Fracture strength of ceramic nozzle blades under thermal shock conditions

The fracture strength of ceramic turbine nozzle blades is evaluated under instantaneous thermal shock working conditions by a newly developed high-temperature thermal cycle cascade tunnel. The facility was designed to be produced an unsteady thermal stress more than 400MPa for ceramic blades and to confirm the fracture strength. The tunnel operates at a constant speed, with heated gas and ambient temperature air, and has the capability to perform thermal fatigue tests. The objective models are SiC and Si3N4 blades with a chord length of 33.6 mm and an aspect ratio of 1.05. The heating and cooling conditions of the test blade were confirmed with steady and transient measurements in the hot gas of 1,400 oC and ambient cooling air. The measurements include surface temperature distributions on the both sides of the blade. The thermal stress distribution of the test blades was predicted by a finite element method under experimental boundary conditions. Typical results of the transient analysis for the test blades are presented and it is shown that the maximum thermal stress occurs at the leading-edge region of the test blades. The two SiC test blades were fractured at a maximum stress range of 550MPa, but any Si3N4 blade was not fractured up to 630MPa. A typical fracture mode of ceramic turbine nozzle blades under instantaneous thermal shock conditions is observed and the fracture mode is presented.