Hot Section Turbine Disk Lifing Philosophies

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The life prediction of critical gas turbine engine components is carried out using a bank of laboratory test data which is also verified by a comprehensive program on full scale fatigue tests. These tests include carcass for containment, canopy for foreign body ingestion, disks for bolt hole and dovetail/firtree root cracks and blades for thermal shock and fatigue.

The life of the majority of these components is limited by low cycle fatigue which eventually is based on ‘initiation’ concepts and first detection of a crack 0.8 mm deep. Safe life design requires a high confidence of 99.9% probability of crack free components. Using safe life design concepts there is a large number of prematurely retired disks from service which have a high potential of useful life left in them.

Damage tolerance or fracture mechanics based lifing concepts attempt to utilize this ‘useful’ life by analyzing the cracks. However, these concepts depend on the accuracy of detecting cracks by Non-Destructive Inspection (NDI) techniques. The probability of detection curves do not follow a fixed statistical distribution to give sufficient confidence level to fix the next inspection interval. Probabilistic and deterministic fracture mechanics based lifing philosophies are overviewed.