Due to the increased use of adhesively bonded plastics as a result of the inherent advantages afforded by bonded rather than mechanically fastened joints, particularly the alleviation of stress risers and stress cracking, there is a need for standard tests by which joints of various substrates and adhesives can be compared. This test method is intended to meet such a need.

This test method is limited to test temperatures below the softening point of the subject adherends, and is not intended for use on anisotropic adherends such as reinforced plastic laminates.

The misuse of strength values obtained from this test method as design stress allowable values for structural joints could lead to product failure, property damage, and human injury.

The apparent shear strength of an adhesive obtained from a given small single-lap specimen may differ from that obtained from a joint made with different adherends or by a different bonding process. The normal variation of temperature and moisture in the service environment causes the adherends and the adhesive to swell and shrink. The adherends and adhesive are likely to have different thermal and moisture coefficients of expansion.

Even in small specimens, short-term environmental changes can induce internal stresses or chemical changes in the adhesive that permanently affect the apparent strength and other mechanical properties of the adhesive. The problem of predicting joint behavior in a changing environment is even more difficult if a different type of adherend is used in a larger structural joint than was used in the small specimen.

The apparent shear strength measured with a single-lap specimen is not suitable for determining allowable design stresses for designing structural joints that differ in any manner from the joints tested without thorough analysis and understanding of the joint and adhesive behaviors.

Single-lap tests may be used for comparing and selecting adhesives or bonding processes for susceptibility to fatigue and environmental changes, but such comparisons must be made with great caution since different adhesives may respond differently in different joints. See Guide D 4896 for further discussion of the concepts relative to interpretation of adhesive bonded single lap joints.