Qualification Tests for Adhesives and Plastic Assemblies

Since the bonding process can affect strength more than the adhesive material itself, the qualification of production assemblies applies just as much to process control as to the design itself. For this reason, test parts must always reflect exact production procedures rather than ideal lab conditions. In particular, any adhesive process change should first undergo the appropriate tests to avoid serious production problems and potential field failures.

Regardless of the type of test, documentation of failed parts should include the percentage of adhesive or substrate failure and the percentage of adhesive coverage at the joints. At a minimum, comparisons of test results to calculations must consider the effects of temperatures, surface finish, air gaps, cleanliness, and especially adhe-sive coverage in the joint.

Short-term tests for plastic parts should at least include impact, thermal shock, environmental resistance, and deformation under application load and temperature.

Long-term creep tests should occur under the application load and temperature for at least 10 percent of the expected product life to permit good extrapolation of results. Fatigue tests for significant variable loads shouldn't exceed 10 times the application cycling rate to provide reliable data.

9.7.1 Thermal Shock Tests

Static thermal shock tests expose bonded or molded items to application, shipping, or storage temperature extremes. Subsequent tests should use the same thermal shock test parts to incorporate any degradation from hidden flaws and to more closely simulate the application. Furthermore, thermal cycles will more quickly ini-tiate the thermoplastic post-mold shrinkage that generally occurs over a long period of time in use. When added to prior assembly stresses, these shrinkage stresses often will initiate cracks during the static thermal tests and will affect the results of later tests. In addition, thermal stresses from materials with different coefficients of thermal expansion could combine with these other stresses to induce failures.

In most cases, the test usually begins with the lowest temperature to avoid potential corrosion at the end of the test from frost formation and to expedite hot shear tests by reducing oven time. The test specimens stay in the cold chamber until reaching equilibrium and then quickly rotate to the heated chamber for a similar period.

After six cycles, any signs of cracks or other failures generally disqualify the assembly from further evaluation.

9.7.2 Impact Tests

A typical impact test setup may include a pendulum of known mass dropped from incrementally higher measured heights until the assembly fails. The sample must securely fasten to the base of the fixture to absorb the full impact at the most desirable point. For a large part mass, dropping the sample vertically to the point of impact from various heights may suffice for failure to occur. With either method, the impact force equals mass times vertical distance. For a small sample mass, a larger known mass may vertically impact a test specimen resting against a hard surface.

Given low enough friction, guides can ensure a consistent and uniform impact for an evenly distributed stress with any of these methods.

The impact method should remain constant for the same assembly to obtain good comparisons of different adhesives or different joint types. However, construction variations between assemblies often force the use of different impact techniques to fully test the joint, while avoiding extraneous deformations or brittle failures that would introduce premature failures unrelated to the joint under test. The same principles apply to plastic coatings or to assemblies that include plastic parts. Ideally, the substrate should fail before the adhesive in bonded assemblies. Otherwise, normal variations in adhesive strength require very large factors of safety to avoid field failures.

Nevertheless, applications with shock requirements would also require testing the complete assembly after component qualification.

9.7.3 Stress Tests

Shear tests on parts at the maximum operating temperature and at room temperature provide a means of checking temperature factors used in stress calculations.

Since plastics and adhesives often don't provide the same strength in all three axes, spin tests for rotating parts at operating temperatures will also permit a check of axial versus radial strength. Production specifications for nondestructive, room-temperature shear tests will then provide confidence that conforming parts will per-form in the application.