Experimental investigations of the effect of thickness on fracture toughness of metallic foils

The effect of thickness on the fracture toughness J_C of metallic foil was investigated experimentally. Double-edge cracked specimens, made of copper foils with thicknesses t ranging from 0.02 to 1 mm, were loaded in tension till fracture. The digital speckle correlation method (DSCM) was used to evaluate the strain fields around the crack tip, allowing determination of the J integral. The fracture toughness defined as the value of the J integral at cracking initiation was shown to first increase with increasing thickness, then to reach a maximum for a thickness of about 0.3 mm and finally to decrease at larger thicknesses. Optical and scanning electron microscopy (SEM) results showed that this significant effect of thickness is essentially attributable to the change in the work required per unit area during crack tip necking and in the work required per unit area during material separation.