Non-Destructive Evaluation of Mechanical Properties of Magnetic Materials
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In order for a magnetic-based NDE method to be useful, statistically valid correlation must be established between mechanical and magnetic properties. Establishing such a correlation requires a set of well-characterized samples that span a range of mechanical properties. The objective then is to determine the extent of mechanical property variation for samples, and then to determine the same for the magnetic properties. Ultimately, it is desired that reproducible correlation be made between a mechanical property, or several mechanical properties, and selected magnetic properties. In the present study, a magnetic based NDE method is described and used to correlate the magnetic and mechanical properties in an ultra low carbon (ULC) steel. In particular, the stiffness of the tensile testing equipment and the limited resolution of load cells prevent the true yield point to be detected. The objective of the present study was to develop a highly sensitive NDE tool to detect small deviations from linearity that occur in the stress-strain curve well below the 0.2% permanent strain, and which is generally used to define the yield point in materials. EXPERIMENTAL DETAILS The composition of the ULC steel, in weight percent, is given in Table I. The average value of yield stress for the given ULC steel is =_-148.1 MPa. Tensile test samples were made from sheet steel 1 mm thick, 19 mm wide, with a 12.5 mm gauge length. Test samples were plastically strained in the tensile testing apparatus at a rate of 0.4mm/s, and the amount of permanent strain was determined by measuring the distance between two marks in the sample gauge after the load was removed. A set of samples with small plastic strains (0%, 0.05% and 0.1%) was obtained. All straining, mechanical, and magnetic measurements were made at a temperature of 298 K. Table I. Composition of ULC steel in weight percent. The remaining trace impurities (Al, Cu, Sn, V, and Ca) were less than 0.08 wt%.
[
C 0.002
N
0.0028
I
P 0.012
I
S 0.008
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F
Mn
Si
Ni
Mo
Nb
Ti
0.13
0.02
0.01
0.01
0.02
0.049
Description of the NDE method: The NDE method used in the present study is a magnetic technique based on the Barkhausen effect and measurement of the hysteresis (B-H) loops.2 -4 The Barkhausen effect comprises of abrupt microscopic changes in magnetization due to discontinuous motion
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Bwmnusni of domain walls. The jumps arise because magnetic
domain walls are pinned by microstructural features o,- s*1Coil X- S;-mple
and consequently the walls jump from one set of pinning sites to the next as the magnetic field is changed. The pinning sites arise from the same features (e.g., precipitates, grain boundaries, dislocations) that
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Fig. 1. Schematic of the system used to obtain magnetic data from a sheet steel tensile sample.
influence the mechanical properties.
Thus, the
Barkhausen signal probes the defect structure as the magnetic field is swept through a hysteresis cycle. The rate of change of flux assoc
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