SUMMER 2021

102 THE DISTRIBUTOR’S

102 THE DISTRIBUTOR’S LINK ROB LaPOINTE FASTENER SCIENCE: SHEAR TESTING – SINGLE AND DOUBLE SHEAR FOR FASTENERS from page 10 FIGURE 2 FRICTIONAL FORCE IS THE PRODUCT OF THE NORMAL FORCE AND THE COEFFICIENT OF FRICTION µ. FIGURE 4 BEARING CONNECTION When a joint is tightened in this manner, there are no shear loads on the fastener itself, even if there are shear loads on the assembly. The shear loads are carried by the frictional force that is created by the tensioned fastener. This type of joint is known as a Slip-critical joint and relies on the friction in the joint rather than the shear properties of the fastener to keep the joint from slipping. The Slipcritical joint is illustrated in figure 3. Notice that the bolt itself does not bear the load needed for the joint to not slip, but rather it is carried by the fiction created through tensioning of the bolt-nut assembly. Fasteners used in the Bearing Connection application must be able to withstand shear loads applicable to the conditions that will be experienced in service. Thus, it is paramount to the performance qualification of these fasteners that shear testing be performed and that the fastener is able to withstand the loads necessary to achieve good performance. Testing fasteners for shear properties is typically done in one of two ways, single shear or double shear. Single shear tests the fastener in one plane and double shear tests the fastener in two planes. Figure 5 illustrates the two types of shear testing. The test chosen depends on the application, specification and configuration of the fastener although, often times, the two tests are interchangeable and either test can provide the necessary data. The principal difference between the two tests is the testing fixture. Single shear creates a shearing force in one plane and double shear creates a shearing force in two parallel planes. FIGURE 3 SLIP-CRITICAL JOINT The second principal way fasteners are used in a Bearing Connection where the bolt itself bears the shear load of the joint rather than the friction. Figure 4 illustrates a bearing connection where there is little or no clamp-load created by tensioning the fastener. This type of joint relies on the shear strength of the fastener to hold the joint in registration rather than relying on the tensile strength of the fastener to create friction in the joint assembly. FIGURE 5 SINGLE SHEAR TEST ASSEMBLY (LEFT) AND DOUBLE SHEAR TEST ASSEMBLY (RIGHT) CONTINUED ON PAGE 158

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