Lap Shear ASTM D3163, ASTM D5868
The purpose of the Lap Shear Test is to evaluate the shear strength of adhesives when used for bonding materials. This test assesses adhesive strength, surface preparation, and environmental durability by examining a single-lap-joint specimen. There are various ASTM single-lap-joint shear tests, such as ASTM D1002 for metal-to-metal bonding, ASTM D3163 for plastic joints, and ASTM D5868 for fiber reinforced plastics against themselves or metal.
ASTM D3163
The test procedure for ASTM D3163 for adhesively bonded rigid plastic involves bonding two specimens together with adhesive and allowing it to cure as per the specifications. The specimens are then placed in a universal testing machine and pulled apart at a rate of 1.3 mm/min (0.05 in/min) until they fail. ASTM D5868 for fiber reinforced plastic requires a loading rate of 13 mm/min (0.5 in/min).
Two specimens, each measuring 25.4 x 101.6 mm (1" x 4"), are bonded together with sufficient adhesive overlap to cause failure in the adhesive, not the substrate. The typical overlaps are 12.7 mm and 25.4 mm (0.5" and 1"). Adhesives are applied as recommended by the adhesive manufacturer.
Data collected from the test includes the maximum, minimum and average values of failure load and the stress at failure in megapascals (pounds force per square inch). The type of failure, whether cohesive, adhesive, or substrate, is also recorded.
ASTM D3163 Significance of use
The use of adhesive-bonded plastics has increased due to the benefits of bonded joints over mechanically fastened joints. These benefits include reduced stress raisers and stress cracking. Therefore, there is a need for standardized tests to compare joints made with different plastic substrates and adhesives. This test method is designed to meet that need. However, this test method has limitations. It is only intended for use at temperatures below the softening point of the adherends being tested and is not suitable for anisotropic adherends like reinforced plastic laminates.
It is important to note that using strength values obtained from this test method as allowable design-stress values for structural joints can lead to product failure, property damage, and human injury. Moreover, the apparent shear strength of an adhesive obtained from a single-lap specimen may differ from that obtained from a joint made with different adherends or by a different bonding process.
Changes in temperature and moisture in the service environment can cause the adherends and adhesive to swell and shrink, and the thermal and moisture coefficients of expansion may vary between them. Even in small specimens, short-term environmental changes can induce internal stresses or chemical changes in the adhesive, permanently affecting its strength and other mechanical properties.
Moreover, predicting joint behavior in a changing environment is more challenging when different types of adherends are used in a larger structural joint than those used in the small specimen. Therefore, the apparent shear strength measured with a single-lap specimen is not suitable for determining allowable design stresses for designing structural joints that differ from those tested without thorough analysis and understanding of joint and adhesive behaviors.
Single-lap tests may be used to compare and select adhesives or bonding processes for susceptibility to fatigue and environmental changes. However, such comparisons must be made with great caution, as different adhesives may respond differently in different joints.
ASTM D-1002
The ASTM D-1002 test method is used to determine the lap shear strength of adhesives for bonding metals through a single-lap-joint specimen. This test can determine adhesive strength, surface preparation parameters, and adhesive environmental durability.
During the ASTM D-1002 test procedure, two metal plates are bonded with adhesive and cured as specified, and then cut into uniform width lap shear specimens. These specimens are placed in the grips of a universal testing machine and pulled at 1.3 mm/min (0.05 in/min) until rupture occurs. The type of failure can be either adhesive (the adhesive separates from one of the substrates) or cohesive (the adhesive ruptures within itself).
The recommended lap shear specimen is 25.4 mm (1”) wide, with an overlap of 12.7 mm (0.5”), and the recommended metal thickness is 1.62 mm (0.064”). The overall length of the bonded specimen should be 177.8 mm (7”), and the specimen failure should occur in the adhesive, not in the substrate. Therefore, the metal thickness and the length of the overlap may be adjusted as necessary, and adhesive is applied based on the manufacturer's recommendations.
The data obtained from the test includes Load at Failure (N or lbf), Shear Strength at Failure (MPA or PSI), type of failure (cohesive or adhesive), and percentage.
ASTM D5868
The ASTM D5868 bonding test for lap shear adhesion plastic has been developed to cater to the growing usage of adhesively-bonded FRP joints instead of mechanically-fastened joints. This is due to the advantages offered by bonded joints, such as stress crack and stress riser alleviation. The purpose of this test is to compare the joints of various FRP substrates and adhesives.
The ASTM D5868 bonding test for lap shear adhesion plastic outlines a lap shear test that measures the bonding characteristics of adhesives used to join fiber-reinforced plastics to themselves and to metals. The test method is applicable to both random and fiber-oriented FRP. It complements the ASTM D1002 test method and extends its application to single-lap shear adhesive joints of FRP adherends.
The ASTM D5868 bonding test generates comparative apparent shear strength data for joints made from different FRP materials. It also provides a means to compare FRP surface treatments. For more information on the significance and use of the ASTM D5868 bonding test, please refer to Guide ASTM D4896.