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·Materials Characterization and testing
Determining Residual Stresses in Ferromagnetic Materials by Barkhausen Noise MeasurementSilvério F. Silva Jr, , Tanius R. Mansur
Brazilian Nuclear Energy Commission
Email: firstname.lastname@example.org, email@example.com
Ernani S. Palma
Pontifical Catholic University of Minas Gerais
MBN measurements were carried out using a microprocessor-controlled Stresstest 20.04 unit and the 144221 general use probe. The Stresstest 20.04 controls the test parameters and data acquisition process during the measurements. The probe incorporates a yoke, to exciting the material with a variable magnetic field and a pick-up coil to detect the corresponding magnetic Barkhausen noise. The material can be excited using a variable magnetic field with frequencies of 10 Hz or 100 Hz and the magnetic Barkhausen noise resulting can be detected from depths of 0.05, 0.10, 0.20, 0.40 e 0.80 mm.
Measurements of stress during the test system calibration were performed with a PL-10-11 strain gage (TML) and residual stress measurements were performed with a FRS-2-11 rosette (TML) respectively. The instrumentation for strain measurements includes a 4 1/2digit microvoltimeter and a power source with a 10-channel selector. Measurements were performed at room temperature.
The material used in this study was ASTM A-515 Grade 55 steel, due to its importance as a structural material for medium and high temperature applications in siderurgical, chemical and petrochemical plants. Tension tests performed in samples of this material according ASTM E 8-96 shown a yield strength of 310 MPa. The calibration test specimens, residual stresses simulation test specimen and welded test specimen were obtained from a 1000mm x 1000 mm x 6,35 mm plate of the material studied. The material surface at the places selected to fix the strain-gages and the rosettes were prepared according to the conventional techniques of strain gage technology.
2.3 Calibration Test Specimens
|Fig 1: Constant stress beam used in the experiments.|
Two beams were machined from a plate of the tested material, with the axis coincident with the directions parallel and perpendicular to the rolling direction of the material, identified as constant stress beams A and B respectively. After machining, the beams were submitted to a heat treatment for stress relieving. The beams were instrumented with two strain-gages each, oriented parallel to the beams axis and arranged in two different places at the beams surface, in order to check the condition of "constant stress" along to the beams axis. During the load test, the difference between the stresses determined by the two strain gauges remains about 0,5%.
Fig 2: Experimental set-up used to test system calibration. Generation of tension (a) and compression (b) stresses in the upper surface of the constant stress beam.gure
Fig 3: Probe holder for Barkhausen probe positioning.
The probe was positioned parallel to the beams axis to maintain the direction of the magnetic excitation field parallel to the beams axis. A special probe holder was developed to conserve the probe in the same position and with the same pressure against the material surface during the calibration step, as shown in Figure 3.
The loads were applied at the extremity of the beams, in an incremental way, using a dead load system, generating tension and compression stresses at the beams surface. To each applied load, the corresponding strains and Barkhausen noise rms value were registered and a curve relating stress level with Barkhausen noise rms value was plotted. Figure 4 shows the calibration curves obtained for the two beams, corresponding to the MBN measurements performed in the directions parallel and perpendicular to the rolling direction of the material .
Fig 4: Calibration curves for the constant stress beams A (a) and B (b), corresponding to the measurements performed in the directions parallel and perpendicular to the rolling direction of the tested material.|
|Fig 5: Results of residual stresses determination using the central hole drilling method and magnetic Barkhausen noise measurements at depths of 0,2mm, 0,4 mm and 0,8 mm.|
|Fig 6: Welded specimen used in the residual stress measurements.|
In the second experiment, measurements of residual stresses were performed in a welded specimen, where the weld seam is perpendicular to the rolling direction of the material, as shown in Figure 6.
In a specific place of the specimen, measurements of MBN rms value were performed in the directions perpendicular and parallel to the weld seam. After this, a rosette for residual stresses determination was installed at this place, with the elements 1 and 3 positioned in the directions parallel and perpendicular to the weld seam respectively. Residual stresses measurements were performed according ASTM E 837-95 standard. The results obtained for the depths of 0,2 mm, o,4 mm and 0,8 mm are shown in Figure 7.
|Fig 7: Residual stress measurements obtained for the Hole-Drilling Strain-Gage Method (HDSGM) and MBN measurements.|
The calibration curves obtained for the constant stress beams A and B shown in Figure 4 (a) and 4 (b), presented two distinct regions, with different behaviour. For low values of the tension and compression stresses, the relation between the rms MBN value and stress magnitude is approximately linear. After a certain level, the changes induced in the MBN value due to stress increasing, diminish until a point where saturation occurs. For the material tested and the experimental set-up used, these points are 50% of the yield strength for tension stresses and 30% of the yield strength for compression stresses. The rms values of MBN obtained for the constant stress beam A are higher than the ones for the constant stress beam B. This behaviour was expected because the axis of the beam A, coincident with the measurement direction, is parallel to the material rolling direction.
The values of residual stresses at the constant stress beam B, determined by the Hole- Drilling strain-gage method and MBN analysis were higher than the expected due to the loads applied to the beam to simulate the presence of residual stresses. Although different in magnitude, the two methods presented the same tendency as the depth increases.
For the welded sample, the principal residual stresses determined by the Hole-Drilling strain-gage method occurred in the directions of the elements 1 and 3 of the residual stress rosette. The measurements of MBN in these directions indicated that this test method was sensitive to the changes in the material stress state but have limitations for high values of compression stresses. In all situations, the values determined by MBN measurements presented values smaller than those obtained by the Hole-Drilling Strain-Gage Method.
The MBN measurements were sensible to the stress changes occurred in the tested material. However, the results obtained for residual stresses measurements by this test method indicate stress levels smaller than the values obtained by the Hole-Drilling Strain-Gage Method. To obtain more conclusive results about the possibilities and restrictions to the use of this test method as a tool for quantitative residual stresses determination in structural materials it is necessary to perform more detailed experiments, with specimens submitted to a different residual stresses levels and calibration conditions.
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