PSI - Issue 14

Tulsi Chouhan et al. / Procedia Structural Integrity 14 (2019) 883–890 Author name / Structural Integrity Procedia 00 (2018) 000–000

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The composite specimen is held in the middle of the incident bar and transmission bar using a suitable lubricant. The lubricant minimizes the friction in between the bars and specimen and holds the specimen in the between the middle of the incident and transmission bar. On the striking of striker bar onto the incident bar, a compressive wave propagates through the incident bar and on contact with the specimen, some portion of the wave is reflected back and rest goes through the specimen into the transmission bar. The part of the reflected and transmitted wave is decided by the impedance mismatch between the material of the bars and specimen. The impedance here refers to the product of density and velocity of sound in the given material. Titanium bars are used to minimize the impedance mismatch only. Since the density of aluminum is low and the resulting impedance mismatch with any high-density bar material like steel could be too high. The strain gauges mounted on midriff of both the incident bar and transmission bar records the strain induced and the same is utilized for the count of stress, strain, strain rate and other properties of interest. The velocity of the striker bar is measured using the optical sensors mounted towards the end of the gas gun barrel. A suitable pulse shaper as recommended in the literature to minimize the noise and to smoothen and modify the incident pulse is used (Vecchio and Jiang 2007). Circular shaped natural rubber (Linatex) having 1.3 mm thickness is used as a pulse shaper. The complete specifications and subtle elements of the set-up are accessible in the literature (Chouhan et al. 2017)(Asija et al. 2017). One-dimensional wave propagation theory is utilized to compute the stress, strain and strain rate induced in the specimen.

Table 2. Specifications of SHPB Set-up Bar properties

Material

Titanium

Density

4430 kg/m3

Modulus of Elasticity

113.8 GPa

Elastic wave speed

5068 m/s

Bar Diameter

16 mm

Length of incident bar

1200 mm

Length of transmission bar

1200 mm

Striker length

240 mm

Strain gauge properties Gauge factor

2.12

Resistance

350 Ohms

Initial excitation voltage

2 Volts

2.3. High strain rate testing

The scheme for high strain rate testing of materials was developed by Kolsky (1949). The mathematical formulations for SHPB testing are dependent on the following assumptions, which are prerequisite for a valid SHPB experiment, as discussed in the literature (Gama et al. 2014). (i) Stress wave propagation in the bar material is in 1-dimension only. (ii) The incident bar-specimen and specimen-transmitter bar interfaces remain plane at all time. (iii) The specimen is in the stress equilibrium after an initial ‘‘ringing-up’’ period. (iv) The specimen is noncompressible. (v) Friction and inertia effects in the specimen are negligible.