Thermo-mechanical fatigue analysis of liquid shim in mechanically fastened hybrid joints for aerospace applications

As resin-infused composite laminates continue to replace alloy components in modern commercial aircraft, liquid-shim is often necessary to compensate for assembly tolerances occurring in mechanically-fastened primary structures. This paper evaluates the structural performance of aerospace-grade liquid-shim in double-bolt composite-aluminium alloy 'hybrid' joints representative of multi-fastener joints found in the primary structures of modern commercial aircraft. Hybrid joints with liquid-shim were subjected to simultaneous thermal and mechanical-fatigue (TMF) loading conditions representative of those experienced over the lifetime of a commercial aircraft. The joints were subjected to 200,000 fatigue load cycles at -59. °C and 100,000 fatigue load cycles at +85. °C to simulate service conditions. Key variables investigated were shim material and shim thickness. The main conclusions from this study are: (i) there was no degradation of the liquid-shim in terms of a loss of mechanical stiffness and, secondly, no significant damage was observed on the bearing-plane; (ii) a reduction in joint stiffness due to the presence of liquid-shim was observed using high magnification 2D digital image correlation (DIC) and the reduction was dependent on the thickness of liquid-shim employed; and (iii) joints with second generation liquid-shim exhibited marginally lower stiffness relative to joints manufactured with third generation liquid shim.