Significance and Use
4.1 This test method is useful in that the joint configuration closely simulates the actual joint configuration of many bonded assemblies. Surface preparation of the adherends can affect the apparent shear strength of the adhesive and can be one of the variables under study.
4.2 This test method is also useful as an in-process quality control test for laminated assemblies. In practice the laminated assembly is either made over size and test specimens removed from it or a percentage of the assemblies are destructively tested.
4.3 The misuse of strength values obtained from this test method as design allowable stress 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.
4.3.1 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.
4.4 The apparent shear strength measured with a single-lap specimen is not suitable for determining design allowable 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.
4.5 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. Review Guide D4896 for further discussion of concepts for interpretation of adhesive-bonded single-lap joint data.
Scope
1.1 This test method (Note 1) is intended for determining the comparative shear strengths of adhesives in large area joints when tested on a standard single-lap-joint specimen and under specified conditions of preparation and testing. Adhesives respond differently in small versus large area joints (Note 2).
Note 1—While this test method is intended for use in metal-to-metal applications, it may be used for measuring the shear properties of adhesives using plastic adherends, provided consideration is given to the thickness and rigidity of the plastic adherends. Doublers or bonded tabs may be required for plastic adherends to prevent bearing failure in the adherends.
Note 2—This variation can be influenced by adhesive density, flow characteristics, cure rate, gel time, carrier composition, entrapped volatiles, volatiles released during cure, etc. and also by cure cycle variables including: temperature, time, pressure, rise rate to temperature, cool-down rate, etc. In addition to the processing variables joint size changes the level of constraint on the adhesive as well as the influence of induced tensile loads at the edge of the joint during mechanical or thermal loading.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.