Resonant-vibration fatigue testing

Abstract

Techniques are described for the fatigue testing of components, such as turbine buckets, used in large steam turbines and generators. Fatigue of these large structures, which have relatively high unit damping, is obtained through the use of resonant-vibration systems. An electromagnetic drive serves as the source of excitation. The drive system is designed to produce the maximum force from a given-size magnet. Control of the amplitudes of vibration, for very long periods, is obtained through the use of an automatic amplitude control, the stability of which maintains the amplitude with a variation of less than 2 percent.

Three specific fatigue-testing techniques described are:

• 1. The fatigue of complete weld sections for large numbers of cycles (500 million), which required dynamic bending moments of greater than 6000 ft-lb to produce failure.

• 2. An apparatus for the fatigue testing of components, such as full-size turbine-bucket dovetails, under combined steady pull and dynamic bending. The tensile load simulates the centrifugal force on the component, and the dynamic bending subjects the component to bending stresses similar to those it may experience if vibration occurs during operation.

• 3. The fatigue of short, stiff turbine buckets in their high vibration modes.