PSI - Issue 28

Sahand P. Shamchi et al. / Procedia Structural Integrity 28 (2020) 1664–1672 Sahand Shamchi et al. / Structural Integrity Procedia 00 (2020) 000 – 000

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3.2. Dynamic tests High rate compression tests were carried out using a split Hopkinson pressure bar, Fig. 2. The setup consists of two titanium Gr.5 bars with a diameter of 16 mm and a length of 2.7 and 1.6 m for incident and transmission bars, respectively. Compressive impact loading was generated using a steel striker with a diameter of 16 mm and a length of 0.8 m. The stress waves were measured using strain gauges located on the incident bar at 1.3 m and the transmission bar at 0.3 m from the bar/specimen interfaces. The output from strain gauges were amplified and directed to a PC oscilloscope from Pico to record the signals for post-processing.

Fig. 2. A view of the split Hopkinson pressure bar system equipped with a high-speed camera.

A rubber pulse shaper with a thickness of 1 mm was inserted to the impact end of the incident bar in order to attenuate the high frequency oscillations on the signal, minimizing the wave dispersion along the pressure bars. All the experiments with SHPB were supported by a Photron Fastcam MINI AX200 high-speed camera equipped with a Nikon 60 mm Micro lens to monitor the progressive failure of the samples. The camera was placed 500 mm away from the specimen, capturing images at 160000 fps with a shutter speed of 1/1500000 sec and an image resolution of 128 pixel ×128 pixel. A high power collimated LED was used to illuminate the specimen surface. The compression fixture used for dynamic tests with SHPB was adapted from Ploeckl et al. [10]. The same fixture was employed for non-standard quasi-static experiments, performed in the servo-hydraulic testing system. Thereby, both test typologies shared the same dimensions, as outlined in table 1. The setup uses a combination of end and shear loadings, to prevent the specimen ends to be crashed under dynamic compressive loading, achieving a valid failure within the gauge section. The rectangular carbon/epoxy specimens were glued into a pair of slotted steel adapters using Scotch-Weld DP 490 adhesive with a shear strength of 30.2 MPa. The assembly was placed into a fixture designed to ensure the alignment between the specimen and the steel adapters during the curing stage of the adhesive, which was in accordance with the instructions of the supplier (24 hours at 23° followed by 1 hour at 80°C). Mechanical clamps were also added to provide an additional restraining force, prohibiting any in-plane misalignment of the sample during the test, Fig. 3(a). The final assembly of the fixture is shown in Fig. 3(b), including two Teflon bearings surrounding the steel adapters and an aligning steel tube to prevent possible bending. Further information on the test fixture can be found in [10]. The specimen´s strain history was acquired directly through a strain gauge bonded on the gauge section of the samples. The stress value was deduced from the recorded stress waves using one-dimensional wave theory, as outlined by Gray III [11].