PSI - Issue 13

S.A. Atroshenko et al. / Procedia Structural Integrity 13 (2018) 1373–1377 S. A. Atroshenko et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 1. Electrical schemas of loading for low-voltage (a) and high-voltage (b, c) cases: AT – autotransformer, REC – rectifier, R ch – charging resistance, C – capacitor, S – spark gap, RC – Rogowsky coil, L – coil, PD – photodiode, OSC – oscilloscope, PT – pulse transformer, FL – the forming line, OD – output device, HVE – a high-voltage electrode.

Fig. 2. Schemes of currents in the coil and sample (a) and of loading of the sample (b).

and fracture of annular samples from copper in the form of thin tapes over a wide range of loading rates (10 4 − 10 6 ) s − 1 are presented.

2. Experimental technique

Three fundamentally di ff erent electric circuits for the realization of the magnetic-impulse loading method were used during our experiments (Morozov et al. (2018)). Electrical schemes of these circuits are presented in Fig. 1. The schemes are implemented on the basis of a generator of short high-voltage pulses, providing the formation of electrical voltages with amplitudes of 10 − 300 kV . The current passing through the coil L induces a current in the ring sample coaxially located on it as shown in Fig. 2, a. The interaction of these currents generates the repulsive force F ( t ) between the solenoid and the ring in accordance with the Ampe`re’s force law. The distributed load q ( t ) (see Fig. 2, b) acting on the inner surface of the annular sample may be calculated by the formula: q ( t ) = F ( t ) / c or may be measured during experiment. Here c is width of the ring sample. The circumferential (tensile) stress is determined by the Laplace formula: σ ( t ) = R 0 q ( t ) / h . (1) Here R 0 is the initial radius of the annular sample and h is its thickness. It should be noted that the determination of the sample failure instant is a fundamental problem in the mechanics of the dynamic destruction of materials. We used developed by us original method of determination of the sample failure instant by the signal from the photodiode ( PD on Fig. 1)for investigating the destruction of ring samples using all three loading schemas. The essence of this method is the fact that the current in the annular sample cant stop instantaneously when it is destroyed. Therefore, a spark appears between the broken parts at the point of discontinuity of the sample at the moment of destruction, and the photodiode should record this flash (Fig. 3). The fracture surfaces of specimens after tension test with di ff erent loading rate were studied in the dark field on the Axio-Observer Z1-M optical microscope. The viscous fracture surface is characterized by a dim grey appearance with characteristic “fibers”. The brittle fracture of the surface is crystalline without visible signs of the plastic deformation on the fracture surface. The percentage of the viscous fracture component S (shear area) (in %) was determined according to the ASTM E 436-03.