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.
Low Plasticity Burnishing (LPB) dramatically improves the damage tolerance of titanium alloy blades, mitigating blade-disk dovetail fretting and blade edge damage in gas turbines. LPB surface treatment of martensitic stainless steels Alloy 450 and 17-4PH subject to corrosion fatigue and pitting in the low-pressure sections of stream turbines has now been investigated. Condensation in the low-pressure steam turbine environment supports corrosion pitting and corrosion fatigue in martensitic stainless steels, primary failure mechanisms driving steam turbine repair and operational expense. Chloride corrosion fatigue results with and without high kf surface damage are compared for LPB, shot peened, and machined 17-4PH; and for ground and LPB treated Alloy 450. The depth and magnitude of compression achieved by the surface treatments are documented. LPB increased the undamaged fatigue strength of 17-4PH by 30% in neutral salt solution, and of Alloy 450 in acidic salt by 50%. In both alloys LPB mitigated damage to the 1 mm depth of compression. The cyclic stress corrosion component of corrosion fatigue was eliminated by the deep LPB compression, effectively restoring the endurance limit lost in active corrosion fatigue in both alloys.
Significant progress has been made in the application of low plasticity burnishing (LPB) technology to military engine components, leading to orders of magnitude improvement in damage tolerance. Improved damage tolerance can facilitate inspection, reduce inspection frequency, and improve engine operating margins, all leading to improved military readiness at significantly reduced total costs. Basic understanding of the effects of the different LPB process parameters has evolved, and finite element based compressive residual stress distribution design methodologies have been developed. By incorporating accurate measurement of residual stresses to verify and validate processing, this combined technology leads to a total solutions approach to solve damage problems in engine components. An example of the total solution approach to develop LPB processing of a 1st stage Ti-6Al-4V compressor vane to improve the foreign object damage (FOD) tolerance from 0.002 in. to 0.025 in. is presented. The LPB process, tooling, and control systems are described, including recent developments in real-time process monitoring for quality control. Performed on CNC machine tools, LPB processing is easily adapted to overhaul and manufacturing shop operations with quality assurance procedures meeting military and industry standards, facilitating transition to military depots and manufacturing facilities.
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