Question

1. (A) Describe how fatigue damage happens in a ductile material. Draw sketches if needed. (B) If we maintain a constant stress σ = 0.9σy (σy is the yield strength) on a steel, will fatigue damage happen? Why?

Answer 1

A)

Fatigue can cause failure in a structure at a load less than the world strength of the material under cyclic or repeated loading.

This can occur in steel Bridges where they are applied with cyclic loading of vehicles which induce stresses lower than the yield stress of the material but of large number of cycles.

Applied stress which may cause fatigue due to cyclic loading may be axial tension or compression, flexural or torsional stresses.

Fatigue failure in ductile materials is brittle in nature. Thus it is sudden and catastrophic.

Fatigue failure occurs in three stages

1. Crack initiation

2. Crack propogation

3. Failure

Endurance limit: It is the maximum stress at which the material never fails even after infinite number of cyclic loading.

B)

Generally the endurance limit of steel is taken as half of the ultimate stress in steel.

0.9Fy if greater than the endurance limit of HYSD steel. So the material will fail at the given cyclic stress under fatigue.