Issue 27

L. Mardavina et alii, Frattura ed Integrità Strutturale, 27 (2014) 13-20; DOI: 10.3221/IGF-ESIS.27.02 18 record thermoelastic data under a reduced sinusoidal load range, e.g. in Fig. 3, ensuring that the crack did not grow during the data collection. They investigate the effects of the applied mixed mode and the R=  min /  max . The reported data at higher R (>0.5) ratios and predominantly mode I loading  K II /  K I = 0.45, when the effect of crack closure on the values of the stress intensity factors is minimised since the crack is fully open, were obtained with an average difference between experiment and analytical solutions of 4.3% and 5% for  K I and  K II respectively which is comparable to the accuracy obtained for the specimens containing slots and also comparable to the results obtained by Dulieu-Barton et al. [11] for central crack. Crack closure and propagating crack studies Thermoelasticity is an ideal method for the study of crack closure, since  K is determined directly from the thermoelastic data. Yet very little work has been done in this area using this technique, probably due to the inaccuracy of manual data processing techniques in the past. Fulton et al. [12] have used their new computer technique to perform a study where experimentally determined stress intensity factors were found to decrease with respect to theoretical values with increasing stress range. This was stated to be possibly due to the effect of crack closure. Other tests have been performed using thermoelasticity by Batchelor [22] using the method developed by Tomlinson et al. [8], where crack closure was studied by measuring the effects on the stress intensity factor of cutting out the wake of a fatigue crack. This was found to be a useful technique, but only one crack was studied and further tests need to be carried out to give greater confidence in the results. Tomlinson et al. 1997 also concluded that if a spatial field of thermoelastic data is used, crack closure effects can be detected. Diaz et al. [19] studying the crack propagation in single edge notched specimens found that the  K I values obtained from thermoelasticity are lower than theoretical ones. The decrease in this difference at high R – ratios pointed to crack closure. Using the compliance change method for determining the opening load for a fatigue crack they found that the stress intensity factor range from thermoelastic data matched perfectly with the effective SIF calculated by compliance change. Marsavina et al. [23] evaluated the effective stress intensity factors for a mixed mode crack, propagated using a successive loading cycle, using thermoelasticity and surface replicas. A sinusoidal loading cycle with applied  K II /  K I =0.45 was used in order to record the thermoelastic data around a crack for determining the stress intensity factors ranges. Fig. 4 shows the results normalized with a theoretical solution for the mode I SIF range. It can be observed that the thermoelastic and surface replica results are lower than the theoretical ones for lower values of R-ratio, and for R > 0.5 the experimental TSA results and the replica ones are in very good agreement with the theoretical values. They concluded that the results for the stress intensity factor range obtained by TSA represent the crack driving force. Recent application of thermoelastic fracture mechanics has been done by Yates et al. [24] in the investigation of techniques for tracking the crack path as it grows and evaluating the strength of the mixed mode crack tip stress field. They demonstrate the suitability of TSA techniques to explore the hypothesis that the direction of fatigue cracks may be governed more strongly by directionality of crack tip plasticity rather than by the magnitude of the elastic stress field. Figure 4 : Theoretical, experimental TSA and replica  K I values against R – ratio. C ONCLUSIONS his review has shown the development of methods to determine stress intensity factors from thermoelastic data. With the availability of new, faster thermoelastic systems it is likely that thermoelasticity will continue to be widely used for crack tip studies. The technique has many advantages over other full-field stress analysis methods T