Quantitative health monitoring of fatigue crack initiation and propagation in aluminium specimen based on electromechanical impedance technique

The process of crack initiation and propagation during fatigue loading in aluminium specimens was quantitatively monitored with the electromechanical impedance (EMI) method. The displacement-controlled fatigue testing mode was used and the stages of crack initiation and propagation consisted of up to 90% and 80% of the total life of the two types of specimen, without and with crack starter notch, respectively. The impedance signatures of the piezoelectric ceramic (PZT) sensors were measured online with the specimens fixed to the fatigue testing machine, and the signatures measured after 200 cycles were selected as the baselines when the area and depth of the indentation were stabilised. The resonant frequency shift Δf and root mean square deviation (RMSD) of the impedance signatures were extracted as damage indicators and compared with the crack length measured from the photographs of the fatigue fracture zone. The results show that at the crack initiation stage, before the generation of a 0.22 mm fatigue crack, the values of Δf and RMSD increased obviously to 0.6 kHz and 11.68%, respectively. The initial pre-crack damages can be effectively monitored with these two indicators. At the crack propagation stage, with the increase of crack length from 0.58 to 9.40 mm, the values of Δf and RMSD both increased monotonically. The increment trend of Δf was evidently similar to that of crack length and the RMSD value was more sensitive to the initial expansion of crack length in the first 5000 cycles. This study confirms that the EMI method can be potentially employed for the monitoring of in-service fatigue damage of engineered products and structures.