Shear fatigue characterization of fire resistant syntactic foam core sandwich beam

Eco-Core is syntactic foam made by high volume percent of flyash and a small amount of phenolic resin. Because of very low volatile content in the mixture, it has demonstrated to be a fire resistant material for composite sandwich structural applications. Its superior mechanical and fire safety properties were established previously. Its fatigue performance under compression, shear and flexure loading are being investigated. Objective of this paper is to establish shear stress–life relationship and failure modes of sandwich beams. The Eco-Core sandwich specimens made of FGI 1854 glass/vinyl ester face sheet were designed to fail in core shear and were tested by shear fatigue loading at a frequency of 2 Hz and load ratio of R = 0.1. The Eco-Core density was about 0.5 g/cm ³ . The fatigue test was conducted at maximum shear stress ( τ _max ) values of 0.7 τ _c to 0.9 τ _c using four-point bend load specimen, where τ _c is the static shear strength of the core. Shear fatigue failure of the core material was found to be three types: shear crack on-set, crack propagation and ultimate shear failure (represented by shear crack linking to top and bottom face sheets and finally interface delamination). These failures were found to be represented by 2, 5 and 7% change in compliance. The fatigue stress–life (S–N) relationship was found to follow the power law equation, τ _max / τ _c = A _o N ^α . Constants of the equation were established for all three modes of failure. Based on 1 million cycles limit, the endurance limit was determined to be 0.66 τ _c , 0.68 τ _c and 0.69 τ _c , respectively, for damage onset, propagation and ultimate failure. The typical shear fatigue failure was a 45° crack in the core, followed by crack propagation to face sheet and finally interfacial delamination between face sheet and core.