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 ordinary bolt as shown in the figure is subjected to tensile forces there are two distinct regions of strength.

   3.(a). Diameter of the threaded portion is less than shank diameter d , 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 the stress.Therefore , large parts of energy is absorbed in threaded portion of the bolts.
   3.(b). Since the diameter of the shaft is more than that of core diameter $d_c$ less energy is absorbed in shank part that of threaded part.

4. So from above discussion threaded portion is the weakest part and the maximum amount of the elastic energy is absorbed in this region.
5. So the ideal bolt will be one,which is subjected to same stress level at different cross sections in the bolt. It is called the bolt of uniform strength. 6. Hence in the bolt of the uniform strength , the entire bolt is stressed to the same limiting value , this resulting in maximum energy absorption.

There are two methods to convert ordinary bolt into a bolt of uniform strength

• Diameter of shank (d) is as same as that of the core diameter of thread ($d_c$):
  when this bolt is subjected to tensile force , the stress in the shank &the stress in the threaded portion are equal.
  

• 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 threaded part.

$.^..$$\frac{\pi(d^2)}{4} - \frac{\pi(d_1^2)}{4} = \frac{\pi(d_c^2)}{4}$ $.^..$$d_1 $= $\sqrt(d^2 - d_c^2 )$

NOTE

A bolt with reduced shank diameter is preferred over a bolt with a axial hole.