LCF Initiated - HCF Propagated Crack Life Estimation of Gas Turbine Bolts

Abstract

This paper discusses the methodology to calculate high cycle fatigue (HCF) crack propagation life of gas turbine bolts and compares two dimensional (2D) HCF crack propagation life to three dimensional (3D) HCF crack propagation life. Gas turbine bolts when exposed to fatigue loading are prone to crack initiation and propagation (structural failure) during operation. In such cases cracks mostly are initiated by low cycle fatigue (LCF) and propagated by HCF. Therefore in current illustration the authors have evaluated crack propagation primarily initiated by low cycle fatigue and propagated by high cycle fatigue. 2D and 3D fracture methodology approaches had been used for analytical evaluation. The authors conclude on the efficacy of both the methods based on the data from the field. The coupling joint bolts located in the engine mid-section, which are used to join compressor rotor with turbine rotor are being considered for crack evaluation studies. The coupling bolts located in mid-section are primarily loaded by high axial bolt pre-loads needed to keep the joint intact, as well as loaded in bending due to rotor gravity sag. The crack propagation life is evaluated and validated with field data using cracked bolt specimen from the field.