17 IMPACT PROPERTIES OF COMPOSITES

Transcription

1 17 IMPACT PROPERTIES OF COMPOSITES 17-1

2 Impact resistance is the ability f a material t absrb and dissipate energies under impact r shck lading. The respnse t impact lads ranges frm lcalized damage t ttal disintegratin. Even lcal damage can be serius since it can lead t delaminatin and ther effects. During impact lading fracture mdes may be significantly different than static tensile failure. This is particularly true fr strain rate sensitive materials. IMPACT MEASUREMENT TECHNIQUES Impact testing is nrmally perfrmed with a swinging r a drpping weight. The bject f these tests is t measure the amunt f energy transferred r "lst" frm the striking bdy. Fr bdies withut flaws r ntches, the transferred energy cnsists f energy t initiate crack and energy t prpagate a crack. The tw cmmn swinging weight impact test methds are the Charpy and Izd methds shwn in Fig In these methds a ntched bar is impacted by a striker-pendulum released frm a standard height. The height f the pendulum swing past the impact is a measure f the amunt f energy absrbed by the impact. The shrter the after-swing the greater the Striking edge 95.3 mm (3.75 in.) Striking edge Specimen Specimen mm (.866 in.) Anvil Specimen supprt Jaw Charpy Izd Figure 17-1 Swinging weight impact testing methds energy absrbed by the specimen and hence the greater impact resistance r tughness. The absrbed energy is expressed per unit crss sectin f specimen. Typical results f Charpy and Izd impact tests n unidirectinal cmpsites and cmmn structural allys are given in Table Amng the cmpsites, impact resistance is strngly influenced by the ductility r strain t fracture f the fiber. This can be seen in Table 17-1 where the S-glass cmpsite has very high impact resistance, while high mdulus carbn fiber cmpsite have lw impact resistance. In recent years the drp-weight methd has becme the preferred technique fr impact testing f cmpsites because a greater range f testing parameters is pssible and the 17-

3 results are mre readily analyzed. A typical drp-weight testing apparatus is shwn in Fig.17-. In the drp-weight test the energy absrbed by structure prir t failure resulting frm high strain rate r impact lad. Drp-weight test stands are equipped with Table 17-1 V-Ntched Charpy and Izd impact f unidirectinal cmpsites MATERIAL IMPACT ENERGY, ft-lbs/in Charpy Izd S-glass/Epxy (55% fibers) B/Epxy (55% fibers) Kevlar/Epxy (6% fibers) AS Carbn/Epxy (6% fibers) HMS Carbn/Epxy (6% fibers) 11 4 T3 Carbn/Epxy Steel T6 Aluminum Ally T6 Aluminum Ally 3 - Electrmagnet Weights Tup Specimen Lad cells High speed camera Interval Timer Interval Timer Figure 17-. Drp-weight impact measuring apparatus 17-3

4 lad measuring devices (usually lad cells) either in the specimen supprting platfrms as shwn in Fig.17- r in the tup. Phtelectric cells are used t time the tup velcity just prir impact. High speed cameras can als be used t recrd the impact events and the prpagatin mde. A typical lad-time curve recrded by these devices is illustrated in Fig The initial rise in the lad-time curves crrespnds t the fracture initiatin P max fiber fracture matrix cracking Lad initiatin Time prpagatin Figure 17-3 Typical lad-time trace fr drp-weight impact test n cmpsite phase. In sme cases a lcal peak is recrded due t matrix cracking. The maximum lad crrespnds t fiber fracture. The prtin f the trace beynd the maximum crrespnds t the prpagatin phase. The area under the lad-time curve, Pdt, is called the impulse. The ttal energy absrbed during impact is t f E = Pvdt (17.1) t where P is the instantaneus lad and v is the instantaneus velcity recrded during the impact. t is the time f initial impact, usually taken as, and t f is the time at fr cmpletin f penetratin. The impact energy can be cnsidered t cnsist f the initiatin energy, E i and the prpagatin energy, E p. The instantaneus velcity v cannt be measured cntinuusly during impact but nly at beginning and end f impact, v and v f, respectively. Hence an apparent energy can be defined as a t f E = v Pdt (17.) A clser apprximatin t the actual energy can be btained by replacing the instantaneus velcity v with the average velcity during penetratin, v 17-4

5 t f E = v Pdt (17.3) Using the relatin between impulse, Pdt and mmentum, mv ( vf ) apprximating the average velcity as ( v v f ) initial velcity is and +, the average velcity in terms f the v E a = v 1 4 E (17.4) where E 1 = mv, the maximum energy available during impact and m is the mass f the tup. We nw have an expressin fr the average tup velcity as it penetrates the specimen in terms f the kinetic energy just prir t impact and the measured impact energy. Cmbining Eqns.(17.3) and (17.4) gives E a E = Ea 1 4 E (17.5) ANALYSIS OF LAMINATE COMPOSITE IMPACT Cnsider a laminate impacted using the three pint bending cnfiguratin shwn in Fig Lad h b L L Y Z q X 1 Figure Cmpsite plate impact test cnfiguratin. Static analysis is perfrmed prir t impact t determine the static flexural prperties. Fr a symmetric laminate laded in ne plane f bending nly, as wuld be the case fr the simply supprted plate shwn in Fig.17-4 the curvature are given as 17-5

6 k x D 11 D 1 D 16 Mx k y D1 D D = 6 k xy D 16 D 6 D 66 (17.6) Frm this result it is apparent that k xy is nn-zer, hence sme twisting will take place. This phenmenn is referred t as lift-ff. Lift-ff will nt ccur in a crss-ply cmpsite as D16 = D6 =. Fr ff-axis lamina the twisting curvature can be minimized fr lng ply lengths. This methd was applied t the study f impact perfrmance f a unidirectinal and crss-ply laminate tested with fiber rientatins ranging frm º t 9º by Mallick and Brutman. The effective flexural mdulus given by E 1 = (17.7) 11 3 Dh 11 was calculated fr the tw laminates and are given in Tables 17- and The experimentally measured flexural mduli and strength are als presented in these tables. Table 17- Effective flexural mdulus fr "unidirectinal" laminate /9/ 4 / S Calculated Measured Strength (ksi) /9 / S Table 17-3 Effective flexural mdulus fr crss-ply laminate ( ) 3 Calculated Measured Strength (ksi) Fr pure flexure in a symmetric laminate { ε } [ ]{ } = z D M z (17.8) 17-6

7 Applying Hke's law t the rthtrpic laminate the stress in k th layer is then k x x ( ) σ = zm Q D + Q D + Q D (17.9) Hence, we can use abve expressin t calculate which layer breaks first. Fig.17-5 shws the stress distributin in each ply thrugh the thickness if the unidirectinal laminate. Ply # Cmpressin Tensin.1" 6 4 σ I m h Figure 17-5 Stress distributin thrugh the thickness if the unidirectinal laminate The uter-layer stress fr "unidirectinal" at varius fiber angles f lading is shwn in Fig The minimum stresses fr bth tensin and flexure ccur near 6º. Nw we can interpret the results f dynamic lading in terms f the static lading. In the instrument dynamic test, a typical lad-time traces fr the impact f the unidirectinal laminate at varius fiber angles are shwn in Fig The drp-weight impact energies calculated frm these results as a functin f fiber rientatin are pltted in Fig17-8. The impact energy is smewhat flat frm fiber rientatins between 3º and 7º. Fr the º rientatin the impact damage manifests itself s an area f delaminatin as represented in Fig.17-9a. The ff-axis impact tests result in fractures that prpagate either parallel r nrmal t the fiber directins, as illustrated in Fig.17-9b. The frmer mde f damage absrbs cnsiderably greater energy that the latter as seen in Fig

8 1 3 s max x 1,psi [ / 9 / / ] s 4 Flexure Tensin q 45 9 Figure Outer-layer stress fr "unidirectinal" laminate at varius angles f lading Figure 17-7 Lad-time trace fr unidirectinal laminate at varius fiber rientatins 17-8

9 1 Drp Weight Impact Energy, E f ft-lb CROSS PLY UNI 45 9 q Figure 17-8 Drp-weight impact energies fr crss-ply and unidirectinal laminates a). º ply b). ff-axis ply Figure 17-9 Impact failure mde depending upn fiber rientatin RELATION BETWEEN INTERFACIAL STRENGTH FRACTURE ENERGY Lamina with high strength cnstituents and lw interfacial bnding generally result in high impact tughness. Laminates cnsisting with high impact tughness plies but lw interfacial strength fail by in delaminatin. Bth debnding and delaminatin are high energy failure prcesses since the vlume f affected material is large. Fig.17-1 is a schematic representatin f the relatin between fracture energy and interfacial strength. Fracture energy r tughness is a maximum when the interfacial strength is minimum. As the interfacial strength is increased the fracture energy is reduced t a critical pint where fiber fracture becmes the dminant failure mechanism and fracture energy increases

10 Delaminatin Fiber tensile failure Fracture Energy Ttal Prpagatin Initiatin Interfacial Strength Figure 17-1 Relatin between fracture energy and interfacial strength FRACTURE MECHANISMS There are numerus failure mechanisms that can ccur individually r in cmbinatin. Each f these mechanisms will cntribute t the fracture prcess with a characteristic fracture energy. These mechanisms include: fiber fracture, fiber debnding, matrix yielding, fiber pullut, ply delaminatin, matrix fracture, and fiber yielding. Varius mdels have been develped t describe each f these mechanisms. Fiber fracture In a discntinuusly reinfrced cmpsite, the energy fr fiber fracture, E Ff per unit area f fracture surface, A c is principally cntrlled by the strain energy stred in the fiber at fracture and the vlume f fibers in the cmpsite. The fiber fracture energy is then E Ff f fu A c V σ l = 6E f (17.1) where l is the fiber length. Fiber pullut The frictinal frce between the fiber and the matrix, which in turn is influenced by the critical fiber length, l c, cntrls the fiber pullut energy,. The pullut fracture energy is then E A Fp f fu c c V σ l = 1 (17.11) 17-1

11 Matrix yielding A mdel fr the absrptin f energy by matrix yielding during cmpsite fracture is represented schematically in Fig In this mdel there is a characteristic distance, X Matrix defrmatin zne D X Fibers Figure Mdel fr matrix yield cntrlled failure ver which plastic defrmatin ccurs during crack prpagatin. The energy f fracture per unit area due t plastic defrmatin f the matrix is E Fm XVmUm A = (17.1) c where U m is the wrk t defrm the matrix per unit vlume. This quantity can be btained frm the area under a lad-deflectin curve fr the matrix. Assuming that the tensile frces in the matrix are transferred thrugh shear frces alne the fiber surfaces the fracture energy per unit area by matrix yielding is E V σ DU = A 4V τ Fm m mu m c f m (17.13) where τm is the shear strength f the matrix. Ductile matrix and large fiber diameters result in tugher cmpsites. BALLISTIC IMPACT AND FOREIGN OBJECT DAMAGE A significant cncern fr cmpsite aircraft panels and structures is freign bjects damage, (FOD) caused by the impact f rivets, hail stnes, pebbles, rck salt, tire tread debris, and birds. This type f damage is particularly serius in gas turbine engines during take-ffs and landings when sudden pwer lsses can be catastrphic. Cmpsite gas turbine engine cmpressr and fan materials are subjected t stringent tests t evaluate resistance t FOD. The cnfiguratin fr FOD testing is shwn in Fig

12 Prelad RTV pellet (6-35 fps) B /Al panel Figure FOD test cnfiguratin fr cmpsite materials In this test an elastmer pellet is impacted at varius velcities against a cmpsite panel preladed in tensin. Typical pssible damage as a functin f velcity is illustrated in Fig Impact velcity, fps Figure Typical damage patterns in cmpsite plates during FOD tests There is a minimum velcity when damage in the frm f delaminatin r matrix crazing first ccurs. At a higher velcity cmpsite cracking, generally by fiber matrix separatin takes place. Higher velcities can cause even greater amunts damage such as partial r cmplete material break-ut. At ballistic impact velcities the pellet can cause a clean shear-ut f material clse t the size f the prjectile. The effect f the impact velcity n the residual strength f the panel is illustrated in Fig As the pre-lad is 17-1

13 increased the residual strength f the impacted panel is seriusly degraded. There is a critical velcity at which the residual strength degradatin is a maximum. Abve this velcity damaged area becmes reduced thus having a smaller effect n residual strength. Nrmalized Residual Strength 1..5 Prelad Ksi Ksi 44 Ksi 9 ksi 15 3 Velcity, ft/sec Figure Effect f velcity and prelad n the residual strength f impacted cmpsite panels