LIFETIME PREDICTION AND IN SERVICE CONTROL OFCARBON FIBER COMPOSITE PRESSURE VESSELS

摘要

Carbon fiber composite pressure vessels are presently in service providing storage for natural gaswhich is used to propel buses and other utility vehicles in Europe and elsewhere. These pressurevessels operate at pressures of up to 200 bars. They are also the best placed candidates for higherpressure applications for storing hydrogen for fuel cells. As with all pressure vessels these compositestructures require periodic evaluation of residual minimum lifetimes in order to remain in service.An analogy has been made between the behaviour of these filament wound structures andunidirectional composite specimens as the fibres in a pressure vessel are subjected to unidirectionalloading. Failure processes in composite structures are diffuse and particularly damaging if thereinforcing fibers fail. The present study sets out to confirm the results of earlier studies that acousticemission can be used to detect fibre failures, which can occur in unidirectional specimens even understeady loads which is analogous to a steady pressure in a vessel. The rate of emission of aunidirectional specimen and also filament wound vessels under a constant pressure obeys a simple lawwhich allows the damage accumulation to be calculated as a function of time. A maximum damagethreshold can be determined experimentally so that master curves corresponding to the damageaccumulated under steady loading conditions over the lifetime required of the composite, can becalculated. A finite element analysis of the process governing this behaviour has shown thatviscoelastic relaxation of the matrix around fibre breaks is augmented by the effects of interfacialdebonding along part of the broken fibre near the break. A comparison between master curves and therate of damage accumulation of any pressure vessel of the same type reveals if it will fail before orafter the desired lifetime. The effects of pressure variations during service have been seen to be minorbut even if this is not the case a comparison with the master curves still allows minimum lifetimes tobe predicted. The technique does not require the vessels to be removed from the vehicle which isimmobilised for a minimum of time.