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R0 compressor blade cracking has curtailed the lives of many F-class gas turbine rotors. The root cause of R0 blade failure has been identified as high cycle fatigue cracking and liberation due to erosion from fogging and compressor washing. R0 compressor blade failure has been the life limiting factor of rotors whose designed lives are 144,000 hours and 5,000 starts, according to engineers at Lambda Technologies Group Surface Enhancement Technologies. Erosion related damage tolerance is limited to 0.010 inch on the leading edge, requiring frequent inspection, blending, and costly downtime
Kyle Brandenburg, N Jayaraman, Douglas Hornbach and Michael Prevéy of Lambda Technologies presented a paper on reducing gas turbine operational costs with engineered residual compression at Power Gen International 2019.
Various corrective actions have been attempted to mitigate R0 blade failure. Among them have been changing foggig and compressor washing methods and frequency, more frequent inspections to detect erosion damage, blending of leading edge required for 0.008 inch erosion damage with a total blend limit of 0.040 inches, blade design changes (P-cut, thickening the LE, etc.), material change (from GTD 450 to Inconel 718), as well as laser peening.
The authors made a case for using Low Plasticity Burnishing to introduce designed compressive residual stresses that would safeguard the blade. Through-thickness compressive residual stress on airfoil blade edges completely mitigates cracking from preexisting damage, the authors say.
LPB is a mechanical process that imparts a designed residual stress field into the surface of components to produce a deep, stable layer of beneficial residual compression. The depth, magnitude and distribution of the compression are designed specifically for each component and application, producing minimal cold work. LPB can sculpt designed levels of compression onto a component surface ranging from a few thousandths of an inch (comparable to shot peening) to over a full centimeter. Through-thickness compression on thin areas, like blade edges, is also possible with LPB.
Leading edge damage induced fatigue cracking can be completely mitigated with -100 ksi compressive residual stress, the authors conclude.
Untreated R0 blades showed compressive residual stress to a shallow chordwise depth of 0.01 inch. Deep compression exceeding -100 ksi was achieved at depths >0.025 inch in research at Lambda Technologies.
LPB Treated R0 Blade -100 ksi compression achieved at the mid-thickness location would completely mitigate fatigue cracking with erosion damage of 0.025 inch. Better than -70 ksi compression achieved would still lead to significantly improved damage tolerance and life improvement with erosion damage of 0.050 inch
Damage tolerance improved achieved via LPB introduced through-thickness LE compression of -100 ksi to a depth of 0.025 inches was:
>15 million cycle life for damage <0.025 inch
>3X improvement over current baseline 0.008 inch limit