Evaluation of Al-specimen fatigue using a “smart sensor”

We develope a new methodology for the characterization and evaluation of precritical crack propagation. Our experiments were performed with Al-alloy specimens that had special thin Al single crystal films, the so-called a “smart sensor” glued to their surfaces. These sensors were used as tools for evaluation of damage developed in the specimen bulk. The specimens were plastified by tensile tests and fatigued by low cycles fatigue load. The on-line measurement for damage characterisation were done for the different stage of fatigue tests on the thin-film sensor. Two techniques were applied for experiments, namely the optical microscopy and pulsed eddy current method. We introducted the fractal dimension Dt of the surface topography as a parameter for evaluation of the fatigue damage. The optical images were evaluated by the use of Gaussian smoothing algorithm and the eddy current signals by use of the autocorrelation function. The two approaches confirmed that the mesostructural changes of the Al-film surface are characteristic for the fatigued material and can be described and evaluated by the fractal dimension quantitatity Dt. It was found that the values of the Dt parameter calculated from the optical images increased as the fatigue damage evolved with the indications of the transition to characteristic pattern. A similar analysis of the pulsed eddy current measurements in tension tests results in similar characteristics. However, in the case of fatigue tests the fractal analysis of the pulsed eddy current signals yields to ambiguous results.