Use of Fracture Mechanics Methods for Establishing Inspection Level for Turbine Wheels

The establishment of inspection criteria for wheels in rotating machinery is frequently a complex issue. On one hand, safety and reliability of the parts and equipment must be assured while on the other, economical use must be made of available materials. In addition to these often opposing criteria, account must be taken of the limitations and cost of non-destructive evaluation methods. One way of considering all these factors and establishing safe inspection and acceptance criteria for wheels is to employ linear elastic fracture mechanics (LEFM) methods. The purpose of this paper is to outline a method for using LEFM to establish allowable stress levels or fracture toughness required in turbine wheel ferritic steels. The methods proposed are discussed in schematic fashion and it is shown that general application of a single “design curve” to many situations is possible if accuracy in specific cases is sacrificed. If this sacrifice is made so that results are conservative then the resulting plots can be used during the design of the wheel, the acceptance of a part or the evaluation of remaining life of an in-service wheel.