Question

Which of the following is/are responsible for reducing the high cycle fatigue life of a component?

• A. increasing the mean stress at constant amplitude
• B. increasing the surface roughness
• C. employing shot peening
• D. absence of sharp corners in the component

Hint:

For fatigue life, remember these simple rules: - {Bad for fatigue:} Tensile mean stress, surface roughness, sharp corners, corrosive environment. - {Good for fatigue:} Compressive residual stress (shot peening), smooth surfaces, rounded corners (fillets).

Answer 1

The Correct Option is A, B

Step 1: Understanding the Concept:
High cycle fatigue life refers to the number of stress cycles a component can withstand before failure when subjected to cyclic loading. We are looking for factors that are detrimental, i.e., they shorten this life. Fatigue failure typically involves crack initiation and propagation.
Step 2: Detailed Analysis of Each Option:
- (A) increasing the mean stress at constant amplitude: For a given stress amplitude, a higher tensile mean stress reduces the fatigue life. This is because the peak stress in each cycle is higher, which accelerates crack initiation and propagation. This relationship is illustrated by various fatigue life models like the Goodman and Gerber criteria. Therefore, this factor reduces fatigue life.
- (B) increasing the surface roughness: Fatigue cracks almost always initiate at the surface. A rough surface contains microscopic notches and valleys that act as stress concentrators. These sites make it much easier for fatigue cracks to nucleate. Therefore, a rougher surface reduces fatigue life.
- (C) employing shot peening: Shot peening is a surface treatment that works by bombarding the surface with small beads. This process creates a layer of compressive residual stress at the surface. Compressive stresses inhibit crack initiation and slow down the propagation of any small cracks that do form. Therefore, shot peening increases fatigue life.
- (D) absence of sharp corners in the component: Sharp corners (like unfilleted shoulders) are geometric stress concentrators. They create high local stresses that can easily initiate fatigue cracks. Designing a component to have an absence of sharp corners (i.e., using generous fillets and radii) reduces stress concentration. Therefore, the absence of sharp corners increases fatigue life.
Step 3: Why This is Correct:
Options (A) and (B) both describe conditions that promote easier crack initiation and/or faster crack growth, thereby reducing the number of cycles to failure. Options (C) and (D) describe methods or design principles used to improve fatigue resistance.