Subject: Machine Design -I

Topic: Curved beams and Thin cylinder

Difficulty: Medium

• Stresses can be classified into two groups, viz., load stresses and residual stresses.
• Residual stresses are those stresses that are independent of external forces. They are usually induced as a result of manufacturing processes and assembly operations. When the machine component with residual stresses is put into service, the load stresses are superimposed on the residual stress.
• Residual stresses are also called internal stresses or locked-in stresses.
• Residual stresses may be harmful or beneficial. If the residual stresses add to the load stresses, they are harmful. On the other hand, if residual stresses are opposite to load stresses and subtract, they are beneficial.

• The residual stresses are induced due to the following factors:

  (i) Manufacturing processes like casting and forging,

  (ii) Machining methods like turning, milling and grinding,

  (iii) Rolling, extrusion and cold working processes,

  (iv) Chemical processes like oxidation, corrosion and electroplating,

  (v) Heat treatment processes like quenching, and

  (vi) Assembly operations involving some misalignment.

• It is observed that it is practically difficult to avoid residual stresses in any component.

• It is very important to consider residual stresses when the component is subjected to fluctuating stresses and failure occurs due to fatigue.
• Residual stresses may either improve the endurance limit of the component or affect it adversely.
• If the residual stresses in the surface of the component are compressive, the growth of a fatigue crack is retarded and the endurance limit is improved. Therefore, residual compressive stresses are purposely induced in the surface of components subjected to fatigue loading. Such parts are subjected to the shot peening process, which build compressive stresses into the surface of the component. This improves the endurance limit of the component.
• Residual stresses are also beneficial in some applications like compound cylinders and press-fitted or shrink-fitted assemblies.