Investigation of Preliminary Factor Leading to First-Third Stage Blade Cracks Initiation, Growth and Their Effects on Aero-Engine Performance

Abstract

The paper presents the preliminary temperature leading to first-third stage blade cracks initiation, growth and their effects on aero-engine performance. The temperatures were determined by Scan Electron Macroscopy installed with optical camera while mechanical loadings at specified temperatures were obtained by means of tensile testing machine. Ti-6Al-4V, AISI 403 cadmium stainless steel and INC 718 were the three blade materials investigated. 1st stage axial and single stage centrifugal compressors; 2nd& 3rd stage axial compressor; and single stage axial gas turbine blades were respectively considered for Ti-6Al-4V, AISI 403 stainless steel and INC 718. The ranges of temperature considered for Ti-6Al-4V, AISI 403 stainless steel and INC 718 were 450 – 550K , 250-500K and 1083-1235K respectively. Chemical compositions and microstructures of the materials were investigated. The observable heat treatment time for all the materials were from 1-8 h. 20 - 70 kN forces were applied on the three materials and their elongations were plotted against the corresponding forces. Results showed that their chemical compositions at room temperature were the same even after several heat treatments while their microstructures were at variance. Stability in the microstructures without cracks was attained at temperature range of 500 to 520K for 1h for Ti-6Al-4V, 450 -480K for 30-45 min for AISI 403 stainless steel and 1083 -1200K for 2 h for INC 718. Beyond these temperatures, crack initiations and growths were noticed for Ti-6Al-4V, AISI 403 and Ni-alloys. In the three blades’ alloy, 𝛾′, 𝛾′′and 𝛿 grain elements for each material showed that their 𝛾′,𝛾 ′′ were thermodynamically stable at room temperature. However, the cause of crack growths was due to changes in grains sizes and the rafting of the precipitates in the blade specimens. The results also indicated that for 2 mm cracks size on stainless steel alloys, the maximum allowable number of cycles for failure to occur is 106and for a crack size of 3 mm on Ti-6Al-4V alloy and 6.5 mm on Ni alloy blades, the maximum allowable number of cycles to failure is 108; beyond these allowable cycles, fracture or rupture becomes inevitable.