA sub-blocks produced notably rougher surfaces (Fig. 7A) than the VA sub-blocks (Fig. 7B shows clear progression bands that where approximately perpendicular to the loading axis and relatively flat). The change in plane often seen with the CA sub-blocks compared to the VA sub-blocks is shown in Fig. 8A. Figure 7 : Local detail of the increasing roughness of the crack path with increasing cycles of CA at R=0.8 in ‘A’ ,  K~1.5 MPa√m. ‘B’ shows an example of a fracture surface produced by the spectrum (similar  K). The CA/VA spectrum is shown in the insert. Figure 8 : ‘A’ shows a 3D image taken of a 0.1mm x 0.1mm section from a coupon tested with CA/VA spectrum (Fig. 8) showing crack path changes between CA and VA crack growth. ‘B’ shows a summary of the measured crack growth rates from CA loading sub-blocks with varying cycles[24], suggesting that for growth below ~2x10 -8 m/cycle the growth rates are inversely dependent on the number of CA cycles applied. T B