Size effect on strength of laminate-foam sandwich plates

Experiments on size effect on the failure loads of sandwich beams with PVC foam core and skins made of fiber-polymer composite are reported. Two test series use beams with notches at the ends cut in the foam near the top or bottom interface, and the third series uses beams without notches. The results demonstrate that there is a significant nonstatistical (energetic) size effect on the nominal strength of the beams, whether notched or unnotched. The o̧bserved size effect shows that the failure loads can be realistically predicted on the basis of neither the material strength concept nor linear elastic fracture mechanics (LEFM). It follows that nonlinear cohesive (quasi-brittle) fracture mechanics, or its approximation by equivalent LEFM, must be used to predict failure realistically. Based on analogy with the previous asymptotic analysis of energetic size effect in other quasibrittle materials, approximate formulas for the nominal strength of notched or unnotched sandwich beams are derived using the approximation by equivalent LEFM. Different formulas apply to beams with notches simulating pre-existing stress-free (fatigued) cracks, and to unnotched beams failing at crack initiation. Knowledge of these formulas makes it possible to identify from size effect experiments both the fracture energy and the effective size of the fracture process zone.