Similarly, “the stresses at the yield point of the thread, based on the area at bottom of thread, when nuts were screwed on without tightening down by spanner against the bolts tightened by spanner, had reduction of 20 percent”.
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”. It is practically concluded that, “most of the stress on the threads will be shear as the tendency in screwing up will be to force the threads off the bolt or nut in a direction parallel to the axis i.e. circumferential due to the friction between the adjacent threads”. Peter (1950) has noted that “when considering the allowable unit stress which may safely be adopted the probability that some of the stock bolts may be wrought iron, instead of mild steel” must not be ignored.
The bolts which assist in the hanging of the runways are exposed to direct tension. It is therefore recommended that conservative approach shall be adopted, with specific reference to the determination of the safe loads for these bolts.