Improving aircraft composite inspections using optimized reference standards Page: 13 of 13
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5. C-Scan Resonance Inspection Results - In order
to eliminate data variations stemming from
changes in probe deployment during testing, a
series of specimens were inspected side-by-side
using the MAUS scanning NDI system in
resonance mode. Existing fiberglass and carbon
step wedge reference standards, along with other
carbon laminates representing fabrication
variability, were inspected along with the
candidate G11 prototype standard (see Fig. 6 for
design). The color coded images provided by the
MAUS system showed that common NDI
responses (i.e. same color in C-scan image) were
obtained for similar thicknesses on all of the
specimens. This provides further evidence that
the G11 material can be used as an NDI
reference standard to support inspections of both
fiberglass and carbon solid laminate structures.
The following set of tasks have been established
to complete the validation of the generic G11 solid
laminate reference standard:
1. Personnel from Boeing's NDT Engineering Dept. will
use the generic standard to support inspections on a
number of in-house solid laminate aircraft structures
with flaws. Additional insights will be solicited from
industry and aircraft inspectors.
2. Solid laminate inspection procedures will be revisited
to determine if any modifications or additions are
necessary to accommodate the use of a generic
3. Optimize the Generic Laminate Design - Issues to
be addressed include minimizing size and weight,
ease of machining, handling, and ease of use in the
field (e.g. identifying locations and thickness of flat-
bottomed holes on the top side of the standard to
allow for proper positioning of the transducer).
While seeking the optimum, yet minimum
number, of composite honeycomb reference standards
needed to conduct inspections on commercial aircraft
structure, this study has determined the honeycomb
construction parameters that have a major effect on
NDI. These results were used to produce a prototype
minimum honeycomb reference standard set. The
reference standard set successfully completed a
preliminary NDI validation phase. Current efforts are
aimed at determining the best and most repeatable
methods for engineering realistic flaws in the reference
An extensive material search, accompanied by
key NDI response studies, has produced a generic solid
composite laminate reference standard that will
accommodate inspections on the full array of fiberglass
and carbon laminates found on aircraft. A prototype
solid laminate standard made from G11 Phenolic
material was demonstrated to provide the same NDI
response as existing carbon and fiberglass standards.
In addition, the G11 material improves on existing solid
laminate standards because it is inexpensive, can be
reliably manufactured and is easy to machine into a solid
laminate standard (i.e. plate with multiple thicknesses).
NDI validation of this material consisted of both pulse-
echo (velocity based) and resonance (acoustic
impedance based) mode inspections.
Overall, this effort will produce a uniform
approach to the inspection of composite structures on
aircraft. Following final validation, field testing, and
design optimization on both solid laminate and
honeycomb reference standards, formal modifications to
appropriate OEM manuals will be addressed. Through
the active participation of the OEM's, this project
represents a harmonized approach by aircraft
manufacturers worldwide. The end result will be more
streamlined inspection set-ups for aircraft maintenance
depots and improved inspections through the use of
optimized NDI reference standards.
This work is a joint effort of the FAA's
Airworthiness Assurance Center operated by Sandia
National Labs and the Commercial Aircraft Composite
Repair Committee (CACRC), Inspection Task Group. In
addition to the authors, key CACRC participants include
Glae McDonald (US Airways), Gerry Doetkott
(Northwest Airlines), Nick Shah (Boeing), John Hewitt
(British Airways), and Bruce Garbett (Airbus Industries).
This work was sponsored by the FAA William J. Hughes
Technical Center under a U.S. Dept. of Transportation
contract. Sandia is a multiprogram laboratory operated
by Sandia Corporation, a Lockheed-Martin Company, for
the U.S. Dept. of Energy under Contract DE-AC04-
1. "Composite Repair NDT/NDI Handbook," SAE
Aerospace Recommended Practice ARP5089,
issued Nov. 1996, prepared by the ATA/IATA/SAE
Commercial Aircraft Composite Repair Committee,
Inspection Task Group.
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Roach, D.; Dorrell, L.; Kollgaard, J. & Dreher, T. Improving aircraft composite inspections using optimized reference standards, article, October 1, 1998; United States. (https://digital.library.unt.edu/ark:/67531/metadc711418/m1/13/: accessed May 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.