PSI - Issue 14

170 Harpreet Singh Bedi et al. / Procedia Structural Integrity 14 (2019) 168–175 Harpreet S. Bedi, Prabhat K. Agnihotri/ Structural Integrity Procedia 00 (2018) 000–000 (Virginia, USA) contact angle instrument. Contact angles of several micro-droplets are measured and the average values are presented and compared.

Figure 1. Schematic of micro-bond test: (a) epoxy micro-droplets adhering to the surface of single carbon fiber filament and (b) specimen undergoing debond test. 2.3 Nanoindentation test TI 950 nanoindenter (Hysitron, USA) is used to carry out nanoindents at a peak load of 1000 µ N. At least seven nanoindents are made on a single fiber composite along a straight line starting from fiber to the matrix through the interphase region as shown in Fig. 2. Load-displacement data is recorded for each indent and analysed to extract the size and modulus of the interphase region.

1 Figure 2. Schematic of nanoindentation testing performed to measure the size and modulus of interphase in single fiber/epoxy composites.

2.4 Micro-bond test After getting the wettability and stiffness properties of different fiber/epoxy composites, time is now to relate these properties to the strength of the interphase. To this end, micro-droplet debond tests are carried with both HCF/epoxy and CNTCF/epoxy composites. Test specimens are prepared as described in previous section (see Fig. 1a) and debond test is performed on a universal testing machine (Shimadzu, Japan) at a crosshead speed of 0.083 µ m/s (see Fig. 1b). Peak force required to debond ( F max ) an epoxy micro-droplet from the fiber is recorded and interfacial shear strength (IFSS) is evaluated using eq. 1.

F

(1)

IFSS



max

π dl

e

1 Reprinted from “Carbon, 132, Bedi, H. S., Tiwari, M., Agnihotri, P. K., Quantitative determination of size and properties of interphase in carbon nanotube based multiscale composites, 181-190, 2018”, with permission from Elsevier.