numero25

F.A. Díaz et alii, Frattura ed Integrità Strutturale, 25 (2013) 109-116 ; DOI: 10.3221/IGF-ESIS.25.16 115 TSA RESULTS o proceed to a direct comparison of thermoelastic results with those previously obtained from the analysis of the compliance traces, Δ K values inferred from TSA (figure 8.A) were also employed to calculate an equivalent opening/closing load according to equation 5. As for the case of compliance-based techniques, the equivalent opening load was plotted for the R -ratios corresponding to every load step and compared to the maximum and minimum applied loads. Results are presented in figure 8.B. Figure 8 : A) Plot showing the variation of ΔK with the R -ratio for a sequence of thermoelastic images corresponding to a 42 mm crack. B) Plot illustrating the variation with the R -ratio of the opening load inferred from a sequence of thermoelastic images. D ISCUSSION he initial aspect investigated was the repeatability of the results based on compliance measurements when using strain gauges bonded at the three locations when applying increasing R -ratio steps. Results for the opening/closing were observed in all the cases to give similar results, with slightly smaller values for the case of the 45˚ strain gauge (figures7.A, B and C). In addition, it was also observed that as the R -ratio increased from 0.04 to 0.3, the opening/closing loads remained relatively constant and in all the cases with values higher than the minimum load. This clearly showed the presence of closure at low R -ratios. However, as the R -ratio increased from 0.3 upwards, the opening/closing load was observed to follow the minimum load indicating a considerably reduction in the closure levels. To check the ability of TSA to successfully infer the effective Δ K , thermoelastic images were captured at the same R -ratio steps simultaneously with compliance experiments. Results for Δ K obtained from the analysis of the thermoelastic images (Fig. 8.A) show the same behaviour as observed in the compliance analysis. As the R -ratio increases the Δ K from TSA tends to approach the nominal Δ K , showing a reduction in the closure level as the R -ratio increases. Moreover, for direct comparison of thermoelastic results with those previously obtained using the strain-offset technique an equivalent opening/closing load was inferred from thermoelastic values of Δ K using equation 5 (Fig. 8.B). Results were observed to follow the same tendency as previous results obtained using the strain-offset technique for increasing R -ratio with very similar values to those obtained using the 45˚ strain gauge. This issue clearly support the ability of TSA to measure the effective Δ K rather that the nominal Δ K . C ONCLUSIONS SA has been presented as a novel technique to quantify the effective stress intensity factor range during cyclic loading. Results from TSA have been compared with those obtained using a standard technique for crack closure measurement namely, the strain-offset technique. Compliance results have clearly shown the presence of crack T T T A) B)