Exploratory Research on a Method for Detecting Shaft Radial Cracks: Severity, Location, and Feasibility

Crack failure is among the most dreaded failures experienced in rotating machinery. It is therefore important to be able to detect a crack before failure occurs and cost effective to know the location and severity of a crack making it possible to predict the behavior and life of the machinery. This dissertation outlines a method of crack detection using the elastic wave created by the snapping shut of a radial crack to determine these characteristics. To determine the feasibility of such a method, preliminary research is performed by examining the behavior of a crack in 4-point-bending. A theoretical solution for the elastic wave behavior is determined by modeling the behavior as the collinear impact between two shafts. A theoretical impact velocity is found using finite element modeling to examine crack geometry. A pendulum experiment is performed in order to examine the validity of the assumed theoretical acceleration at the shaft end. The experimental acceleration response is smaller than the theory because the volume of air caught between the shaft face and the wall has to be expelled. This is explained by Reynolds lubrication equation which proves this hypothesis. An experiment to test the 4-point-bending theory is presented. More work is needed to determine the feasibility of such a crack detection method, such as running a 4-point-bending experiment as the design for which is presented.