Monitoring short fatigue cracks with miniature strain gages |
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Authors: | Yves Verreman |
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Institution: | (1) Ecole Polytechnique de Montreal, Succ “A”, Case Postale 6079, H3C 3A7 Montreal, Quebec, Canada |
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Abstract: | An experimental technique to monitor the length and the opening level of a short fatigue crack is presented. It is based on
the progressive decrease with crack length of the response of miniature strain gages installed on the surface near the crack
plane. A first gage installed close to the crack plane can monitor cracks from 10 μm in depth to half a millimeter where the
response saturates. Other gages at larger distances from the crack plane are less sensitive but can monitor longer cracks.
The response is measured so that it is independent of strain-gage calibration, Young's modulus and Poisson's ratio. The paper
first presents the basic principles and possibilities of the technique as well as a finite-element analysis performed on automatic
welded joints with straight-fronted cracks for which the technique has been developed. The results give a correlation between
gage response, crack length and gage location and the conditions of replacement of a gage reaching saturation. The practical
exploitation of the technique has required further work to derive a continuous calibration of the gage response that includes
corrections to account for the gage finite dimensions and the crack-plane inclination. This calibration is shown to give crack
lengths that compare well with fractographic marks and typical results that have been obtained on short crack growth at the
weld toe are presented. In particular, the resolution of the technique is put into evidence with results on the initial growth
of a 0.1 mm nonpropagating crack. The paper finally points out the distinctive features that appear in current works to adapt
the technique to the growth of semi-elliptical cracks of low and high aspect ratio. |
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