N. Jayaraman

INTRODUCTION OF RESIDUAL STRESSES TO ENHANCE FATIGUE PERFORMANCE IN THE INITIAL DESIGN

High cycle fatigue (HCF) performance of turbine engine components has been known for decades to benefit from compressive surface residual stresses introduced through shot peening. However, credit for the fatigue benefits of shot peening has not been taken into account in the design of components. Rather shot peening has been used primarily to safe guard against HCF damage initiation. Recently, laser shock processing (LSP) and low plasticity burnishing (LPB) have been shown to provide spectacular fatigue and damage tolerance improvement by introducing deep (through-thickness) compression in critical areas. Until now, the fatigue benefits of these new surface treatments have been introduced during repair to improve an existing design. The present paper describes a design methodology and testing protocol * to take appropriate credit for the introduction of beneficial residual stresses into a component design to achieve optimal fatigue performance. A detailed design protocol has been developed that relates the introduction of a residual stress distribution using LPB for targeted HCF performance. This design protocol is applied to feature specimens designed to simulate the fatigue conditions at the trailing edge of a 1 st stage low pressure compressor vane to provide optimal trailing edge damage tolerance. The use of finite element modeling, linear elastic fracture mechanics, and x-ray diffraction documentation of the residual stress field to develop LPB processing parameters is described. A novel adaptation of the traditional Haigh diagram to estimate the compressive residual stress magnitude and distribution to achieve optimal fatigue performance is described. Fatigue results on vane-edge feature samples are compared with analytical predictions provided by the design methodology. The potential for designing reduced section thickness of structural components leading to weight savings is discussed.

Controlled Plasticity Burnishing to Improve the Performance of Friction Stir Processed Ni-Al Bronze

Friction stir welding (FSW) allows the joining of aluminum alloys in ways previously unattainable offering new manufacturing technology. Friction stir processing (FSP) of cast alloys such as Ni-Al bronze eliminates casting voids and improves the properties to that of wrought material. However, the local heating produced by both FSW and FSP can leave a fusion zone with reduced mechanical properties and a heat-affected zone with tensile residual stresses that can be deleterious to fatigue performance. Controlled plasticity burnishing (CPB) is an established surface treatment technology that has been investigated and described extensively for the improvement of damage tolerance, corrosion fatigue, and stress corrosion cracking performance in a variety of alloys. Mechanical CPB processing in conventional CNC machine tools or with robotic tool positioning is readily adapted to industrial FSW and FSP fabrication of components, either simultaneously or as a post process. CPB was applied to FSP Ni-Al Bronze to produce a depth of compression of 2.5 mm and a maximum subsurface magnitude of –150 ksi. The effect of FSP on the fatigue performance in a saltwater marine environment and in the presence of foreign object damage (FOD) was documented with and without CPB processing. FSP was found to increase the fatigue strength of the Ni-Al Bronze by 70% without affecting the corrosion behavior of neutral salt solution. FSW actually produced a more noble material in the acidic salt solution. CPB after FSP mitigated damage 1 mm deep.

Improved High Cycle Fatigue Damage Tolerance of Turbine-Engine Compressor Components by Low Plasticity Burnishing

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 solution approach to solve damage problems in engine components. An example of the total solution approach to develop LPB processing of a first stage Ti-6Al-4V compressor vane to improve the foreign object damage 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 computer numerical control (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.

HCF PERFORMANCE AND FOD TOLERANCE IMPROVEMENT IN Ti-6Al-4V VANES WITH LPB TREATMENT

Mechanical surface treatments that introduce a layer of residual surface compression improve high cycle fatigue (HCF) performance. If the depth of compression extends through the thickness of blade or vane edges, foreign object damage (FOD) tolerance can be dramatically improved. The effect of low plasticity burnishing (LPB) on the HCF performance and FOD tolerance of a first stage Ti6Al-4V turbine engine vane have been investigated in both tension-tension (R=0.1) and fully revered bending (R=-1). Actual vanes from fielded engines and blade-edge feature samples were fatigue tested with FOD simulated by EDM notches. The fatigue strength for LPB processed blades increased over 4-fold for both vanes and vane-edge feature specimens with FOD 0.020 in. deep, and was undiminished by 0.030 in. deep FOD. Assuming a Kt = 3 HCF performance criteria, LPB provided tolerance of FOD up to 0.10 in. deep. The beneficial through-thickness compression was retained even for compressive loading in fully reversed bending. The fatigue and FOD tolerance improvement are shown by linear elastic fracture mechanics modeling to be due to the deep stable compressive layer produced by LPB.

Improved High Cycle Fatigue Damage Tolerance of Turbine Engine Compressor Components by Low Plasticity Burnishing (LPB)

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.© 2006 ASME