Durability of Closed-End Pressurized GRP Filament Wound Pipes under Hygrothermal Aging Conditions. Part II: Creep Tests

Abstract

The creep and damage behavior of GRP filament wound pipes are investigated as a function of the resin ductility. To predict the life time of pipes under low pressure levels and hygrothermal conditions, different methods based on the experimental data and approximations have been used. All these methods involve assumptions that have been discussed and examined carefully. Under creep tests the response of preconditioned pipes by immersion in water at 60°C follows a nonlinear viscoelastic behavior. A damage creep law is, also, assessed followed by microscopic observations to identify the creep damage mechanisms. Two stages are observed: the first describes the initiation of cracks and the second is related to the crack's propagation. Decohesions between the mechanical part of the pipe and the liner followed by microcracking of the liner lead to the failure. Also, the pipes' life is governed by the critical creep strain to failure of the liner. From microscopic observations and physicochemical analysis it appears that pipes' failure results for low pressure levels from mechanical damage and chemical bond rupture, otherwise cracking and delamination lead to the failure. Experimental data show that the use of the creep damage law give the more realistic creep lifetime although this criterion is the most pessimistic one. Predictions based on the Eyring-Zhurkov approach and the nonlinear viscoelastic creep law over-estimate the lifetime because assumptions are not appropriate. Moreover it appears using both microscopic observations and extensometric measurements, that the lifetime, the damage kinetics and the failure mechanisms of the pressurized pipes depend strongly on the matrix ductility.