PSI - Issue 60

212 10 S.K. Chandra et al. / Procedia Structural Integrity 60 (2024) 203–213 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000 associated with larger blunting effect compared to the pre-cracked specimens. This effect is significant for crack initiation characterization, like , and measurements, where pre-cracking is a must. However once the crack starts to propagate the effect of notch radius become insignificant (with crack growth any difference between notched and pre-cracked specimens is lost). Hence, both notch and pre-cracked specimens can be used for crack growth resistance characterisation. In this study, − ∆ , 5 −∆ and ( 5 )−∆ characterize crack growth. Figure 12 gives one example demonstrating that ( 5 ) is only an approximation for, and not a true measure of, . At relatively small ∆ levels, - and ( 5 ) -resistance curves exhibit reasonable matching. However, with increasing ∆ , the plots increasingly deviate and consequently, ( 5 ) , the average of ( 5 ) values in the approximation for the constant CTOA regime, becomes smaller than . As the data in Table 2 show, ( 5 ) values are significantly ( ~11%) lower than . From th e disposition of the ( 5 ) data in this figure, it is also clear that similar results would be obtained with any other plausible alternative criteria.

Fig. 12. Comparison of φ−∆a and φ(δ 5 )−∆a curves of pre-cracked SENT specimen.

4. Summary and conclusions The present study shows that crack growth on 1 mm thick IF steel sheet (showing flat fracture) can be characterized by both − ∆ and δ 5 − ∆ resistance plots. Both CTOA, and δ 5 possess geometry as well as notch root radius independence, and hence qualify as a material parameter. Small size (30 mm width) DENT and SENT specimens can successfully be employed for and δ 5 measurements instead of standard large-size CT and MT specimens, also use of these specimens will simplify the cumbersome CTOA measurement procedure as anti-buckling guides are not necessary for testing such specimens. Even though either SENT or DENT specimens can be used for the critical CTOA, determination, practically, the SENT specimens are better choice. This is because fatigue pre-cracking becomes considerably simpler, only need to monitor one crack tip and allowable ∆ becomes larger. For determining the critical CTOA, and ( 5 ) there is no need to use fatigue pre-cracked specimens, notched specimens can easily be used as they characterize a growing crack. Thus, it further simplifies the CTOA testing procedure as recommended in ASTM E 2472. Increasing crack tip radius increased the initial transition ∆ range, but the value for the critical CTOA remained unaffected. The optical measurement of 5 , by adapting the standard 5 method (that uses an extensometer) was generally successful. However estimating critical CTOA from δ 5 − ∆ plots ( 5 ) , is not an actual measure of standard optical critical CTOA, , as the 5 gauge position does not follow the tip position of the