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refining the prognostic/diagnostic algorithms and building a system for test.
successfully conducted third-party independent testing included acceleration testing
of up to 6G on 6 axes as well as RFI/EMI testing. In-house thermal testing showed
the system to be operational in the specified range of -40 to 85C, including thermal
shock.
effort culminated in a successful flight test of the LAHMP system acquiring data from
three areas on the F-15.
developed patented algorithms to determine structural health from on board sensor
readings. In addition, we designed the health management platform to be fully
customizable for a wide variety of aircraft."
JSF program has CBM features within the aircraft’s design with rudimentary
corrosion sensors installed and strain gauge monitoring of loads on a limited
number of aircraft
loads are then coupled with flight data monitoring to allow parametric usage
monitoring as a tool for CBM across the entire fleet using maneuver recognition
algorithms to determine the loads on aircraft that are not monitored (Reed, JSF CBM
Features, 2007).
USAF Experiences
24
Of various fatigue damage detection technologies being researched to enable CBM
on aerospace structures, Comparative Vacuum Monitoring & acoustic emission are
most mature and are currently marketed as commercial structural health
monitoring solutions – however both have application limitations
CVM has the capability to detect the presence, location and extent of damage, which when
combined with a usage monitoring and a prognostic system could provide a full CBM
capability.
However, at present CVM falls into a grey area between on-line structural health monitoring
and NDT as the sensors are permanently installed but the vacuum and flow detector are
only connected on the ground for off-line damage assessment.
Given the simplicity of this process it still offers significant advantages, especially for
inaccessible structure. However, it is limited to areas of structure where the damage
mechanism is well understood and predictable (localized damage detection)
Nevertheless, it is a elegantly simple concept that is gaining mainstream acceptance from
aircraft OEMs and operators.
Corrosion sensing technology is generally crude - moisture detectors that could be
placed in areas of corrosion prone structure are under development.
However, apart from using UT to measure the reduction in plate thickness, this and all the
present techniques give an indication of the probability of corrosion on the structure that
must be verified by visual inspection and to quantify its extent.
Of the currently developing damage detection technologies, guided waves and
electrical impedance measurement appear to have promise but need to be tested
in realistic structures under environmental conditions.
Fiber Bragg Gratings are also promising for increased structural coverage with
minimal calibration requirements.
Current Technology State
25
Comparative Vacuum Monitoring (CVM)
CVM sensors work on the basis of differential pressure - pressure changes in
a system of small capillaries provide an indication of structural defects
(cracks, corrosion and loss of bonding contact).
Each sensor, which is ~ 125 mm thick, is perforated with fine galleries alternately
containing air and a vacuum. The presence of a crack or other defect in the
monitored material creates a connection between the two types of gallery, altering
the distribution of pressure inside the sensor at this point.
Used by Airbus in acceptance testing of GLARE (GLAss-fibre REinforced
aluminum) composites - a laminate consisting of three layers of Al held
together by intermediate layers of glass-fiber reinforced epoxy resin (A380
upper skins)
Since CVM sensors were permanently attached, inspection was done in a
fraction of the time required by conventional testing methods
each measurement could be reproduced under exactly the same conditions
onerous task of installing and removing sensors only had to be carried out once,
which saved a great deal of time, especially in the less easily accessible parts of the
airframe
H. Speckmann, Materials & Processes - Testing Technology, Airbus
To monitor cracking from the fastener holes, CVM
sensors were positioned inside the lap joint before
riveting began
able to detect cracks of a magnitude of 1-2 mm – cracks
that most other test methods were incapable of detecting
26
CVM Sensitivity & Durability
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