Page 216 - Handbook of Adhesives and Sealants

Quality Control and Nondestructive Tests 187

Some improvement in the tap test can be achieved by using a solenoid
operated hammer and a microphone pickup. The resulting electrical
signals can be analyzed on the basis of amplitude and frequency. How-
ever, the tap test, in its most successful mode, will only measure the
qualitative characteristics of the joint. It will tell whether adhesive is
in the joint or not, providing an acoustical path from substrate to sub-
strate, or it will tell if the adhesive is undercured or filled with air,
thereby, causing a damped path for the acoustical signal. The tap test
provides no quantitative information and little information about the
presence and/or nature of a weak boundary layer.

5.3.2.3 Proof tests. If a high degree of reliability is required, it is
necessary to proof test the production unit. The proof test should sim-
ulate actual service conditions in the manner in which the joint or
structure is loaded and the stress level should be higher than that
expected in service. The duration of the proof test should reflect the
expected life of the joint, but usually this is not possible. The proof
test should be designed so that it is normally a non-destructive test,
unless the bond is unexpectedly weak. Care must be taken to design
the proof test so that it does not overstress the part and cause damage
that will result in a reduced service life.
A common example of a proof test is to apply a cleavage load to a
bonded honeycomb sandwich by placing an instrument between the
face and core and applying a predetermined force perpendicular to the
core. If there is no bond disruption due to this test, it is supposed that
the product will meet all its service requirements. A common proof
evaluation used with sealants is leak testing with a mobile and easily
detected gas such as helium or application of hydrostatic pressures.

5.3.2.4 Ultrasonic inspection. The success of the tap test, although
limited, led to the use of ultrasonics to determine bond quality. Ultra-
sonic methods are at present the most popular NDT technique for use
on adhesive joints. Ultrasonic testing measures the response of the
bonded joint to loading by low-power ultrasonic energy. Short pulses
of ultrasonic energy can be introduced on one side of the structure and
detected on the other side. This is called through-transmission testing.
An unbonded area, void, or high damped adhesive (undercured or
filled with air) prevents the ultrasonic energy from passing efficiently
through the structure.
A number of different types of ultrasonic inspection techniques us-
ing pulsed ultrasound waves from 2.25 to 10 MHz can be applied to
4
bonded structures. The most common methods are:
Contact pulse echo—the ultrasonic signal is transmitted and re-
ceived by a single unit;