38 C. Rudolf et al. temperature concentrations, and the presence of one or more localized field concentrations in the test specimen gage length can bias the deformation behavior and lead to erroneous conclusions. In this work, the challenge was addressed by utilizing independent electro- and thermo-mechanical experiments for current- and temperature-controlled characterizations, respectively. The hardware and testing methodology developed in this research carefully addressed the issues leading to current and temperature hot-spots and strain gradients that may be responsible for many of the differing results reported for tests on the same material [1, 2]. In the sections below, details of the experiments and methodology will be described and discussed along with the test results for pure Cu, Fe, and Ti, and some candidate mechanisms for the observed current effects on the plastic deformation behavior of Ti. Method The critical design requirement for the test system was to achieve uniform current, temperature and strain conditions in the specimen gage section prior to specimen necking. Small specimen size provided greater temperature uniformity, however this meant that electric current had to be supplied to the specimen through the grips. Tensile loading offered the best possibility for achieving uniform conditions in the gage section with minimal effects from potential field concentrations at the specimengrip interface. Tensile experimentation at electric current densities of 10–100’s A/mm2 may require a large current (e.g., 80 A for 100A/mm2, test with a 1 mm diameter specimen), depending on specimen cross-section area, and this gives rise to safety concerns. Currents must be introduced into the specimen, sensitive components and instrumentation must be isolated from the current, and safe test operation must be guaranteed. Experimental design required consideration of the following: 1. Load-frame and gripping modifications to isolate the hot-side of the current and enable passage of large currents through the test specimen safely. 2. Specimen gripping hardware to achieve uniform strain, current density, and temperature conditions in the gage section. 3. Safe and accurate strain and temperature measurements along the gage length using non-contact methods. 4. Effective cooling to dissipate Joule heating in the gage section with potentially large (constant dc) current densities. 5. Time-synchronized data acquisition of signals from many different measurement sources. 6. Independent tensile experiments with zero current and ambient heating to achieve specimen temperature histories identical to those induced by Joule heating with electric current (i.e., thermal control experiments). Test System The methodology utilized two independent experimental setups (Fig. 7.1): an MTI SEMTester load-stage (MTI Instruments, Albany, NY) for tensile testing with applied electric currents, and an Instron ElectroPuls 3000 Test System (Instron, Norwood, MA) with an environmental chamber for tensile testing with controlled temperature histories to match those observed in the applied current tests. The zero-current, temperature history controlled tests with the Instron system provide a baseline for subtracting out thermal effects in the electric current test results to assess electroplasticity effects. The SEMTester and tensile wedge-grip fixturing was modified to safely provide several hundred amps passing through the test specimen. The grip fixture with the positive current connection was electrically isolated from the load-frame and was capable of carrying the maximum tensile load (up to 8900 N). The (earth) grounded grip (load-cell side) was also electrically isolated to prevent any current from passing through the load cell. Numerous ground cables were added to the device for safety. The electrical isolation hardware for the wedge grips had to be mechanically robust enough to handle the maximum loads. The grips were modified to accommodate electrical isolation sleeves and washers made of polyimide and phenolic materials. The insulation hardware provided electrical isolation of the grips and specimen from the SEMTester device while allowing for transmission of tensile loads to the specimen. The computer automated SEMTester load-stage, equipped with an 1100 N (250 lb) load cell, outputs load-displacement data via data-acquisition (DAQ) channels and analog-outputs (Fig. 7.2). A National Instruments (NI) DAQ system with a customized Labview code is employed to collect and synchronize data streams from multiple facility instruments. The system also provides control and monitoring of the electrical current supplied via an Agilent 6600C power supply (Agilent Technologies, Santa Clara, CA). The Instron ElectroPuls system was programmed and controlled using Bluehill Software and provided an analog output of the load which fed into the same NI DAQ to synchronize the load-displacement data with the data streams from the other instruments (video-extensometer and thermocouples).
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