SAMPE Journal - March/April 2016 - 7

Feature Article
In-Situ Structural Health Monitoring of
Composite-Overwrapped Pressure Vessels
S. M. Klute, D. R. Metrey, N. Garg and N. A. A. Rahim
Luna Innovations Incorporated, Blacksburg VA

Abstract
Currently, most composite pressure vessels must be recertified every 2-5 years via hydrostatic testing to confirm
the structural integrity of the pressure vessel. The test requires pressurization in a fluid filled chamber with the
global volumetric expansion compared to acceptance criteria. This requirement poses significant cost and time outof-service issues across many industries. In this work, Luna has teamed with Worthington Industries (WI) - a leading
commercial and military supplier of composite air flasks and contracted designer of 30 year flasks, with the objective
of providing composite flasks with a built-in structural integrity assessment system that will eliminate the need
for hydrotesting recertification. High-definition (HD) distributed strain sensing is used to monitor strain along the
axis of circumferentially-wrapped embedded fiber optic sensors in composite-overwrapped pressure vessels (COPVs)
during qualification testing and following blunt and highly localized damage events. Luna demonstrates that the use
of strain sensors embedded in the composite flask during manufacture will allow rapid assessment of the composite
flask structural integrity on-site, while the flask is still mounted in the rack. In addition to the potential for replacing
hydrotesting, risk associated with the use of 30 year extended service life flasks will be mitigated by utilizing this
efficient health monitoring capability to identify damage and weakened flask structure. The core technology behind
Luna's HD strain measurement systems is Optical Frequency Domain Reflectometry (OFDR) technology, which allows
continuous strain measurements at hundreds of gage locations per meter of fiber. Application of the sensor is directly
integrated into current flask fabrication methods and the technology utilizes standard telecommunication optical
fiber. Therefore, the added cost associated with the embedded sensor and interrogation equipment will be minimal
when compared to the recertification costs currently required.
Introduction
Composite Overwrapped Pressure Vessels (COPVs)
Composite materials are the natural choice for
lightweight construction of pressure vessels. These
materials are highly versatile and can provide
comparable or higher strength than traditional metals
at a fraction of the weight; however, COPVs require
unique design, manufacturing, and test specifications.
Compared to all-metal vessels, composite pressure
vessels require a much more complex mechanical
understanding as a result of the non-isotropic nature of
the material and the interplay between the composite
overwrap and the inner metallic liner. Several
significant differences exist between composite and
metal vessels: 1) Composites of carbon, Kevlar®, and
glass may be subject to a reduction in burst strength
as a result of surface impact, 2) Composites are
vulnerable to an effect known as stress rupture or
static fatigue, defined as a sudden and catastrophic
failure of the overwrap while holding at a stress level
below the ultimate strength for an extended period
of time; this failure mechanism is complex, not well
understood, and difficult to accurately predict prior
to failure, and 3) Quantitative nondestructive testing
methods used to screen for flaws in thick-walled
vessels are not applicable to COPV designs1.
SAMPE Journal, Volume 52, No. 2, March/April 2016

COPV Failure Modes
In the event of a failure, COPVs may experience a
catastrophic release of energy as a result of the high
pressure of the stored gas, posing significant danger to
equipment and personnel. The NASA Pressure Vessel
& Fracture Control Board has identified four primary
failure modes associated with COPVs: 1) Burst from
over-pressurization, 2) Fatigue failure of the metallic
liner, 3) Burst resulting from damage to the metallic
liner or composite damage, and 4) Stress rupture of the
composite overwrap2. Burst from over-pressurization
can be mitigated by material certification and prooftesting during acceptance and recertification testing,
although must also be operationally controlled. Fatigue
failure of the metallic liner can sometimes be mitigated
by inspection and testing. Damage to the metallic
liner or composite may be detected through visual
inspection, although subsurface damage may not be
readily identifiable. Stress rupture data has been used
to develop a stress reliability model which uses specific
characteristics such as survived time at Maximum
Operating Pressure (MOP), expected time at MOP, and the
stress ratio, defined as the stress in the overwrap at the
maximum expected operating pressure divided by the
stress in the overwrap at burst3. Accurately determining
the stress ratio is key to the stress reliability model.
7

Table of Contents for the Digital Edition of SAMPE Journal - March/April 2016