DEVELOPMENT OF A FATIGUE DAMAGE MONITORING SYSTEM USING DIGITAL IMAGE CORRELATION

The prediction of fatigue damage is a challenging task in engineering. This is because fatigue failure occurs unexpectedly during regular working conditions. Most of the time, the applied stresses are well below the yield strength of the material. Therefore, an efficient approach in monitoring fatigue damage and a means of quantifying the severity of fatigue cracks are important, and are the objectives of this study. First, a two-dimensional sub-pixel accuracy deformation measurement algorithm was coded using MATLAB. Attention has been given to improve the algorithm’s computational efficiency and measurement accuracy. Next, validation works were carried out to verify the algorithm, imaging system and the digital image correlation (DIC) technique. The DIC technique was found to be accurate in measuring small displacement of up to 0.01 mm. Tensile tests for mild steel and aluminum alloy samples were also conducted to validate the DIC technique. Through the comparison of the results determined by extensometer and DIC technique, good agreement was achieved as the deviations were found to be less than 3% off the benchmark values. In the second part of this study, fatigue tests were conducted and a quantifiable approach in monitoring the fatigue damage of a notched sample using DIC technique was developed. First, the DIC technique was used to identify the stress concentration area of a notched sample. Then, the condition of fatigue damage around the stress concentration area was monitored continuously using DIC technique. By doing this, drastic changes were observed from the determined first-order shape functions when the average number of cycles were approaching 90% of the total fatigue life. Thus, the operation has to be stopped, so that the final fracture can be avoided. Feasibility study on the DIC technique in monitoring the fatigue damage of a component was also carried out. Based on the real fatigue life of the tested metal samples, fatigue life determined by DIC technique was validated with the benchmark values computed by Paris’ Law with an average variation of 5%. In conclusion, a fatigue damage monitoring system has been developed successfully using DIC technique. Since the DIC technique does not require any knowledge on fatigue analysis, it can definitely become an effective tool to monitor the fatigue damage of a structural component throughout its operational period.