w. Lee et aL. / Composites Science and Technology 66(2006)435-443 He et a.(1994)&a/h=1x10 12 a/h= 2x10 0.8 a/h=1x103 6 h= 5x 6-04-020.00.2 0.6 60 50 He et al. (1994) &a/h=1x10 a/h=1x102 -0.4 -0.20.0 0.2 0.6 Fig 4. (a)Change in sa/sp ratio plotted as a function of a for various putative extension length a normalised with respect to the thickness he outer layer, h(which is also equal to the length of the primary crack) and(b)corresponding change in phase angle of the deflected crack, p [sd and sp: strain energy released due to crack deflection along the interface(d)and that due to crack penetration across the interface(ap), respectively; a: putative crack propagation length(=ad=ap)] force, the effect of which is not the scope of the current fore, it is expected that compressive residual stress in stud. the inner intact layer would be beneficial to crack deflec- From the above result, it is suggested that introduc- tion due to the improved deflection criteria whilst tensile ing residual stress such that r<0, i.e. tensile residual one is unfavourable against crack deflection, especially stress in stiffer intact layer and compressive one in com- as I approaches.0. However, since ga/sp ratio is pliant outer cracked layer, would be disadvantageous insensitive to I when a<0, advantage of introducing for crack deflection as critical toughness ratio, R /Rm, compressive residual stress in the inner intact layer can determined by a/sp ratio via Eq (1), falls as compared be exploited only when the materials combination gives with the case where no residual stress exists(T=0). To a >0, i.e. intact layer is stiffer than the cracked layer the contrary, higher sa/s, ratio, i.e. critical R /Rmra Whilst residual stress has some effects on the sa/sp tio, is resulted in the positive a regime if T>0. There- ratio, there was no appreciable change in the phase anforce, the effect of which is not the scope of the current study. From the above result, it is suggested that introduc￾ing residual stress such that C < 0, i.e. tensile residual stress in stiffer intact layer and compressive one in com￾pliant outer cracked layer, would be disadvantageous for crack deflection as critical toughness ratio, Ri/Rm, determined by Gd=Gp ratio via Eq. (1), falls as compared with the case where no residual stress exists (C = 0). To the contrary, higher Gd=Gp ratio, i.e. critical Ri/Rm ra￾tio, is resulted in the positive a regime if C > 0. There￾fore, it is expected that compressive residual stress in the inner intact layer would be beneficial to crack deflec￾tion due to the improved deflection criteria whilst tensile one is unfavourable against crack deflection, especially as C approaches
1.0. However, since Gd=Gp ratio is insensitive to C when a < 0, advantage of introducing compressive residual stress in the inner intact layer can be exploited only when the materials combination gives a > 0, i.e. intact layer is stiffer than the cracked layer. Whilst residual stress has some effects on the Gd=Gp ratio, there was no appreciable change in the phase an- -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 1.0 1.2 1.4 a / h = 0.01 a/ h = 5x10-3 a / h = 1x10-3 a / h = 2x10-4 He et al. (1994) & a/ h = 1x10-4 α p d -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 0 10 20 30 40 50 60 70 80 90 Phase Angle α a / h = 1x10-2 a / h = 1x10-3 He et al. (1994) & a / h = 1x10-4 a b Fig. 4. (a) Change in Gd=Gp ratio plotted as a function of a for various putative extension length ad = ap = a normalised with respect to the thickness of the outer layer, h (which is also equal to the length of the primary crack) and (b) corresponding change in phase angle of the deflected crack, W. [Gd and Gp: strain energy released due to crack deflection along the interface (ad) and that due to crack penetration across the interface (ap), respectively; a: putative crack propagation length (=ad = ap)]. W. Lee et al. / Composites Science and Technology 66 (2006) 435–443 439