Residual stress analyses in multilayer PP/GFP/PP composite tube

Abstract

The present paper is a contribution in analysing residual stress in multilayer composite tube used for conveying drinking water. The tube is made of three layers spread in one-fourth outer polypropylene layer, one-half central glass-fibre-polypropylene layer, and one-fourth inner polypropylene layer. The contribution of each layer involved in forming the composite tube has been shown through the improvement of yield stress by 80%. Residual stresses have been measured using the Crampton technique and layer removal method. In the parent tube, negative residual stress is obtained for a ratio of tube length to tube diameter L/D0 ≥ 1.4, with a magnitude of 91% significant comparing with the yield stress of the material. The distribution of the residual stress through the wall of the tube has followed a sigmoid curve that is characterised by a three-order polynomial engineering model showing negative stresses at the outer surface and positive stresses at the inner surface. The observation suggests that compressive residual stresses act through 80% of the wall thickness of the tube. Hence, the presence of the central GFP layer permits to equilibrate the distribution of the stresses during the service life of the tube. The residual stress distribution is in good agreement with the evolution of mechanical absorbed energy per unit volume material according to its location within the parent tube. For an equivalent pipe in diameter and thickness, the residual stress in composite pipe is ten times higher than that in PE pipe.