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.

Numerical simulation and experimental verification of fatigue crack propagation in high-strength bolts based on fracture mechanics

“…For engineering analysis, the initial flaw size of the objective studied in this article is generally considered to be 0.5 mm, 12,41 that is, a = c = 0.5 mm. After the initial micro-crack was introduced at the maximum stress position, the mesh around the crack was refined by the FRANC3D mesh algorithm, which can not only make the crack tip transition smoothly to the unmodified grid but also maintain good accuracy near the crack, as shown in Figure 9.…”

“…Based on the M-integral, a more representative stress intensity factor (SIF) solution is provided by FRANC3D to assess crack propagation life, crack propagation path, fracture, and fatigue life, which has been proven very successful in many respects. 12–15…”

“…Based on the M-integral, a more representative stress intensity factor (SIF) solution is provided by FRANC3D to assess crack propagation life, crack propagation path, fracture, and fatigue life, which has been proven very successful in many respects. [12][13][14][15] The fatigue fracture process of high-strength bolts is generally separated into three stages, namely, crack initiation at the bottom of the thread, crack stable propagation along the root of the thread, and rapid fracture stage. The first two stages can basically be considered as the entire bolt life cycle.…”