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Dynamic behavior of a boron carbide aluminum cermet: experiments and observations
Mechanics of Materials 10 (1990) 19-29 Elsevier
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Dynamic behavior of a boron carbide-aluminum cermet: experiments and observations K.T. R a m e s h * and G. Ravichandran Center of Excellence fi~r Advanced Materials', Department of Applied Mechanics and Engineering Sciences, Universi(v of California, San Diego, La Jolla, (M 92093, U.S.A. Received 20 December 1989
Experimental techniques to study and characterize the dynamic behavior of ceramics and ceramic composites are described. A new technique based on the split Hopkinson pressure bar is used to obtain the deformation and failure characteristics of brittle materials at moderately high strain rates. A normal plate impact recovery technique is used to study the evolution of damage under controlled loading conditions and understand the micromechanisms of damage in materials. The normal plate impact recovery technique has the advantage of applying sufficiently high stress levels to initiate damage and yet providing controlled amounts of energy input to avoid catastrophic failure of the material. Hence, the samples can be recovered intact and evaluated for damage using standard characterization techniques such as microscopy and ultrasonics. The damage assessment in the material together with the real-time interferometric measurements serves as a powerful experimental procedure to study processes such as damage initiation, damage evolution, and the failure modes of very hard and brittle ceramics and ceramic composites under stress wave loading. Results are presented for a boron carbide aluminum cermet, a ceramic composite and the role of its microstructure on the failure mechanisms is studied. The cermet exhibits high strength and large strain to failure, in comparison to monolithic ceramics. The basic failure mechanisms in the cermet appear to be axial microcracking due to compression, and transverse cracking generated by compression release and by tension. The cracks are also found to be bridged by ductile aluminum or intermetallics, which together with the microcracking could possibly increase the toughness of the cermet at high loading rates.
1. Introduction
Monolithic ceramics and ceramic composites such as cermets are finding increasing applications, and there is a need to design them for resistance against impact loading. Ceramics and ceramic composites are in general very hard and brittle and hence their failure occur at small strains of less than 1%. An understanding of damage initiation and evolution under dynamic loading conditions is important in the analysis of structures made of such brittle materials. Relatively little work has been carried out in understanding the dynamic behavior of ceramic composites such
* Now at the Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, M D 21218, U.S.A. 0167-6636/90/$03.50 ~ 1990
Elsevier Science Publishers B.V.
as cermets. Past investigations have shown that damage in the form of microcracking a n d / o r microplasticity through dislocation motion play an important role in deformation and failure of monolithic ceramics such as alumina at stresses even below the Hugoniot elastic limit (HEL) under dynamic loading conditions; see Lankford (1981), Yaziv (1985), Longy and Cagnoux (1989), Louro and Meyers (1989), and Raiser et al. (1990). Similar results have been reported for other monolithic ceramics and minerals, such as titanium diboride and quartz; see Grady (1977) and Vanderwalker and Croft (1988). Recently, there have been several investigations on a new class of ceramic-metal composites, boron carbide-aluminum cermets; see Ramesh and Ravichandran (1988), Blumenthal and Gray (1989a,b), Ravichandran and Nemat-Nasser (1989), and Ramesh et al. (1989). These investigations have characterized the
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K . Z Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
dynamic response and failure behavior of this new class of cermets. In this paper, a coordinated experimental program to study and characterize the mechanical response, fracture, and failure modes of ceramics/ ceramic composites under high loading rates is described. Results for a boron carbide-aluminum cermet and recent developments in understanding the compressive failure mechanisms in ceramic composites are presented. The objective of this work is the systematic characterization of the mechanical behavior of ceramic composites under high strain rate loading conditions and the influence of processing and microstructure on mechanical properties. The purpose of the experiments are (a) to develop standard and reliable high strain rate testing techniques for ceramic composites, (b) to provide measurements in real time to quantify the evolution of damage under dynamic loading conditions; (c) to identify the effects of processing on the overall dynamic failure modes, and (d) to explore the role of novel processing techniques in improving the toughness of ceramic based materials. In this paper we discuss two experimental techniques that are used to study the dynamic response of ceramics and ceramic composites: a modified split Hopkinson pressure bar technique (Ravichandran and Nemat-Nasser, 1989) and a normal plate impact recovery experiment (Ramesh and Ravichandran, 1988). The Hopkinson bar tests are used to obtain the deformation characteristics and the failure strength of hard and brittle materials at moderately high strain rates, ranging from 50 to 300 s- 1. The normal plate impact recovery experiments are used to assess the damage evolution and study the micromechanisms of damage under controlled plane wave loading conditions. Results from the experiments are used to assess
Striker Bar
the deformation and failure mechanisms associated with a particular class of boron carbidealuminum cermets developed bv Halverson et al. (1989).
2. Experimental techniques
2.1. Hopkinson bar experiments A schematic representation of the basic split Hopkinson pressure bar (also referred to as the Kolsky bar) is shown in Fig. 1. The striker bar impacts the end, A, of the incident bar at a given velocity, sending an elastic compressive pulse along the incident bar. The pulse duration equals the round-trip time of an elastic bar wave in the striker bar. When the pulse reaches the specimen which is placed between the incident bar and the transmission bar, part of the pulse is reflected back and the remaining part is transmitted through the specimen to the transmission bar. The strain gages at points S 1 and S2 provide time-resolved measures of the signals. The stress levels in the bars are kept sufficiently low, so that all the bars remain elastic. The specimen undergoes dynamic elastic-plastic deformations. From the reflected signal the axial strain in the specimen is estimated. and the transmitted pulse provides a measure of the axial stress in the specimen. Details of this experimental technique can be found in Kolsky (1949) and Foltansbee (1985). One dimensional calculations by Kolsky (1949) show that the strain rate i in the sample can be estimated using i
$1
Strain Gauge er
(1)
/ ~
Transmission Bar
Incident Bar A
2 c0
B
C
! Specimen
Fig. 1. The s c h e m a t i c of a c o n v e n t i o n a l split H o p k i n s o n pressure bar.
$2 Strain Gauge Ct
D
K.T. Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
where l is the length of the sample, c r is the time-dependent reflected strain, and c o is the longitudinal bar wave velocity in the incident bar which is given by
where E i is the Young's modulus and 0 is the mass density of the incident bar material. The axial stress cr in the sample is estimated from At O" = ~ - g t ~ t
(3)
where A s is the cross-sectional area of the sample, and c t is the time-dependent strain in the transmission bar of area A t and Young modulus E~. For the split Hopkinson bar to function as designed, the yield stress of the sample must be considerably smaller than the yield stress of the bars. The ends B and C of the bars then remain essentially flat as the specimen shortens axially and expands radially. To prevent barreling at large strains, the ends of the sample are lubricated. In the classical use of the split Hopkinson pressure bar, for testing ductile metals, the reflected pulse is tensile. When very hard materials such as boron carbide-aluminum (B4C A1), alumina (A1203), partially stabilized zirconia (PSZ), or other ceramics/ceramic composites are being tested in the split Hopkinson compression bar, many serious problems are encountered; see Ravichandran and Nemat-Nasser (1989). The compression pulse is reflected often as compression and sometimes is transmitted completely through the sample with little reflection because of impedance (Oco) match between the bar material and the material being tested. Note that the impedances of high strength steel (material used in the pressure bars) and alumina (a hard ceramic) is within 5%. Therefore, it is not possible, in general, to use the reflected wave for estimating the strain rate in the sample. On the other hand, the transmitted pulse is still a good measure of the stress in the sample.
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2.1.1. Modification of split Hopkinson compression bar for dynamic testing of ceramics/ceramic composites In the present study, the conventional compression Hopkinson bar has been modified for application to dynamic testing of ceramics and ceramic composites which, in general: (1) are extremely hard; and (2) undergo very little strain prior to failing. This new technique involves two essential changes in the conventional split Hopkinson bar technique: (1) a thin copper plate used as a pulse shaper is glued to the end A of the incident bar; (2) the strain is measured directly by attaching strain gages to the sample. The presence of the copper plate introduces a monotonically increasing ramp-like stress pulse in the incident bar. While this limits the strain rates at which the test can be performed, it does preclude the sudden straining of the sample. Furthermore, since the strain is measured directly as a function of time, reliable information is obtained. Together with the measured incident and transmitted pulses, a complete axial stress and deformation history of the sample is produced. Thus, reliable stress strain curves for hard ceramic based materials can be obtained at moderately high strain rates of about 50 300 s ~ This technique has been used successfully to study the deformation and failure modes of a boron carbide-aluminum cermet and a partially stabilized zirconia (Mg PSZ) under dynamic uniaxial stress conditions; see Ravichandran and Nemat-Nasser (1989) and Rogers and Nemat-Nasser (1990). Note that in this technique, both axial and transverse strains of the specimen can be measured during the test, by suitably orienting strain gages on adjacent faces of prismatic samples.
2.2. Normal plate impact recovery experiments To understand the mechanisms of microcracking for the boron carbide aluminum cermet, a normal plate impact recovery technique developed by Kumar and Clifton (1979) is used. The experiments are fully instrumented and very well characterized allowing an unambiguous interpretation of the measurements. The experimental configura-
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K,T, Ramesh, G. Ra~2ichandran / Boron carbide a l u m i n u m t e r m s ;
Momentum trap l:lycr
o ~ U
Laser Interferometer
Specimen Fig. 2. Normal plate impact recovery experiment.
tion is shown in Fig. 2. A thin flyer plate impacts a target made of the material under investigation. The flyer is aligned parallel to the specimen prior to impact using an optical technique introduced by K u m a r and Clifton (1977). The impact velocity is obtained by measuring the times at which five sets of pins that are spaced at known distances apart are contacted by the projectile. The deviation from normal impact (tilt) is assessed by measuring the times at which four pins on the face of the specimen are shorted by the flyer. An impedance matched m o m e n t u m trap is in contact with the rear surface of the specimen so that the principal loading pulse does not reflect from the rear surface and reload the specimen. The compressive loading pulse that propagates through the specimen continues through the m o m e n t u m trap and reflects from the stress free rear surface of the m o m e n t u m trap as a tensile pulse. When this tensile pulse arrives at the s p e c i m e n - m o m e n t u m trap interface, the interface fails and the momentum trap separates. The thickness of the momentum trap is chosen so that the entire loading pulse is trapped within it. Thus, the specimen can be recovered after having been subjected to a single plane compressive step pulse of known duration t i. determined by the round trip time of a longitudinal wave in the flyer and can be expressed as tn-
2h Cl
(4)
where h is the thickness of the flyer and c k is the longitudinal wave speed in the flyer. By providing a suitable gap between the specimen and the m o m e n t u m trap, one can subject the specimen to
a kno,an history of compres,,am, follo'acd bx tension. Flyer and m o m e n t u m trap materials arc selected to remain elastic under conditions of thc test. All interfaces art: lapped flat and aligned parallel so as to permit a plane wave in{erprclation of the experiment. The particle velocit~ at ~he rear surface of the momenltm_l trap is measured using a normal displacentent inlerferometer (N 1)I) introduced by Barker and Holle,~bach (1965L The plate impact recovery experiment is best understood in terms of the distance time (A" l* diagram shown in Fig. 3.. This figure shows the wave-fronts propagating through the flyer, the specimen and the m o m e n t u m trap. The .Y-e diagram as drawn in Fig. 3 shows the actual experimental conditions, in which a gap exists belwecn the specimen and the m o m e n t u m trap. This results in a short tensile pulse, the duration of which corresponds to the time taken for the closure of the gap. This tensile pulse propagates back into the specimen and is also trapped within the m o m e n t u m trap. This spechnen has been subjected to a compressive pulse folh)wed by a shor! tensile pulse. However. only a ~mall region near the impact face is subjected to a state of tension, shown as a shaded area in Fig. 3. In this case.
t
l
T;
\
x
MOMENTUMTP,AP FLYER SPECIMEN Fig. 3. The X - t diagram corresponding to the normal plate impact recovery experiment, including the effect of the presence of a gap between the specimen and the momentum trap.
K.T. Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
both the compressive and tensile pulses are trapped by the momentum trap. The amplitude of the input step compressive pulse % is given by:
= ½0cL
(5)
where O is the mass density and U0 is the impact velocity. The amplitude of the input stress pulse can be controlled by changing the impact velocity. It is assumed that there is no mismatch in the acoustic impedances (0CL) of the flyer, the specimen, and the m o m e n t u m trap. The normal plate impact recovery technique has the advantage of applying sufficiently high stress levels to initiate damage and yet providing controlled amounts of energy input to avoid catastrophic failure of the material. Hence, the samples can be recovered intact and evaluated for damage using standard characterization techniques such as microscopy and ultrasonics. Since this couples the damage assessment in the material together with the interferometric measurements at the back surface of the m o m e n t u m trap, this technique serves as a powerful experimental procedure. Processes such as damage initiation, damage evolution, and the failure modes of very hard and brittle ceramics and ceramic composites under stress wave loading conditions can be investigated.
3. Material The material, a ceramic composite which is used in this study is a 55 vol% boron carbide-45 vol% aluminum cermet obtained from the Lawrence Livermore National Laboratories. The processing technique is described in detail by Halverson et al. (1989). The material is prepared by an infiltration process. Boron carbide particles (sizes ranging from 0.5-2 ~tm) are slip cast to form a porous aggregate and is cleaned chemically. This green body is infiltrated with molten 7075 aluminum. The composite in the as infiltrated state is then heat treated at about 800°C to achieve the desired hardness. The density of this material is 2560 k g / m 3 and it has a hardness of about 1400 k g / m m 2 on the Vickers scale. The
23
longitudinal and shear wave speeds measured for this material using ultrasonic transit time measurements are 11.7 m m / ~ s and 7.2 mm/~xs, respectively. The composite does not have a connected microstructure; i.e. the boron carbide and aluminum phases do not form continuous three dimensional networks. This is in contrast to the material used in a related investigation by Blumenthal and G r a y (1989a,b), where the boron carbide was infiltrated with pure aluminum and the resulting cermet had interconnected phases due to a different processing technique developed by Aksay and co-workers at the University of Washington. The Hopkinson bar specimens are 9 m m diameter cylinders that are 9 m m long, and the plate impact specimens are 25 m m square, each about 6 m m thick. The flyer and the m o m e n t u m trap in the plate impact experiments are made of a Ti6AI-4V alloy. The acoustic impedance of this material is within 5% of that of the cermet. The flyer is also 25 m m square and about 1.5 mm thick resulting in a loading pulse of about 0.5 /~s in duration.
4. Results 4.1. Split Hopkinson bar experiments
The split Hopkinson bar experiments were performed on cylindrical samples placed between two pressure bars, which were made of a maraging steel (yield strength = 2500 MPa). The pressure bars have a diameter of 19 m m ( 3 / 4 in.) and hence an area mismatch of about 4 exists between the sample and the pressure bars. A typical result for the various stress pulses and the strain gage signal for the boron c a r b i d e - a l u m i n u m cermet is shown in Fig. 4. The incident pulse in this test has essentially a ramp profile created by the presence of a copper cushion at the interface between the striker bar and the incident bar (at the end A in Fig. 1) and the pulse duration is approximately 120 p,s corresponding to a striker bar of length 300 mm. Also, note that there is no reflected pulse in this case, which would have been used to estimate the strain rate in the sample in a conventional
K. 7: Ra;nesh, G. Rauichandra;t / Boron carbide aluminum ce,,'nle!
24
r ...... T
r
V
/
,
gpecinlei/
(]age
Strain
;.j i'v'/3 2. 500e-2 ivy - 2. 45000e--3 I 7,'[) 1.C;00e-4 rL - 1. 000~--4
/'
i
r !
L
V/I}
i
~,y
L yi~ i t l e n i
P u l ~
,-~
I. 250e5 3, 985625e5
Y,'O 1.000e-4 i T[_ -1,000~-4 !',',"D 0,250e-3 ivy --i. 639587e--2 -1
l],[} i. ~O0o-4 i IlL -2.25t~--4
I
I
-t
t
I
il-Transmilledl'ulsejli\l%<"~'dC~K'g'~''~-~i L ..... i . . . . . _~_ _St . _ _ J . . . . . . . . TL V× Fig. 4. Typical results for the boron carbide aluminum cermet tested using the modified split Hopkinson pressure bar technique
I
i
I - - - - "
u
.
.
.
.
1 ~
......
I
VI0 1.562e-3 Vx 6.3O625e-3
I
V/D 0,250e4 Vy 2.581503e5
q
2580 t
i
~ 1720
. . . . .
t
I
J 860
•
/
4 I
i
0.0 L
0.3
~
i
0.6 STRAIN(%)
i
i
0.9
Fig. 5. Stress-strain curve for the boron carbide-aluminum cermet.
1.2
i I
K.T. Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
Hopkinson bar test as described in Section 2.1. The sample failed during the compressive loading which is indicated by the interruption in the strain gage reading and the appearance of a tensile re-
25
flected pulse. The tensile reflected pulse is detected by the strain gage S 1 in the incident bar (see Fig. 1), since, when the material disintegrates, the end of incident bar at B corresponds to a free
Fig. 6. Scanning electron micrographs of the surface of the boron carbide-aluminum cermet: (a) fracture surface showing the cleavage cracking and crack bridging; (b) disintegration of boron carbide aggregates and decohesion of particles.
26
K. 7-. Ramesh, G. Ra~2ichandran //Boron carbide-aluminum cermet
Table l Summary of normal plate impact recovery experiments
,--,0.06
~0.04 "~ 0.02 @ 0.00
i
0.0
I
0.2
i
I
0.4
'
I
0.6
'
Experiment
Impact velocity L'. ( m / s )
Stress o,~ (MPa)
t~ (Fs)
K I - 11 KT-12 KT- 13
45 60 100
600 800 1350
0.47 0.50 0.47
I
0.8
Time [ las] Fig. 7. Measured normal velocity profile at the back face of the momentum trap (impact velocity = 45 m/s).
surface. The stress in the sample can be obtained using the transmitted pulse (see Fig. 4) and (3). The strain in the sample can be obtained directly, using the signal from the strain gage mounted on the specimen. The stress-strain curve obtained using the results in Fig. 4 for the boron carbidealuminum cermet is shown in Fig. 5. The stress-strain curve is essentially bilinear, with an initial elastic region whose modulus is about 420 GPa, followed by a more compliant response whose tangent modulus corresponds to about 165 GPa. The axial stress and strain at failure are 2800 MPa and 1.2%, respectively. The strain rate in this test is not a constant but ranges from 50 to 200/s. Figure 6 is a scanning electron micrograph of the fracture surface of the failed sample which shows the cleavage cracking of the boron carbide and the intermetallic phases. Figure 7 shows a detailed view of the process of failure in the boron carbide-aluminum cermet involving the decohesion of the boron carbide particles and the disintegration of the boron carbide aggregates.
the test, the resulting incident compressive pulse would be a step pulse, with a rear surface particle velocity of constant amplitude equal to the impact velocity. The pulse width would then correspond to the roundtrip time of the longitudinal wave in the flyer (assuming no gap exists). Figure 7 shows that the initial compressive pulse reaches the expected amplitude but that the pulse width is tess than the expected value, which is 0.47 bts. The compression pulse is followed by a shorter pulse (about 0.15 ~s) but of considerably smaller amplitude. This is the tensile pulse that propagated back in to the specimen due to the presence of a gap between the momentum trap and the specimen as shown in Fig. 3. This gap is estimated to be about 5 btm (tensile pulse width = g a p / i m p a c t velocity). We note that the tensile pulse that propagated through the specimen has an amplitude of about half that of the original pulse. Quantitative optical microscopy is used to characterize damage in the material. An optical micro-
4.2. Normal plate impact recovery experiments The summary of the normal plate impact recovery experiments on the boron carbidealuminum cermet is shown Table 1. The amplitude of the input step stress pulse ranged from 600 to 1350 MPa; the pulse duration is about 0.5 I*s. The resulting particle velocity measured at the rear surface of the specimen for one of the experiments (KT-11, impact velocity = 45 m / s ) is shown in Fig. 7. If the specimen remained elastic during
Fig. 8. Optical micrograph of the boron carbide-aluminum cermet before impact.
K.T. Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
graph of a specimen that has not been impacted is shown in Fig. 8; it contains only few voids corresponding to 1% by volume. The recovered specimens are sectioned and polished and then examined in an optical microscope. All the specimens for microscopy are prepared using identical procedures. Figures 9(a) and (b) show polished sections of a recovered sample. Both microcracks and extended cracks are observed in the recovered specimens. Cracks occur both parallel and perpendicular to the direction of compressive loading.
27
Transverse cracks occur only near the impact face (see Fig 9(a)) whereas the axial cracks occur throughout the cross section (Fig. 9(b)).
5. Concluding remarks A new technique for determining high strain rate properties of ceramics/ceramic composites based on the split Hopkinson pressure bar was
impact
Fig. 9. Optical micrographs of a recovered specimen: (a) tensile cracks parallel to the impact face. produced by the short tensile pulse in Fig. 7: (b) disintegrated boron carbide particles are connected by axial microcracks.
28
K.T. Rarnesh, G. Ravichandran // Boron carhide.-aluminum cermet
presented. Though the strain rates that are attainable in this technique are an order of magnitude less than the conventional technique, the current technique provides a means of obtaining reliable deformation and failure characteristics of hard brittle materials. This technique has been successfully used in determining the high strain rate behavior of 55 vol% boron carbide-aluminum cermet. The material exhibits high strength and high strain to failure, 2800 MPa and 1.2%, respectively. Though the composite has a strength lower than that of the parent monolithic ceramic (boron carbide), there is an order of magnitude improvernent in the elongation to failure values. The relatively large strain at failure (1.2%) is attributed to the presence of ductile phases including aluminum and associated intermetallics in the microstructure. The reduction in the modulus resulting in a more compliant response which is observed in the stress-strain curve in Fig. 5, and the fracture surfaces shown in Fig. 6(b), corroborate the concept of microcracking in the cermets. It would appear that microcracking also plays an important role in enhancing the strain to failure and as a consequence could possibly increase the toughness of the composite. Microcracking is also accompanied by the bridging of cracks by the ductile aluminum phase or the associated intermetallics. Such bridging is known to increase the toughness of a composite considerably as shown by NematNasser and Hori (1987). The fractograph shown in Fig. 6(b) also suggests the possibility of decohesion of boron carbide particles in the aggregate in the final stages of failure. This would suggest a means for a possible energy absorbing mechanism and hence enhancing the toughness of the cermet. From the results obtained for damage evolution in 55 vol% boron carbide aluminum cermets using the plate impact recovery technique, it appears that the initial compression pulse causes very little damage in the material, with the possible exception of axial cracks. The presence of axial cracks (compression cracks) will not alter the amplitude of the normal compressive plane waves. This is supported by the observation that the compression pulse from the interferometric measurements is nearly a step pulse of the expected amplitude, i.e. the rear surface particle velocity approximates the
impact velocity (see Fig. 7). During the initial compression most of the strain is presumed to be accommodated by the relatively soft intermetallic phases. II is believed that when the compression pulse is unloaded by the reflected wave, residual strains due to the intermetallic phase can cause failure of the boron carbide matrix, generating damage zones. The tensile pulse that propagated through the specimen has a much smaller amplitude, suggesting that the loss of energy of this pulse may be due to the damage evolution in the material. A small gap (less than 5 rsm) between the momentum trap and the specimen produced the short tensile pulse and this pulse could cause the damage to develop further by causing the decohesion within the damage zones. The dark regions that appear in the micrographs shown in Figs. 9(a) and (b) should not be interpreted as voids, but rather as regions of decohesion of boron carbide particles; these particles may have been pulled out during the specimen preparation process (polishing) generating the appearance of a void like structure. The average size of such damage zones are of the size of about 0.5-2 ~m. also the size of the individual boron carbide particles with which the initial aggregate is made. This suggests that the boron carbide matrix is disintegrating, a possible mechanism for final failure as observed in the Hopkinson bar experintents. The transverse cracks which are observed (see Fig. 9(a)) near the impact face probably are due to the local tension arising from the gap between the specimen and the momentum trap, which creates a state of tension near the impact face. The presence of transverse cracks significantly reduces the antplitude of the tensile pulse reflected front the rear surface of the specimen as seen in Fig. 7. The micrographs also show that the damage zones are often clustered, and that they link together to form microcracks as seen Fig. 9(b). The microcracks then link up to form extended cracks that are either axial or transverse, depending on the local dominant stress state; Axial cracks form typically as a result of axial compression as shown by Horii and Nemat-Nasser (1985, 1986), which m the case of plate impact experiments is the compressive plane wave loading.
K.T. Ramesh, G. Raeichandran / Boron carbide aluminum cermet
Acknowledgement T h i s w o r k w a s s u p p o r t e d b y t h e U . S. A r m y Research Office under Contract Nos. DAAL-038 6 - K - 0 1 6 9 a n d D A A L - 0 3 - 8 8 - K - 0 1 1 8 to t h e U n i versity of California, San Diego. The authors are p l e a s e d to a c k n o w l e d g e t h e h e l p f u l d i s c u s s i o n s w i t h P r o f e s s o r S. N e m a t - N a s s e r a n d t h e t e c h n i c a l a s s i s t a n c e o f M e s s r s . J.B. I s a a c s a n d B.A. A l t m a n at the Center of Excellence for Advanced Materials in U C S D . T h e a u t h o r s t h a n k t h e C h e m i s t r y and Materials Science department of Lawrence Livermore National Laboratories for providing the boron carbide-aluminum cermet samples.
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Oxford, England. Blumenthal, W.R. and G.T. Gray (1989b), Characterization of shock loaded aluminum infiltrated boron carbide cermets, in: S.C. Schmidt et al., eds., Proc. of the A P S 1989 Topical Conference on Shock Compression of Condensed Matter,
Elsevier, Amsterdam, 393. Follansbee, P.S. (1985), The Hopkinson bar, Metals Handbook 8, American Society of Metals, Metals Park, OH, 198. Grady, D.E. (1977), Processes occuring in shock wave compression of rocks and minerals, in: M.H. Maghnani and S.-I. Akimoto, eds., High Pressure Research-Geopt~vsical Applications, Academic Press, 389. Halverson, D.C., A.J. Pyzik, I.A. Aksay and W.E. Snowden (1989), Processing of boron carbide-aluminum composites, J. Amer. Ceram. Soc. 72, 775. Horii, H. and S. Nemat-Nasser (1985), Compression induced microcrack growth in brittle solids: axial splitting and shear failure, J. Geophys. Res. 90, 3105. Horii, H. and S. Nemat-Nasser (1986), Brittle failure in com-
29
pression: splitting, faulting and brittle-ductile transition, Phil. Trans. R. So('. London. A-319, 337.
Kolsky, H. (1949), An investigation of the mechanical properties of materials at very high rates of loading, Proc, R. Soc., London, B 62, 676. Kumar, P. and R.J. Clifton (1977), Optical alignment of impact faces for plate impact experiments, J. Appl. Phys. 48, 1366. Kumar, P. and R.J. Clifton (1979), Dislocation motion and generation in LiF single crystals subjected to plate impact, J. Appl. Phys. 50, 4747. Lankford, I. (1981), Mechanisms responsible for strain-rate dependent compressive strength in ceramic materials, J. Amer. Ceram. Soc. 64, C-33. Longy, F. and J. Cagnoux (1989), Plasticity and microcracking in shock loaded alumina, J Amer. Ceram. Soc. 72, 971. Louro, L.H. and M.A. Meyers, Stress-wave induced fragmentation in aluminabased ceramics, in: S.C. Schmidt et al., eds., Proc. of the A PS 1989 Topical Conference on Shock Compression of Condensed Matter, Elsevier, Amsterdam, 465.
Nemat-Naaser, S. and M. Hori (1987), Toughening by partial or full bridging of cracks in ceramics and fiber reinforced composites, Mech. Mater. 6, 245. Raiser, G.F., R.J. Clifton and M. Ortiz (1990), A plate impact experiment for studying microcracking in ceramics, Mech. Mater. 10, 43. Ramesh, K.T. and G. Ravichandran (1988), An ultrasonic evaluation of Damage in cermets, in: D.O. Thompson and D.E. Chimenti, eds., Reeiew of Progress in Quantitative Nondestructive E~aluation 8B, Plenum Press, New York, 1841. Ramesh, K.T., B.A. Altman, G. Ravichandran and S. NematNasser, Failure mode and mechanism in cermets Under stress wave loading, in: K. Salama et al., eds., Advances in Fracture Research 1, Pergamon Press, New York, 811. Ravichandran, G. and S. Nemat-Nasser (1989), Micromechanics of dynamic fracturing of ceramic composites: experiments and observations, in: K. Salama et al., Advances in Fracture Researeh 1, Pergamon Press. New York, 41. Rogers, W.P. and S. Nemat-Nasser (1990). Transformation plasticity at high strain rate in Mg-PSZ, J. Amer. Ceram. Soc. 73, 136. Vanderwalker, D.M. and W.J. Croft (1988), Dislocations in shock-loaded titanium diboride, J. Mater. Res. 3, 76l. Yaziv, D. (1985), P h . D . Thesis, The University of Dayton, Dayton, OH.
19
Dynamic behavior of a boron carbide-aluminum cermet: experiments and observations K.T. R a m e s h * and G. Ravichandran Center of Excellence fi~r Advanced Materials', Department of Applied Mechanics and Engineering Sciences, Universi(v of California, San Diego, La Jolla, (M 92093, U.S.A. Received 20 December 1989
Experimental techniques to study and characterize the dynamic behavior of ceramics and ceramic composites are described. A new technique based on the split Hopkinson pressure bar is used to obtain the deformation and failure characteristics of brittle materials at moderately high strain rates. A normal plate impact recovery technique is used to study the evolution of damage under controlled loading conditions and understand the micromechanisms of damage in materials. The normal plate impact recovery technique has the advantage of applying sufficiently high stress levels to initiate damage and yet providing controlled amounts of energy input to avoid catastrophic failure of the material. Hence, the samples can be recovered intact and evaluated for damage using standard characterization techniques such as microscopy and ultrasonics. The damage assessment in the material together with the real-time interferometric measurements serves as a powerful experimental procedure to study processes such as damage initiation, damage evolution, and the failure modes of very hard and brittle ceramics and ceramic composites under stress wave loading. Results are presented for a boron carbide aluminum cermet, a ceramic composite and the role of its microstructure on the failure mechanisms is studied. The cermet exhibits high strength and large strain to failure, in comparison to monolithic ceramics. The basic failure mechanisms in the cermet appear to be axial microcracking due to compression, and transverse cracking generated by compression release and by tension. The cracks are also found to be bridged by ductile aluminum or intermetallics, which together with the microcracking could possibly increase the toughness of the cermet at high loading rates.
1. Introduction
Monolithic ceramics and ceramic composites such as cermets are finding increasing applications, and there is a need to design them for resistance against impact loading. Ceramics and ceramic composites are in general very hard and brittle and hence their failure occur at small strains of less than 1%. An understanding of damage initiation and evolution under dynamic loading conditions is important in the analysis of structures made of such brittle materials. Relatively little work has been carried out in understanding the dynamic behavior of ceramic composites such
* Now at the Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, M D 21218, U.S.A. 0167-6636/90/$03.50 ~ 1990
Elsevier Science Publishers B.V.
as cermets. Past investigations have shown that damage in the form of microcracking a n d / o r microplasticity through dislocation motion play an important role in deformation and failure of monolithic ceramics such as alumina at stresses even below the Hugoniot elastic limit (HEL) under dynamic loading conditions; see Lankford (1981), Yaziv (1985), Longy and Cagnoux (1989), Louro and Meyers (1989), and Raiser et al. (1990). Similar results have been reported for other monolithic ceramics and minerals, such as titanium diboride and quartz; see Grady (1977) and Vanderwalker and Croft (1988). Recently, there have been several investigations on a new class of ceramic-metal composites, boron carbide-aluminum cermets; see Ramesh and Ravichandran (1988), Blumenthal and Gray (1989a,b), Ravichandran and Nemat-Nasser (1989), and Ramesh et al. (1989). These investigations have characterized the
20
K . Z Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
dynamic response and failure behavior of this new class of cermets. In this paper, a coordinated experimental program to study and characterize the mechanical response, fracture, and failure modes of ceramics/ ceramic composites under high loading rates is described. Results for a boron carbide-aluminum cermet and recent developments in understanding the compressive failure mechanisms in ceramic composites are presented. The objective of this work is the systematic characterization of the mechanical behavior of ceramic composites under high strain rate loading conditions and the influence of processing and microstructure on mechanical properties. The purpose of the experiments are (a) to develop standard and reliable high strain rate testing techniques for ceramic composites, (b) to provide measurements in real time to quantify the evolution of damage under dynamic loading conditions; (c) to identify the effects of processing on the overall dynamic failure modes, and (d) to explore the role of novel processing techniques in improving the toughness of ceramic based materials. In this paper we discuss two experimental techniques that are used to study the dynamic response of ceramics and ceramic composites: a modified split Hopkinson pressure bar technique (Ravichandran and Nemat-Nasser, 1989) and a normal plate impact recovery experiment (Ramesh and Ravichandran, 1988). The Hopkinson bar tests are used to obtain the deformation characteristics and the failure strength of hard and brittle materials at moderately high strain rates, ranging from 50 to 300 s- 1. The normal plate impact recovery experiments are used to assess the damage evolution and study the micromechanisms of damage under controlled plane wave loading conditions. Results from the experiments are used to assess
Striker Bar
the deformation and failure mechanisms associated with a particular class of boron carbidealuminum cermets developed bv Halverson et al. (1989).
2. Experimental techniques
2.1. Hopkinson bar experiments A schematic representation of the basic split Hopkinson pressure bar (also referred to as the Kolsky bar) is shown in Fig. 1. The striker bar impacts the end, A, of the incident bar at a given velocity, sending an elastic compressive pulse along the incident bar. The pulse duration equals the round-trip time of an elastic bar wave in the striker bar. When the pulse reaches the specimen which is placed between the incident bar and the transmission bar, part of the pulse is reflected back and the remaining part is transmitted through the specimen to the transmission bar. The strain gages at points S 1 and S2 provide time-resolved measures of the signals. The stress levels in the bars are kept sufficiently low, so that all the bars remain elastic. The specimen undergoes dynamic elastic-plastic deformations. From the reflected signal the axial strain in the specimen is estimated. and the transmitted pulse provides a measure of the axial stress in the specimen. Details of this experimental technique can be found in Kolsky (1949) and Foltansbee (1985). One dimensional calculations by Kolsky (1949) show that the strain rate i in the sample can be estimated using i
$1
Strain Gauge er
(1)
/ ~
Transmission Bar
Incident Bar A
2 c0
B
C
! Specimen
Fig. 1. The s c h e m a t i c of a c o n v e n t i o n a l split H o p k i n s o n pressure bar.
$2 Strain Gauge Ct
D
K.T. Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
where l is the length of the sample, c r is the time-dependent reflected strain, and c o is the longitudinal bar wave velocity in the incident bar which is given by
where E i is the Young's modulus and 0 is the mass density of the incident bar material. The axial stress cr in the sample is estimated from At O" = ~ - g t ~ t
(3)
where A s is the cross-sectional area of the sample, and c t is the time-dependent strain in the transmission bar of area A t and Young modulus E~. For the split Hopkinson bar to function as designed, the yield stress of the sample must be considerably smaller than the yield stress of the bars. The ends B and C of the bars then remain essentially flat as the specimen shortens axially and expands radially. To prevent barreling at large strains, the ends of the sample are lubricated. In the classical use of the split Hopkinson pressure bar, for testing ductile metals, the reflected pulse is tensile. When very hard materials such as boron carbide-aluminum (B4C A1), alumina (A1203), partially stabilized zirconia (PSZ), or other ceramics/ceramic composites are being tested in the split Hopkinson compression bar, many serious problems are encountered; see Ravichandran and Nemat-Nasser (1989). The compression pulse is reflected often as compression and sometimes is transmitted completely through the sample with little reflection because of impedance (Oco) match between the bar material and the material being tested. Note that the impedances of high strength steel (material used in the pressure bars) and alumina (a hard ceramic) is within 5%. Therefore, it is not possible, in general, to use the reflected wave for estimating the strain rate in the sample. On the other hand, the transmitted pulse is still a good measure of the stress in the sample.
21
2.1.1. Modification of split Hopkinson compression bar for dynamic testing of ceramics/ceramic composites In the present study, the conventional compression Hopkinson bar has been modified for application to dynamic testing of ceramics and ceramic composites which, in general: (1) are extremely hard; and (2) undergo very little strain prior to failing. This new technique involves two essential changes in the conventional split Hopkinson bar technique: (1) a thin copper plate used as a pulse shaper is glued to the end A of the incident bar; (2) the strain is measured directly by attaching strain gages to the sample. The presence of the copper plate introduces a monotonically increasing ramp-like stress pulse in the incident bar. While this limits the strain rates at which the test can be performed, it does preclude the sudden straining of the sample. Furthermore, since the strain is measured directly as a function of time, reliable information is obtained. Together with the measured incident and transmitted pulses, a complete axial stress and deformation history of the sample is produced. Thus, reliable stress strain curves for hard ceramic based materials can be obtained at moderately high strain rates of about 50 300 s ~ This technique has been used successfully to study the deformation and failure modes of a boron carbide-aluminum cermet and a partially stabilized zirconia (Mg PSZ) under dynamic uniaxial stress conditions; see Ravichandran and Nemat-Nasser (1989) and Rogers and Nemat-Nasser (1990). Note that in this technique, both axial and transverse strains of the specimen can be measured during the test, by suitably orienting strain gages on adjacent faces of prismatic samples.
2.2. Normal plate impact recovery experiments To understand the mechanisms of microcracking for the boron carbide aluminum cermet, a normal plate impact recovery technique developed by Kumar and Clifton (1979) is used. The experiments are fully instrumented and very well characterized allowing an unambiguous interpretation of the measurements. The experimental configura-
22
K,T, Ramesh, G. Ra~2ichandran / Boron carbide a l u m i n u m t e r m s ;
Momentum trap l:lycr
o ~ U
Laser Interferometer
Specimen Fig. 2. Normal plate impact recovery experiment.
tion is shown in Fig. 2. A thin flyer plate impacts a target made of the material under investigation. The flyer is aligned parallel to the specimen prior to impact using an optical technique introduced by K u m a r and Clifton (1977). The impact velocity is obtained by measuring the times at which five sets of pins that are spaced at known distances apart are contacted by the projectile. The deviation from normal impact (tilt) is assessed by measuring the times at which four pins on the face of the specimen are shorted by the flyer. An impedance matched m o m e n t u m trap is in contact with the rear surface of the specimen so that the principal loading pulse does not reflect from the rear surface and reload the specimen. The compressive loading pulse that propagates through the specimen continues through the m o m e n t u m trap and reflects from the stress free rear surface of the m o m e n t u m trap as a tensile pulse. When this tensile pulse arrives at the s p e c i m e n - m o m e n t u m trap interface, the interface fails and the momentum trap separates. The thickness of the momentum trap is chosen so that the entire loading pulse is trapped within it. Thus, the specimen can be recovered after having been subjected to a single plane compressive step pulse of known duration t i. determined by the round trip time of a longitudinal wave in the flyer and can be expressed as tn-
2h Cl
(4)
where h is the thickness of the flyer and c k is the longitudinal wave speed in the flyer. By providing a suitable gap between the specimen and the m o m e n t u m trap, one can subject the specimen to
a kno,an history of compres,,am, follo'acd bx tension. Flyer and m o m e n t u m trap materials arc selected to remain elastic under conditions of thc test. All interfaces art: lapped flat and aligned parallel so as to permit a plane wave in{erprclation of the experiment. The particle velocit~ at ~he rear surface of the momenltm_l trap is measured using a normal displacentent inlerferometer (N 1)I) introduced by Barker and Holle,~bach (1965L The plate impact recovery experiment is best understood in terms of the distance time (A" l* diagram shown in Fig. 3.. This figure shows the wave-fronts propagating through the flyer, the specimen and the m o m e n t u m trap. The .Y-e diagram as drawn in Fig. 3 shows the actual experimental conditions, in which a gap exists belwecn the specimen and the m o m e n t u m trap. This results in a short tensile pulse, the duration of which corresponds to the time taken for the closure of the gap. This tensile pulse propagates back into the specimen and is also trapped within the m o m e n t u m trap. This spechnen has been subjected to a compressive pulse folh)wed by a shor! tensile pulse. However. only a ~mall region near the impact face is subjected to a state of tension, shown as a shaded area in Fig. 3. In this case.
t
l
T;
\
x
MOMENTUMTP,AP FLYER SPECIMEN Fig. 3. The X - t diagram corresponding to the normal plate impact recovery experiment, including the effect of the presence of a gap between the specimen and the momentum trap.
K.T. Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
both the compressive and tensile pulses are trapped by the momentum trap. The amplitude of the input step compressive pulse % is given by:
= ½0cL
(5)
where O is the mass density and U0 is the impact velocity. The amplitude of the input stress pulse can be controlled by changing the impact velocity. It is assumed that there is no mismatch in the acoustic impedances (0CL) of the flyer, the specimen, and the m o m e n t u m trap. The normal plate impact recovery technique has the advantage of applying sufficiently high stress levels to initiate damage and yet providing controlled amounts of energy input to avoid catastrophic failure of the material. Hence, the samples can be recovered intact and evaluated for damage using standard characterization techniques such as microscopy and ultrasonics. Since this couples the damage assessment in the material together with the interferometric measurements at the back surface of the m o m e n t u m trap, this technique serves as a powerful experimental procedure. Processes such as damage initiation, damage evolution, and the failure modes of very hard and brittle ceramics and ceramic composites under stress wave loading conditions can be investigated.
3. Material The material, a ceramic composite which is used in this study is a 55 vol% boron carbide-45 vol% aluminum cermet obtained from the Lawrence Livermore National Laboratories. The processing technique is described in detail by Halverson et al. (1989). The material is prepared by an infiltration process. Boron carbide particles (sizes ranging from 0.5-2 ~tm) are slip cast to form a porous aggregate and is cleaned chemically. This green body is infiltrated with molten 7075 aluminum. The composite in the as infiltrated state is then heat treated at about 800°C to achieve the desired hardness. The density of this material is 2560 k g / m 3 and it has a hardness of about 1400 k g / m m 2 on the Vickers scale. The
23
longitudinal and shear wave speeds measured for this material using ultrasonic transit time measurements are 11.7 m m / ~ s and 7.2 mm/~xs, respectively. The composite does not have a connected microstructure; i.e. the boron carbide and aluminum phases do not form continuous three dimensional networks. This is in contrast to the material used in a related investigation by Blumenthal and G r a y (1989a,b), where the boron carbide was infiltrated with pure aluminum and the resulting cermet had interconnected phases due to a different processing technique developed by Aksay and co-workers at the University of Washington. The Hopkinson bar specimens are 9 m m diameter cylinders that are 9 m m long, and the plate impact specimens are 25 m m square, each about 6 m m thick. The flyer and the m o m e n t u m trap in the plate impact experiments are made of a Ti6AI-4V alloy. The acoustic impedance of this material is within 5% of that of the cermet. The flyer is also 25 m m square and about 1.5 mm thick resulting in a loading pulse of about 0.5 /~s in duration.
4. Results 4.1. Split Hopkinson bar experiments
The split Hopkinson bar experiments were performed on cylindrical samples placed between two pressure bars, which were made of a maraging steel (yield strength = 2500 MPa). The pressure bars have a diameter of 19 m m ( 3 / 4 in.) and hence an area mismatch of about 4 exists between the sample and the pressure bars. A typical result for the various stress pulses and the strain gage signal for the boron c a r b i d e - a l u m i n u m cermet is shown in Fig. 4. The incident pulse in this test has essentially a ramp profile created by the presence of a copper cushion at the interface between the striker bar and the incident bar (at the end A in Fig. 1) and the pulse duration is approximately 120 p,s corresponding to a striker bar of length 300 mm. Also, note that there is no reflected pulse in this case, which would have been used to estimate the strain rate in the sample in a conventional
K. 7: Ra;nesh, G. Rauichandra;t / Boron carbide aluminum ce,,'nle!
24
r ...... T
r
V
/
,
gpecinlei/
(]age
Strain
;.j i'v'/3 2. 500e-2 ivy - 2. 45000e--3 I 7,'[) 1.C;00e-4 rL - 1. 000~--4
/'
i
r !
L
V/I}
i
~,y
L yi~ i t l e n i
P u l ~
,-~
I. 250e5 3, 985625e5
Y,'O 1.000e-4 i T[_ -1,000~-4 !',',"D 0,250e-3 ivy --i. 639587e--2 -1
l],[} i. ~O0o-4 i IlL -2.25t~--4
I
I
-t
t
I
il-Transmilledl'ulsejli\l%<"~'dC~K'g'~''~-~i L ..... i . . . . . _~_ _St . _ _ J . . . . . . . . TL V× Fig. 4. Typical results for the boron carbide aluminum cermet tested using the modified split Hopkinson pressure bar technique
I
i
I - - - - "
u
.
.
.
.
1 ~
......
I
VI0 1.562e-3 Vx 6.3O625e-3
I
V/D 0,250e4 Vy 2.581503e5
q
2580 t
i
~ 1720
. . . . .
t
I
J 860
•
/
4 I
i
0.0 L
0.3
~
i
0.6 STRAIN(%)
i
i
0.9
Fig. 5. Stress-strain curve for the boron carbide-aluminum cermet.
1.2
i I
K.T. Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
Hopkinson bar test as described in Section 2.1. The sample failed during the compressive loading which is indicated by the interruption in the strain gage reading and the appearance of a tensile re-
25
flected pulse. The tensile reflected pulse is detected by the strain gage S 1 in the incident bar (see Fig. 1), since, when the material disintegrates, the end of incident bar at B corresponds to a free
Fig. 6. Scanning electron micrographs of the surface of the boron carbide-aluminum cermet: (a) fracture surface showing the cleavage cracking and crack bridging; (b) disintegration of boron carbide aggregates and decohesion of particles.
26
K. 7-. Ramesh, G. Ra~2ichandran //Boron carbide-aluminum cermet
Table l Summary of normal plate impact recovery experiments
,--,0.06
~0.04 "~ 0.02 @ 0.00
i
0.0
I
0.2
i
I
0.4
'
I
0.6
'
Experiment
Impact velocity L'. ( m / s )
Stress o,~ (MPa)
t~ (Fs)
K I - 11 KT-12 KT- 13
45 60 100
600 800 1350
0.47 0.50 0.47
I
0.8
Time [ las] Fig. 7. Measured normal velocity profile at the back face of the momentum trap (impact velocity = 45 m/s).
surface. The stress in the sample can be obtained using the transmitted pulse (see Fig. 4) and (3). The strain in the sample can be obtained directly, using the signal from the strain gage mounted on the specimen. The stress-strain curve obtained using the results in Fig. 4 for the boron carbidealuminum cermet is shown in Fig. 5. The stress-strain curve is essentially bilinear, with an initial elastic region whose modulus is about 420 GPa, followed by a more compliant response whose tangent modulus corresponds to about 165 GPa. The axial stress and strain at failure are 2800 MPa and 1.2%, respectively. The strain rate in this test is not a constant but ranges from 50 to 200/s. Figure 6 is a scanning electron micrograph of the fracture surface of the failed sample which shows the cleavage cracking of the boron carbide and the intermetallic phases. Figure 7 shows a detailed view of the process of failure in the boron carbide-aluminum cermet involving the decohesion of the boron carbide particles and the disintegration of the boron carbide aggregates.
the test, the resulting incident compressive pulse would be a step pulse, with a rear surface particle velocity of constant amplitude equal to the impact velocity. The pulse width would then correspond to the roundtrip time of the longitudinal wave in the flyer (assuming no gap exists). Figure 7 shows that the initial compressive pulse reaches the expected amplitude but that the pulse width is tess than the expected value, which is 0.47 bts. The compression pulse is followed by a shorter pulse (about 0.15 ~s) but of considerably smaller amplitude. This is the tensile pulse that propagated back in to the specimen due to the presence of a gap between the momentum trap and the specimen as shown in Fig. 3. This gap is estimated to be about 5 btm (tensile pulse width = g a p / i m p a c t velocity). We note that the tensile pulse that propagated through the specimen has an amplitude of about half that of the original pulse. Quantitative optical microscopy is used to characterize damage in the material. An optical micro-
4.2. Normal plate impact recovery experiments The summary of the normal plate impact recovery experiments on the boron carbidealuminum cermet is shown Table 1. The amplitude of the input step stress pulse ranged from 600 to 1350 MPa; the pulse duration is about 0.5 I*s. The resulting particle velocity measured at the rear surface of the specimen for one of the experiments (KT-11, impact velocity = 45 m / s ) is shown in Fig. 7. If the specimen remained elastic during
Fig. 8. Optical micrograph of the boron carbide-aluminum cermet before impact.
K.T. Ramesh, G. Ravichandran / Boron carbide-aluminum cermet
graph of a specimen that has not been impacted is shown in Fig. 8; it contains only few voids corresponding to 1% by volume. The recovered specimens are sectioned and polished and then examined in an optical microscope. All the specimens for microscopy are prepared using identical procedures. Figures 9(a) and (b) show polished sections of a recovered sample. Both microcracks and extended cracks are observed in the recovered specimens. Cracks occur both parallel and perpendicular to the direction of compressive loading.
27
Transverse cracks occur only near the impact face (see Fig 9(a)) whereas the axial cracks occur throughout the cross section (Fig. 9(b)).
5. Concluding remarks A new technique for determining high strain rate properties of ceramics/ceramic composites based on the split Hopkinson pressure bar was
impact
Fig. 9. Optical micrographs of a recovered specimen: (a) tensile cracks parallel to the impact face. produced by the short tensile pulse in Fig. 7: (b) disintegrated boron carbide particles are connected by axial microcracks.
28
K.T. Rarnesh, G. Ravichandran // Boron carhide.-aluminum cermet
presented. Though the strain rates that are attainable in this technique are an order of magnitude less than the conventional technique, the current technique provides a means of obtaining reliable deformation and failure characteristics of hard brittle materials. This technique has been successfully used in determining the high strain rate behavior of 55 vol% boron carbide-aluminum cermet. The material exhibits high strength and high strain to failure, 2800 MPa and 1.2%, respectively. Though the composite has a strength lower than that of the parent monolithic ceramic (boron carbide), there is an order of magnitude improvernent in the elongation to failure values. The relatively large strain at failure (1.2%) is attributed to the presence of ductile phases including aluminum and associated intermetallics in the microstructure. The reduction in the modulus resulting in a more compliant response which is observed in the stress-strain curve in Fig. 5, and the fracture surfaces shown in Fig. 6(b), corroborate the concept of microcracking in the cermets. It would appear that microcracking also plays an important role in enhancing the strain to failure and as a consequence could possibly increase the toughness of the composite. Microcracking is also accompanied by the bridging of cracks by the ductile aluminum phase or the associated intermetallics. Such bridging is known to increase the toughness of a composite considerably as shown by NematNasser and Hori (1987). The fractograph shown in Fig. 6(b) also suggests the possibility of decohesion of boron carbide particles in the aggregate in the final stages of failure. This would suggest a means for a possible energy absorbing mechanism and hence enhancing the toughness of the cermet. From the results obtained for damage evolution in 55 vol% boron carbide aluminum cermets using the plate impact recovery technique, it appears that the initial compression pulse causes very little damage in the material, with the possible exception of axial cracks. The presence of axial cracks (compression cracks) will not alter the amplitude of the normal compressive plane waves. This is supported by the observation that the compression pulse from the interferometric measurements is nearly a step pulse of the expected amplitude, i.e. the rear surface particle velocity approximates the
impact velocity (see Fig. 7). During the initial compression most of the strain is presumed to be accommodated by the relatively soft intermetallic phases. II is believed that when the compression pulse is unloaded by the reflected wave, residual strains due to the intermetallic phase can cause failure of the boron carbide matrix, generating damage zones. The tensile pulse that propagated through the specimen has a much smaller amplitude, suggesting that the loss of energy of this pulse may be due to the damage evolution in the material. A small gap (less than 5 rsm) between the momentum trap and the specimen produced the short tensile pulse and this pulse could cause the damage to develop further by causing the decohesion within the damage zones. The dark regions that appear in the micrographs shown in Figs. 9(a) and (b) should not be interpreted as voids, but rather as regions of decohesion of boron carbide particles; these particles may have been pulled out during the specimen preparation process (polishing) generating the appearance of a void like structure. The average size of such damage zones are of the size of about 0.5-2 ~m. also the size of the individual boron carbide particles with which the initial aggregate is made. This suggests that the boron carbide matrix is disintegrating, a possible mechanism for final failure as observed in the Hopkinson bar experintents. The transverse cracks which are observed (see Fig. 9(a)) near the impact face probably are due to the local tension arising from the gap between the specimen and the momentum trap, which creates a state of tension near the impact face. The presence of transverse cracks significantly reduces the antplitude of the tensile pulse reflected front the rear surface of the specimen as seen in Fig. 7. The micrographs also show that the damage zones are often clustered, and that they link together to form microcracks as seen Fig. 9(b). The microcracks then link up to form extended cracks that are either axial or transverse, depending on the local dominant stress state; Axial cracks form typically as a result of axial compression as shown by Horii and Nemat-Nasser (1985, 1986), which m the case of plate impact experiments is the compressive plane wave loading.
K.T. Ramesh, G. Raeichandran / Boron carbide aluminum cermet
Acknowledgement T h i s w o r k w a s s u p p o r t e d b y t h e U . S. A r m y Research Office under Contract Nos. DAAL-038 6 - K - 0 1 6 9 a n d D A A L - 0 3 - 8 8 - K - 0 1 1 8 to t h e U n i versity of California, San Diego. The authors are p l e a s e d to a c k n o w l e d g e t h e h e l p f u l d i s c u s s i o n s w i t h P r o f e s s o r S. N e m a t - N a s s e r a n d t h e t e c h n i c a l a s s i s t a n c e o f M e s s r s . J.B. I s a a c s a n d B.A. A l t m a n at the Center of Excellence for Advanced Materials in U C S D . T h e a u t h o r s t h a n k t h e C h e m i s t r y and Materials Science department of Lawrence Livermore National Laboratories for providing the boron carbide-aluminum cermet samples.
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