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Comparison of Analytical Methods and Test Results for Determining Fastener
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2011-02-23
Analytical analysis of bolted joints requires either the direct calculation of fastener elongation or calculation of the components that determine elongation. Even in instances when tension can be measured directly during testing, calculating elongation can be desirable when direct measurement is impractical. For example, when the clamp load at conditions not tested are desired, calculation of elongation or its components are required to extrapolate the test data. A primary challenge of calculating elongation is translating the stress profile in the head and the engaged threads into equations that can be solved simply. As opposed to the portion of the shank that lies within the grip length, these transition areas can contain complex stress profiles. This is further complicated by the fact that it is the head and nut member that contain most of the features that differentiate the various types of threaded fasteners. As a result there are several published approaches to elongation calculation that contain subtle differences for solving this problem. In this paper we will use two popular calculation methods to compare analytical estimates to actual test measurements of elongation. In light of the challenge of estimating the contribution of the head and nut areas, a test matrix was created to test various head/nut combinations, including some that might not represent practical application. Because the transition areas become more influential as the grip length get shorter tests were conducted at two different ratios of grip length to diameter. In this first phase of testing fully threaded cap screws were used so that the shank would be a constant cross section and the effect of the various head and nut transitions tested could be preserved. Each test was run at two loads as a check of elongation measurement by reviewing the ratio of high/low values.
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Dave Archer
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