Improving aircraft composite inspections using optimized reference standards Page: 6 of 13
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NDT DATA ANALYSIS - The inspection results
were used to identify the important variables which
should be included in composite honeycomb reference
standards. The raw X-Y and C-scan data was analyzed
using a variance analysis. The statistical analysis of the
data was conducted in order to place the effects of flaw
and construction variables into "major," "minor," and
"minimal" categories. The analysis determined the
effect of variables alone (e.g. impact of material
thickness) and in two and three variable combinations
(e.g. impact of core type in combination with laminate
type). The flaw types and construction variables listed
above were assessed. The statistical analysis of the
round-robin test series showed that for typical composite
honeycomb flaws, the dominant factors affecting
inspections are laminate thickness, laminate type, and
honeycomb type. This data indicates that composite
honeycomb reference standards should include the
following variable ranges: laminate thickness (3 ply to 12
ply), laminate type (both fiberglass and carbon), and
honeycomb type (fiberglass and Nomex).
Validation of Minimum Honeycomb Reference
PROTOTYPE MINIMUM REFERENCE
STANDARD SET - The results presented above led us
to the production of a prototype minimum reference
standard set that include the important variables for the
successful inspection of composite honeycomb
structure: laminate thickness, laminate type, and
honeycomb type. The construction characteristics of the
prototype honeycomb set are summarized in Table 1.
Disbonds and delaminations were placed together in a
single standard. Thus, there were eight standards
manufactured: a 3, 6, 9, and 12 ply laminate with carbon
or fiberglass skins and each containing both Nomex and
fiberglass cores. Figure 4 shows the basic honeycomb
VALIDATION TESTING AND RESULTS -
Validation testing on this minimum set was conducted
using the S-9 Sondicator device. After setting up the
equipment on each flaw/skin thickness scenario, the set
of 64 "aircraft" panels were inspected. Amplitude and
phase data were used to assess the viability of the
standards. If the full array of 64 specimens - which
bound the composite honeycomb structure on aircraft -
could be adequately inspected using the minimal
standard set, we will have successfully identified the key
variables and provided justification for excluding other
honeycomb construction variables from the set.
Furthermore, by setting up the equipment on 6 ply
laminates and then inspecting 3, 9, and 12 ply
specimens we determined whether or not exact laminate
thickness matches are required (i.e. the allowable
variation between laminate thickness used in set-up and
laminate thickness in part being inspected).
Carbox Skin Over Both Core T
Nome Core Haf Fberglass Ce Half
00 00 0
2 = 3 = Deaminutions at Differvnt Ply Levels
Figure 4: Sample of Basic Honeycomb
Flaw Laminate Laminate Honeycomb Honeycomb Cell Cell
Type Thickness Type Thickness Size Density
Delam. Carbon 3, 6, 9,12 plies Nomex 1" 3/16" 3 lb.
Disbond Carbon 3, 6, 9, 12 plies Nomex 1" 3/16" 3 lb.
Delam. Fiberglass 3, 6, 9, 12 plies Nomex 1" 3/16" 3 lb.
Disbond Fiberglass 3, 6, 9, 12 plies Nomex 1" 3/16" 3 lb.
Delam. Carbon 3, 6, 9, 12 plies Fiberglass 1" 3/16" 4 lb.
Disbond Carbon 3, 6, 9, 12 plies Fiberglass 1" 3/16" 4 lb.
Delam. Fiberglass 3, 6, 9, 12 plies Fiberglass 1" 3/16" 4 lb.
Disbond Fiberglass 3, 6, 9, 12 plies Fiberglass 1" 3/16" 4 lb.
Table 1: Honeycomb Reference Standards to Be Used to
Set Up NDT Equipment for Inspection Exercise
Signal-to-noise (S/N) results from the panels
indicated acceptable flaw detection over the entire range
of honeycomb types. Thus, the set of eight prototype
honeycomb reference standards described above are
able to support the inspection of honeycomb aircraft
structure. After setting up the NDT instrument on a 6 ply
standard, it was possible to inspect 3 and 9 ply aircraft
panels, however, the flaw sensitivity was not as good as
Variables Addressed inPooveCmsieHeymbSadr St
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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/m1/6/: accessed May 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.