PSI - Issue 14

Neelanchali Asija Bhalla et al. / Procedia Structural Integrity 14 (2019) 564–570 Neelanchali et al/ Structural Integrity Procedia 00 (2018) 000–000

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2.2. Sample Preparation All the test samples were prepared by ultrasonic homogenization method. Prior to mixing, FS was allowed to dry at 100˚C for 12 hours in vacuum oven. The calculated weight percentages of PPG and FS were mixed in excess amount of ethanol and subsequently dispersed with high intensity ultrasonic horn (Ti-horn) at 20 kHz, 1200 W/cm 2 and amplitude of 42%. Various test samples were prepared by varying the specific surface area of FS, while keeping its weight percentage in the solvent fixed at 20% wt. After the ultrasonication process, the samples were placed in a preheated oven at 80ºC to evaporate the excess ethanol from the reaction mixture for further analysis. 3. Theory The SHPB technique comprises of three bars; Incident Bar (IB), Transmission Bar (TB) and the Striker Bar (SB). The schematic diagram of SHPB is shown in Fig.1.

Fig.1: Schematic of the split Hopkinson Pressure Bar (SHPB) experimental set-up

During testing, the striker bar is propelled by compressed nitrogen gas, through a 2m long barrel, before striking the incident bar. A pulse shaper is placed at the impacting surface of the IB, prior to each experiment. The advantage of using a pulse shaper is that it deforms slower than the bar material, thereby resulting in a near trapezoidal loading pulse (ε i ). For each experiment, a fresh pulse shaper was used. When the striker bar hits the IB at the specimen-IB interface, a fraction of the loading pulse is reflected (ε r ) back through the IB and the remaining is transmitted (ε t ) through the TB. The fraction of the loading pulse reflected and transmitted depends upon the acoustic impedance ‘z’ (z = ρc) mismatch between the specimen and the bar materials. The magnitude of these pulses (ε i , ε r and ε t ) are measured with the help of strain gages that are mounted in Quarter Bridge Type-II configuration, in which one active strain gage was attached at the center of each bar. The analytical relations used for determination of specimen strain, strain rate and stress are as follows:

ε̇ s (t) = ( �� � �� ) ε � ( )  s (t) = ( �� � �� ) ∫ ε � ( ). � �  (t) = � � � �  t (t)

(1)

The strain rate,

(2)

The average strain,

(3)

The average stress,