On the ratio of mode ii and i interfacial fracture toughness in layered materials and thin films

Mode I and II interfacial fracture toughness are important design parameters in the application of layered materials and thin films. By taking a straight blister in thin films as a mechanical device a theoretical study shows that their ratios are in the range of 0 < G_IIc/G_Ic ≤ 36 and 0 < K_IIc K_Ic ≤ 6, in terms of energy release rate (ERR) and stress intensity factor (SIF), respectively. For most materials they are in the range of 1 ≤ 𝐺𝐺_IIc/𝐺𝐺_Ic ≤ 13.1153 and 1 ≤ K_IIc/K_Ic  ≤   3.6215, respectively. By examining extensive experimental results in open literature no violation is found.