EFFECT OF LASER PEENING ON FATIGUE PRPOERTIES FOR AIRCRAFT .
EFFECT OF LASER PEENING ON FATIGUE PRPOERTIES FORAIRCRAFT STRUCTURE PARTST. Adachi 1, H. Takehisa 1, M. Nakajima 1, Y. Sano 21 Fuji Heavy Industries Ltd., 1-1-11 Yonan, Utsunomiya, 320-8564, Japan2 Toshiba Corporation, 8 Shinsugita-cho, Isogo-ku, Yokohama, 235-8523, JapanABSTRACTLaser peening is expected to improve fatigue properties more than shot peening,because it is possible to induce compressive residual stresses deeper thanconventional treatments. In this study, we examined the applicability of laser peeningto the aircraft through investigating the effects of laser peening on fatigue propertiesand the formability of aircraft parts.KEYWORDSLaser peening, Residual stress, Fatigue, Aluminum alloy, HelicopterINTRODUCTIONRecently, requirements from aircraft customers are shifting toward the reduction of lifecycle cost (LCC) by the expansion of fatigue life as well as the safety of parts. Forespecially the helicopter, it is a serious problem how to reduce the maintenance cost,since the exchange frequency of numerous parts is comparatively high. Shot peeninghas been applied to many parts with the purpose of improvement of fatigue and stresscorrosion crack resistance for the helicopter since 1970’s. However, some parts,which are used under the most severe condition causes fatigue fracture even if beingconducted by shot peening(1).Laser peening is a new surface improvement technique, which can provide a highercompressive residual stress and a deeper compressive layer than shot peening.Moreover this technology, which is able to give plastic strain more deeply, is expectedto be suitable for shaping the complex curvature of metal parts.Recently, laser peening technology is rapidly advancing in conjunction with lasertechnology development. In the aerospace industry, this surface technology starts tobe applied to the tip of fan blades inside aircraft engines with the aim of saving thefatigue failure by foreign object damage. However there are few applications to fatiguecritical area of aircraft structure parts such as dynamic component of the helicopterand fastener hole areas(2).The purpose of this study is to investigate the applicability of laser peening to fatiguelife improvement and the formability of aircraft parts.Method(1) Laser peening processAs shown in Fig. 1, laser peening is performed in water without the protective coatingsuch as black paint and metallic foil in Japan. The laser source is a Q-switched,frequency doubled Nd:YAG laser operating in the green wavelength range. The theoryof this peening method is the following. A shock wave follows high pressure plasmacaused by pulse laser irradiation on the surface of material and then a compressionstress area is formed by this shock wave propagating into the material. The depth of
the compressive residual stress layer built bylaser peening is deeper than that of shotpeening(3).(Water)LensLaser Pulse(2) MaterialPlasmaSome aluminum forgings have been used innumerous fatigue critical parts of aircrafts withPartsthe aim of improving the material characteristicShock waveof the parts. For this study, 7050-T7452Fig.1 Schematic of laser peeningaluminum alloy hand forging was selected toevaluate the effects of laser peening. Thisalloy has both features of high strength and high resistance to stress corrosioncracking in thick section in comparison with other forging materials as shown inTable1.Table 1 Mechanical properties of typical aluminum alloy hand forging for aircraftstructures(4)Static strengthMaterialThickness(mm)Tensile (MPa)Yield 52.4127.0-152.4421421476365338407Resistance to dual stress (MPa)RESULTS(1) Optimization of laser peening parameterIn order to optimize the processTable2 Laser peening parametersparameters for 7000 series aluminum alloy,Laser energyin this study, we evaluated the residual ConditionPulse densitydensitystress profile, the surface roughness andNo.(pls/mm2)(mJ/mm2)fatigue properties after laser peeningCondition A6236under various conditions.Condition B18260Laser peening was performed under 3ConditionC1888conditions with combination of pulsedensity and pulse laser energy density as shown in Table 2. The specimen, 100 mm 100 mm plate with a thickness of 10mm, is mounted on a fast motor controlled X-Ystage inside a water tank.500-50-100-150-200-250-300-350The residual stress was measured at thecenter of the irradiated area by X-raydiffraction. To obtain in-depth profiles ofConditionAthe residual stress, surface layers of testConditionBpieces were removed by igure 2 shows the result of residualDepth from surfce (µm)stress profiles induced by laser peening.Fig. 2 Comparison of residual stressThe depth of the residual stressprofiles in each conditionimprovement area by laser peening isabout 10 times deeper than that of shot peening whose depth is approximately 0.1 0.2 mm. The compressive residual stress value and the depth of compressive stressarea in the material tended to increase with increase in the laser energy density.
Table 3 shows the average value of surfaceroughness, which were measured at thepeened area and the parameter Ra is thecenter-line average of adjacent peaks. Thisresult indicates the surface roughnesstends to be improved as the pulse densityincreases.Table 3 Average surface roughnessCondition No.Condition ACondition BCondition CAs milled (Ref.)Surface roughness Ra (μm)3.82.82.40.1Table 4 Fatigue test summaryA summary of fatigue test conditions isSpecimenKt 1.0, Thickness 6.0 mmshown in Table 4. Laser peening wasLoadingAxial (Tension – Tension)Frequency10 Hzconducted on all four facets of theStress ratio0.1specimen except for the grip areas. AsMax. stress260 MPa (64%Fty)presented in Table 5, fatigue improvementASTM E466by laser peening was demonstrated in Specification10 ton Fatigue testingCondition A and Condition B. The fatigue Test Machinemachine (MTS)life of Condition B was longer than that ofCondition A. However, Condition C did not show the expansion of fatigue life ascompared with the non-surface treated.In order to investigate the difference of fatigue properties among these conditions,fracture surfaces were observed by a scanning electron microscope (SEM). Therewere two types of crack initiation asTable 5 Results of fatigue testshown in Fig. 3. On one hand, an internalLP ConditionSpecimenFatigue lifefracture type was confirmed on Condition ANo.No.(Cycles)and Condition B. The fracture nucleationA-11930000was located about 1 mm inside from theCondition AA-21909000surface. Such internal crack generation isB-12968000known as a result of the remarkableCondition BB-22760000improvement of fatigue property in theC-1274000fatigue fracture of surface hardeningCondition CC-2218000materials and an internal area has a veryN-1477000Non-surfaceslow fracture nucleation-growth throughtreatedN-2460000fatigue improvement of surface area by(5)laser peening . On the other hand, asurface fracture type as a typical fatigue fracture was confirmed on Non-surfacetreated and Condition C. This shows that fatigue improvement by compressiveresidual stress in surface area was not enough when specimens were laser peenedon Condition C. And then, because the surface of Condition C is rougher thanNon-surface treated (Table 3), it was expected that the fatigue life at Condition C wasshorter than Non-surface treated. Therefore, it was confirmed that Condition B is themost suitable laser peening condition for 7000 series aluminum alloys from the viewwhere the compressive residual stress was more important than the surfaceroughness on fatigue resistance.(a) Condition B(b) Condition CFig. 3 Fatigue fracture surface of laser peened coupon specimens
Maximum stress (MPa)(2) Open hole coupon testsTo demonstrate the expansion of fatigue Table 6 Notched fatigue test summarylife by laser peening with Condition BSpecimenKt 2.5around fastener hole, tension-tensionLoadingAxial (Tension – Tension)coupon fatigue tests were conducted withFrequency10 Hznotched specimens. A summary of theStress ratio0.1ASTM E466fatigue tests is shown in Table 6. The Specificationspecimen has a typical fasten-hole6.35 0.025structure in the shape of a dog bone asshown in Fig. 4. Laser peening wasRolledconducted on both areas around a hole in25.0directionthe specimen. Shot peening was applied to50.050.0compare the effects each other. The shot200peening intensity was 0.006A, which was(Unit : mm)achieved using a cast steel shot and acoverage rate of 200%. Figure. 5 shows the Fig. 4 Notched coupon specimen(Thickness: 6mm)result of the comparison of notchedtension-tension fatigue tests of untreated,N on-surface treated240shot peened and laser peened specimens.Shot peened220As shown in Fig. 5, improvements of fatigueLaser peened200life by laser peening tended to increasewith a decrease of the maximum stress.180Fatigue cycles to failure for the laser160treated specimens increased more than140approximately 20 times compared with the120non-surface treated and about 5 times over100as compared with the shot peened at the1.0E 041.0E 051.0E 061.0E 07maximum stress of 140 MPa. The fatigueNum ber of cycles to failure (C ycles)life improvement by laser peening on holeFig. 5 Result of notched fatigue testscoupon fatigue test is greater than that ofun-notched fatigue test.(3) Component testIn order to evaluate the possibility offatigueimprovementwiththecomponent level, fatigue tests of anPitch hornactual-size element, which simulate apart of the helicopter were conducted.The pitch horn, which is a part offlight control system for the main rotorof helicopters (see Fig. 6) wasFig. 6 Pitch horn of helicopterselected for the component test(6).The pitch horn in actual helicopters is loaded by about 5Hz high cycle fatigue loadduring flight. The actual-size element test was also conducted 5Hz fatigue load underthe condition applying a simple sinusoidal load. The element test specimen was madeof 7050-T7452 aluminum alloy as well as the coupon test. One specimen wasuntreated and the other specimen was treated by laser.The test equipment and test set up are illustrated in Fig. 7. The restraint condition inthe joint portion of the test specimen was simulated an actual part. The test specimenwas fastened with two bolts, and the opposite side was a pinned joint. The sinusoidaltest load of /-1800 kgf was applied via a rod to the pinned joint. The rod simulated a
pitch link.Fatigue MachineThe untreated specimen was testeduntil a failure with a visible crack due tofatigue. The crack on the untreatedElement test specimenspecimen was found at a location (1)shown in Fig. 8(a). The number ofLoad directioncycles to failure was about 160,000. For Fastened with boltsFixtureanother specimen, laser peening wasconducted at the location of A and B inFig. 7 Test set upFig. 8(b). The location A and B werehigh stress areas predicted by the stress analysis and the preliminary strain survey.The laser peened specimen was tested until a failure and the number of cycles tofailure was about 210,000. The visible crack was found at a location (2) shown in Fig.8(b). The result of component test demonstrated that the fatigue strength of a stressconcentration area was improved by laser peening. The fracture point in laser peenedspecimen shifted from the high stressed area to the low stressed area.Consequently the improvement of fatigue life due to laser peening was verifiedthrough these tests. If not only high stress area but also low stress area is treated bylaser peening, the expected life expansion of the element specimen would be thesame as coupon test results.(1)(2)BA(a) Untreated test specimen(b) Laser peened test specimenFig. 8 Results of component testFig.9 Laser peen formed specimen1.E 05Number of pulses(4) Laser peen formingTo confirm the formability of laserpeening at a coupon level, specimensof a sheet metal (7475-T761 Al alloy)were peened on some conditions andthe radius of the peened specimencurvature were measured. Figure.9shows a photo of the laser peenedsheet material. All specimens wereformed successfully in convex singlecurvature without showing buckling. Asindicated in Fig.10, the radiuscurvature tended to decrease withincrease in the number of pulses andthe minimum contours’ curvature issmaller than that of the wing tip areaon small aircrafts such as business jets.In this test, the good formability of laser1.E 041.E 030100020003000Radius curvature (mm)Fig.10 Laser peen formability4000
peen forming was verified. By the optimization of process parameters and peeningpatterns, this forming method is expected to become the effective method, which canform the wing skin in complex configuration and straighten some parts.ConclusionThe optimum process parameters of laser peening for 7000 series aluminum alloysand the improvement of fatigue life by laser peening was studied and the followingresult were obtained:(1) The fatigue cycles to failure for the laser treated specimens increased more thanapproximately 20 times compared with the non-surface treated and about 5 times overas compared with the shot peened at maximum stress of 140 MPa.(2) The improvement of fatigue life by laser peening on the hole type fatigue test isgreater than that of the un-notched fatigue test. This indicates that it is more effectivefor the improvement of fatigue life to apply this new surface technology to the stressconcentration area such as fastener hole.(3) The improvement of fatigue characteristics due to laser peening wasdemonstrated by the component fatigue test.(4) The forming method by laser peening has a great potential to form the wing skinand straighten various parts.AcknowledgmentThis research was conducted under the Society of Japanese Aerospace Companies,Inc. contract.References(1) T. Adachi, M. Shimanuki, H. Taguchi et al. “Laser Peening Technology for AircraftStructure Parts” Proceedings of AHS International Meeting on AdvancedRotorcraft Technology and Life Saving Activities. Heli Japan 2006(2) T. Adachi, M. Shimanuki, H.Taguchi et al. “Study of Fatigue Improvement by LaserPeening for Aircraft Structure Parts” Proceedings of 2006 KSAS-JSASS JointInternational Symposium on Aerospace Engineering, 2006(3) Y. Sano, “Residual Stress Improvement by Laser Peening without ProtectiveCoating and its Applications” Proceedings of the 65th Japan Laser ProcessingSociety, 2005(4) “Metallic Materials Properties Development and Standardization” NationalTechnical Information Service, 2003(5) “Fractography” The Society of Materials Science, Japan, 2000(6) H. Taguchi, M. Obukata, T. Adachi et al. “Application Study of Fatigue Improvementby Laser Peening for Life Limited Dynamic Components” Proceedings of AHSInternational Meeting on Advanced Rotorcraft Technology and Life SavingActivities. Heli Japan 2006
Shock wave Fig.1 Schematic of laser peening Fig. 2 Comparison of residual stress profiles in each condition Table2 Laser peening parameters the compressive residual stress layer built by laser peening is deeper than that of shot peening(3). (2) Material Some aluminum forgings have been used in numerous fatigue critical parts of aircrafts with
Fig. 1 & 2: laser peening (left) and shot peening (right) basic concepts 1-Effects of Laser Peening and Shot Peening on material The effects of laser peening and shot peening on the processed material can be grouped in three categories: Redistribution of residual stresses Surface topography modification Cold work 1-1 Residual stress
development of high peak power short pulse from Nd:YAG laser along with its peening application. It presented the design scheme of laser and the characteristic of laser beam transmission. Zhu [15] et al. discussed the influence of laser shock peening on surface morphology and mechanical property of Zr-based bulk metallic glass.
effects was investigated. We adopted a laser peening meth-od that can be used to treat metals without a protective coating [9, 10], which can induce a compressive residual stress in metals by increasing the coverage. In the estimation of the effects of laser peening i.e., the performance of laser peening, magnitude of compres-
Laser Shock Peening (air) An extension of conventional peening Laser peening provides – Highly compressive surface residual stress – Deep layer of compressive residual stress – Smooth surface – Excellent process control Laser near field image Laser peened aluminum
Laser Peening . 28% Increase on Top . 21% increase on Bottom . Hardness Levels for FSW 2195 increased proportionally with number of Laser Peening layers . The polishing that takes place prior to microhardness can wipe mitigate all hardness effects associated with the Shot Peening Process
Keywords Inconel 625, laser shock peening (LSP), microstructure, nanohardness, surface roughness 1. Introduction Laser shock peening (LSP) also known as laser shock processing is an innovative laser-based surface processing technique used to modify the surface of a metallic material, which causes compressive residual stresses and microstructural
The net result of shot peening is often a noticeable improvement in fatigue properties (Ref. 5). Shot peening under a prestress can produce an even higher level of compressive stresses (Ref. 3). Laser-shock peening (LSP) was first used by Battelle Columbus Laboratories in 1974 (Ref. 6). In this process, the
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