PSI - Issue 14

P R Krishna Mohan et al. / Procedia Structural Integrity 14 (2019) 176–183 P R Krishna Mohan/ Structural Integrity Procedia 00 (2018) 000–000

178

3

second dip roller andthe exit squeeze roller. Spacers are used to control the gap between the two nip rollers that will help in maintaining the required thickness of the prepreg sheet. An oil bath circulatory system is used to regulate the temperature of the resin bath. 3. Preparation and Testing of material Prepreg is fiber pre-impregnated with epoxy. The prepreg sheets are cut into 275x275 mm each of 0.25 mm thickness, and they were laid up in the mold. We have consolidated the prepregs into panels of required thickness as per ASTM standards for different tests. The prepregs are compression molded at 0.7 MPa in a hydraulic hot press. The curing cycle used by Rahul and Kitey (2016) comprises of low-temperature curing of 85 °C for 3 hours and a higher temperature curing of 140 °C for 12 hours, was adopted here.

3.1. B-stage curing

B-stage curing of the prepreg is carried out at room temperature in a non-vacuum and vacuum environment. We have partially cured the prepreg at room temperature for about 9 hours, and then it is cut into sheets. These prepregs shall be called as non-vacuum prepregs for future references. Another prepreg sheet is cured partially at room temperature in a vacuum environment for 48 hours, and then it is cut into sheets. These prepregs shall be called as vacuum prepregs for future references. All the prepregs are stored in a deep refrigerator maintained at -18°C.

3.2. Fiber Content

To find the volume fraction of fibres, v f in the composite, Thermo Gravimetric Analysis (TGA) ‘SDT Q600’ apparatus is used, which uses a matrix burn off method. We have chosen the heating cycle used by Yee and Stephens (1996) for this purpose. From the TGA test, the weight ratios are obtained to calculate the percentage of fiber volume using Equation (1) and assuming no voids are present.

f ρ ρ

= f f v w

100 ×

(1)

C

Here, ρ is the density, and w is the mass/weight percentage, and subscripts f and c represent the fiber and composite, respectively. ρ c is the theoretical density and is calculated by using Equation (2), where ρ m is the density of matrix which is found by relative density test.

100

=

f ρ

(2)

f w w ρ ρ +

m

c

m

3.3. Void Content The percentage void content is estimated using ASTM D2734 and is given by Equation (3), ρ e is the experimental density of composite which is obtained from relative density test as per ASTM D792.

ρ ρ

−

v v

c

e

100 ×

=

(3)

c ρ

3.3. Test setup

We have conducted the uniaxial tensile testing for laminates made of 0°, 90° and [±45] S layups using in-house