Page 219 - Handbook of Adhesives and Sealants

190 Chapter Five

This method is applicable to honeycomb sandwich structures as well
as metal and non-metal joints.
Thermal transmission methods are relatively new techniques for
adhesive inspection. Heat flow is determined by monitoring the sur-
face temperature of a test piece immediately after external heating or
cooling has been applied. Subsurface anomalies will alter the heat flow
pattern and, thereby, affect the surface temperature. The surface tem-
perature difference can be detected by thermometers, thermocouples,
or heat sensitive coatings. Liquid crystals applied to the joint can
make voids visible if the substrate is heated.
Thermal transmission testing is an excellent way for detecting var-
ious types of anomalies such as surface corrosion under paint before
the corrosion becomes visually evident. Thin single layer structures,
such as aircraft skin panels, can be inspected for surface and subsur-
face discontinuities. This test, as shown in Fig. 5.4, is simple and in-
expensive, although materials with poor heat transfer properties are
difficult to test, and the joint must be accessible from both sides. For
nonmetallic materials, the defect diameter must be on the order of
four times its depth below the surface to obtain a reliable thermal
indication. For metals, the defect diameter must be approximately
eight times its depth. Some bright surfaces such as bare copper and
aluminum do not emit sufficient infrared radiation and may require
application of a dark coating on their surface.
Thermal wave inspection is also a relatively new technique for stud-
7
ying adhesive disbonding. With this method, heat is injected into the
test object’s surface from a hot gas pulse. The resulting surface tem-
perature transient is analyzed to determine the bond quality in nearly
real time. The surface temperature transients are sensed using a non-
contacting, emissivity independent infrared sensor or video camera.
This method is not adversely affected by surface blemishes or rough-
ness.
Table 5.5 shows the types of defects that can be detected with var-
ious NDT techniques. Table 5.5a shows the correlation of NDT results
for built-in defects for laminate panels, and Table 5.5b shows a similar
correlation for honeycomb structures. A universal NDT method for
evaluating all bonded structures is not currently available. Generally,
the selection of a test method is based on:

Part configuration and materials of construction
Types and sizes of flaws to be detected
Accessibility to the inspection area
Availability and qualifications of equipment and personnel