National Aeronautics and Space Administration

www.nasa.gov

Background

Damage tolerance life (safe-life) is a 

fracture control approach that assumes 

the existence of cracks of a size that can 

be reliably detected by an established 

nondestructive evaluation (NDE) method 

used to inspect the COPV liner prior to 

service. The intent of damage tolerance 

life is to demonstrate that cracks at 

or below this size will not grow to a 

depth equal to the thickness of the liner 

(leak) or to a length that is predicted to 

become unstable. ANSI/AIAA S-081A1 

requires that metallic COPV liners possess adequate damage 

tolerance life that is demonstrated by analysis or test. Details 

on testing and analysis are specified in S-081A.1

Issue

Many COPVs are designed with a liner that responds 

elastically for all pressure and combined load conditions 

after autofrettage. For these elastically responding liners, 

S-081A allows demonstration of damage tolerance life based 

on linear elastic fracture mechanics (LEFM). 

However, the use of LEFM methods may not be appropriate if 

the liner is relatively thin compared to the inspectable crack 

depth (for example, a detectable crack depth of 0.025 inch in 

a liner that is only 0.04-inch thick, resulting in a ligament that 

is 0.015 inch). The crack growth rate through the ligament 

may be faster than that predicted by LEFM for two reasons: 

• Plasticity: Even though far-field analysis shows that 

thin liners are responding elastically, local plasticity in the 

ligament must be considered. LEFM assumes that the cracktip plastic zone is small relative to the size of the crack. 

Regions of local yielding (remaining ligament or local stress 

concentrations) adjacent to cracks may create plasticity 

that renders LEFM analysis inappropriate. For example, 

liner embossing (plastically straining the liner into the 

overwrap during autofrettage) could lead to highly localized 

strain concentrations, affecting the growth of cracks in the 

remaining ligament. 

• Microstructure: When the remaining ligament is relatively 

small, the ligament thickness may be on the same order as 

microstructural features, such as grain size. The use of crack 

growth rate data obtained from standardized tests (such as 

that supplied in NASGRO) may not be representative of crack 

growth in the ligament. Instead, tests based on ASTM Standard 

E-647 (Appendix X3) guidelines that 

address the measurement of small 

fatigue crack growth rates may be more 

appropriate. 2

These “small-scale” effects could 

result in the inappropriate use of LEFM 

methods and existing crack growth 

rate data resulting in an unconservative 

estimate of damage tolerance life. 

Path Forward

Additional understanding will be 

required to establish limitations on the 

use of LEFM-based analysis and test methods for elastically 

responding COPV liners. This effort will require a better 

understanding of localized strain fields and microstructural 

considerations for applicable combinations of material, 

material processing, autofrettage and subsequent response 

characteristics, NDE method, and crack depth for each aspect 

ratio. Evaluation of these parameters and their relative 

importance to damage tolerance life would allow guidelines 

for the appropriate use of LEFM for thin liners. 

Small-scale effects are not limited only to elastically 

responding liners. For COPVs with far-field plastic response, 

coupon or vessel testing is required per S-081A. Such tests 

may be adequate to address small scale effects by test if the 

test captures the effects of plasticity and microstructure in 

the remaining ligament; however, test methods and guidance 

for ensuring that worst-case conditions were present during 

coupon or full-scale tests would need to be developed. 

The NESC will be developing specific guidelines for damage 

tolerance life analysis and test that addresses crack growth 

in these small uncracked ligaments that include localized 

plasticity and microstructure effects. This may result in the 

need for additional qualification tests to quantify crack growth 

in flight COPVs with thin liners. 

References

1. ANSI/AIAA S-081A-2006, Space Systems-Composite Overwrapped 

Pressure Vessels (COPVs), American Institute of Aeronautics and 

Astronautics, VA. 

2. ASTM Standard E647-15, Standard Test Method for Measurement of 

Fatigue Crack Growth Rates, ASTM International, West Conshohocken, PA.

Damage Tolerance Life Issues in COPVs with Thin Liners 

Typical spherical COPV

During composite overwrapped pressure vessel (COPV) qualification, damage tolerance life analysis and/or 

tests may not adequately address crack growth for COPVs with thin liners, and additional qualification tests 

beyond those in current requirements documents may be needed to quantify crack growth. 

NASA Engineering and Safety Center Technical Bulletin No. 16-02