DETERMINATION OF RESIDUAL STRESSES IN A LAMINATED THERMOSET COMPOSITE USING THE INCREMENTAL SLITTING METHOD

Residual stresses are those stresses present in a material in the absence of any external loading. For fibre-reinforced plastic (FRP) composites, residual stress development during processing can cause significant fabrication and in-service performance problems resulting in part distortion, matrix cracking, delamination, adverse effects on the stress-strain behaviour of the material [1-5], and reduction in fracture toughness [6], impact and environmental resistance. In this paper, measurements of residual stress in [0 degrees(2)/90 degrees(2)](4s) laminates fabricated from SE84 LV carbon fibre-reinforced epoxy have been undertaken using the incremental slitting approach. The technique involves machining a slit of increasing depth in a rectangular coupon such that stresses normal to the plane of the slit are relieved. The resultant back-face deformation is used to determine the residual stresses in the coupon prior to machining. Residual stresses from incremental slitting measurements have been compared to measurements made using the layer removal technique and predictions using a semi-coupled transient-thermal and structural analysis. The incremental slitting technique has been demonstrated to be a technique suitable for measuring residual stress in thin (similar to 0.3 mm) plies of a [0 degrees(2)/90 degrees(2)](4s) laminate. Results derived using a constant stress approximation approach were in good agreement with residual stresses predicted using a semi-coupled transient-thermal and structural model and measured using the layer removal technique. Although the incremental slitting technique requires a computationally intensive data reduction methodology, once the methodology has been established it is easy to modify for different laminate configurations and is not restricted by the geometric limitations stipulated by classical laminate theory. Experimentally, accurate machining of thin plies in their entirety without over or under machining is virtually impossible to perform and is a severe limitation of the layer removal method. Incremental slitting overcomes this limitation by only requiring a narrow slit of material to be removed, which can be performed much more accurately.