INVESTIGATIONS BY NONDESTRUCTIVE INSPECTION TO DETERMINE THE SIZE OF NATURAL DEFECTS IN LARGE FORGINGS OF TURBOGENERATORS

For more than 10 years German turbine manufacturers and forgemasters h ave have been studying the fracture mechanics behaviour of natural fla ws in turbine rotors. One approach of these studies consists of determ ining the size of flaws by conventional and computer-assisted ultrason ic inspection methods. The tests are being conducted on a total of 22 representative reject forgings which feature individual as well as res olvable and non-resolvable group indications. The fracture mechanics b ehaviour of the flaws is being studied on large notchless specimens un der conditions of quasi-operational pulsating tensile stresses up to r oughly 25,000 load cycles. Test results show that for a ''worst case a nalysis'' under a fracture mechanics acceptance check of small indicat ions, the size of the flat-bottom hole determined by the DGS method ca n be assumed to be the maximum possible flaw width. Larger indications exceeding twice the wave length of the test frequency must be adequat ely defined by determining the half-value width of the echodynamics cu rve. Computer-assisted mechanised ultrasonic inspection methods have p roved to be suitable for detecting large individual indications and fl aw regions with group indications.