Which process reduces the fatigue strength of rotating or reciprocating equipment, such as shafts?

Which process reduces the fatigue strength of rotating or reciprocating equipment, such as shafts?

• A. Annealing
• B. Shot peening
• C. Electroplating
• D. Hardening

Answer: (C)

Fatigue life for a rotating shaft depends heavily on the condition of the surface layer and any stresses locked into it. An electroplated coating can introduce tensile residual stresses and, often, a coating with porosity, microcracks, or imperfect adhesion. Under repeated bending or torsional cycles, these surface flaws become crack initiation sites and paths for crack growth, so the shaft reaches failure after fewer cycles. Hydrogen embrittlement from certain plating processes can also weaken the metal and hasten cracking. Because of these issues, electroplating tends to reduce fatigue strength. In contrast, other processes have different effects: shot peening creates beneficial compressive residual stresses that impede crack initiation and usually increases fatigue strength; hardening raises surface hardness and improves resistance to crack initiation up to a point; annealing softens the material and relieves stresses, which can either help or hurt fatigue life depending on the prior state.

Fatigue life for a rotating shaft depends heavily on the condition of the surface layer and any stresses locked into it. An electroplated coating can introduce tensile residual stresses and, often, a coating with porosity, microcracks, or imperfect adhesion. Under repeated bending or torsional cycles, these surface flaws become crack initiation sites and paths for crack growth, so the shaft reaches failure after fewer cycles. Hydrogen embrittlement from certain plating processes can also weaken the metal and hasten cracking. Because of these issues, electroplating tends to reduce fatigue strength.

In contrast, other processes have different effects: shot peening creates beneficial compressive residual stresses that impede crack initiation and usually increases fatigue strength; hardening raises surface hardness and improves resistance to crack initiation up to a point; annealing softens the material and relieves stresses, which can either help or hurt fatigue life depending on the prior state.