ABSTRACT
The design theory of fasteners has, over the years, been influenced by experimental work aimed at defming yield, proof, and failure criteria for a single fastener. The main task was to estimate the tensile strength of the threaded portion of the shank, which was difficult to do with the elementary equations such as, for instance, those discussed briefly in Chapter 33. Over the years a large number of fasteners made from well-characterized materials were tested and manufacturers were required to repeat many of these tests in order to build-up technical confidence in the product. While a great many test results have also been published, our need for constant vigilance of quality and improved calculational procedures has not gone away. One important practical result, however, has remained essentially unchanged, indicating that the yield load on the fastener in pounds divided by the stress area results (essentially) in a theoretical stress at yield. At the same time many actual applications show that the fastener seldom sees the yield stress. The basic requirement in fastener design is not necessarily to predict the exact yield stress but to select the appropriate torque Mt and the required preload F i with a rational margin of design safety. The comparison of Fi values with published strength data for a particular fastener size offers, at this time, the most reliable design alternative.
