Investigations by non-destructive inspection to determine the size of natural defects in large forgings of turbogenerators

Abstract

For more than 10 years German turbine manufacturers and forgemasters have have been studying the fracture mechanics behaviour of natural flaws in turbine rotors. One approach of these studies consists of determining the size of flaws by conventional and computer-assisted ultrasonic inspection methods. The tests are being conducted on a total of 22 representative reject forgings which feature individual as well as resolvable and non-resolvable group indications. The fracture mechanics behaviour of the flaws is being studied on large notchless specimens under conditions of quasi-operational pulsating tensile stresses up to roughly 25,000 load cycles. Test results show that for a “worst case analysis” under a fracture mechanics acceptance check of small indications, the size of the flat-bottom hole determined by the DGS method can be assumed to be the maximum possible flaw width. Larger indications exceeding twice the wave length of the test frequency must be adequately defined by determining the half-value width of the echodynamics curve. Computer-assisted mechanised ultrasonic inspection methods have proved to be suitable for detecting large individual indications and flaw regions with group indications.