National Aeronautics and Space Administration

www.nesc.nasa.gov

Current Obstacles to COPV 

Mechanical Model Validation

Mechanics models and FEA of 

COPVs developed by manufacturers 

have not always been adequate to 

provide accurate general deformation 

response and to pinpoint areas of 

stress concentration in the composite 

shell and liner. This lack of accuracy 

has been an obstacle to determining 

risks associated with failure modes, 

such as stress rupture and fatigue 

crack growth. Key phenomena in 

the understanding of COPV liner 

and composite response include 

overwrap stress-deformation states, 

liner mechanics, and liner/overwrap 

interface mechanics. Accurate 

quantification of the interference 

strain between the liner and overwrap 

is difficult to capture without 

measurement and model correlation. 

While closed-form solutions and FEA models with 

simple liner-overwrap interface assumptions may be 

calibrated to conservatively bound hoop strain response, 

they cannot accurately capture the complete multiaxial stress and deformation state to simultaneously 

correlate with all axial, circumferential, and volumetric 

deformation measurements, especially in the presence 

of an interface gap. The cited reference identifies ways 

in which measurements and model correlation can be 

performed. Global measurements taken from axial linear 

variable differential transducers (LVDTs), belly bands, and 

volumetric measurements, along with local measurements 

of axial and hoop strain from strain gages and laser 

profilometry measurements, were all demonstrated to 

be helpful in understanding the complex mechanical 

response of the COPV. COPVs are classified into 3 levels, 

and guidelines for measurements are suggested.

Best Practices for

Validation of COPV Models 

Three levels of measurements are recommended based 

on design burst safety factors and are intended to serve as 

guidelines for measurements on flight pressure vessels.

Level 1: Burst factor > 3.0

Determine composite and liner 

response based on analysis of the 

vessel per the as-built specifications 

and demonstrated burst pressure. 

Alternatively, determine composite 

and liner response based on closedform analysis of a measured fiber 

strain response (nominal or local) as 

a function of pressure to burst.

Level 2: 2.0 < Burst factor < 3.0

Determine composite and liner 

response based on fully verified FEA. 

Measurements needed as a function 

of applied internal pressure include:

1. Global measurements: Axial 

elongation by LVDT and internal 

volume growth.

2. Local measurements: Hoop 

and axial strain at equator and other carefully referenced 

positions by foil strain gages and/or full-field methods of 

optical metrology.

Level 3: Burst factor < 2.0

Determine composite and liner response based on fully 

verified finite element model. Measurements needed:

1. Global measurements: Axial elongation by LVDT and 

internal volume growth.

2. Local measurements: Hoop and axial strain at 

equator and other carefully referenced positions by foil 

strain gages and/or full-field methods of optical metrology.

3. Interior Laser Profilometry: Unwound liner, 

wound liners prior to overwrap cure, wound liner postoverwrap cure prior to autofrettage, and cured COPV post 

autofrettage.

References

Thesken, J.C., et. al., Composite Pressure Vessel Working 

Group (CPVWG) Task 4: A Theoretical and Experimental 

Investigation of Composite Overwrapped Pressure Vessel 

(COPV) Autofrettage, December 19, 2013. TM-2014-218260.

Global and local deformation measurements should be incorporated into the composite overwrapped 

pressure vessel (COPV) design and analysis process to allow correlation of these measurements with finite 

element analysis (FEA) models. This correlation improves understanding of liner, liner/overwrap interface, and 

composite deformation response in COPVs. The improved accuracy reduces error in subsequent analyses, 

such as fracture, fatigue, and stress rupture that are critical for COPV qualification. 

COPV Mechanical Model Validation 

LVDT Belly band cable

Strain

gauges

Cable extension 

transducer

Model correlation study test fixture.

NASA Engineering and Safety Center Technical Bulletin No. 14-03