Prediction of transverse fatigue behavior of unidirectionally reinforced metal matrix composites

PREDICTION OF TRANSVERSE FATIGUE BEHAVIOR OFUNIDIRECTIONALLY REINFORCED METAL MATRIX

COMPOSITES

Reji John*, Dennis J. Buchanan* and James M. LarsenMaterials and Manufacturing Directorate, Air Force Research Laboratory (AFRL/MLLN), Wright

Patterson Air Force Base, OH 45433-7817*Structural Integrity Division, University of DaytonResearch Institute, Dayton, OH 45469-0128

(Received March 30, 1998)(Accepted in revised form August 22, 1998)

Introduction

Unidirectionally reinforced metal matrix composites (MMC) are targeted for use in many aerospaceapplications which require high specific strength and stiffness at elevated temperatures [1,2]. Suchapplications include blings and disks. The primary weakness of a component made of unidirectionallyreinforced MMC is its susceptibility to transverse loads. The strength of the component in the transversedirection is significantly lower than that in the longitudinal direction under monotonic, sustained andfatigue loading conditions [1–12]. Figure 1 shows the effect of orientation of the reinforcement on thefatigue behavior of unidirectionally reinforced SCS-6/Ti-24Al-11Nb [10,11]. Note that [0] and [90]imply longitudinal and transverse loading, respectively. The data correspond to tests conducted at roomtemperature with load ratio (5minimum load/maximum load)5 0.1 in lab air. The tensile strengthunder transverse loading is' 20% of tensile strength under longitudinal loading and' 43% of thematrix tensile strength. Similarly, the threshold stress under transverse loading is also significantlylower than the [0]8 MMC and the matrix. Hence, replacement of monolithic components with MMCcomponents requires that the transverse strength of the MMC should be predicted accurately. This paperdiscusses the applicability of a net-section based model to predict the fatigue behavior of [90] MMCunder transverse loading.

Materials and Experimental Results

The data analyzed in this paper were obtained from Buchanan et al. [9], Gambone [10] and Hall et al.[11]. Two types of MMC are discussed in this paper: (1) Ti-24Al-11Nb (wt%) unidirectionallyreinforced with silicon carbide fibers (SCS-6TM), SCS-6/Ti-24Al-11Nb [10,11] and (2) Ti-6Al-2Sn-4Zr-2Mo (wt%) unidirectionally reinforced with silicon carbide fibers (Trimarc-1TM), Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo [9]. SCS-6TM is a silicon carbide fiber with a carbon core and two layers of exteriorcarbon coating ('1 mm thick/layer). The diameter of SCS-6TM fiber is ' 142mm. Trimarc-1TM is alsoa silicon carbide fiber with a Tungsten core and three layers of exterior carbon coating ('1 mmthick/layer). The diameter of Trimarc-1TM fiber is ' 127 mm. SCS-6/Ti-24Al-11Nb is a 8-plycomposite fabricated using a foil/fiber/foil technique through hot isostatic pressing (HIP) [10,11]. The

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average thickness of the SCS-6/Ti-24Al-11Nb specimens was' 1.9 mm and the volume fraction offibers, Vf ' 0.32. Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo is a 10-ply composite fabricated using the wirewinding technique [9]. The average thickness of the Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo specimens was'2.26 mm and the volume fraction of fibers, Vf ' 0.29. SCS-6/Ti-24Al-11Nb was tested in laboratoryair at 21°C with frequency5 1–5 Hz and load ratio (5minimum load/maximum load) R5 0.1.Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo was tested in laboratory air at 163°C with frequency5 3 Hz and R50.1.

The prediction of the fatigue behavior of the MMC requires the characterization of the fatiguebehavior of the corresponding matrix. In Refs. [9–11], fatigue data from fiberless (neat) matrices werereported. These matrix plates were fabricated using the procedure identical to that used for the MMC.Accordingly, Ti-24Al-11Nb foils and Ti-6Al-2Sn-4Zr-2Mo wires used to make the matrix plates. Thematrix specimens were cut from the plate consistent with the direction of the layup of the matrix in theMMC. For Ti-6Al-2Sn-4Zr-2Mo, the length of the specimens was aligned perpendicular to the wirewinding direction. Figure 2 shows the data for Ti-24Al-11Nb [11] and Ti-6Al-2Sn-4Zr-2Mo [9] fromtests conducted at R5 0.1 in laboratory air. Ti-24Al-11Nb was tested at 21°C and Ti-6Al-2Sn-4Zr-2Moat 163°C. The following equation was used to describe the fatigue behavior of the matrix.

Figure 1. Effect of orientation of reinforcement on fatigue behavior of SCS-6/Ti-24Al-11Nb. Data from Refs. [10] and [11]. Fibervolume fraction, Vf ' 0.32.

Figure 2. Fatigue behavior of fiberless Ti-6Al-2Sn-4Zr-2Mo and Ti-24Al-11Nb.

FATIGUE OF METAL MATRIX COMPOSITES1530 Vol. 39, No. 11

sm,max5 sm,th 1 (sm,u 2 sm,th)eC1(logNf)C2 (1)

wheresm,max5 maximum matrix stress,sm,th 5 matrix threshold stress,sm,u 5 matrix ultimate tensilestrength, Nf 5 cycles to failure, and C1 and C2 are constants. These values are shown in Table 1 for thetwo fiberless matrices. The constants C1 and C2 were obtained based on data from relatively few fatiguetests as shown in Fig. 2. Since, fatigue data are typically associated with significant scatter (2–3X),additional tests are required to adequately characterize the fatigue behavior of the neat matrices.

Typical fracture profiles of failed SCS-6/Ti-24Al-11Nb and Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo spec-imens are shown in Figures 3 and 4, respectively. These figures show that the failure surfacecorresponds to the narrowest matrix ligament between the fibers. The surfaces of the failed matrixligaments between the fibers are approximately perpendicular to the loading direction. Based on theseobservations, a net-section based model was used to predict the fatigue behavior of transversely loadedMMC as described next.

Net-Section Model

The schematic of a unit cell for a rectangular arrangement of fibers is shown in Fig. 5 [8,9]. In thisfigure, Rf 5 radius of fiber, B5 thickness of composite, Bp 5 average ply thickness5 B/n, n 5number of plies, and s5 center-to-center fiber spacing along the length of the specimen. Using loadequilibrium, the far-field applied stress for the composite,s90, can be related to the stress in the matrixligament,slig as,

s90 5 slig Fn, (2)

where,

TABLE 1Constants Used to Describe Fatigue Behavior of Fiberless Matrix.

Matrix sm,u (MPa) sm,th (MPa) C1 C2

Ti-24Al-11Nb (21°C, R50.1) 512.6 349.8 20.001761 5.254Ti-6Al-2Sn-4Zr-2Mo (163°C, R50.1) 840.0 325.0 20.05367 2.751

Figure 3. Fracture profile of [90]8 SCS-6/Ti-24Al–11Nb.

FATIGUE OF METAL MATRIX COMPOSITES 1531Vol. 39, No. 11

Fn 5 1 22Rf

Bp. (3)

Using the geometry parameters shown in Fig. 5, the volume fraction of fibers, Vf can be derived as,

Vf 5pRf

2

Bps. (4)

Note that s5 0.197 and 0.195 mm for SCS-6/Ti-24Al-11Nb and Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo,respectively. Substituting Eqn. (4) in Eqn. (3), we obtain,

Fn 5 1 2 Î4Vf s

pBp. (5)

Figure 4. Fracture profile of [90]10 Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo.

Figure 5. Schematic of the net-section model.


For a square arrangement of fibers with s5 Bp, Eqn. (4) becomes identical to that proposed by Wallset al. [6]. The net-section based model has been used to successfully predict the transverse creep-rupture[8] and fatigue crack growth parallel to fibers [13,14] in various MMC systems.

Thus, combining Eqns. (1) and (2), the composite maximum stress,s90,max versus Nf can bepredicted using Eqn. (6).

s90,max5 { sm,th 1 (sm,u 2 sm,th)eC1(log Nf)C2} Fn (6)

Using the same approach, the MMC ultimate tensile strength,s90,uand the MMC threshold stress,s90,th

can be predicted using Eqn. (7).

s90,u 5 sm,u Fn and s90,th 5 sm,th Fn (7)

Results and Discussion

The transverse fatigue behavior of the MMC predicted using Eqn. (6) is compared with the data fromSCS-6/Ti24Al-11Nb and Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo in Figs. 6 and 7, respectively. The proposedmodel predicts the following features satisfactorily:

(1) Average tensile strength of the [90] MMC (5s90 at Nf 5 1),(2) The threshold stress of the [90] MMC, and(3) The overall trend of the maximum stress versus cycles (S-N) behavior, i.e. near-flat S-N behavior

relative to that of the matrix.

The proposed model assumed that (1) the matrix ligament between the fibers is always perpendicularto the loading axis and (2) the fibers are arranged in perfectly aligned rows. As shown in Figs. 3 and4, some of the matrix fracture surfaces are not perpendicular to the loading axis and the fibers show astaggered arrangement. The influence of these features can be estimated assuming an increase in thematrix net-section. The prediction based on 10% increase in the matrix ligament surface is shown asdashed lines in Figs. 6 and 7. The dashed lines appear to correlate better with the data at low stressesor longer lives. This implies that the orientation of the fracture surface with respect to the loading axis

Figure 6. Fatigue behavior of [90]8 SCS-6/Ti-24Al-11Nb.l from Hall et al.f & e from Gambone.e 5 specimens stoppedprior to failure.


could depend on the maximum stress or stress range. Detailed fractographic studies (similar to thatdiscussed in Ref. [14]) are required to determine these effects.

Comparing the difference betweensu andsth for the matrix and the corresponding composite, weconclude that the fatigue range (5su 2 sth) for the composite loaded in the [90] orientation is only'40% of that of the fiberless matrix. The resulting near-flat S-N trend is correctly predicted by themodel as shown in Figs. 6 and 7. This S-N behavior of the [90] MMC is also associated with significantscatter ('5X) in the measured Nf for s90 # 200 MPa. Hence, a damage tolerance based design fortransverse loading may be impractical for some MMC systems. In these components, the transversestresses should always be less thans90,th predicted by Eqn. (7).

For typical MMC with Vf ' 0.3, the stress-correction term Fn ' 0.40 implying thats90,u' 0.4sm,u

ands90,th' 0.4sm,th, i.e. the transverse tensile strength and threshold stress (and other parameters suchas creep-rupture strength) of the MMC component is' 60% lower than that of the correspondingmonolithic component. Consequently, the use of unidirectionally reinforced MMC is restricted tocomponents with low transverse loads.

The proposed approach neglected the:

(1) load-carrying capacity of the fibers,(2) influence of residual stresses due to processing,(3) non-uniform stress distribution in the matrix ligament between the fibers,(4) potential for cracks emanating from the fiber/matrix interface, and(5) influence of non-uniform fiber distribution.These issues are discussed next.

The normalized tension stress-strain curve for the two MMC systems [11,15] under transverseloading is shown in Fig. 8. A “knee” (onset of change in slope) in the curve occurs at' 68% of ultimatestrength for SCS-6/Ti-24Al-11Nb and' 40% of ultimate strength for Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo.Interestingly, these stresses are close to the MMC threshold stress indicated by the data and predictedby the model in Figs. 6 and 7. The onset of the change in slope corresponds to the stress required toexceed the residual stresses and the fiber-matrix bond strength [4,16]. Using edge replicates, Johnsonet al. [4] showed that this change in slope was also associated with beginning of the separation between

Figure 7. Fatigue behavior of [90]10 Trimarc-1/Ti-6Al-2Sn-4Zr-2Mo.l & L from Buchanan et al.L 5 specimens stoppedprior to failure.


the fiber and the matrix. Hence, the proposed method of neglecting the fibers can be expected to be validfor all composite stress levels greater than the stress corresponding to the onset of debonding at thefiber/matrix interface.

The mismatch of coefficient of thermal expansion between the matrix and the fiber results insignificant residual stresses in the MMC after consolidation [2,17]. The residual stresses in the hoopdirection are typically tensile in the matrix and range from 250 to 450 MPa depending on the propertiesof the fiber and the matrix [17]. Hall et al. [11] reported that the residual stresses decrease during fatigueloading. This decrease in residual stress will depend on the applied stress and stress range. The tensileresidual stresses will increase the effective mean stress and load ratio in the matrix ligament in theMMC. If the effective mean stresses exceed the yield stress of the matrix, the processing-inducedresidual stresses will be zeroed during the first load/unload cycle. In addition, the stress distribution inthe matrix ligament may not be constant as assumed in the proposed model (see Fig. 5). These variousissues require a detailed micromechanical analysis of a unit cell [16] under various loading conditions.

The fiber packing factor, 2*Rf/Bp 5 0.55–0.60 for the TMC systems studied during this investiga-tion. The stress concentration at the hole edge (5maximum stress/slig) is ' 1.4–1.5 for this fiberpacking factor [18]. Hence, in some TMC systems cracks could emanate from the fibers/matrixinterface during transverse fatigue loading, as reported by Castelli [19] of SCS-6/Ti-15V-3Cr-3Al-3Sn.The proposed model does not account for such crack growth. If crack growth occurs during fatigueloading, the measured fatigue life of the composite can be expected to be lower than the life predictedby the proposed model. Buchanan et al. [15] reported that the global fracture surface is not alwaysperpendicular to the loading axis as shown in Figs. 3 and 4. The orientation of the fracture surface withrespect to the loading axis appeared to be sensitive to the non-uniformity of the fiber distribution andfeatures such as fiber touching. Work is in progress to analyze the influence of the fiber distribution onthe observed scatter in the transverse fatigue life of the MMC [15].

Summary

A net-section based model was used to successfully predict the overall isothermal fatigue behavior ofunidirectionally reinforced MMC subjected to transverse loading. The model correctly predicts therelatively flat transverse fatigue behavior (S-N) of the MMC including the ultimate strength during

Figure 8. Normalized stress versus normalized strain curve for the two MMC systems. Data from Hall et al. [11] and Buchananet al. [9].


tension loading and the threshold stress. The threshold stress is close to the stress corresponding to theonset of debonding between the fiber the matrix during transverse tension loading. The near-flattransverse fatigue behavior (S-N) implies that a damage tolerance based approach may be inapplicableunder transverse loading for some MMC systems.

Acknowledgments

This research was conducted at the Materials and Manufacturing Directorate, Air Force ResearchLaboratory (AFRL/MLLN), Wright-Patterson Air Force Base, OH, USA. R. John and D. J. Buchananwere supported under on-site contract number F33615-94-C-5200. The authors gratefully acknowledgethe assistance of Mike Shepard in obtaining the fracture profiles.

References

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1253, American Society for Testing and Materials, Philadelphia, PA (1996).3. M. G. Castelli and J. Gayda, inReliability, Stress Analysis, and Failure Prevention, DE-55, ASME, pp. 213 (1993).4. W. S. Johnson, S. J. Lubowinski, and A. L. Highsmith, inThermal and Mechanical Behavior of, Metal Matrix and Ceramic

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(AFRL), Wright-Patterson Air Force Base, OH 45433-6533 (1990).11. J. A. Hall, A. Peralta, R. L. Hollars, D. Harmon, M. A. Finefield, M. R. James, and D. B. Marshall,Damage Tolerance

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12. P. J. Cotterril and P. Bowen, J. Mater. Sci. 31, 5897 (1996).13. R. John, N. E. Ashbaugh, and A. F. Lackey, Scripta Metall. Mater. 35, 711 (1996).14. R. John, N. E. Ashbaugh, and A. F. Lackey, J. Comp. Tech. Res. to be submitted (1998).15. D. J. Buchanan, R. John, and J. M. Larsen, Mater. Sci. Eng. to be submitted (1998).16. J. L. Koupa and N. E. Ashbaugh, Composites Eng. 5, 569 (1995).17. D. Coker, F. Boller, J. L. Kroupa, and N. E. Ashbaugh,FIDEP2 User Manual to Micromechanical Models for

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Prediction of transverse fatigue behavior of unidirectionally reinforced metal matrix composites