Test method ASTM D5868 is applicable to single-lap adhesive bonded shear joints in Fibre Reinforced Plastic (FRP) adherends. Adhesive joints in FRP applications typically need to function in a state of shear, where forces act parallel to the cross-section of the adhesive, causing the adherend surfaces to slide in opposite directions. The single-lap shear test measures the shear strength of a specified adhesively bonded lap joint between FRP plates or between FRP and metal plates. The ASTM D5868 method can be used for random and fiber oriented FRP composites. The bond shear strength depends on the adhesive and adherend materials, surface preparation, curing achieved, and environmental conditions. This test is useful for the characterization and comparative analysis of different adhesives, FRP materials, and surface treatments.
The ASTM D5868 single lap shear test is performed on two rectangular adherend pieces of FRP, bonded by an adhesive at the single-lap joint. Testing is conducted on any standard testing machine capable of applying a controlled tension force. The composite specimen is held vertically and firmly, without slip or damage, by a pair of self-aligning grips. The specimen is subjected to tension forces along the centreline, by pulling from the opposite ends till the lap-joint fails. The ASTM D5868 test for FRP, specifies a crosshead speed of 0.5 inch/min, with a tension loading rate of 1200 to 1400 psi of shear area per minute. The stress value at joint failure, as well as the type of failure is recorded.
Video 01: Tensile shear strength testing of adhesives
For ASTM D5868, the specimens are typically fabricated from rectangular pieces each with a length of 100 mm, and width of 25 mm, with an adhesive layer thickness of 0.75 mm. The length of overlap can vary between 12.7 mm and 25.4 mm. The thickness of each piece would depend on the FRP type.
The ASTM D5868 adhesive shear strength is reported in pounds per square inch (psi).
Test method ASTM D5868 is applicable to single-lap adhesive bonded shear joints in Fibre Reinforced Plastic (FRP) adherends.