Growth of Interfacial Cracks in Sandwich Beams

ABSTRACT

Debonding between core and facings is a common failure mode of sandwich structures that can severely damage the load-carrying capacity of the structure. The objective of this work is to study the effect of debonding in double cantilever beam specimens made of aluminum facings and PVC foam cores. The configuration follows the standard ASTM D5528-94a peel test. Four PVC foam core materials under the commercial name Divinycell H with densities 60, 80, 100, and 250 kg/m3 are considered. In each case debonding is introduced between the core and the adhesive at the loaded facing of the beam. Linear elastic fracture mechanics is used to model interfacial crack growth and crack kinking into the core. Due to the different mechanical properties of the adjoining materials mixed-mode loading conditions dominate in the neighborhood of the crack tip. Results for the stress and displacement fields are obtained using a finite element computer code. The energy release rate and both opening- and sliding-mode stress intensity factors for interfacial and core cracks are calculated and found that they can be approximated as linear functions of crack length. From results of stress intensity factors in conjunction with the maximum circumferential stress criterion it was obtained that for weak interfaces debonding grows along the interface. On the contrary, for strong interfaces, crack kinks into the core, followed by rapid curving. After a small initial curved depth h∞ the crack becomes parallel to the interface. It was obtained that h∞ is inversely proportional to the modulus of elasticity of the core material and independent of the core thickness.

Keywords:

Sandwich beams, Debonding, Foam cores, Finite elements, Linear elastic fracture mechanics, Strain energy release rate.