90/95 POD Radiography Concern for COPVs and Metal Tank Welds 

Radiographic inspections of welds in all-metal tanks and composite overwrapped pressure vessel (COPV) 

liners that are used to establish initial crack size in damage tolerance assessments are not as rigorous as 

previously understood. As a result, the 90/95 percent probability of detection (POD) requirements in S-0801

and S-0812

 are not met when this inspection method is used. Development of new inspection methods will 

require about three years. During that time, additional review and assessment of the fracture margin may 

be needed to support waivers and provide a better understanding of weld cracking risk associated with an 

individual tank design.

NASA Engineering and Safety Center Technical Bulletin No. 19-02

National Aeronautics and Space Administration

www.nasa.gov

Longitudinal

Crack

Transverse

Cracks

Longitudinal

Crack

Transverse Cracks

Longitudinal

Root Crack

Transverse Cracks

Transverse Cracks Longitudinal Root Crack

Illustration of metal weld cracks Illustration of notional radiographic inspection of metal weld cracks

Discussion

Damage tolerance life (safe-life) for COPVs and metallic tanks 

is a deterministic damage tolerance approach required by 

S-081 and S-080. It assumes the existence of cracks of a size 

that can be reliably detected by an established nondestructive 

evaluation (NDE) method used to inspect the liner/tank prior to 

service. The intent of damage tolerance life is to demonstrate 

that cracks at or below this size will not grow to failure during 

the service life. S-081 and S-080 require that this initial crack 

size be determined from the sensitivity limit of the 90% POD 

at a 95% confidence level.

For volumetric inspections of tank welds and domes, tank 

manufacturers typically use radiography. The majority use a 

crack depth of 60% of the thickness (0.6t, identified as special 

radiography) and specific inspectors and processes have been 

certified by NASA to be able to detect cracks of that size with 

POD of 90/95 in accordance with NASA-STD 50093

. The larger, 

0.7t crack depth is occasionally used and requires less rigor 

under NASA-STD 5009.

Recent radiography studies have concluded that 0.6t and 

0.7t cracks are not as consistently detectable as previously 

understood. Detectability of cracks on the film (or digital 

radiography) is sensitive to several parameters: the need 

for double-wall inspection on close-out welds, separation 

distance of the tank walls, incidence angle, wall thickness, and 

exposure time. These parameters have not been included in 

certification tests in the past, but were found to be important in 

crack detection. Implementation of more stringent certification 

tests that capture these parameters will take about three 

years since new image quality aids will need to be developed, 

qualified, and implemented. It is understood that these planned 

improved double-wall radiography methods may not be 

achievable in all tank designs. In the near term, certifications 

will continue in the current protocol to establish that heritage 

capability has not changed.

As a result, manufacturers that are currently certified to 0.6t 

or 0.7t radiography may not be able to detect cracks of that 

size with 90/95 POD, so the risk of missing a crack larger than 

0.6t or 0.7t is higher than previously understood. This risk has 

been present in previous flights, but was not appreciated until 

recent studies of radiography techniques were completed.

Since the damage tolerance analysis or test assumes crack 

sizes associated with radiography, additional analysis and tests 

may be needed to understand fracture risks associated with 

cyclic and sustained load crack growth. Tests of larger initial 

crack sizes or better understanding of analytically-derived 

fracture margin and critical initial flaw size analysis may be 

needed to support waivers against the 90/95 POD requirements in S-081 and S-080 and provide a better understanding 

of the risks associated with individual tank designs. 

References

1. American Institute of Aeronautics and Astronautics 

(AIAA) S-080 Space Systems - Metallic Pressure Vessels, 

Pressurized Structures, and Pressure Components

2. AIAA S-081, Pressure Vessel Standards Implementation 

Guidelines

3. NASA-STD-5009 Nondestructive Evaluation Requirements 

for Fracture-Critical Metallic Components

For information contact Dr. Grimes-Ledesma at 

lorie.r.grimes-ledesma@jpl.nasa.gov