The standard method ASTM D3983 describes the test for measuring the shear modulus and rupture shear stress of non-rigid adhesives in bonded joints. The test method employs thick rigid adherends with a short overlap length, to minimize peel stresses and ensure uniform distribution of shear stresses across the adhesive bond in the overlap area. Some tensile and compression stresses are unavoidable in the joint, hence pure shear strength of the adhesive cannot be obtained by this method. The measured non-rigid adhesive shear strength values can be used for the structural design of joints and for research purposes. The ASTM D3983 test specimens can be prepared using wood, metal, or composite specimens bonded by adhesives having shear moduli not exceeding 700 MPa (100 000 psi). Generally, the ratio of adherend tensile modulus to adhesive shear modulus at the joint needs to be greater than 300 to 1. The test is not suitable for adhesives with high shear modulus after curing or those involving the removal of volatiles during curing.
The ASTM D3983 test uses thick adherends with an overlap section designed to minimize peel stresses on the adhesive. The specimen is configured as two rectangular thick adherend bars bonded together with a layer of adhesive, with a zig-zag notch (vertical cut-horizontal cut-vertical cut) splitting the bars into two lap-jointed pieces, enabling their movement in opposite directions when pulled longitudinally in opposite directions. The shear stress gets exerted on the adhesive layer in the overlap area between the two staggered vertical notches. For the pieces to be pulled apart longitudinally, two loading holes are provided 80 mm apart along the longitudinal axis for insertion of loading pins. The pins are suitably connected to any standard testing machine capable of applying a controlled tension force.
Specimens for ASTM D3983 may be prepared by bonding two adherend bars of the required size together and then cutting the notches and drilling the holes for the gripping pins. Alternatively, two large sheets can be bonded together and the required specimen is cut from the larger bonded sheets. Another approach is to machine the adherends to the required shape prior to bonding. Adequate surface preparation is needed followed by application of the correct thickness of the adhesive and curing. The cleaned sample is then subject to tensile loading. The specimen is held vertically and firmly, without slip or damage, by a pair of self-aligning grips and tension forces applied by the machine through the loading pins along the specimen’s centreline, by pulling from the opposite ends at the specified crosshead speed. The displacement of the adherend bars under load is measured using a dual transducer slip gauge or an extensometer .
Video 01: Adhesive lap joint shear strength test
The specimen for the ASTM D3983 test has an overall length of 110 mm, a width of 25 mm, and an overlap length of 5 mm. Each adherend’s thickness is at least 6 mm and the bondline thickness is typically 0.5 mm. The slots should be 1.5 mm wide. Tensile loading is via two 12.7 mm diameter pins inserted in holes 80 mm apart.
The ASTM D3983 adhesive strength and shear modulus is expressed in megapascals (Mpa).
The standard method ASTM D3983 describes the test for measuring the shear modulus and rupture shear stress of non-rigid adhesives in bonded joints.