Issue 48

H.S. Bedi et alii, Frattura ed Integrità Strutturale, 48 (2019) 571-576; DOI: 10.3221/IGF-ESIS.48.55 573 Interfacial characterization In order to evaluate the shear strength of the fiber/matrix interface, micro-droplet debond tests are carried out with both HCF/epoxy as well as CNTCF/epoxy composites. Schematic of sample preparation is shown in Fig. 2a where the ends of a single CF filament (unsized or CNT grafted) are first attached to a channel section stand with the help of a quick setting adhesive. Epoxy matrix is prepared by mixing epoxy besphenol resin with polyamine hardener in a ratio of 1:2 by weight. As soon as a drop of matrix is poured on a fiber it disintegrates into multiple micro-droplets as shown in Fig. 2a. Figure 2 : Schematic of single fiber micro-droplet debond test: (a) polymer micro-droplets adhering the surface of single carbon fiber filament. (b) Test specimen undergoing debond test. (c) Representative load ( F ) vs. displacement ( δ ) curve for the debond test. Afterwards, the specimens are allowed to cure at atmospheric conditions for at least 24 hr followed by post curing in an electric oven at 60°C for 4 hr. During this period of curing, matrix droplets set and get hard, adhering the fiber surface to the best of their potential. Debond test is then performed on a universal testing machine (Shimadzu, Japan) as illustrated in Fig. 2b. Crosshead speed is varied from 0.05 to 5 mm/min and load ( F ) vs. displacement ( δ ) data is logged corresponding to each speed and for each sample. A typical load/displacement curve obtained from such a test is shown in Fig. 2c. Peak force ( F max ) required to debond a micro-droplet from the fiber is obtained from these curves and interfacial shear strength (IFSS) is then evaluated using Eqn. (1). max   e F IFSS dl (1) where, d is the fiber diameter and l e is the embedded length of a polymer micro-droplet on the carbon fiber (see Fig. 2b for detail). The small rise after the abrupt drop in load ( F ) corresponds to frictional sliding effects. R ESULTS AND DISCUSSION oad or stress transfer from matrix to fiber in CFRPs critically depends on the strength of the fiber/matrix interface. Single fiber micro-droplet debond test is a simple way to measure the interfacial shear strength (IFSS) in CFRP composites. Figs. 3a and 3b show the variation of maximum debond force F max as a function of varying embedded length l e of polymer micro-droplet for each fiber/matrix composite. The results of debond test are fitted well with linear interpolation, indicating the direct proportionality between peak load and embedded length of the micro- droplet. Therefore, IFSS of each composite is evaluated from the slope of a linear fit to the experimental data (markers in Figs. 3a and 3b) and average IFSS with standard deviation is shown in Fig. 3c. On increasing the loading speed from 0.05 to 5 mm/min the IFSS of HCF/epoxy composite rises from 50 MPa to 54 MPa (Figs. 3a and 3c) by 8%. Similarly, the IFSS of CNTCF/epoxy composite increases by 6% from 66 MPa (at 0.05 mm/min) to 70 MPa (at 5 mm/min) (see Figs. 3b and 3c). It is observed from Figs. 3a and 3b that at higher speed of loading the slope of the linear fit is higher than the one obtained at lower speed. This corresponds to an increase in IFSS with loading speed irrespective of the type of fiber (HCF or CNTCF) used to reinforce epoxy. This could be attributed to increased yielding of the matrix due to strain hardening effects at higher load speeds, resulting in more energy required to fracture [15]. The energy to fail the composite is determined from area under the load-displacement curve (Fig. 2c) which shows an increasing trend when the rate of loading is increased and/or CNTs are grafted on fiber surface. L