Background

COPVs are often designed with a bond between the liner and 

composite. The purpose of the bondline is to provide load transfer continuity between the liner and overwrap during pressurization and depressurization cycles throughout the lifetime of a 

COPV. In the cylinder region the liner and overwrap longitudinal 

strains are often similar; therefore, the bondline is not highly 

strained in shear. However, longitudinal strains are not similar in 

the dome, leading to development of bondline shear stress. This 

shear stress can concentrate in the liner at geometric transitions 

such as at a liner thickness taper near the boss.

Bondline Strain Mechanisms

If the liner taper does not smoothly transfer load into the 

overwrap from the liner, stress concentrations can result in both 

the liner and the bondline. For example, if the taper is too short, 

then geometric stress concentrations in the liner occur near 

the thin end of the liner taper along with an abrupt increase of 

adhesive shear stress between the liner and overwrap as the liner 

thickness increases. These stress concentrations can result in 

larger plastic strains than intended in both the liner and adhesive 

and when these large plastic strains occur at the same location 

in the liner and the adhesive, the liner deforms independently 

from the overwrap. This allows the plastic strain in the liner to 

localize and the resulting strain spike can increase quickly with 

additional deformation. The figure shows large plastic strains in 

the adhesive associated with the strain spike in the liner can lead 

to failure of the adhesive, increasing the independence of the 

liner. A similar plastic strain concentration in the liner can occur 

in regions where the composite and liner are unbonded due to a 

manufacturing error.

Recommendations to Mitigate Bondline 

Strain Spikes

Liner strain concentrations from adhesive and liner yield interaction or manufacturing defects can lead to crack nucleation and 

growth or development of a liner buckle. To evaluate the risk, the 

margin of safety should be determined at design burst. If it is positive, then examine strain distributions for evidence of alignment 

of adhesive and liner yield. If the adhesive is predicted to yield or 

disbond at a location concurrent with net section liner yielding, 

perform one of the following:

1. Explicitly model the bondline with elastic-plastic properties 

and re-evaluate the development of the liner strain spike. Determine the magnitude of any strain spike that develops in 

this new model. If adhesive strains approach the shear failure 

criterion of the bondline, then a local disbond should be modeled and strain spikes re-evaluated. 

2. Add a disbond only at the location where the adhesive exceeds yield and determine the magnitude of any strain spike 

that develops in the liner. 

Evaluating and Mitigating Liner Strain Spikes in COPVs

Unexpected cracking and leaking in bonded composite overwrapped pressure vessel (COPV) liners occurring in recent test 

programs have been attributed to liner strain spikes observed through measurement and predicted by analysis. Diminished 

load transfer between the liner and composite overwrap can lead to localized excessive liner yielding in the dome section. 

This diminished constraint can occur due to yielding of the adhesive or a manufacturing unbond defect. COPVs should be 

assessed for susceptibility to this new failure mode.

NASA Engineering and Safety Center Technical Bulletin No. 20-07

National Aeronautics and Space Administration

www.nasa.gov For information contact Dr. Grimes-Ledesma at lorie.r.grimes-ledesma@jpl.nasa.gov

09/18/2020

Note that simulating a disbond over the entire bondline either by 

releasing nodes or diminishing shear modulus is not necessarily 

conservative. To evaluate the significance of the strain spike for all 

pressure conditions of the COPV, include the magnitude of the strain 

spike in all required verification activities associated with crack 

nucleation, crack growth, and liner buckling failure modes in ANSI/

AIAA S-081B Space Systems-Composite Overwrapped Pressure 

Vessels (sections 5.2.13 Fracture Control Design, 5.2.14 Fatigue 

Life Design, 5.2.6 Negative Pressure Differential Design, and 

5.2.10 Stability Design). The potential for local normal deflection 

reversals (oil-canning) at a disbond should be considered in crack 

nucleation and growth failure modes. 

If the magnitude of the liner strain spike is too large to be robust to 

these failure modes, then the design can be modified to reduce the 

shear stress in the adhesive below yield. For example, increasing 

the taper length could be considered. In addition, process control 

measures should be implemented to ensure that the risk of 

unbonds is acceptably low. 

Analytical Results: Explicitly Modeled Elastic-Plastic

Adhesive, Disbond Not Included

2.5 1.25

2.0 1.0

1.5 0.75

1.0 0.50

0.5 0.25

0.0 0.00

-0.5 -0.25

0.0 0.2 0.4 0.6 0.8 0.85 1.0

Normalized Arclength

Normalized Liner Total Strain

Normalized Adhesive Plastic Shear Strain

1⁄2 Cylinder Dome

Liner Longitudinal

Strain Spike

Large Adhesive

Plastic Shear Strain

Taper Start

Cylinder/Dome Transition

Equator

1⁄2 Cylinder Dome

0.0 0.4

Liner Arclength

Liner Longitudinal

Strain Normalized by 

Peak Liner Hoop Strain

Liner Thickness Taper

1.0