13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 6. a) Typical load-compliance offset curves for as-welded and heat treated specimens. b) Comparison between experimental values of U and Newman’s prediction. The solid line in Fig. 6b was calculated using Newman’s plasticity-induced closure stress equation [12, 13] and represents an upper bound of the effective stress intensity factor range ratio. Experimental data above this upper bound indicate the existence of a crack opening mechanism. Tensile residual stresses are believed to act as such a mechanism in the as-welded specimens, since tensile residual stresses are typically found in CA6NM welds. To further assess the presence of tensile residual stresses, an as-welded compact tension specimen with a crack length of 20 mm was incrementally loaded from 500 N to -12000 N and the crack opening displacement was recorded (Fig. 7). A closure load of -1000 N was found with a 2 percent compliance offset criterion. The existence of a negative closure load is another strong indication that tensile residual stresses are acting on the crack by preventing it to close at positive loads. A similar behavior has been observed in the welds of three different kinds of steels where tensile residual stresses caused the crack to remain fully open at a load ratio R = 0, resulting in higher fatigue crack growth rates than that of the base metal [5]. Figure 7. As-welded specimen load-COD curve from a single compression excursion (a = 20 mm). a) b)
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