PSI - Issue 13

Sheikh Muhammad Zakir et al. / Procedia Structural Integrity 13 (2018) 1244–1249 Sheikh Muhammad Zakir et al. / Structural Integrity Procedia 00 (2018) 000–000

1246

3

glass surfaces. It is difficult to accurately measure the strain history during tests due to brittle nature of glass and gauges can easily fell off the glass surface due to surface chipping off. The total of 20 static compression experiments was performed to report the reliable stress-strain data. The high-speed camera with framing rate of 100,000 fps was used to capture the real-time failure process of specimens during static tests, in static loading the specimen after reaching saturation fails abruptly, therefore, we were unable to capture the start of cracks and its development process.

Figure 1 Static test arrangement.

2.3. Dynamic compression experiments The dynamic compression experiments on ALS glass specimens were completed employing the Kolsky bar (modified Split Hopkinson Pressure Bar, SHPB) apparatus. Figure 2 presents the overall tests set-up for high strain rate tests used in current research. The three bars namely incident, transmitted and striker of diameter 19 mm and made of maraging steel was used in experiments. The bars have density, elastic modulus and Poisson’s ratio of 8088 kg/m 3 , 190 GPa and 0.29, respectively. The tungsten carbide platens of matching impedance were placed between the incident and transmitted bars and glass specimen to avoid bars plastic deformation and indentation marks due to glass fragments. The petroleum jelly was applied between the platens and glass specimens to minimize the friction shear effects at the interface. The deformation of the glass specimens at constant loading rate is achieved by using the composite (aluminum + annealed copper) pulse-shaper, placed at the incident bar face. The strain signal from gauges 2 and 3 are utilized to measure the strain-rate and stress-time history in specimens using proper Kolsky bar equations [12]. The glass is brittle material like ceramics and gauges on the bars cannot accurately give the strain in the specimen, because of their very low strain to failure. In all tests, the strain-time history was recorded directly from the gauges pasted on the specimens. A high-speed camera with the framing rate of 200,000 frames per second with two synchronized flashlights is used to capture the real-time failure process of the glass specimens, triggered by a signal from the strain gauge 1. By adjusting the intensity of lights and exposure time, the best possible images were recorded during tests. During the signal recording process of the oscilloscope, the images are captured during the exposure duration to accurately correspond the stress history in the specimens. 3. Results and discussion In the static compression tests, at two strain-rates of 2.5 × 10 -4 s -1 and 2.5 × 10 -3 s -1 the stress is calculated from the load data recorded by loadcell and strain is measured using strain gauges pasted on the specimens. It is observed during tests that strain measurement is little difficult due to the brittle behavior of glass, during the loading process the cracks on the surface can result in dropping off of the gauges. However, it was made sure in tests to record the full strain history in specimens to get the stress-strain plots at static loading. For the dynamic compression tests at an average strain-rate of 650 s -1 , the pulse-shaping method is used to impart the linear ramp loading to the specimens for uniform deformation. The dynamic stress equilibrium in tests is checked by dynamic equilibrium coefficient R(t) introduced by Ravichandran and Subash [12], this is important to accurately report the ALS glass specimen’s failure process and linked stress history. The plateau region on the reflected pulse confirms that specimens deform at constant strain rate before failure, as shown in figure 3 (a).