Experimental analysis of stresses in bonded, pultruded composite structures

Abstract

Pultruded composites are usually manufactured by pulling fibres that have been immersed in resin through a heated die. The material is layered, usually with the main component being a thick layer of unidirectional material (UD). This is often sandwiched between outer layers of a randomly orientated material. In the current work the outer layers comprise a chopped strand mat (CSM) material stitched on a woven roving mat, known as complex mat (CM). For protection purposes, a thin layer of material, known as a ‘surface veil’, that consists of thermoplastic fibres wetted with polyester resin is deposited on the surface. Investigations into bonded pultruded butt strap joints have been carried out by the authors showing that the joint efficiency was poor. This was attributed to damage initiating in the UD/CSM material interface, as a result of poor interlaminar mechanical properties, exacerbated by the large stress discontinuities at the adhesive/surface veil/CM and CM/UD interfaces. The failure essentially results in a detachment of the surface of the pultruded material. To identify a means of increasing the joint efficiency it is necessary to establish the stress distribution in the joint particularly at the interfacial layers in the pultruded material to define how the load is carried and transferred to each layer. To do this an experimental technique known as thermoelastic stress analysis (TSA) is used. TSA is a non-contact technique that provides a full field map of the stresses on the surface of a component. The application of TSA to composite materials is described in the paper. To obtain quantitative stress data from TSA it is necessary to ‘calibrate’ the materials; the calibration approach is also described in the paper. The TSA data provides a layer-by-layer map of the stresses from the joint interface on the surface through the complex mat layer into the UD. This approach has provided a new insight into pultruded joint behaviour and enabled improved validation of Finite Element Analysis (FEA)