Improving aircraft composite inspections using optimized reference standards Page: 9 of 13
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3. Pulse Laminate Standards for Pulse-Echo (Velocity-
Based) Testing - Based on the velocity results, it
was determined that for velocity-based equipment
the standard should be made from a material with a
median velocity (0.115 in/ps). Ease of manufacture,
material cost, and ease of use are important factors.
The basic design approach is to machine flat-
bottomed holes in a plate which is large enough to
accommodate scanner heads. This plate design will
be less susceptible to breakage than the existing
4. Laminate Standards for Resonance Testing -
Velocity measurements alone do not allow for proper
resonance equipment set-up. Furthermore,
resonance testing requires that the equipment be
set-up on laminates with similar thickness to the part
being inspected. Thus, the necessary laminate
reference standard(s) should have the appropriate
material property. The key property may be
acoustic impedance, Z, where p = density and
Thickness in mils of
5 40 120 300 600
15 60 1804 00 80
20 80 220 450 900
30 100 260 500 1000
Z = p X Velocity
Figure 6: Generic Composite
5. Search for Suitable Material - Based on the above
observations, a search was performed to locate a
material with the appropriate velocity and density
(thus, acoustic impedance) properties. Other
desirable attributes were that the material be
inexpensive, easy to machine, and able to be
reliably produced. Table 2 lists candidate materials
along with the data from the current carbon and
fiberglass material which we attempted to match.
For resonance mode inspections, a close match with
acoustic impedance is necessary. Also, attenuation
characteristics are important. In order to
accommodate inspections through thick laminates
(0.25" - 0.5" thick), the allowable material
attenuation was 8-10 dB relative to the existing step
Based on cataloged property values, a number of
materials were selected to go through the prototype
fabrication and testing process. An extensive study
of Phenolic materials was performed and two types,
G10 and G11, were proven to be excellent
candidates. They both provide close matches to the
critical material properties and have low attenuation
relative to carbon.
6. Generic Standard Design - A laminate standard
design, which includes thickness ranges from one
ply (0.010") to 1.0", was developed. Prototype
laminate standards, as shown in Figure 6 below,
were fabricated from candidate "generic" materials
listed in Table 2. The results below address the
material our team is proposing as the new laminate
standard: G11 Phenolic.
7. Manufacturing Specifications - Through-transmission
ultrasonic inspections of G11 showed that it can be
manufactured as very pure material with very little
porosity (basically none was measured). Ultrasonic
C-scans showed less than 2 dB's variation in
response across the entire 12" X 12" area. The G11
Phenolic type can be uniformly and repeatably made
and is readily and inexpensively available. In the
machining process, the plates were first faced to
assure a uniform thickness and depth accuracy in
the flat-bottomed holes. Next, the flat-bottomed
holes were machined as per Figure 6. Thickness
areas as low as 0.010" (0.990" deep hole) could be
produced in the Phenolic material.
NDI Validation Test Results
Through-transmission ultrasonics (TTU), pulse-
echo ultrasonics (Quantum device from NDT
Engineering Inc.) and resonance (Bondmaster device)
inspection techniques were applied to the prototype
laminate standards in order to measure the material
properties and to assess the prototype standards use on
simulated aircraft structure. Following is a summary of
the inspection results.
1. Velocity and Attenuation Measurements - TTU was
used to measure velocity and attenuation in both the
candidate and existing laminate standards.
Attenuation measurements were made in an
immersion tank to produce consistent and
repeatable test results. Relative attenuation values
shown in Table 2 are as compared with the Boeing
ST8870 step wedge at 0.2" thick and the Boeing
ST8871 step wedge at 0.5" thick.
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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/m1/9/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.