This means that the environment becomes less effective at enhancing the crack growth rate for lower applied stresses, or smaller crack sizes. Detailed measurements suggest that this behavior is due to the build-up of corrosion products within the crack that serve to wedge the crack shut. This process is consistent with the enhanced emissions that occur at low pressure during AE testing of cylinders. This property defines the crack-tip stress intensity factor at which the crack becomes unstable and leads to catastrophic failure in a given material. Since the minimum detectable crack size for AE technology has not yet been determined, it is convenient to express predicted remaining cylinder life as a function of the initial crack size.
Results indicate that the remaining life is relatively sensitive to the initial crack size.
Specifically, an initial crack that is 25 percent of the cylinder wall thickness corresponds to a remaining life of about 95 years, while a deeper crack of 40 percent of the wall thickness corresponds to a cyclic life of only five years. The primary advantage of the fracture mechanics approach is that these properties, when expressed in terms of crack-tip stress intensity factor, are independent of geometry. This feature enables laboratory measurements, made on conveniently sized specimens, to be transferred to full-scale structures, through appropriate analysis. In essence, this lets one compute the cyclic life for any structure by integrating the crack growth kinetics from some initial stress intensity factor, defined by the minimum detectable crack size, to some final stress intensity factor, defined by the materials fracture toughness. This procedure has been implemented for the case of a steel CNG cylinder.