Effects of temperature on tensile strength of Kevlar reinforced composite materials.Introduction
Since 1972, Kevlar has been brought to the market and applied to many fields of work. Kevlar is known for its high strength compared to its weight. Kevlar is used to improve the strength of composite materials. According to the past researches, most researchers found that some Kevlar's distinguished qualifications e.g. high stress, high Young modulus, etc. could not be fully utilized when applying with composite material. (Yue and Sui). There are two possible assumptions to explain the cause of this problem as follows:
a. The connection between Kevlar and Matrix is not strong enough to transfer the stress between Kevlar and Matrix.
b. The Kevlar damage caused by heat while in the processing process or in working filed made the Kevlar qualifications change.
From the previous researches, there were some studies about the effect of temperature on Kevlar. The researchers, Hindeleh and Abdo, studied the temperature effect on Kevlar by heating Nitrogen at over 150[degrees]C for 15 minutes. Parimala and Vijayan tested Kevlar-49 at the temperature of 350[degrees]C from 0.5-260 hours. From those studies, only high constant temperature was applied to Kevlar. Any change in the temperature was involved in the studies. In aerodynamic, Kevlar is mixed with polyester resin and epoxy resin in order to make the wing model. Kevlar used in this case helps improving the strength of polyester resin and also epoxy resin. In real situation, it is true that the temperature changes from time to time. Moreover, changing in altitude also causes in temperature change. Consequently, the objective of this research is to study how the changing in temperature affects the strength of composite material in order to make the wing design more complete. Some variables e.g. temperature, temperature change, period of time, etc. will be studied. The result will be summarized in terms of mathematic equation illustrating the relation among these variables. This equation will help improving the quality of wing design.
Kevlar aramid fiber was commercialized by DuPont Company in 1972 as an industrial fiber product. "Kevlar" is a registered trademark of Du Pont Co. The word 'aramid' is a generic term for 'a manufactured fiber in which the fibre-forming substance is long chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings', as defined by the US Federal Trade Commission. (H.H. Yang 1991) The discovery of Kevlar aramid fiber began in 1965 when S.L. Kwolek, a Du Pont research scientist synthesized a series of para-oriented super-rigid molecular chain and a fiber of ultra-high modulus could be made from a para-oriented symmetrical polymer molecular. Kevlar has a unique combination of high strength, high modulus, toughness and thermal stability.
Table 1 compares the properties of Kevlar 29 and Kevlar 49 to other yarns, such as a glass, steel, wire, nylon, polyester, polyethylene and carbon. Compare to Kevlar, nylon and polyester have relatively low modulus and intermediate melting points. Polyethylene has a high initial modulus, which is offset by its relatively low melting point. Kevlar was developed for demanding industrial and advanced-technology applications. Currently, many types of Kevlar are produced to meet a broad range of end uses, e.g. continuous filament yarn, staple, engineered short fiber, pulp, spun yarn, needlepunched felt, paper, woven fabric, cord, and narrow webbing. Fig. 1 presents an SEM micrograph of basic Kevlar aramid filaments.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Fig. 2 illustrates a typical stress-strain curve of Kevlar aramid yarn comparing to other industrial filament yarns. These curves show that Kevlar yarn has a break tenacity of 22-23 gpd (gms/denier1). This is five times more than that of steel wire and two times more than that of nylon, polyester, or glass fibers. Kevlar yarn also has an unusually high initial modulus of 475 gpd, which is about twice more than that of steel wire or fiberglass, four times more than that of high tenacity polyester, and nine times more than that of high tenacity nylon.
The Kevlar strands were separated from that of Woven fabrics, one strand of Kevlar consists of approximately 20,000 filaments. During the heat treatment, the specimens were suspended with the two ended support, and all parts were not contacted with the surface of the oven in order to protect any parts of specimens from burning. Only the middle portions of specimen which were constantly subjected to the heating and cooling were used for tensile testing both in heat and cold treatment. The Kevlar strands were treated under the following varied conditions, e.g. at - 20[degrees]C, 100[degrees]C, 200[degrees]C for durations of 2, 4, 8, 16, 32 hours in the normal atmosphere. All the heat treatments conditions were completed in a hot air oven BINDER FD-240, and cold treatments conditions were completed in Ultra-Low Temperature Freezer model MDF-U6086S. The tensile testing had been immediately experimented after completing the treatment conditions in order to summit the accurate results.
After the specimens were randomly selected, the Universal Testing Machine (UTM) INSTRON Model 5566, which can apply the maximum load of 10 KN with a load cell capability of 500 N, was used for measuring the tensile strength. The vertical movement of the crosshead of UTM was controlled at the speed of 1 mm. per minute for all tensile tests.(CRE Test) gauge length of 20 mm was used in tensile test. 25 specimens for each case were tested. The results were captured by the data acquisition unit of the UTM. The data were recorded in tension and extension, which depend on time base. Fig. 3 shows a failure of the sample specimen under tensile testing.
[FIGURE 3 OMITTED]
Result and Discussion
The average of tensile strength for non-treated and treated Kevlar are summarized in Table 2 and the average of strain for as-received and treated Kevlar are summarized in Table 3. The variation in mechanical properties with temperature are shown in Figs. 4 and 5.
[FIGURE 4 OMITTED]
Similarly, fig. 5 shows the correlation between the average tensile strain for non-treated and treated Kevlar in varied temperature and duration of treatment. There is no significance between the average tensile stain of heat-treated specimens and that of non-treated specimens. The highest difference was 2.71% at a heating duration of 16 hrs, and the average one is 0.50% in all duration. At the temperature of 100[degrees]C and 200[degrees]C, there is a negative correlation between tensile strain and temperature. The average tensile strain of heat-treated specimens differed from that of non- treated specimens with approximately 7.56% and 15.13% under 100[degrees]C and 200[degrees]C respectively.
[FIGURE 5 OMITTED]
From the study, Kevlar lost its strength when temperature increased. The property of the Kevlar itself may be the cause of this behavior. This assumption was strongly supported by Kevlar Dupont technical information saying that as temperature increase, there is an immediate weight reduction, corresponding to water loss.
The result got from this study will be beneficial for the work that requires light, high strength and high temperature resistant plane surface. Kevlar is appropriate for using as a reinforce material for composite materials.
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(1) Sumpun Chaitep, (2) Chaiy Rungsiyakull and (3) Pipatpong Watanawanyoo
(1&2) Department of Mechanical Engineering, Faculty of Engineering, Chiang Mai University, Chiang Mai, Thailand 50200
(3) College of Engineering, Rangsit University, Pathum Thani, Thailand 12000
Table 1: Comparative Properties of Kevlar vs. Other Yarn (Du Pont Co. 1999) Specific Tenacity Modulus Break Density [10.sup.3] [10.sup.6] Elongation lb/[in.sup.3] psi psi % KEVLAR19 0.052 424 10.2 3.6 KEVLAR49 0.052 435 16.3 2.4 Other Yarns S-Glass 0.090 665 12.4 5.4 E-61ass 0.092 500 10.5 4.8 Steel Wite 0.280 285 29 2.0 Nylon-66 0.042 143 6.0 18.3 Polyester 0.050 168 2.0 14.5 HS Polyethylene 0.O35 375 17 3.i High-Tenacity 0.065 450 32 1.4 Specific CTE (1) Decomposition Tensile [10.sup.-6]/ Temperature Strenght (1) [degrees]F [degrees]C [10.sup.6] in. KEVLAR19 8.15 -2.2 427-482 KEVLAR49 8.37 -2.7 427-482 Other Yarns S-Glass 7.40 +1.7 850 E-61ass 5.43 +1.6 730 Steel Wite 1.00 +3.7 1,500 Nylon-66 3.40 -- 2-4 Polyester 3.36 -- 256 HS Polyethylene 10.7 -- 119 High-Tenacity 6.93 -0.1 3,500 Carbon (1) The weight in grams of 9000 meters of yarn Table 2: Tensile strength of non-treat and treated Kevlar Tensile Strength (N) (Standard deviation) Temp 2hr d1ir 8hr 1 hr 32hr Non-treated 302.69 (8.58) -20[degrees]C 300.19 289.99 303.89 285.80 296.78 (17.19) (21.51) (13.07) (9.53) (23.07) 100[degrees]C 281.01 254.03 283.52 276.99 279.75 (13.32) (9.60) (13.29) (15.34) (17.91) 200[degrees]C 259.30 258.52 252.02 250.37 244.61 (18.52) (13.97) (18.18) (15.20) (17.03) Note: Each data represented for 15 measured values Table 3: Tensile strain of non-treat and treated Kevlar Tensile Strain (%) (Standard deviation) Temp 2 hr 4 hr 8hr 16hr 32hr Non-treated 15.58 (0.38) -20[degrees]C 15.70 15.70 15.70 15.70 15.70 (0.51) (0.51) (0.51) (0.51) (0.51) 100[degrees]C 14.35 14.35 14.35 14.35 14.35 (0.57) (0.57) (0.57) (0.57) (0.57) 200[degrees]C 13.36 13.36 13.36 13.36 13.36 (0.85) (0.85) (0.85) (0.85) (0.85) Note: Each data represented for 15 measured values