Shear testing your fasteners for quality control
What is shear force?
Shear is the force that causes two touching parts of the same body to slide parallel to their plane of contact. Testing a material’s shear strength is important, as it determines your choice of tooling, ease of processing and product quality. Those working with composite materials advocate for adhesive bonding when putting components together, yet fasteners remain popular. The bottom line: shear testing should always include your fasteners.
What do we mean by shear strength?
Sheer strength is the stress that fractures the material in the plane of material cross-section. There are several test methods for determining the shearing characteristics of a material. Using a standard tensile test machine, shear testing is usually performed by pulling on the part or joint arranged in a double shear configuration.
Shear testing your fasteners explained:
In service, the fastener is loaded in either axial tension or transverse shear. Shear loading is much more typical. Axial tensile testing of fasteners is rather straightforward and requires minimal specialized fixturing.
Not with shear testing. Depending on the design of the joint, the shear loading is single or double shear. If the fastener is subjected to double shear, it can carry twice as much load. Also, double shear results in symmetrical loading, which reduces bending effects in both the fastener and the structure. However, a structural design might not allow this configuration.
In double-shear testing, the diameter of the fastener can range from a few millimetres to several centimetres. Because of this, some fixtures for testing large-diameter fasteners are relatively large and expensive.
Testing fixtures
Fixtures for shear testing can consist of a base, an anvil and a blade. The fastener can be placed across the anvil and sheared into three pieces by a compressive force applied to the blade. The relative thicknesses of the anvil supports and the blade, along with the tolerances of other critical dimensions, are defined in the standard. This fixture has existed for years and performs accurately.
One problem concerns metallic fasteners – specifically some of those favoured by the aerospace industry – which are being manufactured in stronger materials. Test the shear strength is proving difficult, as the materials used to fabricate the anvil and blade have to be tougher than the fastener material. Right now, fixtures are usually made from high-hardness tool steel.
Used less often are double-shear designs. This is when the fastener passes through holes in the anvil and blade, instead of semicircular cutouts. This fixture configuration is specified in British Standard BS EN 28749 (ISO 8749).
Single-shear testing of joints is common, but single-shear testing of the fastener itself is less so.
Think About Quality control
As manufacturers turn to composite materials, the need for reliable shear testing has never been greater. Fasteners should be a priority in quality control; after all, that’s what holds your product together. With shear testing, you’re maximising your processes and ensuring your parts and fasteners do their intended job.
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