Effect of Conductivity Between Fasteners and Aluminum Skin On Eddy Current Specimens. Abstract

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John Harmon
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1 Effect f Cnductivity Between Fasteners and Aluminum Skin n Eddy Current Specimens David G. Mre Sandia Natinal Labratries Federal Aviatin Administratin Airwrthiness Assurance ND Validatin Center Albuquerque, New Mexic 8785 (55) M. C. Hutchinsn Being Cmmercial Ai P.. Bx 77 M / S 9U-EA Pk 5 '-a$ Seattle, Washingtn ~ ~! FZ (26) b i d Abstract The Federal Aviatin Administratin Airwrthiness Assurance Nndestructive nspectin Validatin Center (FAA-AANC) and Being Cmmercial Airplane Grup are currently develping a study pertaining t the detectin f cracks in multi-layered aluminum sheets. The specimen panels mdel pertinent aspects f the lap splice jints fr Being 77 aircraft, Line Numbers Upn initial characterizatin f the specimen panels, it became clear that signals prduced frm a sliding prbe at fastener sites were nt representative f an in-service lap splice, and therefre, culd nt be used in a prbability f detectin experiment. This paper discusses specimen characterizatin and steps taken t make the specimens useful fr nndestructivetechnlgy assessment. Backgrund The FAA-AANC was established at Sandia Natinal Labratries by the FAA William J. Hughes Technical Center, Atlantic City, N.J. in August 99 and is hused in a hangar at the Albuquerque nternatinal Airprt. AANC's majr rle is t the study and validate nndestructive inspectin techniques and assess advanced inspectin technlgies. During a fleet survey f a Being 77 aircraft in September f 994, cracks were fund in the lwer fastener rw, lwer skin (Figure ) in a high cycle time aircraft. Because the primary crack lcatin was cvered by tear straps, a lw frequency eddy current prcedure was develped t detect this type f cracking fim the exterir f the aircraft. n Nvember 8, 994 Being Cmmercial Aircraft Grup issued Service Bulletin 77-5A77 which specified the inspectin f the lwer fastener rw f 77 lap splices fr line numbers 292 thrugh During a 995 ND Reliability Wrkshp spnsred by the FAA, the AANC was asked t assist the Seattle Aircraft Certificatin ffice in develping Prbability f Detectin (PD) curves fr primary inspectin called ut in the Service Bulletin. This wrk was supprted by the United States Department f Energy under Cntract DE-AC4-94-AL85. S UMf

2 Prtins f this dcument may be illegible in electrnic image prducts. mages are prduced frm the best available riginal dcum?lt

3 DSCLAMER This reprt was prepared as an accunt f wrk spnsred by an agency f the United States Gvernment Neither the United States Gvernment nr any agency theref, nr any f their emplyees, make any warranty,expms r implied, r assumes any legal liabiliq r respnsibilityfr the accuracy, cmpleteness, r usefulness f any infnnab'n, apparatus, prduct, r prcess disclsed, r represents that its use wuld nt infringe privately wned rights. Refmnce herein t any specific cmmercial prduct, pmcess, r service by trade name, trademark, manufactuq r therwise des nt necessarily crrstitute r imply its endrsement, recmmendatin,r favring by the United States Gvernment r any agency theref. The views and pininsf authrs expressed herein d nt necessarily state r reflect thse f the United States Gvernment r any agency theref.

4 Design and Prductin f Lap Jin, Specimens The specimens were designed t mdel pertinent aspects f an in-service 77 lap splice. Each test specimen cnsists f ne mck dubler, fur mck tear straps and tw aluminum skins riveted tgether with a 76.2 mm (") verlap (Figure 2). Prgrammed fatigue cracks were grwn in aluminum sheets and placed as the lwer skin. The spacing f the tear straps was varied t simulate manufacturing tlerances. Since the tear straps create a majr surce f nise fr the eddy current inspectin, the number f tear straps per unit length f lap splice was dubled. This gives an inspectr a larger number f tear strap inspectin sites with a minimum number f samples. The eddy current inspectin is als effected by the skin thickness f the lap splice. Therefre, the tp tw layers f the specimens (upper skin and dubler) had tw thickness values -.8 mm (.72") and 2. mm (.8"). The prgrammed fatigue cracks were created fim starter ntches at select lcatins. The lwer skin panels were then cycled until the desired length f fatigue crack was reached. Starter ntches were remved by drilling the final hles fr fastener installatin. n an attempt t create the relatively lw electrical cnductivity between the specimen skin and fastener which is bserved n aged in-service lap jints, at least eight hurs were t elapse between drilling and cuntersinking the fastener hle and fastener installatin. A petrleum base lubricant was als applied t each hle prir t fastener insertin. / d D - ~ ~ Secndary Cracks in Lwer Skin Figure. Crack lcatins in the Being lap jint. Figure 2. Panel design t simulate Being lap jint. Characterizatin f Lap Jint Specimens Three criteria were used t judge hw well the test specimens mdeled the eddy current respnse f an aged in-service lap jint. First, des the eddy current signal frm unflawed fastener lcatins n the specimens cmpare t similar sites n an aircraft? Secnd, is there a similar amunt f variatin in eddy current signal strength frm a sample f unflawed specimen sites as cmpared t a similar sample f aircraft sites? Third, des the eddy current signal strength at flawed fastener sites in the test specimens match the signal strength frm aircraft sites with similar length flaws? n making these cmparisns it was fund that sme flawed and unflawed sites in the specimens did prduce eddy current signal strengths cmparable t aircraft sites.

5 Hwever, mst sites prduced a smaller than expected signal. f curse this meant that the amunt f signal variatin amng similar sites n the specimen panels is much greater than fund n an aircraft. t was cncluded that the specimen panels had nt been prperly lubricated. Therefre, the generatin f artificially aged fastener sites was nt btained n the test specimens. Figure dcuments the characteristics f a rivet site with an abnrmal cnductivity between the rivet and hle. This impedance plane signal is nt representative f sites fund n aircraft. Remanufacturing f Lap Jint Specimens T remedy the signal respnse discrepancy fund during the initial characterizatin, each tp and bttm rw f rivets was remved. Fllwing is the remval prcess develped fr these specimens: ) remve the back side f the rivet using an air pwered metal shaver and 2) drive ut each rivet using a rivet press. After this prcess is cmplete, the specimens were re-inspected. Each hle and rivet was then cated with Sharpie@permanent marker. Petrleum lubricant was applied t each site and rivets were then driven back int the existing hles. The specimens were re-inspected using the lw frequency eddy technique and data acquisitin sftware. The re-inspectin data was cmpared t the riginal data. The purpse f this cmparisn was t dcument that remval and re-installatin f rivets des substantially change the impedance plane signal and the mst significant factr affecting riginal respnse in the lap jint specimens was the result f pr lubricant during rivet installatin. Figure 4 displays the results f the re-manufacturing prcess (remving the rivet, applying permanent marker t the hle and rivet, adding petrleum lubricant t the rivet hle and re-installing the rivet). The impedance plane signal is representative f fasteners fund n actual aircraft..b f R e a l (vlts) i Figure. Signal frm rivet that exhibits high cnductivity between the hle and rivet. -., R e a l (vlts) Figure 4. Signal frm the same rivet site after remanufacturing. Cnclusins T successfully implement a prbability f detectin experiment, specimens must simulate.he respnse f an in-service aircraft inspectin. The electrical cnductivity between the fasteners and skin in a high cycle aircraft are different frm newly manufactured nes, thus, lap jint specimens must be artificially "aged" t mdel the eddy current respnse f an in-service aircraft. The remanufacturing steps described in this paper are ne methd t apprximate the crrect electrical cnductivity between the fastener and skin f an in-service lap jint.

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