NESC identifies critical improvements for aerospace pressure vessel standards

NESC identifies critical improvements for aerospace pressure vessel standards
by Clarence Oxford
Los Angeles CA (SPX) Apr 05, 2024

The Structures Technical Discipline Team (TDT) at NASA's Engineering and Safety Center (NESC) has recently focused on composite overwrapped pressure vessels (COPVs), uncovering a crucial vulnerability in current specifications and testing standards that may underestimate their structural life. This finding is poised to enhance the safety and efficacy of aerospace missions by refining COPV evaluation processes.

COPVs, essential for containing fluids or gases in aerospace applications, rely on a metallic liner for containment and a composite overwrap for strength. The discovery revolves around the operational pressure cycles these vessels undergo, including an initial overpressure phase, or autofrettage, which pre-stresses the liner. Subsequent cycles can vary between elastic and elastic-plastic responses. However, the current standard for evaluating COPV durability, ANSI/AIAA-S-081B, may not fully account for the impacts of these cycles, particularly the initial autofrettage.

NESC's study, titled "Unconservatism of Linear-Elastic Fracture Mechanics Analysis Post Autofrettage" (NASA/TM-20230013348), presents experimental and analytical research that highlights the accelerated crack growth in COPVs subjected to autofrettage. By comparing tests on specimens with and without autofrettage cycles, researchers found a significant discrepancy in crack progression rates, suggesting that the existing standard's approach might be too optimistic.

Further, the study leverages finite element analysis to illustrate how autofrettage can induce a plastic deformation region ahead of the crack, potentially leading to less effective crack closure and higher growth rates under cyclic stress. This evidence points to a need for revising the ANSI/AIAA S-081B standard to better reflect the actual stresses and deformations experienced by COPVs, thereby ensuring more accurate life predictions and operational safety.

The implications of these findings extend beyond immediate engineering adjustments. As NASA increasingly engages with commercial partners for aerospace ventures, the Structures TDT underscores the importance of adaptive standards that reflect the unique demands of emerging technologies and partnership models. This approach aims not only to safeguard missions but also to preserve institutional knowledge and foster hands-on engineering experience among NASA's workforce.

Ensuring the safety and success of future space missions hinges on continuous improvement and collaboration across the aerospace community. NESC's work in advancing COPV standards exemplifies this commitment, offering a pathway to more reliable and secure aerospace operations.