Material of connecting rod bolt ,importance of elongation, nature of stresses in this component and why should these bolts be replaced after some time in service

The connecting rod bolt in service is subjected to: 
a. A dynamic tension loading due to centrifugal force of the mass of connecting rod 
rotating with the crank pin 
b. A dynamic tension loading owing to inertial forces of the reciprocating mass of the 
piston which is fluctuating with angular displacement of the crank and having the 
peak value at an instant of 360˚ after the firing TDC in a cycle of operation 
c. A dynamic shear stress at the parting of the two halves of the bearing housing 
Dowel pins with fitted bolts or serrations at the face or both are used to reduce shear loading  on bolts and possibility of fretting. 
Bolts should be constructed of materials having high resilience and should not be stiffer w.r.t  bearing housing. 

Pretension of the bolts should be regarded as the single most important factor which 
contributes towards the fatigue life of the material of the bolt. Pretension must be kept high 
enough, so that the increase in stress owing to dynamic loading remains within the range of 
stress already given by pretension. 

Some routine checks on this part are (rejection criteria of the bolt) 
a. Check for corrosion by acidic lube oil, discard if any present on shanks 
b. Check the length of the bolt against a new or bolt tolerances. If longer, yielding of 
the material should have taken place. Renew the bolt in this circumstance 
c. Check for mechanical damage, especially on shanks 
d. Check for fractures by NDT 
e. Check the landing faces for uneven tightening 
f. Discard the bolt when either designated life, over speed failure or piston seizure has 
occurred