technique on a single specimen with a particular method for evaluating the R-curve for HSLA steels [8,9]. Stable crack initiation in this case is similarly depicted from the local minimum (or maximum) of the potential drop value and, as in the case of ME technique, may not give clear local extreme values when conditions of fracture change from brittle to ductile [5]. As expected, results obtained from testing specimens at several loading rate (or impact velocity) have shown that the change of slope in the PD-t diagram can be used to evaluate critical crack behaviour, and if compared to similar changes of slope in the MF-t diagram, or vice versa, can eliminate doubts and provide a better understanding of both diagrams and measurement techniques. Some preliminary investigations have already been discussed [10]. Therefore, the two techniques were implemented in a Charpy instrumentation that gave independent and simultaneous records of ME and PD signals and the results were not only compared to check their validity, but to portrait the essence of superimposing all the benefits that characterize a certain testing technique. MATERIAL AND EXPERIMENT Micro-alloyed steels have a wide range of use in gas and oil pipelines, storage tanks, pressure vessels, vehicles, cranes, and metal structures in general. The tested steel is micro-alloyed with Nb and Ti and is obtained by controlled rolling and accelerated cooling, providing a ferrite-pearlite microstructure. With a yield stress of 411 MPa, the material is very ductile, even at lower temperatures, with a nil-ductility temperature below -80°C. The chemical composition (in wt. %) is shown in Table 1. TABLE 1 CHEMICAL COMPOSITION IN WEIGHT PERCENT C Si Mn P S Al Cu Cr Ni Mo Nb Ti 0.08 0.20 1.12 0.027 0.011 0.033 0.065 0.027 0.019 0.010 0.026 0.017 The ferritic-pearlitic steel exhibits upper shelf values for ductile type of fracture at room temperature. This type of fracture is evident from load-time or load-displacement data. All the tests were performed at room temperature, owing to certain limitations of the PD technique [9]. The specimens were tested using initial energy levels in the range of Eo=26÷70 J, enabling different impact loading rates between vo=1.69÷2.75 m/s. The fracture toughness of the material is affected by the impact velocity, and in ductile materials, the fracture mechanism is controlled by the strain field. This property may increase or decrease, depending on the loading rate [9,11]. The introduced fatigue cracks, that also influence the fracture toughness, are within the range: a/W=0.47÷0.57. The standard V-notched Charpy specimens were cut from a 12 mm plate perpendicular to rolling direction. All specimens were pre-cracked by high frequency fatigue. Since potential drop measurement requires additional specimen preparation, specimens were prepared in the manner explained by Grabulov [9]. Fig. 1 shows the prepared specimen on the anvil of the Charpy machine with the required instrumentation. Very thin wire connections for potential signal output were resistance-spot-welded to the specimen at asymmetric positions in respect to the notch (locations A, Fig. 1), being the optimal position. Apparently, the spot welding technique was performed by selecting inadequate welding parameters, resulting in loose contacts. The output wires were produced of steel, Ni, or Ni-Cr alloy, so some connections had to be re-connected in an unfavourable manner, by soldering. The heavier input Cu-wires originating from the DC power source were connected to the specimens by bolts (position III), securing a firm contact. The nominal input DC electric current of 30 A was required in order to produce output potential drop values ranging from several to at least 10 mV. This input DC value was selected as a minimum, and for some specimens it was increased up to 40 A, and even 50 A in order to amplify the weak output PD signal. Another limiting effect appeared from these wire connections as well. Being rather large and massive, they influenced the inertia characteristic of the specimens, and produced a problem that has yet to be solved – interrupted specimen fracture at higher loading rates, because of mutual contact and/or collision of these wire-connections with the inclined anvil
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