In the manufacture of automobiles, metallic bolts are used in large volumes ranging in application from high strength required for structural assembly to securing lightweight parts to the vehicle. Whatever the application they are used for, testing is critical to determine if they will be suitable for the application and/or to ensure they are meeting the quality standard.
Metal Bolt Testing
Quality Control for High Volume Production
Bolts are typically produced in extremely large quantities, testing needs to be completed as quickly as possible to ensure the product can be shipped.
Bolt testing accessories enable proof tests, axial and wedge tensile tests, and cone stripping tests on most standard bolts, screws, studs, and nuts. They are compatible with most universal and tensile testing machines. Available accessories consist of: Bolt holders, washers, short bolt adapters, nut proof loading assemblies, and holder reducing assemblies.
Each bolt holder has a key slot that allows the operator to quickly and correctly load the specimen. A recessed circular area in the bottom of each bolt holder ensures that the washer will fit snugly (sheet metal alignment device is used on Series E holders). This reduces loading time and occurrences of slippage, because a secure, centered seating is ensured for each test.
Short Bolt Lengths
The length of each type of bolt can vary hugely depending on the application. Many are quite short, which can make gripping these for a tension test extremely challenging. The typical tension test setup for bolts uses a high strength plain and threaded washer, the thickness of these two components can often mean that there is not sufficient shank left exposed to test.
"Top Hat" or "Dumbell" type accessories protrude out of the bold holder fixtures allowing shorter lengths of bolts to be tested.
Nut Proof Loading
As well as testing the bolt, the nuts that secure to the bolts also need to be tested. This requires attaching the nut to a mandrel that is stronger than the threads of the nut to determine what the maximum load of the threads within the nut are.
Using high strength nut proof load assemblies allows mounting into the standard bolt holders to complete this test quickly and easily.
Measuring Strain and Multi Step Loading
For some testing, an extensometer may be required to accurately determine the yield strength and stiffness of the bolt, however, the extensometer needs to be removed during the test to prevent damage to the instrument due to high energy failures. To do this after yield has been determined, the test needs to return to zero load and then continue the test once removed to test through failure.
In their application, bolts are often subjected to shear forces, these can be in single shear or in double shear. It is important to understand how the bolts will perform under these shear forces to ensure there are no product failures.
Instron’s single or double shear fixtures can attach simply to any universal testing machine. The double shear test is completed in compression against a fixture supporting either side of a punch, whereas the single shear is conducted in tension as the bolt is mounted through the fixture. These tests are typically performed until the initial yield in shear, further elongation may cause damage to the fixtures.
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