INTERFACIAL SHEAR STRENGTH OF SINGLE FILAMENT CARBON FIBER REINFORCED COMPOSITES.

Transcription

1 m INTERFACIAL SHEAR STRENGTH OF SINGLE FILAMENT CARBON FIBER REINFORCED COMPOSITES.

2 : 16 7 : 4.1 INTRODUCTION It is widely accepted that bonding between fiber and matrix plays an important role in stress transfer mechanism in fiber reinforced composite materials and hence influence the mechanical properties of the composite materials, specially interfacial shear strength and transverse tensile strength [ ]. It has also been recognized that the tensile properties such as longitudinal tensile strength and composite toughness etc. are indirectly affected by the adhesion between fiber and matrix [ ]. Single filament composite (SFC) specimens are becoming increasingly recognized for their usefulness in studying the micromechanics of interaction between a fiber and its surrounding matrix. Ultimately one would like to transfer the improved understanding of micromechanics in SFCs into the higher fiber volume fraction of 'real' composite materials. Interest in single filament composite testing systems is also generated from the desire to establish a scientific understanding of the load transfer behaviour of the interface. As discussed in chapter 1, the fragmentation test [126,132] in which a single fiber is embedded in a polymer and broken into small pieces is the most realistic test from the

3 : 168 : point of view of the interfacial stress. The fiber is neither push nor pulled directly, and so fiber's poison effects are similar to those occurring in a fiber composite. Unlike the other methods, it produces only one result for the interfacial shear stress, which is the average for the many fragments produced. With the fragmentation test, y HI can be found using the following expression,.- (4.1) c where tv\ is the fiber tensile strength (MPa) at 1 (fiber C critical length) and d is the fiber diameter ( jum )lc is the final fragment length. This can be taken as a valuable indication of the ability of the interface to transfer stresses. When lc/d ratios from various sources are to be compared, one has to be cautious that the average fragment length very often stands for critical length (1 ) in the above equation. In the present work this method was used for evaluation of interfacial shear strength in different carbon fiber epoxy matrix systems selected for present studies.

4 : 169: This chapter presents the conditions required to get unquestionable sound X values and the applications of the technique to some experimental and commercial carbon fiber epoxy systems. 4.2 INTERFACIAL SHEAR STRENGTH OF LABORATORY TREATED HIGH MODULUS CARBON FIBERS EPOXY SYSTEMS EVALUATED BY SINGLE FILAMENT FRAGMENTATION TEST As discussed above, interfacial shear strength of carbon fibers in fiber reinforced polymer composites can be better determined by single filament fragmentation test. Particularly, when there is need for comparing behaviour of fibers with different surface treatments with resin of different end functional groups. Under present work, the interfacial shear strength (IFSS) of different carbon fibers with commercial LY 5052 (DGEBA) resin was evaluated. As a first step, single filament reinforced epoxy coupens were made with non surface treated high modulus X340 carbon fibers and Araldite LY 5052 epoxy resin. Figure 4.1 shows the sequence of fragmentation of single filament composite made with X 340 high modulus carbon fibers and epoxy Araldite LY 5052 resin. From these photographs, fiber critical length (1 ) was measured by using standard microscale graduated slide which was

5 Fig. 4.1 Typical sequence of fragmentation process in single filament fragmentation test for X340 (HM) carbon fibers with LY5052 epoxy resin. i 7 o

6 : 171: also photographed under the microscope at sane magnification (Final scale 1.0 cm = 0.01 cm = 0.1 mm) shown in fig.4.2. The critical length (1 ) so measured was used in equation (4.1). Fiber tensile strength (ofr ) at this critical length was measured from the graph of tensile strength vs. different gauge length (chapter 3) and fiber diameter was measured by using Scanning Electron Microscope (SEM). These values were used in equation (4.1) to calculate interfacial shear strength. The results are compiled in Table 4.1. Following this, single filament coupens were made with surface treated high modulus carbon fibers, treated in the laboratory. Figures 4.3(a), 4.3(b) and 4.3(c) show the fragmentation sequences of these composites. Interfacial shear strength for all such composites were determined which are included in table 4.1. As evident from this table, the fiber critical length decreases and as a consequence, the interlaminar shear strength increases with increase in surface treatment of carbcn fibers upto 3 hrs. of treatment. For higher time of surface treatment, say for six hours the critical length was very low as reported in the table 4.1. Close examination of the fracture pattern revealed that the increase in interfacial shear strength is not only due to the increased bonding between fiber and matrix but is also dependent on the state of the

7 171 Fig. 4.2 Photograph of standard microscale graduated slide.

8 : 173 TABLE 4.1 Fiber critical length and interfacial shear strength of different carbon fiber epoxy systems as evaluated by fragmentation data Sr. No. Fiber type Matrix system Critical length ild IFSS (MPa) 1. X340 LY HY (UT) 2. X340 LY HY (1 hr) 3. X340 LY t- HY (3 hrs) 4. X340 LY HY (6 hrs)

9 Fig. 4.3(a) Typical sequence of fragmentation process in single filament fragmentation test for HM1 carbon fibers with LY 5052 epoxy resin.

10 Fig. 4.3(b) Typical sequence of fragmentation process in single filament fragmentation test for HM3 carbon fibers with LY 5052 epoxy resin. 175-

11 Typical sequence of fragmentation process in single filament fragmentation test for HM6 carbon fibers with LY 5052 epoxy resin.

12 : 17 7 : matrix near the interface and mode of stress transfer. The failure has to be interfacial type. In case of carbon fibers having excessive active surface groups, the fiber matrix bonding becomes too strong. The interface becomes brittle and matrix cracking takes place near the interface. Under such circumstances fiber breaks instead of fiber/matrix interfacial failure. It meant that if fiber-matrix bonding is too strong, the transfer of load to the fiber through shear forces at the fiber/matrix interface does not take place. This is the necessary condition for using single filament fragmentation test to measure Interfacial shear strength. It meant that though the fiber/matrix bonding is too strong in this case, evaluated critical length i low. It further suggested that sudden failure of the epoxy matrix in single filament coupen should not take place. 4.3 INTERFACIAL SHEAR STRENGTH OF COMMERCIAL HIGH STRENGTH CARBON FIBER REINFORCED EPOXY RESIN EVALUATED BY SINGLE FILAMENT FRAGMENTATION TEST Following successful studies with high modulus carbon fibers, single filament composite coupens were made with commercially available high strength carbon fibers, such as T300, RK carbon fibers and IPCL carbon fibers. These fibers had commercial surface treatment and sizing given by the

13 : 178: manufacturer. Figures 4.4(a), 4.4(b), 4.4(c) 4.4(d) and 4.4(e) shows the photographs of the fragmentation sequences of the composites made with these fibers. Fiber critical length was measured for these systems and interfacial shear strength was calculated as discussed before. Interfacial shear strength of these systems is shown in Table 4.2. As evident from the Table 4.2 T300 carbon fibers exhibit highest value of interfacial shear strength with LY 5052 (DGEBA) system than IPCL carbon fibers. Even for IPCL carbon fibers, 6K carbon fibers exhibit higher interfacial shear strength than IPCL (3K) carbon fibers and epoxy LY 5052, though latter fibers possess more surface groups than former carbon fibers. It means the chemical surface groups present on carbon fibers are not solely responsible for the interfacial shear strength. There are other factors as well which contribute to interfacial shear strength. 4.4 EFFECT OF THE FIBER TREATMENT TEMPERATURE ON INTERFACIAL SHEAR STRENGTH It is mentioned quite often in the literature that heat treatment temperature of the fibers alters the surface reactivity. Surface of high strength carbon fibers are more reactive than highly ordered surfaces of high modulus carbon

14 Fig. 4.4(a) Typical sequence of fragmentation process in single filament fragmentation test for T300 carbon fibers with LY 5052 epoxy resin. 173

15 190 Fig. 4.4(b) Typical sequence of fragmentation process in single filament fragmentation test for RK carbon fibers with LY 5052 epoxy resin.

16 Fig. 4.4(c) Typical sequence of fragmentation process in single filament fragmentation test for IPCL(3K) carbon fibers with LY 5052 epoxy resin. /«/

17 Fig. 4.4(d) Typical sequence of fragmentation process in single filament fragmentation test for IPCL(6K) carbon fibers with LY 5052 epoxy resin. I si

18 IS 3 Typical sequence of fragmentation process in single filament fragmentation test for IPCL(12K) carbon fibers with LY 5052 epoxy resin.

19 : 184 TABLE 4.2 Fiber critical length and interfacial shear strength of different carbon fiber epoxy systems as evaluated by fragmentation data Sr. No. Fiber type Matrix system Critical length {1d IFSS (MPa) 1. T300 (S) 2. IPC1 (3K) 3. IPC1 (6K) 4. IPC1 (12K) 5. RK (HT) LY HY LY HY LY HY LY HY LY HY

20 : 18 5 : fibers. In order to study this and their response to interfacial shear strength by fragmentation technique, unidirectional fiber reinforced coupens were made with high strength HT carbon fibers, intermediates modulus carbon fibers and high modulus carbon fibers having heat treatment temperatures of 1200 C, 1600 C and 2800 C. These fibers after heat treatment temperatures had been given commercial surface treatment and sizing treatment by the manufacturer. The interfacial shear strength of the composites made with these fibers and epoxy Araldite LY 5052 by fragmentation technique. Figures 4.5(a), 4.5(b) and 4.5(c) show the fragmentation sequences of these composites. The interfacial shear strength of the composites as calculated from these tests are given in Table 4.3. As evident from the table 4.3, the interfacial shear strength (IFSS) of high strength carbon fiber composite is higher than of those made with high modulus carbon fibers. Even intermediate modulus carbon fibers exhibit higher IFSS than high strength carbon fibers but the difference is marginal. This confirms that the surface reactivity of the carbon fibers decreases with heat treatment temperature. However, the value of IFSS of composites made with commercially treated carbon fibers reported in section 4.3 and 4.4 are lower than those reported in section 4.2. In order to resolve this point, test coupens

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22 Fig. 4.5(b) Typical sequence of fragmentation process in single filament fragmentation test for IM carbon fibers with LY 5052 epoxy resin. 137

23 Fig. 4.5(c) Typical sequence of fragmentation process in single filament fragmentation test for M40B carbon fibers with LY 5052 epoxy resin. 138

24 : 189: were made with high strength carbon fibers and high modulus carbon fibers having been given the same degree of surface treatment (almost nil) in the laboratory. These fibers were given no sizing treatment. Table 4.4 shows the interfacial shear strength of the composites made with these fibers. Figures 4.6 and 4.1 show the fragmentation sequence of these composites. As evident from the table 4.4, IFSS of the composites made with high strength carbon fibers is higher than those made with high modulus carbon fibers. The value are also of the same order as reported in section 4.2. These fibers were not having any sizing on them. Therefore this study brings out two points. (i) High strength carbon fibers possess higher surface reactivity than high modulus carbon fibers. (ii) The low IFSS of commercial carbon fibers show that sizing treatment protects the surface. It partially reacts with the surface groups on the carbon fibers and hence the IFSS of the composites made with commercial carbon fibers is lower than those mace with freshly surface treated in the laboratory.

25 Fig. 4.6 Typical sequence of fragmentation process in single filament fragmentation test for T300E carbon fibers with LY 5052 epoxy resin. I9o

26 : 191 TABLE 4.3 Fiber critical length and interfacial shear strength of different carbon fiber epoxy systems as evaluated by fragmentation data Sr. No. F iber type Matrix system Critical length (1c> mm IFSS (MPa) 1. D (HT) LY HY M-40-B LY HY IM LY HY TABLE 4.4 Fiber critical length and interfacial shear strength of different carbon fiber epoxy systems as evaluated by fragmentation data Sr. Fiber No. type Matrix system Critical IFSS length (1c) mm (MPa) 1. T300 ( US ) 2. X340 (HT) LY HY LY HY