Lambda Technologies Application Notes

Aircraft Propulsion Application Notes

Engine Components: Low Plasticity Burnishing (LPB™) improves foreign object damage (FOD) tolerance and high cycle fatigue endurance limits while completely mitigating cracking along the trailing edge of the Ti-6Al-4V Alloy F402 First Stage Low Pressure Compressor (LPC1) Vane used in the U.S. Marine Corps V/STOL tactical strike aircraft.

(Excerpt) Lambda Technologies’ development of the LPB™ procedure for the Ti-6Al-4V F402 1st stage low pressure compressor (LPC1) vane was initiated through a Phase II SBIR Contract with NAVAIR, the Naval Air Systems Command. The purpose of this program was to substantially extend the life of the F402 LPC1 engine vane as a result of improving FOD tolerance and increasing fatigue strength by imparting through-thickness residual compression along the vane’s trailing edge.

The F402-RR-408 engine employs Ti-6Al-4V 1st stage low pressure compressor (LPC1) vanes which are subject to both high stresses and FOD generated by airborne debris during takeoff. Inspection and maintenance costs adversely impact flight readiness and significantly increase the total cost of ownership and operation of the F402 engine. Applying the LPB process to the F402 LPC1 vane improves FOD tolerance of damage of up to 0.100 in. deep and increases fatigue strength by a factor of 5 with complete mitigation of fatigue failure when tested at the design stresses. LPB™ offers a significant reduction in the cost of aircraft ownership and improved fleet readiness. (See the Ti-6Al-4V Alloy F402 Engine First Stage Low Pressure Compressor (LPC1) Vane Application Note)

Engine Components: Low Plasticity Burnishing (LPB™) mitigates pitting, diminishes foreign object damage (FOD), and improves damage tolerance and high cycle fatigue (HCF) life while reducing the replacement costs of the 17-4 PH Stainless Steel First Stage Compressor Blade in the T56 Turboprop Engine.

(Excerpt) Lambda Technologies’ development of the LPB™ procedure for the 17-4 PH Stainless Steel 1st Stage Compressor Blade was initiated through a Phase II SBIR Contract with NAVAIR, the Naval Air Systems Command. The purpose of this LPB™ program was to substantially improve the life of a T56 engine blade as a result of improving FOD tolerance and fatigue strength by imparting through-thickness residual compression along the blade’s leading edge. LPB™ treated blades have a deep compressive layer, which resists crack initiation.

High cycle fatigue (HCF), corrosion fatigue, and general corrosion tests were performed on LPB™, SP and low stress ground (LSG) 17-4 PH specimens. The effects of these surface treatments on the HCF strength, damage tolerance, and salt water corrosion fatigue behavior were studied using both thick section and blade-edge feature specimens, as well as actual retired T56 1st Stage Blades. The results demonstrated that LPB™ dramatically improved the HCF and corrosion fatigue performance of 17-4 PH by producing compressive residual stresses to a depth of 0.040 in. with low associated cold work. T56 blades treated in this manner were able to withstand 0.050 in. FOD along the blade edge with significantly less detriment to fatigue life when compared to baseline or shot peened specimens. (See the 17-4 PH Stainless Steel T56 Engine 1st Stage Compressor Blade Application Note)

Engine Components: Improved Ti-6Al-4V Fan Blade Foreign Object Damage (FOD) Tolerance

(Excerpt) Aircraft engines are subject to high cycle fatigue (HCF) that can lead to engine failures. Further, foreign object damage (FOD) caused by the ingestion of debris into the engine, creates crack initiation sites that exacerbate the effects of HCF. At a minimum, this damage can lead to increased inspections and additional maintenance. At worst, the combined effect of HCF and FOD may cause catastrophic engine failures.

In a recent U.S. Navy sponsored cooperative study, low plasticity burnishing (LPB) was found to greatly enhance FOD tolerance in the first stage Ti-6Al-4V fan blades used in the F404 engine that powers the F18 Fighter. The LPB™ process resulted in deeper compression when compared to shot peening, and provided comparable compression to laser shock peening (LSP) at a much lower cost.

The LPB™ process was utilized to create through-thickness compression of maximum magnitude along the lower half of the blade’s leading edge to a distance of 0.25 in. back from the edge of the blade as indicated in the photograph to the right. Initial FOD tolerance was documented by developing fatigue S-N curves using actual blades both with and without LPB™ protection. Fatigue strength was compared to a baseline S-N curve developed for as-received blades without artificial FOD. LPB™ processed as-received blades produced fatigue strengths 30 ksi higher than non-processed blades. This increase in the fatigue strength for LPB™ processed blades approaches the yield strength of the material. (See the Improved Ti-6Al-4V Fan Blade Foreign Object Damage (FOD) Tolerance Application Note)

Engine Components: Inconel 718 Low Cycle Fatigue

(Excerpt) The threat of low cycle fatigue (LCF) failure greatly increases the cost of inspection, maintenance, and operation of turbine engines. Surface enhancement, or the introduction of a layer of compressive residual stress, is one of the few practical and affordable ways of improving the LCF performance of engine components. Low plasticity burnishing (LPB™) has demonstrated spectacular improvements in LCF for Inconel 718 at a turbine operating temperature of 1000°F through the introduction of deep compressive residual stresses.

LPB™ has been demonstrated to produce compression of far greater depth and magnitude than shot peening. Additionally, LPB™ can be applied using conventional CNC machine tools in a manufacturing environment. LPB™ can produce compression exceeding 0.040 in. in turbine engine alloys such as IN718. In recent studies supported by NASA, variations in crack shape (aspect ratio, a/c) resulting from processing-induced residual stress distributions in IN718 were observed post-test via the heat-tinting method. Two examples of heat-tinted crack surfaces, photographed post-test, are presented. The depth of compression for LPB™ far exceeds that achievable by shot peening, and the amount of cold work can be lower than shot peening and laser shocking. Controlled low cold working provides stability of compressive stresses at elevated temperature and in the event of mechanical overload as may occur during operating conditions for turbine components.

LPB™ can be applied with CNC machine tool controlled single point tools to disk bores, web regions, and with special tooling to dovetail slots. Caliper tooling provides processing of both sides of thin blade sections simultaneously. CNC controlled caliper tooling has been applied successfully to fan and compressor blades for both aircraft engine compressors and ground based power generation turbine blades. (See the Inconel 718 Low Cycle Fatigue Application Note)

Engine Components: Mitigation of Fretting Fatigue

(Excerpt) Often, turbine engine components are retired from service before “full life” is reached. Turbine disks are a typical example. One of the most common reasons for turbine disk retirement is accumulated fretting damage in the dovetail slots of the disks. Fretting damage on such components is often difficult to characterize and analyze, but is usually the result of movement between two metallic surfaces in contact with each other. As such, prudence often dictates that the components be removed from service before they reach their potential design life. Due to the long lead times and the high costs associated with replacing this hardware, it would be desirable to have proven solutions to avoid, minimize or repair fretting damage. One such solution would provide surface treatments to mitigate the effects of fretting damage by producing a layer of compressive residual stress that will be retained at high temperatures.

Low plasticity burnishing (LPB™) has been demonstrated to provide deep, controlled high compression that improves the fatigue life of fret damaged Ti-6Al-4V test specimens compared with shot peened specimens, even after thermal exposure. (See the Mitigation of Fretting Fatigue Application Note)

Engine Components: Turbine Engine Sustainment

(Excerpt) The threat of high cycle fatigue (HCF) failure greatly increases the cost of inspection, maintenance, and operation of legacy turbine engines. As these components continue to age, the probability of fatigue failure steadily increases. Surface enhancement, or the introduction of a layer of compressive residual stress, is one of the few practical and affordable ways of improving the HCF performance and damage tolerance of legacy engine components. While simple shot peening has long been considered an obvious option, spectacular improvements in damage tolerance have been demonstrated through the introduction of deep, stable, and even through-thickness compression by alternative surface enhancement technologies such as laser shock peening (LSP) and low plasticity burnishing (LPB™) in fan blades.

Low plasticity burnishing has been demonstrated to produce compression of comparable depth and magnitude to LSP. However, LPB™ is applied using conventional CNC machine tools in a manufacturing environment. LPB™ can produce compression exceeding 0.040 in. in turbine engine alloys such as Ti-6Al-4V and IN718, as shown in Figure 1. The depth of compression far exceeds that achievable by shot peening, and the amount of cold work can be lower than laser shocking. Controlled low cold working provides stability of compressive stresses at elevated temperature and in the event of mechanical overload as may occur during a foreign object damage (FOD) event. (See the Turbine Engine Sustainment Application Note)

Aircraft Structures Application Notes

Propeller Blades: Low Plasticity Burnishing (LPB™) mitigates stress corrosion cracking and improves corrosion fatigue strength while increasing the service life and reducing the total maintenance cost of the 7076-T6 Propeller for the U.S. Navy's maritime patrol aircraft.

(Excerpt) The P-3C is a land-based, long-range, anti-submarine warfare (ASW) patrol aircraft, which has effectively served the U.S. Navy since July of 1962. The veteran aircraft is set in motion by four Allison T56-A-14 Turboprop engines and 13½’ paddle-blade propellers. Evidence suggests that stress corrosion cracking (SCC) can occur on the propeller blade’s internal taper bore. Cracking caused by SCC can lead to fatigue failure and possible liberation of the propeller blade.

Currently, to suppress SCC in the tapered bore, shot peening is used to impart compressive residual stresses (RS) and prolong the short-term life of the bore. The relatively high intensity shot peening leaves a dimpled surface which requires a reaming of the tapered bore after the shot peening, to reduce the surface roughness and maximize contact between the bronze bushing and the tapered bore surface. Shot peening is also performed as a rework process at various stages of the propeller blades life that requires further reaming of the taper bore. Further machining of the end-face of the blade is also necessary to produce the proper interference fit of the bushing. The entire process is time consuming and labor intensive and therefore costly. Furthermore, the reaming and machining performed successively at required maintenance intervals ultimately reduces the service life of the blade as a result of metal removal from the taper bore and the blade end-face.

Lambda Technologies’ development of the LPB™ rework procedure for the 7076-T6 P-3 Propeller Taper Bore was initiated through a Phase III ID/IQ Contract with NAVAIR, the Naval Air Systems Command. The goal of the project was to produce compression similar to that produced by shot peening yet with a surface finish that would not require further reaming of the taper bore. Lambda Technologies tasks included determining the LPB™ processing requirements, developing the appropriate compressive residual stress (RS) distribution, performing corrosion and fatigue tests via “verification and validation” testing, designing and fabricating the tooling, LPB™ processing taper bores for component testing and, finally, preparing a production implementation plan. (See the 7076-T6 Aluminum Alloy Propeller Taper Bore Application Note)

Main Landing Gear: Surface treatment program improves the damage tolerance of a 300M steel main landing gear component by development of an engineered residual stress (RS) distribution to mitigate stress corrosion cracking (SCC) and fatigue failure through the use of Low Plasticity Burnishing (LPB™) in combination with conventional shot peening.

(Excerpt) Degradation processes such as foreign object damage (FOD), fretting, active corrosion fatigue, corrosion pitting, and stress corrosion cracking (SCC) are recognized to seriously affect military airlift cargo aircraft landing gear. In order to contend with the landing of a maximum payload, large military cargo aircraft are equipped with main landing gear composed of 300M steel, used extensively for landing gear due to its high strengthand fracture toughness. Unfortunately, like other high-strength steels, 300M is vulnerable to corrosion fatigue and stress corrosion cracking (SCC) which, if left unresolved, can lead to catastrophic consequences in an airlifter’s landing gear.

Researchers at the Air Force Research Laboratory (AFRL) investigated the use of LPB™ on 300M steel. AFRL's Materials and Manufacturing Directorate (AFRL/ML) compared the results to those obtained using conventional shot peening (SP), the current state of the art. The tests indicated that LPB™ imparts persistent compressive residual stresses in 300M steel surfaces that more effectively improved fatigue strength and damage tolerance and eliminated SCC. As a result of the AFRL’s investigation of the use of LPB™ on 300M steel, Lambda Technologies developed and qualified an engineered residual stress (RS) distribution to mitigate SCC and fatigue failure in a landing gear mechanism through the use of LPB™ in combination with conventional shot peening. High cycle fatigue tests conducted on baseline, shot peened, and LPB™ treated 300M samples overwhelmingly indicate that the deep surface compression from LPB™ dramatically improved fatigue strength. (See the Military Airlift Cargo Aircraft Main Landing Gear Application Note)

Aircraft Structures: Aging Aircraft

(Excerpt) Corrosion and corrosion related fatigue of structural components threatens the readiness of military aircraft and greatly increases the cost of maintenance of virtually all legacy aircraft. Annual costs exceeding one billion dollars are currently incurred for inspection and maintenance by the US Air Force alone, and will only escalate as aircraft continue to age. For some legacy aircraft, spare parts are at a premium, if available at all. Redesign of components or introduction of new alloys is generally prohibitive. Surface enhancement to restore fatigue strength is a practical, affordable solution for many critical components.

Low plasticity burnishing (LPB™) has been demonstrated to provide deep, controlled high compression with low cold working that dramatically improves the fatigue life of corroded structural components. Application of LPB™ to 7075-T6 aluminum can restore the fatigue strength lost to salt spray pitting corrosion. The fatigue data in the accompanying figures show that LPB™ can fully restore the endurance limit of 7075-T6 aluminum exposed to aggressive salt fog pitting for either 100 or 500 hours. Further, the depth of compression produced by LPB™ exceeds the depth of the corrosion pits, virtually eliminating the pits at fatigue nucleation sites and, thus, restoring the endurance limit to the unpitted condition. The deep compressive layer resists crack growth, providing an order of magnitude improvement in the fatigue life at stress levels above the high cycle fatigue (HCF) endurance limit. Comparable fatigue strength and life improvements are achieved in 4340 steel. (See the Aging Aircraft Application Note)

Aluminum and Titanium Alloys: Friction Stir Weld Finishing

(Excerpt) Friction stir welding (FSW) offers great promise as a means of joining dissimilar alloys and producing welds with superior metallurgical properties and strengths. Fasteners may be eliminated from many aerospace applications. Significant cost reductions and novel combinations of materials and designs may be achieved by FSW.

Although FSW is performed at temperatures well below the melting point of the alloys, complex thermal and plastic strain gradients are developed during the stirring process. Residual stress distributions develop, which vary with distance from the fusion line and depth through the weldment stir zone. Tensile residual stresses have been observed in both aluminum and titanium alloys in the stir zone. Graphs of the residual stress parallel to the weld line as a function of distance across the weld are shown for 7050-aluminum and Ti-6Al-4V in the accompanying figures. These local tensile regions can leave the otherwise high integrity weld susceptible to stress corrosion cracking (SCC), high cycle fatigue (HCF) failure, or a combination of corrosion fatigue.

Furthermore, the finish produced in FSW is often too rough for use in aerospace applications, requiring additional processing. The surface roughness is a function of the FSW tooling design and the welding parameters. Surface finish requirements can also limit the production rate and therefore cost of the FSW operation. A simple fly cutting operation to remove the excess flash followed by low plasticity burnishing (LPB™) can produce a surface which is both highly compressive to resist fatigue and SCC, and has a surface finish superior to the original plate, meeting all of the requirements for aerospace applications. (See the Friction Stir Weld Finishing Application Note)

Engineering Application Notes

Fatigue Design: A patented methodology*, the Fatigue Design Diagram (FDD) analysis provides a means of incorporating compressive residual stress distributions into the designs of metallic components necessary to achieve optimal fatigue performance and to mitigate typical damage conditions.
(*U.S. Patent No. 7,219,044; foreign patents pending.)

(Excerpt) During the design process of a metallic component, applied stresses are generally determined using finite element analysis (FEA) to ensure the part can withstand the applied loads either in yielding or fatigue. To reduce the applied stresses, designers can modify the geometry by adding material in critical regions or by using a material with more desirable properties. Utilizing the FDD method, the designer can reliably use compressive residual stresses (RS) to offset the applied stresses thereby improving performance. Incorporating compressive RS in metallic components has long been recognized to enhance fatigue strength. However, credit is seldom taken for RS in design because RS may not be stable or reproducible and a reliable method of including RS in design has not been available.

Lambda Technologies’ patented FDD was applied throughout a sequence of contracts with NAVAIR, the Naval Air Systems Command. A recent SBIR contract (Navy Topic N05-026, Design Tools for Fatigue Life Prediction in Surface Treated Aerospace Components) supports transition to commercial software providing a reliable method of designing beneficial, compressive RS magnitude in components for optimal fatigue performance.

Lambda Technologies’ FDD enables: (a) prediction of fatigue behavior in the presence of damage, (b) prediction of fatigue behavior in the presence of both damage and RS, and more importantly, (c) provides a design guideline to determine the compressive RS magnitude needed to achieve a target damage tolerance. As an additional benefit, the use of the FDD allows reduced use of excess material which is a common practice in traditional “over” design. This not only provides a significant weight savings in the part but also serves to lower production costs. Furthermore, the FDD method provides a cost savings in view of the fact that with enhanced service life it will no longer be necessary to maintain and replace parts as frequently, thereby reducing labor as well as costly aircraft and machine downtime. (See the Fatigue Design Diagram Application Note)

Medical Implants Application Notes

Medical Implants: Low Plasticity Burnishing (LPB™) improves high cycle fatigue performance and eliminates the occurence of fretting-induced fracture in the Exactech M-Series Modular Hip Prosthesis by producing beneficial, compressive residual stresses sufficient to protecting the tapered region of the implant's neck segment.

(Excerpt) Total hip replacement surgery is often required to alleviate pain and improve the function of hips damaged from disease or fracture. It is estimated that over 300,000 hip replacement surgeries are performed each year in the United States. Modular total hip prosthesis (THP) systems afford surgeons the flexibility to choose properly sized prosthesis subcomponents to treat a wide spectrum of diverse patients with various hip defects and injuries. However, modular THP subcomponents are vulnerable to fretting at the tapered connections causing a debit in the fatigue strength and a reduction in the functional life of the prosthesis.

The purpose of the surface enhancement program for Exactech Inc., and Lambda Technologies was to substantially extend the life of the Ti-6Al-4V Exactech M-Series Modular Hip Prosthesis Neck Segment by imparting deep compression in the tapered region of the neck with LPB™ to increase fatigue strength and eliminate the occurrence of fretting-induced fatigue. X-ray diffraction measurements of RS distribution and fatigue testing of LPB™ processed neck segments validated Lambda Technologies LPB™ treatment process. LPB™ processing of the neck taper increased fatigue strength by approximately 40% and eliminated the occurrence of fretting-induced fatigue. (See the Total Hip Prosthesis Application Note)