The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, SPONSORING/MONITORING AGENCY ACRONYM(S)AFRL/MLLMN SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)Materials and DISTRIBUTION/AVAILABILITY STATEMENTApproved for public release; distribution is unlimited. SUPPLEMENTARY NOTESThis work has been submitted to Elsevier for publication in the International Journal of Fatigue. One of the authors is a U.S. Government employee. If published, Elsevier may assert copyright. If so, the United States has for itself and others acting on its behalf an unlimited, nonexclusive, irrevocable, paid-up royalty-free worldwide license to use, modify, reproduce, release, perform, display or disclose the work by or on behalf of the Government. ABSTRACT (Maximum 200 WordsThe fretting fatigue performance of Ti-6Al-4V after isothermal exposure was explored in test coupons in low plasticity burnished (LPB), shot peened (SP), and electropolished (ELP) baseline conditions. In the current study, fretting fatigue data and fractography are presented along with in-depth residual stress profiles, both before and after the isothermal exposure. Surface roughness data for each of the three surface conditions are reported. For the studied fretting configuration, it was found that both the shot peening and LPB process improved the fretting fatigue performance over the baseline electropolished condition. In the LPD'd case the fretting damage was largely ameliorated by the burnishing process. The fretting fatigue performance of Ti-6A1-4V after isothermal exposure was explored in test coupons in low plasticity burnished (LPB), shot pcened (SP) and electropolished (ELP) baseline conditions. In the current study, fretting fatigue data and fractography are presented along with in-depth residual stress profiles, both before and after the isothermal exposure. Surface roughness data for each of the three surface conditions are reported. SUBJECT TERMSFor the studied fretting configuration, it was found that both the shot peening and LPB process improved the fretting fatigue performance over the baseline electropolished condition. In the LPB'd case the fretting damage was largely ameliorated by the burnishing process.
Low plasticity burnishing (LPB) is now established as a surface enhancement technology capable of introducing through-thickness compressive residual stresses in the edges of gas turbine engine blades and vanes to mitigate foreign object damage (FOD). The “fatigue design diagram” (FDD) method has been described and demonstrated to determine the depth and magnitude of compression required to achieve the optimum high cycle fatigue strength, and to mitigate a given depth of damage characterized by the fatigue stress concentration factor, kf. LPB surface treatment technology and the FDD method have been combined to successfully mitigate a wide variety of surface damage ranging from FOD to corrosion pits in titanium and steel gas turbine engine compressor and fan components. LPB mitigation of fretting-induced damage in Ti–6Al–4V in laboratory samples has now been extended to fan and compressor components. LPB tooling technology recently developed to allow the processing of the pressure faces of fan and compressor blade dovetails and mating disk slots is described. Fretting-induced microcracks that form at the pressure face edge of bedding on both the blade dovetail and the dovetail disk slots in Ti-6-4 compressor components can now be arrested by the introduction of deep stable compression in conventional computer numerical control (CNC) machine tools during manufacture or overhaul. The compressive residual stress field design method employing the FDD approach developed at Lambda Technologies is described in application to mitigate fretting damage. The depth and magnitude of compression and the fatigue and damage tolerance achieved are presented. It was found that microcracks as deep as 0.030in.(0.75mm) large enough to be readily detected by current nondestructive inspection (NDI) technology can be fully arrested by LPB. The depth of compression achieved could allow NDI screening followed by LPB processing of critical components to reliably restore fatigue performance and extend component life.
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 1 st 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.
High cycle fatigue (HCF) strength and the resistance to foreign object damage (FOD) can be improved by the use of mechanical surface treatments like shot peening and low plasticity burnishing (LPB) to introduce beneficial surface layers of compressive residual stress. In this paper, results from an extensive study of the relative effects of these two surface treatments on the residual stress, cold work distributions, HCF performance, and FOD tolerance of alloy Ti-6Al-2Sn-4Zr-6Mo (Ti-6-2-4-6) are presented. The compressive layer produced by LPB is shown to be stable even after thermal exposure to 371C. Blade-edge bending fatigue specimens were designed to simulate the leading edge of an integral bladed rotor (IBR) compressor blade. FOD was simulated by controlled size notches introduced on the specimens using electrical discharge machining (EDM). Both disk and blade simulation specimens with 0.5 mm (0.020 in) deep FOD had HCF strengths after LPB over 4-times higher than 8A shot peening. The HCF performance after LPB was relatively unaffected by FOD up to 0.75 mm (0.030 in) deep. FOD up to 2.5 mm (0.10 in) in depth after LPB decreased the fatigue strength only nominally. If the traditional design criterion of Kt (notch sensitivity factor) of 3 were to be used, LPB effectively mitigated FOD damage up to 2.5 mm (0.10 in) deep.
The benefits of applying low plasticity burnishing (LPB) to 17-4PH Stainless Steel (H1100) on both the fatigue and corrosion fatigue performance were compared with the shot peened (SP) and low stress ground (LSG) conditions. LPB treatment dramatically improved both the high cycle fatigue (HCF) performance and fatigue strength. The baseline LSG and SP treatments showed similar fatigue strengths of about 150 ksi (~1035 MPa), while LPB treatment improved the fatigue strength by about 30%. Introduction of an EDM notch of a o = 0.010 in. (0.25 mm) and c o = 0.030 in. (0.75 mm) simulating a semi-elliptical surface foreign object damage (FOD), decreased the fatigue strength of both SP and LSG by nearly 80%, while LPB helped retain much of the fatigue strength at the levels comparable to baseline material without FOD. Corrosion fatigue strength (in the presence of active corrosion medium of 3.5% NaCl solution) of the LSG material showed a drop of nearly 33% from the baseline material without corrosion; LPB material showed corrosion fatigue strength nearly the same as the baseline material without corrosion. While the introduction of a simulated FOD on the LSG dramatically decreased the fatigue strength to less than 15 ksi (~100 MPa), LPB retained nearly 90% of the fatigue strength of the baseline material without corrosion. Mechanistically, the effect of corrosion and FOD resulted in early crack initiation and growth, thus resulting in a dramatic decrease in fatigue performance. Despite the existence of s imilar corrosion conditions, the deep compressive surface residual stresses from LPB treatment helped to mitigate the individual and synergistic effects of corrosion fatigue and FOD.
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