Mechanics Leading to Stress Rupture Failure in Carbon Fiber Composite Overwrapped Pressure Vessels (COPVs)

An improved understanding of the mechanics of stress rupture has been developed by recognizing that the process of damage accumulation and failure is largely the same for both burst failures produced under quasistatic loading conditions and under constant hold pressure stress rupture. Both failures are tensile failure of the carbon fibers in a localized region such that the remaining fibers are overloaded and can no longer support the applied stress. The key parameter leading to failure in both cases is fiber stress. For both quasistatic burst testing and stress rupture testing, fiber stress is initially accumulated in the same manner because of increasing quasistatic pressure. The remaining increase in fiber stress in a quasistatic burst test is simply the result of increasing pressure, while for stress rupture under conditions of constant pressure, the increase in fiber stress is a result of redistribution of stress from the matrix into the fibers and from damage mechanisms such as broken fibers. This simplistic model explains phenomena observed in stress rupture tests where some vessels fail at short times, others at later times, and others not at all. It also explains why stress rupture failures are more likely to occur shortly after reaching the hold pressure, with decreasing failures as a function of time after reaching the hold pressure. Micromechanics modeling is used to show how the stress is transferred from the matrix to the fibers during a hold and is used to estimate the maximum amount of stress that can accumulate in a unidirectional laminate. Further, the modeling is used to show the relationship between measured composite strain and fiber stress. Applying this new understanding, composite strain measurements can provide insight into the fiber stress, and the increase that occurs under constant pressure conditions as the result of viscoelastic effects. Fiber stress increases based on strain measurements for composite overwrapped pressure vessels (COPVs) with multi-directional lamina are provided and compared against the previously estimated maximum fiber stress increase for a unidirectional laminate. Historically, due to a poor understanding of stress rupture physical mechanisms, accelerated life testing has been relied on to estimate stress rupture reliability of carbon fiber COPVs, and these methods have recently been shown to have significant limitations. The improved physical understanding of the mechanics of stress rupture presented here provides an alternate method for estimating stress rupture reliability based on the strength distribution of COPVs, and it overcomes the limitations of accelerated life testing.