QUANTITATIVE MEASUREMENT OF FATIGUE CRACK GROWTH IN NOISY ENVIRONMENTS IN REAL TIME WITH ACOUSTIC EMISSION

INTRODUCTION

In November of 1998 I reported on the results of a fatigue test on a section of railroad bridge containing fatigue cracks. The bridge section was cut out of the bridge and fabricated into a four point bend specimen for fatigue testing in a large servo-hydraulic testing machine. The earlier report showed that the AESMART 2000 acoustic emission instrument was successful in detecting the initiation of new fatigue cracks in the specimen and confirming that the original cracks did not grow.

Since that time a final report authored by A. Shakoor Uppal and Greg Garcia at the Transportation Technology Center in Pueblo Colorado entitled "Nondestructive Evaluation Technologies for Steel Bridge inspection" has been released. This report shows that there was excellent agreement between the summation of AE counts and fatigue crack length. These results will be shown in this report.

The Specimen

Figure 1 shows a section of an I beam removed from the bridge with its upper flange removed. The load platens of the 650 kip test machine were in contact with the web of the beam and what looks like grips from the test machine were primarily used to keep the specimen aligned with the load head. Since the cylinders from the test machine were unrestrained a lot of lateral movement of the specimen occurred during testing which resulted in an enormous amount of extraneous noise as the test proceeded. In addition to the AESMART 2000 instrument, traditional parameter based acoustic emission instrumentation was used during the test. The traditional instrumentation was so swamped with noise that no meaningful results were obtained.

Acoustic Emission Results

I have tested many specimens and structures over my lifetime but never encountered a test with as much extraneous noise as this test. In spite of the high background noise present the filtering capability of the AESMART 2000 allowed us to record the initiation and growth of fatigue cracks in this specimen in real time. In addition excellent correlation between the summation of acoustic emission counts and crack length was observed. Figure 2 shows these results. Note in figure 2 that quite periods occur where no crack growth or AE is observed, followed by very active AE and crack growth. This is consistent with a crack growing in a redundant structure and in the process causing stress relief and redistribution of stresses and subsequent crack arrest. Redistributed stresses cause new crack initiation in an adjacent area.

Figure 2

CONCLUSIONS

These results show that the real time modal analysis capability of the AESMART 2000 Acoustic Emission instrumentation can be used to detect and measure fatigue crack initiation and growth in a quantitative manner in the presence of lots of extraneous noise. Traditional acoustic emission instrumentation used on the same test failed to achieve the same results.

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