Fracture of drawn tungsten wires
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D. LEE is with General Electric Company-Corp. Research and Development, Schenectady, New York 12301. Manuscript submitted November 15, 1974. METALLURGICAL TRANSACTIONS A
I. EXPERIMENTAL METHOD The material used was 0.0025 in. (0.0064 cm) diam tungsten wire that had been warm reduced by nearly 99.9 pct in cross section through the multiple die drawing method. All the tension tests were made in an Instron testing machine at a cross head speed of 0.2 in./min (0.51 cm/min) at room temperature, at least in duplicate. The "bend-and-stretch" test was carried out by making a complete loop around a 0.006 in. (0.015 cm) diam molybdenum mandril and pulling the wire to fracture. A schematic diagram showing the essential features of the setup is shown in Fig. 1. Scanning electron microscopy (SEM) samples were prepared by inserting segments of broken wires into 0.010 in. (0.0254 cm) diam holes drilled in an aluminum pedestal and coating with silver glue at the bottom of samples. A Cambridge Stereo Scan-30 KV was used throughout this work. II. TEST DATA AND ANALYSIS Representative engineering stress vs displacement curves obtained under two different loading conditions are shown in Fig. 2 together with the low magnifica-
WIRE diameter
BDENUM MANDRIL 2 cm diameter
Fig. 1-Schematic diagram showing the main features of "bend-and-stretch" test used to pull 0.0064 cm diam tungsten wire. VOLUME 6A, NOVEMBER 1975-2083
Fig. 2—Engineering stress vs displacement of cross head relationships for samples tested under (a) simple tension and (b) "bend-andstretch" loading conditions. Profiles of fractured samples are also shown in the figures.
tion photographs of fractured samples. The reduction in area at fracture under the simple tensile testing condition was nearly 45 pet; this value was decreased to about 35 pct under the "bend-and-stretch" testing condition. Moreover, the load at fracture in simple tension test was 2.22 lbs (1 kg), corresponding to a true fracture stress of about 830,000 psi (5723 MPa). The total strain to fracture based on the extension of crosshead is also about 2.4 pet. The corresponding value in the "bend-and-stretch" test could not be readily estimated. Based on this tension test data, a few qualitative remarks can be made on the deformation behavior of drawn tungsten wires. First of all, the wires have a remarkable degree of ductility as indicated by their capacity to deform locally. This is also shown in the "bend-and-stretch" test where wire can be bent read2084-VOLUME 6A, NOVEMBER 1975
ily over a small radius without causing fracture. For example, if it is assumed that the wire deforms as a rigid body and the neutral axis is in the middle of the wire diameter, the maximum fiber strain is, Fig. 3(a), Ex, max = b ,, c = 0.29.
[1]
This implies that either the elongation values observed in simple tension tests cannot be directly applied to the case of simple bending or the wire may never deform as a rigid body so that the dimension y = b — c was used incorrectly. Support for the latter argument can be made in
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