Experimental study on macro-slipping in composite single-lap bolted joints with metal inserts

Abstract

In a preloaded single-lap bolted joint, load transfer between the assembled parts takes place through friction between the parts and then, after slipping, by bolt bearing stress/shear. The maximum load before slipping can therefore be increased by improving the load transfer by adherence at the contact interface of the assembled parts. This can be achieved by optimizing the level of preload in the assembly and maximizing the friction between the assembled parts. In the case of composite parts, with low mechanical properties in the out-of-plane direction, high performance assemblies can be achieved by adding inserts into the holes. This paper proposes a macroscopic analysis of the assembly on load transfer by adherence and friction. An experimental approach is carried out to quantify the influence of the surface condition at the interface of assembled parts (metal inserts) on the load transfer. Several techniques are used to modify the characteristics of the contact interface: laser texturing with a concentric circular pattern or a cross hatch pattern, and the addition of a friction shim at the interface. The analysis is carried out on the macroscopic scale of the assembly, based on post-mortem observations of the surfaces in contact, to validate the global behaviour of the assembly. Compared to results for a machined surface, these results show an increase in the friction coefficient and therefore in the pre-slipping load for all the configurations proposed. However, the interposition of a friction shim provides interesting results, with an increase in the friction coefficient (+205 % for the static friction coefficient) compared to the reference surface condition. The effect of a disassembly-reassembly operation is also studied and the benefits of adding a friction shim to the load transfer are validated by a tensile test on a single-lap composite bolted joint.