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The strength of the bolt which is subjected to direct tension is unexpected. This tension is function of amount of initial tension which is induced during tightening down the nuts by hand with common tools.
It is important to note that when the nuts are simply screwed by hand, without excessive tightening through tools, "the stresses at the yield point of thread, based on area at bottom of thread, were fairly consistent"…
It is important to understand that the failure of bolt is the characteristics of the minimum residual stresses of the bolt material. During an exercise, consistent stresses were derived for ordinary black, and turned and fitted bolts upon their respective failure. As per Peter (1950) experimentation, "the yield point of thread, however in the case of the black bolts was approximately 30 percent, which is lower than yield point obtained for turned and fitted bolts". It is important to specify that in all the failures the damage initiated from the root of the thread, however there were instances when the threads stripped.
It has been experimental proven by Peter (1950) that the "strength of a bolt in direct tension is greatly affected by the amount of initial tension induced when the nuts were made spanner tight; this is a factor which is uncontrollable from design perspective. The strength is dependent upon resistance of the threads against stripping". ...
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