Subject: Machine Design -I

Topic: Basic properties of Machine design and theories of failure

Difficulty: Medium

Fluctuating Stresses:

• The components are subjected to forces, which are not static, but vary in magnitude with respect to time. The stresses induced due to such forces are called Fluctuating Stresses.
• The most popular model for stress–time relationship is the sine curve.
• There are three types of mathematical models for cyclic stresses:

(a) Fluctuating or alternating stresses,

(b) Repeated stresses and

(c) Reversed stresses.

• Stress–time relationships for these models are illustrated in Fig.

(a) Fluctuating or alternating stresses (Ref Fig a):

• The fluctuating or alternating stress varies in a sinusoidal manner with respect to time. It has some mean value as well as amplitude value.
• It fluctuates between two limits—maximum and minimum stress.
• The stress can be tensile or compressive or partly tensile and partly compressive.

(b) Repeated stresses (Ref Fig b):

• The repeated stress varies in a sinusoidal manner with respect to time, but the variation is from zero to some maximum value. The minimum stress is zero in this case and therefore, amplitude stress and mean stress are equal.

(c) Reversed stresses (Ref Fig c):

• The reversed stress varies in a sinusoidal manner with respect to time, but it has zero mean stress.