Case Studies of Fatigue Life Improvement Using Low Plasticity Burnishing in Gas Turbine Engine Applications

Abstract

Surface enhancement technologies such as shot peening, laser shock peening (LSP), and low plasticity burnishing (LPB) can provide substantial fatigue life improvement. However, to be effective, the compressive residual stresses that increase fatigue strength must be retained in service. For successful integration into turbine design, the process must be affordable and compatible with the manufacturing environment. LPB provides thermally stable compression of comparable magnitude and even greater depth than other methods, and can be performed in conventional machine shop environments on CNC machine tools. LPB provides a means to extend the fatigue lives of both new and legacy aircraft engines and ground-based turbines. Improving fatigue performance by introducing deep stable layers of compressive residual stress avoids the generally cost prohibitive alternative of modifying either material or design. The x-ray diffraction based background studies of thermal and mechanical stability of surface enhancement techniques are briefly reviewed, demonstrating the importance of minimizing cold work. The LPB process, tooling, and control systems are described. An overview of current research programs conducted for engine OEMs and the military to apply LPB to a variety of engine and aging aircraft components are presented. Fatigue performance and residual stress data developed to date for several case studies are presented including: • The effect of LPB on the fatigue performance of the nickel based super alloy IN718, showing the fatigue benefit of thermal stability at engine temperatures. • An order of magnitude improvement in damage tolerance of LPB processed Ti-6-4 fan blade leading edges. • Elimination of the fretting fatigue debit for Ti-6-4 with prior LPB. • Corrosion fatigue mitigation with LPB in Carpenter 450 steel. • Damage tolerance improvement in 17-4PH steel. Where appropriate, the performance of LPB is compared to conventional shot peening after exposure to engine operating temperatures.

Improving the Surface Finish of Concave and Convex Surfaces Using a Ball Burnishing Process

“…The ball is in contact with the surface just for burnishing it, but it can freely rotate on itself, because the values of friction forces are very small. As it happens in the cutting process, plastic deformation is produced on the entire surface because the tool is constantly impacting on the workpiece according to Roettger [2], Klocke and Liermann [3], Prevey et al [4], Nemat and Lyons [5], and Yen and Altan [6].…”

Efecto de los parámetros del proceso de bruñido por rodillo en el campo de esfuerzos residuales

“…Debido a que los máximos esfuerzos residuales compresivos están ligados con el aumento en la dureza sub-superficial, resistencia a la corrosión bajo tensión, resistencia a la fatiga y resistencia a la fluencia, entre otras propiedades [15], [16], [26], [27], es importante determinar cuál es el efecto de los parámetros de bruñido de entrada sobre el campo de esfuerzos residuales, el máximo esfuerzo residual compresivo y la profundidad de éste. El máximo esfuerzo residual compresivo y su profundidad como función de la fuerza, el avance y el número de pases, son analizados en las figuras 8 y 9.…”

An Empirical Model of Low Plasticity Burnishing to Improve Surface Roughness and Hardness

“…The values of exponents thus obtained for these groups are presented in table 6. On substitution of these exponents in equation (6) and subsequent transformation, the model for surface roughness can be written as, Ra = A (D 0.101 Rai 0.899 ) (7) The constant A was evaluated by substituting values of variables and was found equal to 0.3806.…”

“…Fatigue life of AL 7075-T6 is at least 100 times greater for LPB caused due to reduction in corrosion mechanism [5]. The investigation for IN718, Ti-6Al-4V, 17-4Ph steel have been carried out to find out the relation between percentage of cold work, depth of compressive layers and residual stresses [6]. LPB (for Ti-6Al-4V) is used to improve high cycle fatigue damage tolerance of turbine engine compressor components [7].…”