Subject: Machine Design -I

Topic: Basic properties of Machine design and theories of failure

Difficulty: Medium

• A schematic diagram of a rotating beam fatigue testing machine is shown in Fig.
• The specimen acts as a ‘rotating beam’ subjected to a bending moment. Therefore, it is subjected to a completely reversed stress cycle.
• The specimen is rotated by an electric motor.
• The number of revolutions before the appearance of the first fatigue crack is recorded on a revolution counter.
• In each test, two readings are taken, viz., stress amplitude (Sf) and number of stress cycles (N).
• These readings are used as two coordinates for plotting a point on the S–N diagram. This point is called failure point.

• The results of these tests are plotted by means of an S–N curve.
• The S–N curve is the graphical representation of stress amplitude (Sf ) versus the number of stress cycles (N) before the fatigue failure on a log-log graph paper.
• The S–N curve for steels is illustrated in Fig. Each test on the fatigue testing machine gives one failure point on the S–N diagram. The S–N diagram is also called Wohler diagram