Abstract: To contribute to a better knowledge of the rheological characteristics of high modulus Kevlar fibers (type 0.95. TWA2) intended for use in the ...
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Temperature effects on the creep and dynamic behaviors of Kevlar prestressing cables S. Kaci and A. Khennane
Abstract: To contribute to a better knowledge of the rheological characteristics of high modulus Kevlar fibers (type 0.95 TWA2) intended for use in the prestressing industry, experimental results on the creep behavior, as well as on the dynamic behavior in tension as a function of temperature, are presented. The creep test rig is of original design and was built entirely in the laboratory. It is composed of a rigid frame, a load control mechanism, and an anchorage system perfectly suited to the material tested. The experimental results on the dynamic behavior as a function of temperature are obtained by means of a visco-analyser (METRAVIB). The frequencies scanned by this apparatus range from 7.8 to l000 Hz. The temperatures studied range from –30°C to 100°C. Key words: Kevlar, composite cables, creep, temperature, visco-analyser, conservation dynamic modulus. Résumé : Dans le but de contribuer à une meilleure connaissance des caractéristiques rhéologiques du kevlar de haut module (type 0,95 TWA2) destiné à la précontrainte, des résultats expérimentaux sur le fluage d’une part, et sur le comportement dynamique en fonction de la température d’autre part, ont été présentés. Le dispositif de fluage est d’une conception originale et entièrement construit au laboratoire. Il est composé d’un bâti rigide, d’un mécanisme de contrôle de la charge et d’un ancrage parfaitement adapté au matériel étudié. Les résultats expérimentaux sur le comportement dynamique en fonction de la temperature sont obtenus à l’aide d’un viscoanalyseur (METRAVIB). Les fréquences étudiées avec cet appareil s’etalent de 7,8 à 1000 Hz. Quant aux températures étudiées, elles s’étalent de –30°C a 100°C. Mots clés : Kevlar, câbles composites, fluage, température, viscoanalyseur, module de conservation dynamique.
Introduction Owing to their lightness, good mechanical properties, and an excellent resistance to corrosion, composite materials have gained a wide acceptance in high technology industries such as aeronautics, space, and ship building. With further research and development, this trend is to continue into the future, and will extend to other fields. In the building industry, the use of technologically advanced materials is at the research and development level. An example is the experimental bridge built in Germany (Gerritse et al. 1987). It is a concrete bridge prestressed with Kevlar tendons. Since its completion, the bridge has been under strict control and monitoring. This in-situ survey will supplement the laboratory investigations (Gerritse et al. 1986a; Gerritse et al. 1986b; Koning et al. 1987; Kaci 1989; Erki 1992; Hamelin 1992), and will contribute to a better knowledge of the overall behavior of these new materials. Indeed, the relative high cost and the lack of knowledge about their mechanical behavior have hampered development in this area. The introduction of composite cables in the prestressing industry presents technological advantages. In addition to
good mechanical performance such as a high tensile strength and small time-dependent losses (Kaci 1995), composite cables have excellent resistance to corrosion. According to a survey conducted by a team of American experts (Meier 1989) on prestressed structures built in North America in the last decades, most of the metal tendons, especially those used in cable stayed bridges, are in an advanced state of corrosion. In the present work, the results of the laboratory investigations into the creep behavior of composite cables intended for use in the prestressing industry, as well as their dynamic behavior as a function of temperature in order to determine the conservation dynamic modulus, E′, of the composite, are presented. The conservation dynamic modulus, E′, is the real part of the elastic dynamic modulus, E*, which is a complex number written in the form: [1]
E* = E′(1 + jη)
with [2]
η = E′′/E′
where E′′ is the damping modulus and η the damping factor. These constants are very useful when characterizing the viscoelastic behavior of the composite.
Received December 28, 1995. Revised manuscript accepted November 22, 1996.
Material studied (Kevlar type 0.95 TWA2 cord)
S. Kaci and A. Khennane. Institut de Génie Civil, Université de Tizi-Ouzou, Tizi-Ouzou, 15000, Algeria.
The material tested is a twisted high modulus Kevlar cord impregnated with a thermo-setting resin of polyester urethane. The composite thus obtained has a cylindrical shape of diameter 0.95 mm. It is highly anisotropic and can be manufactured in a greater length. The plait is obtained from two groups
Written discussion of this article is welcomed and will be received by the Editor until October 31, 1997 (address inside front cover). Can. J. Civ. Eng. 24: 431–437 (1997)
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Fig. 1. Tension stress–strain curve of a Kevlar cord (type 0.95 TWA2).
Fig. 2. Fracture surfaces of a filament of Kevlar type 49 fiber obtained with an electric microscope.
constituted each of four strands (the first group is twisted Swise and the second Z-wise). Each of these strands is made of an assembly of 768 parallel Kevlar type 49 fibers, and has a linear mass of 1270 × 10–7 kg/m. Such an assembly of twisted fibers allows the best possible conservation of the mechanical characteristics of the Kevlar fibers. The impregnating resin, in addition to the lubricating role it plays between the fibers, helps to conserve the cylindrical shape of the cord. Further, because of its elastic bonding to the fibers, it enhances the
flexibility of the composite. The percentage of fibers by weight in the composite is about 80%.
Mechanical characteristics in tension The mechanical characteristics in tension are obtained using an Adamel–l’Homargy tension test device (model D.Y.25), which is equipped with a high resolution optical extensometer and a high force measuring device. The diameter of the © 1997 NRC Canada
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Fig. 3. Creep experimental setup. (1, frame; 2, anti-vibration device; 3, passive anchorage (pulley); 4, metallic plate; 5, cylindrical weights; 6, active anchorage (pulley); 7, universal beam (U80); 8, universal joint; 9, jack; 10, crank of jack; 11, specimen; 12, dial gauge.)
Fig. 4. Creep machine.
Fig. 5. Active anchorage.
test specimen is measured with a profile projector (SASEM P500), and has an average value of 0.95 mm. The results are as follows: average tensile strength: 2201 MPa average ultimate strain: 2.13% The stress–strain curve is shown in Fig. 1 (Kaci 1989). The curve is linear up to rupture and is similar to that of an elementary Kevlar fiber. The examination of the rupture surfaces with a scanning electric microscope revealed that before rupture, the plait starts to open first (Fig. 2). This results from the rupture of the polyester–urethane matrix, which leaves the strands free to dissociate from each other. Then the fibers constituting the strands © 1997 NRC Canada
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Fig. 6. Passive anchorage.
Fig. 7. Creep of a Kevlar cord (type 0.95 TWA2) at ambient temperature (20°C) and 30% rupture stress.
start to stretch and move apart from each other to break up into many elements of different diameters, which, in turn, will rupture as their strengths are exceeded. This, of course, causes the rupture at different sections.
Creep test Test choice The choice of a creep test under uniaxial tension is dictated by
the nature of the material and its future use as a prestressing tendon. Because of its fibrous, hence anisotropic, nature, the material presents a very low resistance to transverse actions such as shear, bending, and tension. As a result, creep tests under these actions are inadequate. Furthermore, the high flexibility of the cord has no effect under these actions. Uniaxial tension is therefore the most suitable choice, since it ensures a uniform stress distribution in the section of the cord, thus eliminating any transverse actions. © 1997 NRC Canada
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Kaci and Khennane Fig. 8. Relative creep of a Kevlar cord (type 0.95 TWA2) at ambient temperature.
Fig. 9. Logarithm of dynamic conservation modulus as a function of temperature and frequency.
Experimental setup The rigid frame The frame is made of universal beams (U80) and designed to ensure that the stress is negligible and would not induce any creep or displacement that could interfere with those of the specimen tested. The frame stands on a very stiff base (labeled
1 in Fig. 3) isolated from the ground by anti-vibration devices (labeled 2). Anchorages Preliminary tests have been carried out on Kevlar cords using different means of griping. However, none of them turned out to be satisfactory. This is due to the serration of the anchorages © 1997 NRC Canada
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Fig. 10. Dynamic conservation modulus as a function of temperature and frequency.
that dig into the specimen, resulting in extremely high compressive and shear stresses that the material cannot withstand because of its low resistance to transverse forces. Casting the ends of the specimen in a resin also turned out to be unsatisfactory because of the slip, which was noticed during testing. Since the test specimen should not undergo any transformations either in its properties or in its shape, and no slip should be noticed during testing, an anchorage based on a system of pulleys (labeled 3 and 6 in Fig. 3) was then chosen (Figs. 5 and 6). The first tests carried out with this new anchorage proved to be very satisfactory (Kaci 1989). The strains of the cord around the pulleys are uniformly reduced to a negligible value. Furthermore, the straining occurs completely in the free length of the specimen. Loading system The loading system was also designed and built in the laboratory. It is made of cylindrical weights (labeled 5 in Fig. 3) standing on a steel plate (labeled 4) which is interdependent with the active anchorage (labeled 6) by means of a universal joint (labeled 8). The steel plate and the weights are originally supported by a jack (labeled 9). Once the jack is disactivated by a crank (labeled 10), the entire load is instantly transmitted to the specimen (labeled 11). Data recording Depending on the load level, different displacement measuring devices are used. At low load levels, the displacements of the specimen are measured by means of a dial gauge (labeled 12 in Fig. 3) attached to the active anchorage. At high load levels, they are, instead, measured with either a cathetometer or an optical extensometer.
Creep test results The test results are shown in Fig. 7. It can be seen that after 8 h, most of the creep (80%) is completed. When plotted in semilogarithmic coordinates (Fig. 8), the creep of Kevlar fibers is linear in ln(t), where t denotes time. Therefore, it can be approximated with a linear function of the form: [3]
ε = εt/εi = A ln(t) + B
where εt is the total strain at time t; εi is the instantaneous strain; and A and B are constants independent of time, but were found to be dependent on temperature for an equivalent relaxation test (Kaci 1989, 1995).
Dynamic behavior under tension as a function of temperature Experimental setup Tests on the dynamic behavior in tension at various temperatures are carried out in order to determine the effect of temperature on the conservation modulus, E′, of Kevlar fibers. These tests are necessary, since E′ cannot be obtained with static tests such as creep or relaxation. The specimens tested are Kevlar cords 50 mm long and 0.95 mm in diameter. They were placed in a vertical position and tested with a visco-analyser (METRAVIB). The frequencies scanned by this apparatus range from 7.8 to 1000 Hz and the temperatures range from –30°C to 100°C. This apparatus consists of a vibrator which exerts a sinusoidal strain on the specimen. The strain is measured with a displacement measuring device. At the other end of the sample, the applied force is measured. The sample is surrounded by an electrical oven to ensure that it is heated to the right temperature. Cooling is ensured by circulating air in © 1997 NRC Canada
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contact with a tank full of liquid nitrogen. The results are presented in Figs. 9 and 10. Test results It appears from Figs. 9 and 10 that for any given temperature, the higher the frequency, the greater is the dynamic conservation modulus. Its maximum is reached at the higher frequencies. However, at the low frequencies, the dynamic modulus is small, and most of the strain must be due to some damping effects. At the medium frequencies, a continuous increase in the dynamic modulus is observed. Furthermore, it appears that temperature has a major effect on the values of dynamic modulus. For any given frequency, the dynamic modulus decreases with increasing temperature.
Conclusions Experimental evidence on the creep behavior and the dynamic behavior in tension as a function of temperature of a Kevlar cord, type 0.95 TWA2, intended for use in the prestressing industry has been presented. The uniaxial stress–strain curve of the composite (Kaci 1989) was found to be similar to that of an elementary Kevlar fiber. It is linear up to rupture with a low ultimate strain. Furthermore, the composite presents a high elastic modulus and a high tensile strength. However, the uniaxial stress–strain curve does not present a plastic yielding, thus reducing the ductility of the composite. Therefore, it may be necessary to use a high safety factor. The creep behavior was investigated using an original test rig specially designed for the material tested. The creep of the composite was found to increase logarithmically in the first 8 h after loading. The dynamic behavior in tension of the composite as a function of temperature was studied using a visco-analyser. It was found that temperature has a major effect on the dynamic conservation modulus E′. Therefore, thermal effects should be taken into account when modeling the creep or relaxation behavior of Kevlar fibers, as well as in other experimental studies.
References
Kevlar cable/fibre-reinforced plastic tube structural system. In Proceedings of the First International Conference on Advanced Composite Materials in Bridges and Structures. Edited by K.W Neale and P. Labossiere. Canadian Society for Civil Engineering, Montreal, Que., pp. 445–454. Gerritse, A., and Schurhof, H.J. 1986a. Aramid reinforced concrete (ARC). Technical contribution to the RILEM 3rd Symposium, Sheffield, England. Gerritse, A., and Schurhof, H.J. 1986b. Prestressing with aramid tendons. Technical contribution to International Federation of Prestressed Concrete 5th Congress, New Delhi, India. Gerritse, A., Schurhof, H.J., and Maatjes, E. 1987. Prestressing concrete structure with arapree. Technical contribution to International Association of Bridges and Structures Engineering (IABSE) Symposium, Paris, France. Hamelin, P. 1992. Poutres de béton de polymère précontraintes par câbles composites verre-epoxy. In Proceedings of the First International Conference on Advanced Composite Materials in Bridges and Structures. Edited by K.W Neale and P. Labossiere. Canadian Society for Civil Engineering, Montreal, Que., pp. 201–209. Kaci, S. 1989. Câbles composites pour la précontrainte: étude de la relaxation. Doctorate thesis, Universite de Bordeaux 1, Bordeaux, France. Kaci, S. 1995. Experimental study of mechanical behaviour of composite cable for prestress. ASCE Journal of Engineering Mechanics, 121(6): 709–716. Koning, G., and Wolff, R. 1987. Heavy duty composite material for prestressing of concrete structures. Technical Contribution to International Association of Bridges and Structures Engineering (IABSE) Symposium, Paris, France. Meier, U. 1989. Reparation des poets avec des materiaux composites à hautes performances. Journees d’etude sur les Materiaux Composites, Ecole Nationale des Ponts et Chaussees, Paris, France, October 25–26.
List of symbols A, B E′ E′′ E* t εi εt η
material constant dynamic conservation modulus damping modulus dynamic elastic modulus time instantaneous strain total strain a time t damping factor
Erki, M.A. 1992. Creep relaxation characteristics of a prestressed
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