On-line monitoring of crack propagation

A method has been developed to monitor crack growth within chemical process plant. The technique depends on detecting changes in the strain field on the accessible outer metal surface opposite the growing crack. The strain distribution has been measured using an array of resistance strain gauges and the data have been logged by a microcomputer. In the plane of the crack a 20% change in the surface strain resulted from 40% through-thickness penetration by a semi-elliptical crack. The problem of temperature and pressure fluctuations may be overcome by ‘normalizing’ all gauge readings. The relationship between strain and crack length must be known for each structure geometry encountered. Photoelastic analysis may be used to confirm the strain distribution.