Unconservatism of ANSI AIAA S081B for Damage Tolerance Life Analysis of COPVs

Composite overwrapped pressure vessels (COPVs) consist of a metallic liner to contain the commodity and an outer composite overwrap for strength. COPVs generally have an initial high-pressure cycle, called autofrettage, followed by operational cycles. The autofrettage pressurization will plastically deform the liner while the overwrap remains elastic. The depressurization following autofrettage will compress the newly deformed liner and result in far-field compressive stresses. The subsequent operational cycles can either be elastic or elastic-plastic. Damage tolerance qualification standards for COPVs have been established to minimize risk. These standards allow damage tolerance for elastically responding liners to be demonstrated by crack growth life analyses that ignore the influence of the autofrettage cycle on elastic crack growth. Experimental tests conducted on uniaxial coupons provided evidence that the allowed approach can be unconservative for conditions that are representative of COPV liner operation. Digital image correlation (DIC) measurements were performed to characterize the crack opening displacement during the uniaxial test. Elastic-plastic finite element analyses that were validated with the DIC measurements were used to evaluate the influence of biaxial loading in COPV liners. The combined experimental and numerical results indicate that the autofrettage cycle blunts the crack tip and the resulting plastic deformation prevents the crack surfaces from contacting under the compressive loads. The open and blunted crack tips increase the cyclic loading range that can contribute to crack growth. Conventional crack growth life analyses that omit the influence of the autofrettage cycle can underestimate damage tolerance life by more than a factor of 3.