Improving aircraft composite inspections using optimized reference standards Page: 8 of 13
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The following set of tasks have been established to
complete the validation of the minimum honeycomb NDI
reference standard set:
1. Finalize the standard fabrication process by
determining the optimum way to engineer flaws.
The S-9 Sondicator device, the Bondmaster device,
and thru-transmission ultrasonics in an immersion
tank will be applied to make this assessment. A
minimum of 18 dB attenuation will be required at the
2. After completing item #1, the suite of 64 aircraft
panels will be revisited with the S-9 Sondicator (both
A-scan and C-scan mode). Appropriate OEM
inspection procedures and manufacturer equipment
calibration procedures will be followed. An alarm
bx (as opposed to alarm threshold) will be set and
flaws in the standard will be assessed. Next, the set
of 64 aircraft panels will be inspected following
equipment set-up on the honeycomb reference
standards. If acceptable detection is achieved on
the array of flaws then we will infer that the
important construction variables have been included
in the standard set.
3. Field Tests - The prototype honeycomb reference
standard set will be delivered to United Airlines and
Northwest Airlines to study how they function in the
field. They will be evaluated on damaged
honeycomb structure removed from aircraft (on as-
available basis) and on honeycomb structure
currently on aircraft.
4. Design Optimization - The final design will minimize
the overall size of each standard and will provide for
the fewest number of separate honeycomb
standards. The final specimen size must
accommodate probe deployment on both good and
flawed structure and eliminate any edge effects or
effects from adjacent flaws.
TASK 2: COMPOSITE LAMINATE STANDARDS
The goal of this effort is to establish a single,
generic composite laminate reference standard that will
accommodate inspections on the full array of fiberglass
and carbon laminates found on aircraft. Optimally, we
would like to substitute a single material for both carbon
and fiberglass solid laminate inspections. The material
would need to provide the same NDI response to both
carbon and fiberglass. In addition, in order to improve
on existing solid laminate standards, the material should
be inexpensive, reliably manufactured and easy to
machine into a solid laminate standard (i.e. plate with
The first step in this task was to apply thru-
transmission ultrasonics to the series of existing Boeing,
Douglas, and Airbus laminate specimens (step wedges
of various materials at different thicknesses) in order to
measure the key velocity, acoustic impedance, and
attenuation characteristics in the laminates. A
subsequent material search identified what appears to
be an excellent candidate as a generic solid laminate
reference standard material. Testing to date has
determined matches in key velocity and acoustic
impedance properties, as well as, low attenuation
relative to carbon laminates. Furthermore, comparisons
of resonance testing response curves from the G11
Phenolic prototype standard was very similar to the
resonance response curves measured on the existing
carbon and fiberglass laminates. Resonance tests on
three carbon composite standards showed that
variability across "similar" standards was similar to the
variability observed between G11 and carbon or
fiberglass. Additional insight from experienced aircraft
inspectors is needed to make a final assessment of the
viability of G11 material as a suitable generic solid
Search for a Generic Solid Laminate Material
The following issues were addressed to arrive at
the G11 generic material candidate.
1. Attenuation Data - A significant number of the
attenuation values varied substantially in a single
step wedge (common material). Numerous factors
affect attenuation measurements and this parameter
is difficult to use to correlate one laminate with
another. In fact the carriers indicated that they use
laminate standards to set up their equipment
(functionality) but not to establish flaw call "levels."
Attenuation in the laminate standards doesn't
exactly represent the actual part on the aircraft.
Inspectors base flaw calls on consistency across the
part being inspected (in-situ measurements
determine appropriate signal levels). However, this
parameter does provide a basis of comparison with
existing laminate standards. We want to match the
attenuation of the existing laminates and not induce
additional attenuation through the introduction of a
new generic material.
2. Velocity Data - Longitudinal velocity data was
acquired using 1 MHz, 2.25 MHz, and 5 MHz
transducers. The velocity data was very consistent
across each step wedge and even similar from one
material to another. The maximum difference
between the minimum and maximum velocities for
all OEM standards including fiberglass and carbon
materials was less than 10%. The velocities ranged
from 0.112 in/ps to 0.120 in/ps. These results are
logged in Table 2 and produced the target values
shown for our generic material.
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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/m1/8/: accessed May 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.