PSI - Issue 14

M.K. Singh et al. / Procedia Structural Integrity 14 (2019) 475–481 M. K. Singh, R. Kitey / Structural Integrity Procedia 00 (2018) 000 – 000

477

2 . Materials

To prepare the composites first diglycidyl ether of bisphenol-A (DGEBA) epoxy resin of density 1.15 gm/cm 3 is heated at 40 0 C to remove the entrapped air bubbles. The resin is then mixed with milled glass fibers of requisite amount. Mixture is stirred manually and sonicated using ultra sonication probe. This is followed by degassing the solution at 25-inch Hg vacuum. The process is repeated until the fibers are uniformly dispersed. Curing agent methyl tetra hydrophthalic Anhydride (MTHPA) is then liquefied by heating at 120 0 C for ~20 min and mixed short fiber reinforced resin along with a small amount of accelerator, 2,4,5-tris[(dimethylamino)methyl]-Phenol. The mixture is poured into wax coated horizontal aluminium mould. The curing cycle consists of heating the material at 85 0 C for 3 hours followed by another cycle of heating at 120 0 C for 12 hours. This high temperature cured sheets are kept on flat surface at room temperature for a couple of weeks prior to machining into flexural and fracture test samples. In this investigation resin and hardener are mixed in 100:100 ratios by weight. The 16 μm diameter short fibers of three different lengths, 1/4”,1/8” and 1/32” , are considered in this study where they are embedded into the matrix at 3% volume fraction.

3. Experimental methods and measurement

3.1. Flexural test

The quasi-static flexural tests are conducted by following ASTM standard D790-17. Samples of dimension 100 mm x10 mm x 5 mm are machined and loaded symmetrically in three-point bend fixture along thickness direction as illustrated in Fig.1.Tests are performed in displacement controlled mode at a cross head speed 0.1 mm/min by using universal testing machine INSTRON 3345 at an ambient temperature of 25 0 C. Flexural modulus (E f ) of composites is measured from the initial slope of stress vs. strain curve. Flexural stress (σ f ) is calculated by using the following equation.

   

  

2

3PL

B                   4 L L L 

(1)

1 6 

 

f

2

2WB

In the above equation L, W and B represent the specimen’s span, width and thickness, respectively. The load and the mid- span deflection is presented by P and δ, respectively. Correction factor term in stress calculation for accommodating the specimens of large span to depth ratio is included in the bracket of Eq. (1). Flexural s train (ε f ) is calculated by employing the following equation.

2 6 B L 

 

(2)

f

Fig.1. Configuration for flexural test specimen