NDE of Carbon Fiber Composite Pressure Vessels

Abstract

The use of advanced composites in pressure vessels requires means of periodically evaluating residual minimum lifetimes in order for them to remain in service. Tests based on the prediction of crack propagation are not applicable for composite vessels as, in the absence of stress raisers or localised damage, the failure processes are diffuse and particularly damaging if the reinforcing fibers fail. Acoustic emission has been used to detect this type of damage. At a constant pressure it has been shown that even unidirectional carbon fiber reinforced resin continues to emit which reveals a continuing failure process involving the failure of fibers. The rate of emission under a constant pressure has been shown to obey a simple law which allows the damage accumulation to be calculated as a function of time. A maximum damage threshold can be determined experimentally so that master curves corresponding to the damage accumulated by a pressure vessel under constant pressure over the lifetime required can be calculated. A comparison between these master curves and the rate of damage accumulation of any other pressure vessel of the same type reveals if it will fail before or after the desired lifetime. The effects of pressure variations during service have been seen to be minor but even if this is not the case a comparison with the master curves still allows minimum lifetimes to be predicted. The technique does not require the vessels to be removed from the vehicle which is immobilised for a minimum of time.