1) When bolts are subjected to shock and impact loads the resilience of the bolt is the most important design consideration to prevent the breakage at the thread.
2) Here, resilience is defined as the ability of the material to absorb energy when deformed elastically and to release this energy when unloaded.
3) When the ordinary bolt as shown in the figure is subjected to tensile force, there are two distinct regions of strength.
(a) The diameter of threaded portion $d_c$ is less than the shank, diameter $d_1$ the threaded portion is subjected to stress concentration. Therefore stress induced in the threaded portion is more than the stress induced in the shank portion. The energy absorbed by the material is proportional to the square of stress. Therefore, large parts of energy are observed in the threaded portion of bolts.
(b) Since the diameter of the shank is more than that of core diameter $d_c$ less energy is observed in the shank part that of the threaded part.
4) So from the above discussion threaded portion is the weakest part and the maximum amount of elastic energy is observed in this region.
5) So the ideal bolt will be 1, which is subjected to the same stress level at different cross sections in the bolt. It is called the bolt of uniform strength.
6) Hence in the bolt of uniform strength, the entire bolt is stretched to the same limiting value, thus resulting in maximum energy absorption.
There are two methods to convert ordinary bolt into a bolt of uniform strength: -
(i) The diameter of the shank (d) is the same as that of the core diameter of the thread ($d_c$)
When this bolt is subjected to tensile force, the stress in the shank and the stress in the third portion are equal.
(ii) The other method is one in which the cross-sectional area of the shank is reduced by drilling a hole
The diameter of the hole d, is obtained by equating the cross-sectional area of the shank to that of the threaded part.
$\therefore \dfrac \pi 4d^2-\dfrac \pi4d_1^2=\dfrac \pi 4d_c^2$
A bolt with reduced shank diameter is preferred over a bolt with an axial hole.