Measurement of Fatigue Crack Propagation by Eddy Current Method

Abstract

The eddy current method was applied to the detection of fatigue crack propagation of notched specimens under rotating bending. The practical device to apply the eddy current method and the test results are described. The depth of fatigue cracks can be measured continuously by the eddy current method, and the accurate value of crack depth is obtained without breaking the test pieces. The method can be applied to ferromagnetic substance as well as to paramagnetic substance, though the detecting probe must be varied so as to suit the materials. A linear relation is obtained between the crack depth and the response of the eddy current circuit irrespective of the environment in which fatigue tests are carried out. The circumferential crack propagation can also be measured by rotating the test piece once or by reciprocating the detecting probe over the notch at several circumferential positions, and the latter method shows better agreement with the crack depth measured by temper coloring. The sensibility of the crack detection is 0.1∼0.05mm. The maximum detectable crack depth is more than 2.5mm in the case of carbon steel (ferromagnetic substance) and is more than 4.0mm in the case of Al alloy (paramagnetic substance).