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Research Papers

Piezo-Barkhausen Emission as an Indicator of the Fatigue Limit of Steel

[+] Author and Article Information
S. A. Guralnick

Department of Civil, Architectural,
and Environmental Engineering,
Illinois Institute of Technology,
Chicago, IL 60616
e-mail: guralnick@iit.edu

F. Nunez

Department of Civil Engineering,
Pontificia Universidad Javeriana,
Bogota D.C. 110231, Colombia

T. Erber

Department of Physics,
Department of Applied Mathematics,
Illinois Institute of Technology,
Chicago, IL 60616

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received October 28, 2014; final manuscript received May 28, 2015; published online June 15, 2015. Assoc. Editor: Ashraf Bastawros.

J. Eng. Mater. Technol 137(4), 041004 (Oct 01, 2015) (9 pages) Paper No: MATS-14-1210; doi: 10.1115/1.4030759 History: Received October 28, 2014; Revised May 28, 2015; Online June 15, 2015

The fatigue properties of two variants of AISI 1018 steel samples were measured in a series of 33 experiments using new kinds of magnetic diagnostics. An MTS-810 servohydraulic test machine applied sinusoidal fully reversed (R = −1) loads under strain (Є) control in the range of 0.0008 (Є) 0.0020. In 28 experiments, the number of cycles to fatigue failure Nf varied between 36,000 < Nf < 3,661,000. By contrast, in five runs extending over 107 cycles, the specimens showed no detectable signs of weakening or damage. The corresponding “S-N” or classical Wöhler plots indicated that the transitions from fatigue failure to nominally infinite life (i.e., the fatigue limit) occurred at strains of about Є = 0.0009 and Є = 0.0010, respectively, for the two types of steel. Every loading cycle of each test was instrumented to record continual values of stress and strain. Flux gate magnetometers measured the variations of the piezomagnetic fields near the specimens. A 1000-turn coil surrounding the test pieces detected the piezo-Barkhausen pulses generated by abrupt rearrangements of their internal ferromagnetic domain structures. Analyses of the magnetic data yielded four independent indices each of which located the fatigue limits in complete agreement with the values derived from the Wöhler curves.

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Figures

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Fig. 1

(a) Stress versus strain diagram for AISI 1018 steel types A and B and (b) S-N or Wöhler diagram for AISI 1018 steel types A and B. The asymptotes are carried over to Fig. 1(a) to emphasize that the fatigue limits lie below the elastic strain range.

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Fig. 2

Piezo-Barkhausen pulses in a fatigue test with type B steel. The peak strain is |Єmax| ∼ 0.0014; the time-to-cycle conversion is 1.8 × 104  T (hr) = N (load cycles).

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Fig. 3

Piezo-Barkhausen pulses in a fatigue test with type B steel near the fatigue limit. In this case the sample did not break: ǀЄmaxǀ ∼ 0.0009, N > 107.

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Fig. 4

Piezo-Barkhausen pulse fluctuations versus maximum strain amplitude. The peaks of the curves coincide with the fatigue limits of the two types of steel.

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Fig. 5

Stress–strain hysteresis of steel type B during the cycle interval 1,010,368 ≤N≤ 1,029,799 at peak strains of ǀЄmaxǀ ∼ 0.0010. Negative stress values indicate compression (1 ksi = 6.895 MPa). Negative strains indicate contraction.

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Fig. 6

Variation of piezomagnetic field versus strain. The magnetic field units are milliGauss (10−7 T); positive values mean B⇀ points from the specimen to the fluxgate probe. The cycling interval and strain range are the same as in the caption of Fig. 5.

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Fig. 7

Cumulative piezo-Barkhausen pulse distribution versus strain for fatigue test T-018-12 with type A steel. The necking index Q is defined in the inset.

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Fig. 8

Plots of Q ratios versus applied strain. The curves have minima at values corresponding to the fatigue limits.

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Fig. 9

Illustrative plot of partial area U versus time t. The inset shows a typical piezo-Barkhausen pulse profile of a fatigue test.

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Fig. 10

Integrals of the piezo-Barkhausen pulse traces (AISI 1018 steel A). The ordinate values are normalized so that all results can be displayed on a single graph.

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Fig. 11

Integrals of the piezo-Barkhausen pulse traces (AISI 1018 steel B)

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Fig. 12

Average amplitude of the piezo-Barkhausen pulses versus maximum strain in fatigue tests

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Fig. 13

Piezo-Barkhausen pulses versus time for a 1018 steel sample, type B. Maximum strain ǀЄmaxǀ = 0.0011 (test T-020-13).

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Fig. 14

Single sawtooth pattern of piezo-Barkhausen pulses

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Fig. 15

(a) Hysteresis loops of the piezomagnetic field versus strain for test T-022-12. The graph is an overlay of all the traces generated during the sixth hour of the test. (b) Fan-out of the hysteresis loops just before failure during the tenth hour of running ∼17,322 cycles.

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Fig. 16

(a) Stress–strain and piezomagnetic field hysteresis loops recorded during hour 230 of the fatigue life test where the maximum strain, ǀЄmaxǀ ∼ 9 × 10−4, is below the fatigue limit; test T-005-13. The narrow stress–strain loop is actually an overlay of 21,550 loading cycles. This repetitive pattern shows no signs of microstructural changes. However, the spread of the piezomagnetic hysteresis loops is the result of structural transformations. (b) Stress–strain and piezomagnetic field hysteresis loops recorded after 223 hrs of additional running time past the data acquired in Fig. 16(a). Below the fatigue limit the graphs seem to be invariant under time translation.

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Fig. 17

Selected piezomagnetic hysteresis loops of a steel sample subjected to load cycles below the fatigue limit; test T-005-13. The six coincident interior loops were recorded about 7 × 104 cycles after the other traces.

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Fig. 18

Three-dimensional display which shows the sinusoidal strain variations generated by the loading machine as a function of time. The distribution of the piezo-Barkhausen pulses is indicated by the vertical signal spikes. The necking zone diagram in Fig. 7 is the result of projecting all the pulses onto the vertical plane shown in the upper right part of Fig. 18.

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Fig. 19

(a) Perspective views of piezo-Barkhausen pulse profiles (left panel) and piezomagnetic-strain hysteresis loops (right panel) evolving in time. The traces show the magnetic signals generated during cycles 10,000–10,005 of a fatigue test above the fatigue limit: test T-017-12. (b) Both graphs are similar to those shown in Fig. 19(a); but these magnetic signals were generated by a test run at loadings below the fatigue limit: test T-008-13.

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