Page 218 - Handbook of Adhesives and Sealants

Quality Control and Nondestructive Tests 189

area. Shear waves can also be introduced into the structure with a
5
wedge shaped transducer. This technique is effective in analyzing
sandwich structures.
Bonded structures that are ultrasonically tested by the immersion
method often use a C-scan recorder to record the test. This recorder
is an electrical device that accepts signals from the pulser/receiver
and prints out a plan view of the part. The ultrasonic search unit is
automatically scanned over the part. The ultrasonic signals for bond
or unbond are detected from built-in reference standards. C-scan NDT
techniques are used extensively by aircraft manufacturers to inspect
bonded parts.
One of the oldest and best known ultrasonic testing system is the
Fokker Bond Tester originally manufactured by the Fokker Aircraft
Company in Germany. This method uses a sweep frequency resonance
method of ultrasonic inspection. Some degree of quantitative analysis
is claimed with the Fokker Bond Tester in the aircraft industry. Table
5.4 indicates the degree of correlation between predicted and actual
bond strength of the Fokker Bond Tester compared with two other
NDT instruments.

5.3.2.5 Other NDT methods. Radiography (x-ray) inspection can be
used to detect voids or discontinuities in the adhesive bond. This
method is more expensive and requires more skilled experience than
ultrasonic methods. The adhesive must contain some metal powder or
other suitable filler to create enough contrast to make defects visible.

TABLE 5.4 Correlation between Predicted and Actual Bond Strengths Made by
Three Ultrasonic NDT Instruments 1
Nondestructive test instrument
Coinda
Adhesive Fokker, scope, Stubmeter,
Type of bond material lb/in. 2 lb/in. 2 lb/in. 2
Overlap ............ FM-47 950 1,300 1,400
Overlap ............ HT-424 360 1,100 950
Overlap ............ FM-58 740 1,175 1,050
Overlap ............ Metlbond 4021 730 600 1,150
Honeycomb ......... FM-47 250
Honeycomb ......... HT-424 200
Overlap ............ FM-47 750
Honeycomb ......... Epoxy-phenolic 20 to 150
Overlap ............ Redux 775 570

95% confidence limits.