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Assemblies are often subjected to dynamic environments including vibration, shock, and/or thermal cycling. Such conditions can lead to fastener loosening and result in increased maintenance and failure. Previous work shows that transverse or shear loading of threaded products provides the most severe environment for loosening [1]. As a result, joints oriented such that fasteners are parallel to the direction of loading provide better resistance to dynamic failure. However, there are situations where this orientation cannot be achieved or, more often, loading is multi-directional.
Today, there exist numerous techniques for minimizing fastener loosening. Fasteners with locking features can be divided into four general groups: (1) free running pre-load independent locking fasteners, (2) free running pre-load dependent locking fasteners, (3) prevailing torque locking fasteners, and (4) chemical locking. In addition, there are a number of assembly design considerations which can lead to improved resistance to vibration-induced loosening. These include fastener orientation and joint shape details which minimize fastener movement, slip, and/or shear stress levels. A recent review [1] on vibration and shock induced loosening examines the existing literature, locking products, design considerations, and testing standards. Kerley [2,3] applied the principles of retroduction to the problem of vibrationinduced loosening. He proposed that shear force calculations could be used to quantify loosening and specify fastener placement. His experiments were performed with an inertial loaded compound cantilever beam with one fastener. Since the focus of his work was to demonstrate the application of retroduction, no data was presented which quantified shear force levels that cause loosening for given fastener pre-loads. Also, the fastener position was not varied in any of the experiments.
The compound beam assembly apparatus was originally used by Haviland [4, 5] with direct loading to demonstrate loosening. This apparatus provides a realistic vibration test because it represents the most common type of structure that causes shear loading on a fastener. It includes joints and panels of most major buildings, aircraft, cars, homes, and household appliances. In addition, tests with an inertial loaded compound beam assembly apparatus provide more realistic and repeatable results than the more severe existing standard tests, such as MIL-STD-1312-7A [6] and NAS 1675 [7], which generate shock and impact loads.