BEARINGBYPASS LOADING ON BOLTED COMPOSITE JOINTS

Transcription

1 NASA TECHNCAL MEMORANDUM L BEARNGBYPASS LOADNG ON BOLTED COMPOSTE JONTS [NASA-TM-89153) BEARNG-BYPASS LOADXNG ON ~ ~ n - 2 ~ ~ 3 7 BOLTED COBPQSTE JONTS (NASA) 34 p Avil: NTS HC i3/flp AO? CSCL lld uncls S3/ J. H. CREWS, JR. nd R. A. NAlK MAY 1987 Ntionl Aeronutics nd Spce Administrtion lngley Reserch Cnter Hmpton, Virgini 23665

2 SUMMARY A combined experimentl nd nlyticl study hs been conducted to investigte the effects of simultneous bering nd bypss loding on grphite/epoxy (T3/528) lminte. Tests were conducted with test mchine tht llows the bering-bypss lod rtio to be controlled while single- fstener coupon is loded to filure in either tension or compression. Test coupons consisted of 16-ply qusi-isotropic grphite/epoxy lmintes with centrlly- locted 6.35-mm bolt hving clernce fit. Onset-dmge nd ultimte strengths were determined for ech test cse. Next, finite element stress nlysis ws conducted for ech test cse. The computed locl stresses were used with pproprite filure criteri to nlyze the observed filure modes nd strengths. An unexpected interction of the effect of the bypss nd bering lods ws found for the onset of compression-rected bering dmge. This interction ws cused by decrese in the bolt-hole contct rc nd corresponding increse in the severity of the bering lods. The mount of bolt-hole contct hd significnt effect on locl stresses nd, thus, on the clculted dmge-onset nd ultimte strengths. An offset-compression filure mode ws identified for lminte filure under compression bering-bypss loding. This filure mode ppers to be unique to compression bering-bypss loding nd, therefore, cnnot be predicted from simple tests. 1

3 NTRODUCTON n the pst, composite joints hve often been designed using rther simple metls-bsed procedures without encountering serious problems. n most such cses, the structurl design strins hve been limited by dmge-tolernce considertions, nd t low structurl strin levels, the joints hve been dequte. However, s tougher composites come into use, the design strins will rise nd structurl joining requirements will become more criticl. Anlyticl design procedures for joints should be bsed on sound understnding of the response of composite mterils under loding conditions similr to those in multi-fstener joints. Within multi-fstener joints, fstener holes my be subjected to both bering lods nd lods tht bypss the hole, s shown in figure 1. The rtio of the bering lod to the bypss lod depends on the joint stiffness nd configurtion. As the joint is loded, this bering-bypss rtio t ech fstener remins nerly constnt until dmge begins to develop. n generl, different bering-bypss rtios produce different filure modes nd strengths for ech fstener hole. The lminte response cn be studied by testing single- fstener specimens under bering-bypss loding, but such tests re usully difficult. The first objective of the present pper is to describe reltively simple pproch for the bering-bypss testing of single-fstener coupons. This pproch uses two hydrulic servo-control systems to pply proportionl bering nd bypss lods to specimen with centrl hole. The second objective of this pper is to present bering-bypss strength dt for wide rnge of bering-bypss rtios in both tension nd compression. Previous dt in the 2

4 literture hve been rther limited, especilly for compression. The test specimens were mde of T3/528 grphite/epoxy in 16-ply qusi-isotropic lyup. The bering lods were pplied through clernce-fit steel bolt hving nominl dimeter of 6.35 mm. The test results re presented s bering-bypss digrms for dmge-onset strength nd for ultimte strength. The corresponding dmge modes were determined by rdiogrphing ech specimen fter testing. The third objective of this study ws the nlysis of the bering-bypss test results using the locl stresses round the bolt hole, computed for combined bering nd bypss loding. These stresses were clculted using finite element procedure tht ccounted for nonliner bolt-hole contct. The corresponding contct ngles were clculted, s well s the locl stress distributions. The stresses were used to compute the dmge-onset nd ultimte strengths for the rnge of bering-bypss test conditions. The computed strengths were used to discuss the test trends. NOMENCLATURE C d ' 'b P P 'b bolt-hole clernce, m hole dimeter, m pplied lod, N bering lod, N bypss lod, N nominl bering stress, MP S np nominl net-section bypss stress, MP r, polr coordintes, m, deg 3

5 t W X,Y B e2 U rr *e e specimen thickness, m specimen width, m Crtesin coordintes, m bering-bypss rtio bolt-hole contct hlf-ngle for single contct, deg secondry bolt-hole contct hlf-ngle for dul contct, deg rdil stress component, MP tngentil stress component, MP BEARNG BYPASS TESTNG Test Procedure The test specimen configurtion nd loding combintions re shown in figure 2. The grphite/epoxy specimens were mchined from single [/45/9/ -45]2s pnel. The bolt holes were mchined using n ultrsonic dimond core drill. They were then crefully hnd-remed to produce clernce of.76 mm with the steel bolts. This clernce, 1.2 percent of the hole dimeter, is typicl of ircrft joints. The test system used in this study is shown schemticlly in figure 3. The center of the specimen is bolted between two bering-rection pltes tht re ttched to the lod frme using two lod cells. The ends of the specimen re then gripped nd loded independently by two servo-control systems (clled "upper" nd "lower" in figure 3). Any difference between these two end lods produces bering lod t the centrl bolt hole. This bering lod is mesured 4

6 by the lod cells under the bering-rection pltes. The end lods re synchronized by common input signl; s result, constnt bering-bypss rtio is mintined throughout ech test. A photogrph of the pprtus is shown in figure 4. This photogrph shows the friction grips tht lod ech end of the specimen nd the hed of the steel bolt tht ttches the specimen to the bering-rection pltes. Only smll portion of the specimen edge is visible. Notice tht the bering-rection pltes re bolted to the bering lod cells, llowing either tension or compression bering lods. During compression, the bering-rection pltes prevent specimen buckling. Although not visible in this photogrph, hrdened steel bushings were used between the bolt nd the bering-rection pltes. These 12.7-mm bushings were mchined for sliding fit, llowing the bolt clmp- up force to be trnsmitted to the locl region round the bolt hole. This rrngement ws equivlent to hving clmp-up wsher directly ginst the side of the specimen, s ws used in references 1, 2, nd 3. For the present tests, the bolt ws finger tightened (bout.2 Nm torque) to produce very smll clmp-up force ginst the specimen. The loding nottions for tension nd compression testing re shown in figure 2(b). All tests were conducted t rther slow loding rte of 3.75 N/s. The results re reported in terms of nominl bering stress Sb nd nominl net-section bypss stress S np equtions: clculted using the following sb Pb / td S np -Pp/t(w-d) 5

7 where t is specimen thickness nd w is the width. The bering-bypss rtio / is defined s B 'b ' 'np Throughout ech test, the specimen deformtion ws mesured by displcement trnsducers. These trnsducers were mounted symmetriclly on the front nd bck of the bering-rection pltes, see figure 4. (These pltes were mde from nonmgnetic 347 stinless steel so they would not ffect the trnsducers.) The trnsducer rods rested on smll brs tht were cemented to the specimen slightly bove the grip line. This rrngement provided mesurement of the reltive displcement between the bering-rection pltes nd the specimen. These mesurements were used to determine the onset of dmge. The bering nd bypss lods were plotted ginst the specimen displcement, shown for typicl cse in figure 5. Both the bering nd bypss curves hve smll initil nonlinerity (probbly due to vrying bolt-hole contct) but grdully develop nerly liner response. At higher lod levels, the curves grdully develop second nonlinerity, which indictes dmge t the bolt hole, s mentioned in reference 3. An offset of.1d ws selected to define the dmge-onset lod, s indicted in figure 5. Some specimens were unloded fter the dmge-onset lod level nd were then treted with n X-ry opque dye-penetrnt nd rdiogrphed to determine the dmge-onset mode. Test Results Figure 6 shows rdiogrphs of four dmge-onset modes. For tension dominted loding, the dmge developed in the net-section tension (NT) mode, 6

8 figure 6(). The gry shdows show delmintions nd the drk bnds indicte ply splits. The tension-rected bering (TRB) nd compression-rected bering (CRE!) dmge modes re quite similr, s expected, nd pper to be delmintion dominted. The net-section compression (NC) mode involves rther discrete dmge zones extending from the hole. This dmge ws probbly cused by microbuckling in the plies. The mesured Sb nd S vlues corresponding to dmge onset re np plotted ginst one nother in figure 7, s so-clled bering-bypss digrm. Ech open symbol represents the verge of three tests nd the tick mrks indicte the rnge of the mesured strengths, plotted long lines of constnt /3. The dt in figure 7, lso given in Tble 1, were tken from reference 4. The right side of figure 7 shows tension results for four /3 vlues (, 1, 3, ). The symbol on the S xis represents the ll bypss loding in tension np (/3 - ). The NT next to the symbol indictes net-section tension dmge. As expected, ll the test cses with NT dmge cn be represented by stright line nd, thus, show the liner interction discussed in references 3 nd 5. This linerity suggests tht the locl stresses due to bering loding nd those due to bypss loding ech contribute directly to filure. The "bering-cutoff" line ws drwn through the /3 - dt point. The dmge-onset strengths for the ll bering tension cse (/3 - ) nd the ll bering compression cse (fi - -) differ by bout three percent. However, the bering-cutoff line used for tension does not pper to pply for compression. The CRB dmge mode ws found t / for much lower strength level. The compressive bypss lod hd somewht unexpected effect on the onset of bering dmge. The bering filure for combined bering nd bypss loding ws nlyzed in reference 3 nd resulted from decrese in the bolt-hole contct ngle cused by the 7

9 compressive bypss lod. For the ll bypss compressive loding (B - -O), NC dmge initited t -422 MP, which is much lrger thn the expected compression strength for this lminte. This suggests tht "dul" bolt-hole contct developed, llowing the lod to trnsfer cross the hole, nd, therefore, produced higher strength. This will be discussed lter in n nlysis of the bolt-hole contct. The bering-bypss digrm for the ultimte strengths (solid symbols) is shown in figure 8. To evlute the progression of dmge from onset to ultimte filure, the dmge-onset curves re replotted from figure 7. A generl comprison of these two sets of results shows tht the specimens filed immeditely fter dmge onset when n ll bypss loding (p - nd -) ws used. n contrst, for the ll bering loding (/3 nd -), the specimens filed t considerbly higher lods thn required to initite dmge. Also, when bering nd bypss lods were combined, the specimens showed dditionl strength fter dmge onset, more so in compression. A comptison of the dmge modes indicted in figure 8 shows tht the onset-dmge mode ws the ultimte filure mode in most cses. The exception occurred for the compressive bering- bypss loding. For B - -3, the dmge initited in the CRB mode but the specimen filed in different mode, referred to here s the offset-compression (OSC) mode. Figure 9 shows specimen tht filed in the OSC mode. The filure is typicl of compression filure but developed wy from the specimen net- section. The light region ner the frcture is solid film lubricnt tht trnsferred from the bering-rection plte when the specimen filed. The offset of the filure from the net section is believed to be cused by the CRB delmintion dmge extending beyond the clmp-up re round the hole, thereby inititing compression filure wy from the hole.. This trnsition from the 8

10 CRB dmge-onset mode to the OSC filure mode could lso hppen in multi- fstener joints nd, therefore, my be n dditionl compliction when joint strength predictions re mde for compressive lodings. STRESS ANALYSS n this section, first, the stress nlysis procedures re briefly described. Then, stresses t the hole boundry re shown for selected combintions of bering-bypss loding in tension nd compression. These stress results re then used in the next section to clculte the specimen strengths for the vrious dmge modes observed in the bering-bypss tests. Finite Element Procedures The finite element procedures used in this study were presented nd evluted in reference 6. When bolt clernce is used, s in the present study, the contct ngle t the bolt-hole interfce vries with pplied lod, s shown in figure 1. Using the inverse technique described in reference 6, this nonliner problem is reduced to liner problem. n this technique, for simple bering loding, contct ngle is ssumed nd the corresponding bering lod is clculted. This procedure is repeted for rnge of contct ngles to estblish reltionship between contct ngle nd bering lod. n the present study, this technique ws extended to include combined bering nd bypss loding. For ech bering-bypss rtio p, the combined bering nd bypss loding ws expressed in terms of bering stress Sb nd p. Thus, for the procedure ws identicl to tht used in reference 6. This procedure ws 9

11 repeted to estblish reltionship between contct ngle nd bering-bypss loding for ech p vlue in the test progrm. These clcultions were done using the NASTRAN finite element code. This code is well suited for the inverse technique becuse the contct of the bolt nd the hole cn be represented using displcement constrints long portion of the hole boundry. Displced nodes on the hole boundry were constrined to lie on circulr rc corresponding to the bolt surfce. This represent rigid bolt hving frictionless interfce with the hole. A very fine two-dimensionl mesh ws used to model the test specimen. Along the hole boundry, elements subtended less thn 1 of rc. As result, the contct rc could be modeled very ccurtely. Stress Results Figure 11 shows the hole-boundry stress distributions for three tension loding cses. The first cse is n ll bypss tension loding (B = ); the hole-boundry tngentil stress ogb is shown s dsh-dot curve. t hs the expected pek t 8-9, the specimen net-section, The vlue of this obb pek cn be used to predict the pplied lod for dmge onset in the NT mode, s demonstrted in reference 2. The second cse in figure 11 represents n ll bering tension loding (j3 = -) of 4 MP with zero clernce between the bolt nd the hole; ueb nd Q rr re shown s dshed curves. The pek vlue of the Q rr curve governs the bering dmge onset. The urr curve lso indictes contct ngle of bout 82 ; the pek vlue in the ge curve occurs slightly beyond the end of the contct ngle. The third cse in figure 11, the solid 1

12 curves, lso represents n ll bering cse with Sb - 4 MP, but with the.76 mm clernce used in the tests. Comprison of the orr curves for the two ll bering cses shows tht the clernce reduced the contct ngle by bout 2 nd incresed the orr bering stress. Thus, clernce tends to decrese bering strength. Hole-boundry stresses for tension bering-bypss loding re shown in figure 12. These results correspond to bering stress Sb of 4 MP with three tension bypss stress S np S np levels. The urr curves show tht incresing the bypss lod, cused the contct ngle to increse. However, the pek D vlue chnged very little for incresing S suggesting tht tension rr np bypss loding hs little influence on the bering strength. n contrst, incresing the tension bypss stress levels produced higher corresponds to lower net-section tension strengths. odd peks, which Hole-boundry stresses for compression bering-bypss loding re shown in figure 13. Agin, the bering stress Sb ws 4 MP. The urr curves show tht incresing the compressive S np vlue produced smller contct ngles nd correspondingly higher pek urr vlues. This shows tht the vlue of S np influences the bering strength, s shown previously in figure 7. The u tensile peks were nerly the sme for the rnge of S vlues but the orr np compressive peks vried widely. The curve for S - 5 MP illustrtes dul np contct; notice the D contct stress ner rr The contct ngle trends discussed in figures 12 nd 13 re summrized in figure 14. The contct ngles, B1 nd B2 defined by the insert, re ee 11

13 shown for rnge of tension nd compression bypss lodings with Sb - 4 MP ncresing the tensile bypss loding incresed 61, while incresing the compressive bypss loding hd the opposite effect. The smll jog in the curve t S "P - is cused by the smll difference between tension-rected bering nd compression-rected bering. Notice tht dul contct initited for compressive bypss stress of bout 45 MP. The secondry contct ngle 62 incresed rther bruptly s Additionlly, the decresing the compressive S exceeded this vlue. np trend for 61 reversed when dul contct developed. Dul contct provides pth for the pplied lod to "bridge" cross the hole rther thn divert round it. This reduces the stress concentrtion in the net section nd produces higher net-section compression strength thn with no dul contct, STRENGTH CALCULATONS As previously mentioned, lminte strengths for bering-bypss loding were clculted using the locl stresses t the bolt hole. These clcultions were used to study the strength trends for the rnge of loding combintions nd the different filure modes. The dmge-onset strengths were clculted using the pek hole-boundry stresses, s in reference 2. The onset of dmge ws ssumed to occur when the pek hole-boundry stress reched criticl vlue for ech dmge mode. For ech mesured dmge-onset strength, the pek stress ws clculted nd the verge of the peks in ech dmge mode ws ssumed to be the criticl vlue for tht dmge mode. The following criticl vlues were clculted for ech dmge mode: 869 MP for NT; -76 MP for TRB; -88 MP for 12

14 CRB; nd -817 MP for NC. The solid cumes in figure 15 represent the dmge- onset strengths clculted in this mnner. The verge dmge-onset strength dt from figure 7 re replotted s open symbols in figure 15. The clculted curves gree with the dt trends for strength. Also, the clculted dmge modes gree with those discussed erlier. This demonstrtes tht dmge-onset strengths cn be predicted from the pek hole-boundry stresses if criticl stress vlue is known for ech dmge mode. The ultimte strengths for bering-bypss loding were clculted using the well-known point-stress criterion (reference 7). The stress distribution clculted long the observed direction of dmge growth ner the hole ws compred with the lminte strength to determine chrcteristic dimension. This filure theory ws fit to the test dt for ech filure mode to determine n verge chrcteristic dimension. As explined in reference 4, the following chrcteristic dimensions were determined for ech filure mode: 2.44 mm for NT; 2.22 mm for TRB; 3.87 mm for CRB; 8.89 mm for OSC; nd 2.54 mm for NC. The strength clcultions must gree with the dt verges for ech filure mode. As expected, the clcultions lso greed with the trends for the strength dt, s shown by the dsh-dot curves nd solid symbols in figure 15. The correltion between the strength clcultions nd the strength mesurements in figure 15 suggests tht combined nlyticl nd experimentl pproch could be used to predict bering-bypss digrms from few tests. Such tests could be conducted for the ll bypss ( - ) nd the ll bering (p - eo) cses to determine the criticl mteril strength prmeters, which could then be used with stress nlysis to construct curves for the more complicted cses of bering-bypss loding. This pproch seems vible for tension bering-bypss loding becuse only two filure modes re involved. But for compression loding, there re three ultimte filure modes. The OSC mode 13

15 occurs only for combined bering nd bypss loding, so this filure mode cnnot be evluted using simple loding. Therefore, strength clcultions for compression would require bering-bypss tests for t lest one jl vlue to find the OSC mteril strength prmeter. CONCLUDNG REMARKS A combined experimentl nd nlyticl study hs been conducted to investigte the effects of combined bering nd bypss loding on grphite/epoxy (T3/528) lminte. Tests were conducted on single-fstener specimens loded in either tension or compression. Test specimens consisted of 16-ply, qusi-isotropic grphite/epoxy lmintes with centrlly-locted hole. Bering lods were pplied through steel bolt hving clernce fit. Onset dmge, ultimte strengths, nd the corresponding filure modes were determined for ech test cse. A finite element procedure ws used to clculte the locl stresses round the bolt hole. A dul-control test system, described in this pper, ws used to successfully mesure lminte strengths for wide rnge of bering-bypss lod rtios in both tension nd compression. The tension dt showed the expected liner interction for combined bering nd bypss loding where the dmge developed in the net-section tension mode. However, the bering dmge-onset strengths in compression showed n unexpected interction of the effects for the bering nd bypss lods. Compressive bypss lods reduced the bering onset strength. The compressive bypss loding decresed the bolt-hole contct rc, which incresed the severity of the compressive bering lods. Compressive 14

16 bering-bypss loding lso produced n off-set compression (%) filure mode, not previously reported in the literture. The present stress nlyses showed tht combined bering nd bypss loding cn hve significnt influence on the bolt-hole contct ngles. These contct ngles hd strong influence on the locl stresses round the hole; they must, therefore, be ccurtely represented in strength clcultion procedures tht re bsed on locl stresses. Although the trends in the bolt-hole strength for combined bering nd bypss loding were ccurtely clculted using locl stresses together with filure criteri, some bering-bypss testing is required to predict the compression response. The lminte response under compression bering-bypss loding involved the OSC filure mode, which cnnot be evluted by simple loding conditions. Therefore, predictions of lminte bering-bypss strengths my need to be combined with selected bering-bypss testing to ccount for the off-set compression filure mode. 15

17 REFERENCES 1. Crews, J. H., Jr.: "Bolt-Bering Ftigue of Grphite/Epoxy Lminte," Joining of Composite Mterils, ASTM STP 749, K. T. Kedwrd, Ed., Americn Society for Testing nd Mterils, 1981, pp Crews, J. H,, Jr. nd Nik, R. A.: "Filure Anlysis of Grphite/Epoxy Lminte Subjected to Bolt-Bering Lods," Composite Mterils: Ftigue nd Frcture, ASTM STP 97, H. T. Hhn, Ed., Americn Society for Testing nd Mterils, 1986, pp Crews, J. H., Jr. nd Nik, R. A.: "Combined Bering nd Bypss Loding on Grphite/Epoxy Lminte," Composite Structures, Vol. 6, 1986, pp Nik, R. A.: "An Anlyticl nd Experimentl Study of Clernce nd Bering- Bypss Lod Effects in Composite Bolted Jointstus Ph.D. disserttion, Old Dominion University, Norfolk, Virgini, August Hrt-Smith, L. J.: "Bolted Joints in Grphite/Epoxy Composites, NASA CR , Ntionl Aeronutics nd Spce Administrtion, Jnury Nik, R. A. nd Crews, J. H. Jr.: "Stress Anlysis Method for Clernce- Fit Bolt under Bering Lods," AAA Journl, Vol. 24, No. 8, August 1986, pp Whitney, J. M. nd Nuismer, R. J.: "Stress Frcture Criteri for Lminted Composites Contining Stress Concentrtions," J. Composite Mterils, Vol. 8, July 1974, pp

18 Tble 1 Lminte strengths for bering-bypss loding. Bering - bypss rtio, B Dmge -nse t Sb(MP) S (MP) Mode np Ultimte Filure Tens ion 34 NT 33 NT NT NT NT NT Q) 542 TRB 812 TRB ComDression NC -422 NC crb/nc osc CRB osc 528 CRB 853 CRB 17

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34 Stndrd Bibliogrphic Pge 1. Report No. NASA TM Government Accession No. Bering-Bypss Loding on Bolted Composite Joints 7. Author(s) J. ti. Crews, Jr. nd R. A. Nik 9. Performing Orgniztion Nme nd Address Ntionl Aeronutics nd Spce Administrtion Lngley Reserch Center Hmpton, VA Sponsoring Agency Nme nd Address Ntionl Aeronutics nd Spce Administrtion Wshington, DC Recipient s Ctlog No. 5. Report Dte My Performing Orgniztion Code ~~ ~~~ 8. Performing Orgniztion Report No. 1. Work Unit No Contrct or Grnt No. 13. Type of Report nd Period Covered T p c h n i c l r n 14. Sponsoring Agency Code 15. Supplementry Notes 16. Abstrct A combined experimentl nd nlyticl study hs been conducted to investigte the effects of simultneous bering nd bypss loding on grphite/epoxy (T3/528) lminte. Tests were conducted with test mchine tht llows the bering-bypss lod rtio to be controlled while single-fstener coupon is loded to filure in either tension or compression. Test coupons consisted of 16-ply qusi-isotropic grphite/epoxy lmintes with centrlly-locted 6.35-mm bolt hving clernce fit. Onset-dmge nd ultimte strengths were determined for ech test cse. Next, finite element stress nlysis ws conducted for ech test cse. The computed locl stresses were used with pproprite filure criteri to nlyze the observed filure modes nd strengths. An unexpected interction of the effect of the bypss nd bering lods ws found for the onset of compression-rected bering dmge. This interction ws cused by decrese in the bolt-hole contct rc nd corresponding increse in the severity of the bering lods. The mount of bolt-hole contct hd significnt effect on locl stresses nd, thus on the clculted dmge-onset nd ultimte strengths. An offset-compression filure mode ws identified for lminte filure under compression bering-bypss loding. This filure mode ppers to be unique to compressi on beri ng-bypss loding nd, therefore, cnnot be predicted from simple tests. 17. Key Words (Suggested by Authors(s)) Lnii nte Bo1 t Bering Grphi te/epoxy Strength 19. Secirity Clssif.(of this report).. l l f l P d Stress nlysis Composites Joint 18. Distribution Sttement Uncl ssi f ied - Unl i mi ted Subject Ctegory Security Clssif.(of this pge) 21. No. of Pges 22. Price.. Unclssified 33 A 3 NASA Lngley Form 63 (June 1985) For sle by the Ntionl Technicl nformtion Service, Springfield, Virgini 22161