Evaluation of fatigue damage on the mechanical properties of fiber reinforced cement pastes

CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 879-888, 1985. Printed in the USA. 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.

EVALUATION OF FATIGUE DAMAGE ON THE MECHANICAL PROPERTIES OF FIBER REINFORCED CEMENT PASTES

H. Nayeb Hashemi I, M. D. Cohen 2 Northeastern University Boston, Massachusetts 02115 and T. Erturk 3 Massachusetts Institute of Technology Cambridge, Massachusetts 02139

(Communicated by R.E. Philleo) (Received May 14, 1985) ABSTRACT Type III Portland cement samples reinforced with 0.01 volume fraction of chopped steel fibers were subjected to cyclic loading in tension with zero minimum stress and maximum stress amplitudes varying from 0.72 to 1.38 of the first crack strength of the composite. The number of cycles-to-fracture at different stress amplitudes (S-N curve) indicated good fatigue resistance in stress amplitudes representing up to 80% of the range between first crack and ultimate composite strengths. A number of specimens cycled at a stress amplitude 38% above the first crack strength were monotonically fractured in tension following different levels of prior fatigue damage. Pre-cycled specimens exhibited a rapid drop in Young's modulus, substantial increase in ultrasonic attenuation, but insignificant decay in composite strength and post-cracklng ductility. It was concluded that fatigue damages, short of propagating interface cracks to the ends of the fibers, would not adversely affect the composite strength and post-cracking fiber pull-out behavior.

INTRODUCTION Because of the low fiber volumes utilized for workability considerations in the fresh state, only modest improvements in strength or stiffness are i. 2. 3.

Department of Mechanical Engineering Department of Civil Engineering On sabbatical leave from Dept. of Metallurgical Engineering, Middle East Techmical University, Ankara, Turkey. 879

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Vol. 15, No. 5 H.N. Hashemi, et al.

achieved by incorporating metal, ceramic or polymer fibers in cementitlous composites. On the other hand, good post-cracking energy-absorblng properties, where conventional descriptions of fracture toughness are not an appropriate measure, can be obtained through low volume fiber reinforcement of cementitlous materials. The first crack strength of the composite is a function of several parameters such as fiber volume fraction, orientation, aspect ratio, interracial bond strength, Young's modull of the matrix and fibers, and fracture surface energy of the matrix, and it may be lower or higher than that of the matrix itself (i). Likewise, post-cracklng strength can be lower or higher than the first crack strength (2). With relatively low volume fractions utilized (usually in the range of 1-3% for steel fibers), modest improvements are obtained in both first crack and post-cracklng composite strengths. Toughness, as measured for example, by the area bounded by the stress-strain curve, on the other hand, can be improved by several orders of magnitude hy the addition of fibers. Thus, unlike other composite systems with a high volume of fibers, where strengthening, stiffening, and other properties are of major concern, here it is the increase in ductility through fiber pull-out following the cracking of the matrix that is of major concern. Most concrete structural components are subjected to fatigue or repeated loading during their lifetime. Even though significant research has been performed on understanding the mechanisms of both static and dynamic fatigue in brittle solids, little is known about the effects of prior cycling on subsequent engineering properties. This is especially true for fiber-reinforced cement, mortar or concrete. The objective of the present study is to investigate those effects on the static tensile, microstructural and ultrasonic properties of fiber reinforced portland cement pastes. EXPERIMENTAL PROCEDURE In order to eliminate any complications that might have arisen due to cement/aggregate bond, neat cement paste was used in the present study. The commercial grade Type III portland cement had a calculated phase composition (from Bogue's equation) of C3S = 51.8%, C2S = 20.5%, C3A = 8.9% and C4AF = 8.3%. The 1-day, 3-day, and 7-day compressive strengths were 3060 psi (21.1 MPa), 4820 psi (33.24 MPa) and 6050 psi (41.72 MPa), respectively, as determined by ASTM C39. Initial and final sets, as determined by Gillmore Apparatus ASTM C266, were 2 hours and 4 hours, respectively. The commercially obtained AISI i010 low-carbon steel fibers, chopped from plates for surface roughness, averaged 0.33 x 0.63 mm in rectangular cross section and 25 mm in length. A water/cement ratio of 0.4 was used, and specimens were cast with a nominal fiber volume fraction of 0.01. Mixing, casting and curing procedures were carried out as follows: Dry cement powder was placed in a large mixing bowl, and steel fibers dispersed. This was followed by pouring water at 23°C for 30 seconds, and careful hand mixing for 4 minutes. After a 2-mlnute rest period, mixing was resumed for an additional 5 minutes using a small double-blade mixer at medium speed. Cement was then cast in dog-bone shape molds ASTM C 190, covered with moist paper towels, and sealed with sheets of polyethylene. During casting, the molds were vibrated to ensure compaction. Two hours after casting, excess cement was scraped off and the top of the samples smoothened, and again specimens were covered as described above. After 24 hours, specimens were demolded, labeled and stored in water saturated with

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881 STEEL FIBER, REINFORCED CEMENT, FATIGUE, TENSILE STRESS

lime at 23°C for 14 days. (Water was saturated with llme to avoid leaching of Ca(OH) 2 from inside the specimen.) On the 14th day, samples were washed and stored in Jars containing acetone for preservation. The 14-day old specimens were servohydraulic testing machine, at (R=Gmin/Gmax=0). Specimens failed relatively rapid, progressive fiber using standard procedures.

cycled sinusoidally at 5Hz in an MTS a mean load of 0.5 of the maximum load at the end of their fatigue lives by a pull-out. The S-N curve was determined

Effect of prior fatigue damage on tensile properties were studied using a stress amplitude (amax) of 2.9 MPa. Several specimens cycled at this stress amplitude for 5,000, i0,000, 15,000, 20,000, 25,000 and 30,000 cycles were taken out of machine for measurement of their subsequent tensile properties. The expected fatigue life of specimens at this stress were between 2.5 x 104 and 3.1 x 104 cycles giving an average value of 2.7 x 104 . Surfaces of the specimens were then polished using 200 grade emery sand-paper. Type FAB-37-35-SI3 strain gages were mounted using EPY-150 epoxy. Specimens were then tested in monotonic tension at a stress rate of 4.5 N/sec. A half-bridge circuit was used to measure axial strain. After the specimens were fractured in monotonic tension, they were sectioned longitudinally (perpendicular to the fracture surface) to expose the embedded fibers for fractographic examination. Specimens were sectioned by slowly cutting 2 mm deep notches on the sides with a diamond saw, followed by static compression using pointed dies. Samples were then prepared similar to previously used procedures (3, 4) and both matrix and fiber/matrix interfacial regions were examined in AMR-1000 and Cambridge Stereoscan ii00 scanning electron microscopes. In order to measure the level of degradation of the material due to fatigue, an ultrasonic set-up shown in Figure i was constructed. The set-up consisted of a pulse generator, transmitter and receiver transducers, an oscilloscope, and a clamping device which exerted a "saturation pressure" from the transducer to the specimen. The saturation pressure is the pressure above which the received signal becomes insensitive to the pressure. Previous research (5, 6) showed that the amplitude of the received signal depends on the pressure of the transducer to the specimen and if it is less than the saturation pressure, the signal may vary as much as 100%, rendering the attenuation measurement meaningless. Any change in amplitude of the received signal relative to the virgin specimen is an indication of damage, mainly in the forms of matrix and interface cracks. The change of attenuation was measured by generating a pulse with amplitude V T and transmitting it through the specimen perpendicular to the loading direction. The received signal V N can be defined as:

(1)

V N = FIF 2 V T exp (-~N L)

where F 1 and F 2 are impedence between transducers and the cement specimen, a N is the attenuation after N cycles and L is the thickness of the specimen. For a virgin specimen, the received signal is, V 0 = FIF 2 V T exp (-~0 L) The change

in attenuation

(2) (eN-a0)

can be found by dividing

Equations

(i)

882

Vol. H.N. Hashemi,

35

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'

o

' '''E

~ Composite Strength

~30

0 • 0 0

K o

I~

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F,rst Croc~ Strength

2.0

15, No. 5

et al.

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x

u ,,

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08

u) t5

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t0 4

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~06

NUI~BER OF CYCLES, N

Figure i.

Schematic of Ultrasonic Set-Up

and (2) and assuming impedences are not changed, V0 VN

(3)

= exp (~N-~0)L

or, (~N-
(V0) VN

(4)

where A~ has the unit neper/mm. RESULTS The number of cycles-to-fractUre (S-N amplitudes (Omin=0) is presented in Figure

curve) at different stress 2. Each point in Figure 2

Generotor

Ch Z

3,

A

TrQnsm#tter Tronsducer

-Clomplnq System

Receiver Tronsducer

"

7

T

SCHEMATIC OF ULTRASONIC SET-UP

Figure 2.

S-N Curve for Type III Portland Reinforced With 0.01 Volume Fraction of Chopped, Rectangular Cross-Sectlon, Low Carbon Steel Fibers

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883 STEEL FIBER, REINFORCED CEMENT, FATIGUE, TENSILE STRESS

represents an average of at least 4 specimens. The first crack strength, Ofc=2.1 MPa, and the ultimate composite strength Oc=3.1 MPa, are marked in the Figure signifying approximately 48% strengthening above the first crack strength in monotonic tension. The first crack and ultimate composite strengths were obtained from stress-strain curves of four virgin specimens; an abrupt increase in stress level upon matrix cracking identified the first crack strength. The highest stress amplitude employed in cyclic loading was omax = 2.9 MPa, 38% above the first crack strength, and representing 80% of the stress range between the first crack and ultimate composite strengths, i.e. ( o c- Ofc)/( Cm8 x- Ofc)=0.80. At this stress amplitude, failure occured at 2.7 x i0 ~ cycles, Figure 2. The lowest stress amplitude employed in the present study (1.51 MPa) was 0.72 of the first crack strength of the composite. Here, specimens did not fail even after 2 x 106 cycles. The decrease in the relative elastic modulus, as determined from the engineering stress-strain curves, in specimens cycled to different degrees of damage is shown in Figure 3. A rapid decay in modulus during the first 104 cycles is observed. Both post-cracking strengthening and pull-out behavior were affected insignificantly by pre-fatiguing. I

I

I

I t0

I t5

I

I

I

t .0(

Figure 3. Loss in Apparent Elastic Modulus Due to Pre-Cycling at o max=2.9 MPa.

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0.6

u.I > J

0.4

0.2

O 0

I 5

.,

I 20

1 25

,,

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N U M B E R OF C Y C L E S

Figure 4 indicates the increase in ultrasonic attenuation upon cycling at two stress amplitudes, Omax = 2.9 and 2.62 MPa. It is clear that cycling increases the attenuation, and specimens cycled with a higher stress amplitude show higher damage. The ultimate composite strengths for specimens cycled to dl£ferent levels of their fatigue lives at a 8tress amplitude of 2.9 MPa are presented in Figure 5. No appreciable decay in composite strength is observed.

884

Vol. 15, No. 5

H.N. Hashemi, et al. ]

I 0

O0

0 0

50

o~ t 10

~°z

OB

25

~

b~ ~20

C'J 0

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m

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Figure 4. Increase in Sound wave Attenuation Due to Pre-Cycling at Omax=2.62 MPa and omax=2.9 MPa

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t5

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i

0

2 0.5

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30 x t0 3

NUMBER OF CYCLES Figures 6 and 7 show a fiber, at two dlfferent magnifications, emerging from the fracture surface of a specimen pre-fatigued 2 x 104 cycles with a stress amplitude of 2.9 MPa and sectioned longitudinally as described before. Note the presence of matrix cracks emanating from the fiber and also pronounced interface delamination. Such radial cracks were not observed in unfatigued specimens under similar magnifications. Figure 8, and its close-up, Figure 9, show large and small cement particles remaining on the surface of the fiber. Occasionally observed large and unpatterned particles, such as the ones seen on the left side of the fiber existed in groups or bundles. (See also Figures i0 and ll). In addition to the presence of large particles, fiber surfaces were coated, almost uniformly, wlth minute and colloidal particles.

N,o

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i

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0

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NUMBER OF CYCLES Figure 5.

Change in Composite Strength With Precycling at omax=2.9 MPa.

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885 STEEL FIBER, REINFORCED CEMENT, FATIGUE, TENSILE STRESS

Generally, the surface particles do not resemble the plate-llke hexagonal calcium hydroxide Ca(OH) 2 crystals commonly observed at fiber/matrix interfaces in cementitious composites (3, 7). Energy dispersive x-ray analysis of these particles indicated Ca/Sl ratios of 1.5-2.0 indicative of a family of hydrated calcium silicate gels, a product resulting from the reaction of tricalcium and dicalcium silicates with water. These products are known to be responsible for strengths of cements and bond strength of fiber cements due to their very high surface areas of about 250 m2/g (8, 9). Figure 12 depicts an interface crack in a plane sectioned perpendicular to the fracture surface of a specimen pre-fatlgued 104 cycles at a stress amplitude of 2.9 MPa. Figure 13 shows the same crack along the interface near the end of the fiber. DISCUSSION AND CONCLUSIONS Steel fiber reinforced cement exhibited good fatigue resistance at stress amplitudes up to 38% above the first crack strength and penetrating 80% of the range between the first crack and ultimate composite strengths, Figure 2. On the other hand, a rapid decay in the elastic modulus, Figure 3, and a substantial increase in ultrasonic attenuation, Figure 4, both indicative of substantial prior fatigue damage, have been observed. Yet, the composite strength is not affected substantially by pre-cycling, Figure 5. Even though a somewhat more pronounced decay in post-cracking fiber pull-out was observed in monotonic tension for pre-cycled specimens, this too was not substantial. Batson and co-workers (i0) observed an increase in flexural strength i n fiber-reinforced concrete subjected to prior flexural fatigue, without providing, however, an explanation for the observed behavior. Yoshimoto, Ogino and Kawakami (ii) observed similar increase in some of their unrelnforced concrete specimens, the others showing no decay. Also, similar to the present study, they observed reduction of Young's modulus with prior fatigue. These investigators hypothesized that fatigue cracks forming in the cement paste could relieve the cement/aggregate bond cracks or coarse aggregate fissures. Clearly, whether healing or damaging, cyclic loading of cementitlous materials involve extensive cracking of the interfaces and the matrixes. In specimens cycled at stress amplitudes below the first crack strength, fatigue cracks most likely grow in the matrix followed by delamination of fiber-matrix interfaces. On the other hand, damage at stress amplitudes above the first crack strength should consist mainly of interfacial degradation, i.e., propagation of cracks along the interface. SEM micrographs clearly indicate interface delamination in pre-fatigued samples, Figures 6-8. Argon, Hawkins and Kuo (i) demonstrated that the composite reaches its ultimate strength when the cracks at the interfaces travel to the ends of the fibers. Thus, since the ultimate strength of the composite is attained at the end of maximum probable damage, any prior damage short of this should not affect the composite strength, Figure 5. Post-cracking ductility, on the other hand, being controlled by the interfacial frictional characteristics, appears to be somewhat more degraded by pre-cycling. Fatigue of steel fiber reinforced cement at stress amplitudes exceeding

886

Vol. 15, No. 5 H.N. Hashemi, et al.

r

Figure 6:

Fiber emerging from the fracture surface of a specimen pre-fatigued 2xlO 4 cycles.

Figure 7:

Magnification of Figure 6

the first crack strength of the composite appears to involve mainly the delamination of the fiber/matrix interfacial region, followed by relatively rapid fiber pull-out. Cycling below the first crack strength, however, should involve damage accumulation in the matrix and at the fiber/matrlx interface, leading to longer fatigue lives. It appears that fatigue damages short of propagating the interfacial cracks to the fiber ends do not adversely affect the composite strength and post-cracking ductility, even though the material undergoes substantial loss in elastic modulus.

Figure 8:

Large and small cement particles remaining on the surface of a fiber

Figure 9:

Magnification of Figure 8

Vol. 15, No. 5

887 STEEL FIBER, REINFORCED CEMENT, FATIGUE,

. . . . . . .

TENSILE STRESS

.

f

6

%

Figure i0:

Large and small cement particles remaining on the surface of a fiber

Figure ii:

Large and small cement particles remaining on the surface of a fiber

ACKNOWLEDGEMENTS The authors would llke to express their gratitude to Professor A. S. Argon, Department of Mechanical Engineering, M.I.T., for useful discussions.

h

Figure 12:

Plane perpendicular to fracture surface of specimen pre-fatigued 10 4 cycles

Figure 13:

Same crack (figure 12) along the interface near the end of the fiber.

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Vol. 15, No. 5 H.N. Hashemi, et al.

REFERENCES 1.

Argon, A.S.; Hawkins, 1707-1716, 1979.

G.W.;

Kuo,

H.Y.,

J.

of

Mat.

Sci.,

14,

pp.

2.

Hannant, D.J., Fibre Cements and Fibre Concretes. John Wiley P. 13, 1978.

3.

Erturk, T., and Tokyay, M. in J. Carlson and N.G. Ohlson (Eds.), Mechanical Behavior of Materials - IV, Vol. i (Proceedings 4th Int. Conf. on Mech. Behav. of Mat., Stockholm, Sweden, August 1983), pp. 507-515.

4.

Cohen, M.D., Campbell, E., and Fowle, W., Proc. 7th Int. Conf. on Cem. Micr., pp. 360-381, March 1985.

5.

Nayeb Hashemi, H.; Lee, S.S.; and Williams, J.H., J. of Non-Dest. Eval. V. i, No. 3, pp. 191-199, 1980.

6.

Nayeb Hashemi, H.; Williams, J.H.; and Lee, S.S., J. of Non-Dest. Eval. V. i, No. 2, pp. 137-147, 1980.

7.

Page, C. L., Composites V. 13, pp. 140-144, 1982.

8.

Soroka, I., Portland Cement Paste and Concrete, MacMillan Press, 1979.

9.

Power, T.C. Proc. Symp. Chem. Cem. Washington, 2, pp 577-613, 1960.

i0. Batson, G.; Ball, C.; Bailey, L.; Landers, E.; and Hooks, ACI Journal, pp. 673-677, November 1972. ii. Yoshimoto, A.; Ogino, April 1972.

S.; and Kawakami,

M., ACI Journal,

pp. 233-240,