Elastic compliance of four-point bend specimens comprising two linear-elastic materials bonded with a thin layer

The four-point bend "sandwich" specimen, comprising two identical linear-elastic substrates joined by a thin elastic layer, is widely used in the study of strength and toughness of bimaterial joints loaded under nominal mode I loading. As an aid to crack monitoring in such specimen geometries, the linear-elastic compliance based on load-line displacement, back-face strain, and crack-opening displacement has been determined for a wide range of substrate/layer material combinations using finite-element analyses. Calculations for bimaterial systems, with elastic moduli ratios varying from 0.2 to 5 and with joining layer thicknesses between 100 and 500 μm, show that for crack sizes between 0.1 to 0.7 of the specimen width, the compliance is significantly different from that of the bulk substrates, even when the layer is very thin. It is concluded that crack-opening displacements are preferable for the monitoring and detection of interfacial and near-interfacial cracks in this specimen geometry, as the compliance based on these displacements is the least sensitive to errors from either measurement site or crack location.