Shear loading involves applying a force coplanar with the cross-section of a structure, which causes the internal layers to slide past each other in a parallel manner. There are various configurations of shear loading— one of which is the double shear.
This article delves into the concept of double shear loading, covering its strength characteristics, calculations, comparison with single shear loading, and its role in material strength testing.
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Understanding Double Shear Loading
Double shear loading involves subjecting a structure to a force that generates shear stress along two distinct planes within the material. In this scenario, the applied load or force is evenly distributed between two parallel cross-sections. This commonly occurs when a component is sandwiched between two other structures using a fastener, such as a bolt or pin, aligned in such a way that the load is shared between them.
To illustrate, consider the diagram below showing a double-sided pin connection:
In the diagram, a pin is used to connect a rod to a hinge, which has a double support to the ground. Since the rod is positioned between the two supports, applying an upward axial load ‘P’ on the rod induces two internal reaction shear forces on the pin. These forces are directed downward and act on two parallel cross-sections of the pin.
Double Shear Stress Formula
In general, the shear stress in a double shear loading can be calculated using the following formula:
- τdouble = shear stress in a double shear loading [N/m2]
- V = internal shear force [N]
- A = cross-sectional area of the shear plane [m2]
- P = applied external force [N]
In the given example of a double-sided pin connection above, the formula for the average shear stress becomes:
- d = diameter of the pin [m]
Double Shear vs Single Shear
While the load gets evenly distributed across two parallel shear planes in a double shear loading, in a single shear, the load is handled by only one shear plane. These are illustrated in the diagram below.
Because the reaction to an applied force in a double shear loading is evenly split between two shear planes, the magnitude of the shear stress developed is reduced by half compared to single shear, which only has one shear path. As a result, double shear connections provide enhanced strength and superior resistance against shearing forces. This makes them the preferred choice for heavy-duty applications, especially those that require a high safety factor.
Since the fastener can withstand double the shear force, it is also possible to reduce the number of fasteners required. In addition, a double shear configuration excels in maintaining alignment and resisting unbalanced loading. These advantages make double shear generally more favorable than the single shear configuration.
However, it is important to note that using the double shear configuration necessitates the use of two clips in order to sandwich the component. As a result, the required length of the bolt increases due to the inclusion of two clip thicknesses in the grip length.
Double Shear Testing
The double shear configuration can also be used to test the shear strength of a material specimen, specifically its ultimate shear stress. The ultimate shear stress refers to the maximum shear stress a material can endure before failing.
In double shear testing, lateral shear forces are applied to the specimen in two separate parallel planes using a universal testing machine (UTM) until failure results. This test method is commonly employed to evaluate the performance of adhesive bonds, welds, riveted joints, and other mechanical connections in order to assess their shear strength, deformation characteristics, and overall structural integrity.
A double shear test fixture consists of three blades with centrally located transverse holes. Two blades are kept stationary to support the specimen, while the third blade, known as the plunger, is moved vertically in a parallel plane, effectively shearing the specimen.
The schematic diagram below shows the typical setup for the double shear testing process: