Evaluation of Multiple Crack Propagation Behavior in a Gas Turbine Blade Under Thermal Fatigue Condition

Frequent start-stop operation of a gas turbine sometimes causes coating cracks on the surface of the first stage blades. Since the coating cracks tend to propagate into the substrate, accurate prediction of crack propagation behavior is important to make a reasonable judgment for the repair or replacement of blades. This paper describes the development of a crack propagation prediction methodology for first stage blades, which includes evaluation of the stress intensity factor K, and estimation of temperature distribution and stress distribution between the outer surface and the inner cooling hole of blades. Since the coating cracks of blades reveal themselves as multiple parallel cracks with narrow intervals, a primary concern was directed to establish evaluation method of K values of multiple parallel cracks. For this purpose, a series of finite element analyses were performed for multiple edge cracked plate with finite width subjected to a crack face pressure expressed as σ=σ_n(x/W)ⁿ, where x is a distance from the mouth of the crack and σ_n is the stress at plate surface (x=W). The result was used to make approximate equations for the influence coefficient G_n, defined as K=G_nσ_n(a/W)ⁿ(πa)^1/2, where a is a crack length. Temperature distribution was evaluated by observation of microstructure (gamma prime phase) change of a first stage blade used in a power plant for more than 20,000 hrs. Thermal stress distribution was calculated by finite element analysis using the evaluated temperature distribution. These results and the approximate equation of G_n were used to evaluate the crack propagation behavior of the blade surface, together with the fatigue crack propagation data of blade material that had been obtained in other studies.