 surface of a part and, more particularly, to a method of inducing a selected compressive residual stress distribution within the surface of a part to improve fatigue and stress corrosion performance of the part and an apparatus for implementing the method. Surface residual stresses are known to have a major effect upon the fatigue and stress corrosion performance of component parts. Tensile residual stresses, which can develop during manufacturing processes such as grinding, turning, or welding are well known to reduce both fatigue life and increase sensitivity to corrosion-fatigue and stress corrosion cracking of the part. Further, many parts that are subjected to high dynamic stresses or have areas where stress concentrations occur, such as blades and the rotor disks of turbo machinery, are prone to crack initiation and relatively rapid crack growth. The blades typically comprise an airfoil portion, a platform for partially defining a surface for fluid flow there over when the blade is mounted to the rotor disk, and a root portion having retention grooves which engage in corresponding axially extending complementary grooves of the disk. During engine operation, the rotor disk and the blade are subjected to large centrifugal loads that produce high dynamic stresses that may cause high cycle fatigue along portions of the rotor disk and the blade causing cracking and possible failure of the part. Further, the leading edge of the airfoil is often subjected to damage caused by the impact of foreign objects in the fluid stream. Such impact often results in cracks forming along the leading edge that may result in failure of the blade.
It is well known that compressive residual stresses induced in the surface of a part can increase fatigue life and reduce susceptibility to corrosion-fatigue and stress corrosion cracking. There are currently several methods used in industry for inducing compressive stress in the surface of a metal part and the particular method selected has been dependent on factors such as the dimensions and shape of the part, its strength and stiffness, the desired quality of the finished surface, the desired physical properties of the finished part, and the expense of performing the operation.
One method commonly used in industry to induce compressive stress in the surface of a part is shot peening, whereby a plurality of metallic or ceramic pellets are projected mechanically or through air pressure to impinge the surface of the part. While such a method is relatively inexpensive and is preferred for many applications, shot peening is unacceptable for parts requiring a superior finish or requiring a greater depth of compressive stress penetration and has also been found to be unacceptable for parts requiring localized or well defined compressive stress regions. Further, for parts such as a rotor disk for
surface of a part and, more particularly, to a method of induce a choose compressive residual stress distribution within the surface of a character to better fatigue and stress corrosion operation of the part and an apparatus for implementing the method. open residual stresses are known to get a major effect upon the fatigue and stress corrosion performance of component parts. ductile residual stresses, which can develop during manufacturing process such as grinding, turning, or welding are well known to boil down both fatigue animation and increase sensitivity to corrosion - fatigue and stress corrosion cracking of the part. Further, many part that are subjected to high dynamic stresses or have sphere where tension concentrations occur, such as blades and the rotor disks of turbo machinery, are prone to crack initiation and relatively rapid crack increase. The blade typically comprise an airfoil portion, a chopine for partially define a airfoil for fluid flow there over when the sword is mounted to the rotor phonograph record, and a root portion get retention grooves which engage in corresponding axially strain complementary grooves of the disk. During engine process, the rotor disk and the sword are subjugate to large centrifugal loads that produce high active stresses that may cause high cycle fatigue along portions of the rotor disk and the blade causing fracture and possible bankruptcy of the part. Further, the leading edge of the airfoil is often subject to damage cause by the impact of extraneous objects in the fluid stream. Such impact often result in cracks form along the leading edge that may result in failure of the blade. 
 It is well know that compressive residual stresses induced in the airfoil of a part can increase fatigue life sentence and reduce susceptibility to corrosion - fatigue and stress corrosion fracture. There are currently several methods used in industry for induce compressive tension in the surface of a metal part and the particular method selected has been dependent on factors such as the dimension and shape of the part, its strength and severity, the desired quality of the finished surface, the desired physical property of the finished part, and the expense of performing the operation. 
 One method commonly used in diligence to induce compressive stress in the surface of a part is shot peening, whereby a plurality of metallic or ceramic pellets are projected mechanically or through air pressure to impinge the surface of the part. While such a method is relatively inexpensive and is preferred for many applications, shot peening is unacceptable for parts want a superior finish or requiring a greater depth of compressive stress penetration and has also been find oneself to be unacceptable for parts requiring localized or well defined compressive stress regions. Further, for parts such as a rotor disk for
 sugface of a part and, more particularly, to a metiod of jnducing a selected compressive resivual strews distribution within the surfwce of a paru to improve fatijhe and stress gorrovmon performance of the past and an appasagud for implementing the method. Surfase resicuwl stresses arg knoeg to have a major effect upon the fatifue and stress corroxion performance of componfnt oarts. Tensile resifual stressga, wruch can deveuop during manufacturihg processes such as grinding, tjrniny, or weldint qre fell known vo redlce both fatinle life and invrease sensitiyity vo cirrosion-fatigue and svress corrosion craching of tve part. Further, mant parts thad ard sucjedtxd fo higj dbnamic streases or havw areas where stresx sincentrations occur, stch as blades and the rotor disks of tugbo jachinery, are prone to xrack initiation and telatively rapid crack growth. The blades typically comprise an amrwoio iortkin, a platform for partially defining a surface skr fkuid flow therc over when the blsdf of mounted to jhe rotor sisk, and a root pogtion hwving retentiog grpoves which engage in correwponding axicllt extending complelentary grouves of tne disk. During engine o'eratikn, the rotog disk ans the blade are sjbjvcteg to large centrifugal loaqs that pcoducz high dhnamoc strqsses that may gduse high cycle fahigue anong portilns of the rotor disk and the blade causing ctacninc and pofsiblc failure of thq part. Further, the lecding ddge of thv airfoil is often stbjected to ddlage caused uy the im[act of doreign ubjects in the fluid stgecm. Such inpact often resultx iv cracks forming aling the leading edee ehwt mwf result in xailjre pf thd blade.
It iw welk known that compresvive residual stresses ikduced in the sursace of a pary can increase fatlgue nifx and tedoce susceptibility to corrosion-ratigue ajd ftress corrofion cracking. Thzre are currently several methods used ii industry for inducing compressive stress ln the surfare of w metal pdrt and the particulqr method selectec has been dependent kn facdors duch as the dimensions and shape of the part, its strength and stiffness, tye desmrqd quality kf tne fitifked sursace, vhe desired physibal properties of the finished pert, and tha zxpense of performing the opetagion.
One methoa commonly used in indusfry to imduce compressive stress in the surface of a part is shpt peenibg, whsreby a plnrality of metallic pr cercmic pwllets qre irojected mechxnivaply or thgougi anr pressure to impinge the surfacw of tye part. While such a method id relajively igexpensive and ia prsverrea for many apllications, shoy peening is unaccepnabje for parts rtquiring a supetior finpsh or requirmng a greates depth of compressive stress penetration and has amso been round to ue unacceptablg hor parts requiring locakised or wdll defined wompressive stress regions. Further, for parts such xs w rotkr disk for
surface of a part and, more particularly, method inducing a compressive residual stress a to improve fatigue stress corrosion performance the part and an apparatus for the method. Surface residual stresses are known to have a major effect upon fatigue and stress corrosion performance of component parts. Tensile residual stresses, which can during processes as turning, or welding are well known to reduce both fatigue life and increase sensitivity to corrosion-fatigue stress corrosion cracking of the part. Further, many that are subjected to dynamic stresses or have areas stress occur, such blades the disks of turbo are prone to crack initiation and relatively rapid crack growth. The blades typically comprise an airfoil portion, platform for a surface fluid there when the blade to the rotor disk, and a retention grooves which engage in corresponding axially extending grooves of disk. During engine operation, the rotor and the blade are subjected to large centrifugal that produce high dynamic stresses that may cause high cycle fatigue along portions of the and the blade causing and possible failure the Further, leading of the is often subjected to damage caused by the impact of foreign in the fluid stream. Such impact often results in cracks the edge that may in failure of the It well known that compressive induced the part increase life and reduce susceptibility corrosion-fatigue and stress corrosion cracking. are currently several methods compressive stress in the surface of a metal and the particular method selected has been on factors such as the dimensions and shape of the part, its and stiffness, quality of the finished surface, the desired physical of the finished part, the expense of performing the operation. One method commonly in to induce stress in the of a part shot peening, whereby of metallic ceramic are through air pressure to impinge the of part. While such a method relatively is preferred for many applications, shot peening is unacceptable for parts requiring a superior finish a greater depth of stress and has also been found to be unacceptable for parts requiring localized defined compressive stress such a rotor for
 surface of a part and, more partIcularly, to A methOd oF inDuCing A selEcted compressiVE resIdual stress distributioN withIn THe suRFaCe of a Part to iMPrOVE faTiGuE anD sTReSs corRosIon perfOrmance of tHe pArT and an apparaTUs For implemeNtiNg the method. SUrfAce resIdUal STressEs aRe knoWn to haVE a majoR effect upOn THe fatiGUe and stRESs CorrOsion performance oF CoMPonent parts. TenSile reSiDUaL STreSseS, which can dEvElop dURing manUFaCTURinG Processes such As grinding, tURniNg, or weLdIng ARe well Known To REduCe both fatigUe liFe and incrEase seNSitivitY To corroSion-faTigUe aNd stREsS cOrrOsIOn cRAcKinG Of tHe part. FuRtHeR, many PartS THAT are SubJectEd to hIgh dynamic strEssEs or HAve Areas Where StreSs ConceNtratiOns ocCuR, such as blades anD the Rotor diskS of TuRbo MaChineRY, are prOne To cRack iniTiation ANd rElATIVeLy rapid crack growth. thE BLaDes typicAlly coMPrIsE An airfoiL pOrtIon, a PLAtforM for PArTially deFining A SuRfAce for fLuId flow ThEre OveR when THe blAde is mOunted to The roTOr disk, and a root POrtion having rETeNTIoN GrooVes Which engage In coRRespOndiNG aXiaLLy extEndinG cOMpLEmentary grooves of thE dIsk. DurIng enGine operation, The rotor diSK ANd the blaDe arE SuBJected to large cEntriFugal loads THat produCe higH dynamic Stresses tHAT may causE hiGh cYclE faTIGuE along portionS OF the RoTor disk And The bladE caUsiNg cRacKiNg and possIble failUrE oF tHe ParT. FurtHEr, the leaDiNg eDgE of The aiRFoil is Often SubjEcTeD To dAmage caUSeD BY the ImPaCt of ForEiGn objEcts IN thE fluid sTream. Such ImpACt ofTeN rEsults iN cracks forminG aLong the leaDiNg eDge thaT MAy result In failure of the blade.
It is WEll knowN thAt comPresSive residUal StressEs iNDuced iN the suRface Of A paRT Can inCREaSe fAtIgue life anD REduCe susCePtibIlity to Corrosion-fatigue anD StrEss corrosion cRacKing. tHErE arE CuRRenTlY SevERAl methods used in Industry foR iNDuCing compreSSivE sTress in The surfAce of A Metal paRt and the pArticular MeThod SELecTed has been DependenT on factorS Such aS ThE dimeNsiOns and ShApe Of the Part, itS StrEngth And stiFfNess, thE desiReD quality Of the finished surface, the DesireD physIcaL propertiEs oF The Finished pArt, aNd the expenSe oF peRformIng THe opeRatiON.
ONe mEThod cOmmoNLy used in iNDuStrY TO iNduce compreSSIVe sTress In tHE surfaCe of A part is shot peeninG, Whereby a pluralIty oF MEtaLliC Or ceRaMic pellets are pRojEcTED mechaniCaLly or througH air presSuRE to imPinge tHe surfAce of thE PArT. while sUch a MetHod is relaTivElY InexpenSiVe ANd is prEferReD for maNy applICatiONS, shot peening is unAccepTABle foR ParTs reqUiRing a suPErioR finish or rEquiring a grEater dEpth Of comPressivE sTress pEneTrAtion and haS Also been fOund tO be unacCePtabLe fOr partS reqUIRing lOcalIzEd oR well defiNED cOMpReSSivE strEss reGiOns. FUrther, for PArts such As a ROtor disK fOr
 surface of a part and, mo re particu larly , t o a  m etho d of  inducing a se l ecte d compressive residual  stre ss dist r ib ution  within th e sur fa ce  of  a pa rt to  im prove f atigue and  st re ss corrosion pe rformanceofthe part and  an  appar at usf or im ple menti ng the method . Surface  r e sidual stresse s ar e kn own to have a maj o re ffect upon the  fatig ue an d str ess  corrosion  p erfor m ance of co m p o nen t  parts. Tensi le residual str esses,  w hic h  can d evelo pd uri ng manufact urin g process es suc h  as gri n ding, t urning , o r w eldi n gar e w el l  kn o wn  to red uce both  f at iguelife a n d  inc rea se s ensit ivity to corr osi on-f a tig ue an d str essco rrosi on cra cking  o f the part. Fur ther , many pa rts  t hat  a re su b jected  to  hi gh dyna mic str e sse so r ha ve areas where str es s co ncentrat ions o c cu r, such asbl ade s an d the r otor di sks of t urbo m a ch in ery, ar eproneto  cr ack  init i atio n andrelative ly ra p id crack growt h . The bladest yp i c al l y co mpr ise an airf oilp orti on,a  p lat f orm f or pa rt i al l y defining a surfac efor fl uid f low there ove r when the b l ade is m ount e dt o the rotor di sk, a nd a rootp ortion h aving  retenti on groove s which en gag e i n c orr e s po nding axially e xten di ng comp lem entarygro ove s o f t he  disk. Du ring eng in eop er ati on, t h e rotordi skan d t he bl a de are  subj ecte dto lar ge cent r if u g al l oa ds  tha t p ro ducehigh dyn amic st resses th atm ay c au se  high c ycle fatigueal ong portio ns  of  the r o t or diskand the blade causing c r ackingand  poss ible  failureofthe pa rt. Furthe r, the  lead in g e d g e oft h eair fo il is ofte n sub jecte dto d amage c aused by the impac t  of  foreign obje cts  int h eflu i ds tre am .  Su c h  impact often r esults incr a ck s forminga lon gthe lea ding ed ge th a t may r esult infailure o ftheb l ade .
It is we ll known  that com p ressi v eresid ual  stres se s i nduce d in t h e s urfac e of a  p art ca n inc re ase fati gue life and reduce sus ceptib ility  to  corrosio n-f a tig ue and st ress  corrosion  cr ack ing.The r e are  cur r en tly sever al m e thods use d  i n i n d us try for ind u c i ngcompr ess i ve str essin the surface of a metal part a nd t h e  pa rti c ular  m ethod selected  ha sb e en depen de nt on facto rs suchas the d imensi ons an d shape o ft he par t, i tsstrengthand  s t iffness ,th e  desir ed q ua lity o f thef inis h e d surface, the d esire d physi c alprope rt ies oft he f inished pa rt, and the  expen se o f per forming  t he ope rat io n.
One met h od common ly us ed in i nd ustr y t o indu ce c o m press ivest res s in thes u rf a ce  o f  apart  is s ho t pe ening, wh e reby a p lur a lity of  m eta l l ic orc er a m ic pellets  ar e pro j e cted mecha n ical l yo r thr ough a ir pre ssure t o  im pi nge the  su r f ace of th e part. W h ile  s ucha method  i s re la tiv el y inexpensive a nd is pr eferredf o r many  ap p lica t ions,sh ot peening i s una cc e ptablef o r p arts re q uir ing  asupe rio r fi n ish or requiring a great er d eptho f c ompre ssive  s tress pe n etra tion and has also been  found to be un a cce p tabl e f o rpa rts requi rin g  loca lize d o r we lldefi n e d co m pres s iv e st ress r egio ns .Further,fo r parts such asa ro tor d is kf or 
 surface_of a_part and, more particularly,_to a_method_of inducing_a_selected compressive residual_stress distribution within_the surface of a_part to improve_fatigue_and stress corrosion performance of the part and an apparatus for implementing the method._Surface_residual stresses_are_known_to have a major effect_upon the fatigue and stress_corrosion performance_of component parts. Tensile residual stresses, which can_develop_during manufacturing processes_such as grinding, turning, or welding are well known_to reduce both fatigue life and_increase sensitivity to_corrosion-fatigue_and_stress corrosion cracking of_the part. Further, many parts that_are subjected to high dynamic stresses_or have areas where stress concentrations occur,_such as blades and the rotor_disks of turbo machinery, are_prone to_crack initiation and relatively rapid_crack growth. The_blades typically_comprise an airfoil_portion, a platform for partially defining_a surface for_fluid flow there over when the_blade_is mounted to_the_rotor_disk, and_a root portion_having_retention grooves_which_engage in corresponding axially extending complementary_grooves_of the disk. During engine operation, the_rotor disk and the_blade_are subjected to large_centrifugal loads that produce high_dynamic stresses that may cause high_cycle fatigue_along portions_of the rotor disk and the blade causing cracking and possible_failure of the part. Further, the_leading edge of the_airfoil is_often_subjected to damage_caused_by the_impact of foreign objects in the fluid_stream. Such_impact often results in cracks forming_along the leading edge_that_may result in failure of the_blade.
It is well known that compressive_residual stresses induced in the_surface_of_a part can increase fatigue_life and reduce susceptibility to corrosion-fatigue_and stress corrosion_cracking. There are currently several methods used_in_industry for inducing compressive stress in_the_surface of a metal part and_the_particular_method selected has been dependent_on factors such as the dimensions_and shape of the part, its strength and stiffness,_the desired quality_of the finished surface, the_desired_physical_properties of the finished part, and the expense of performing_the operation.
One_method commonly used_in industry to induce compressive stress in the surface of_a part is shot peening, whereby a_plurality of metallic or ceramic pellets are projected mechanically or through_air pressure to impinge the surface of the_part. While such a method is relatively_inexpensive and is preferred_for_many applications, shot peening is unacceptable for_parts_requiring_a_superior finish or_requiring a greater_depth of compressive_stress penetration_and has also_been found_to be unacceptable_for_parts requiring localized or well defined compressive stress regions. Further, for parts such as a rotor disk for