  Detecting mechanical fatigue of metallic components is always a challenge in
industries. In this work, we proposed to monitor the low-cycle fatigue of a
6061 aluminum alloy based on internal friction (IF) measurement, which is
realized by a quantitative electromechanical impedance (Q-EMI) method using a
small piezoelectric wafer bonded on the specimen. Large strain amplitude
(3.3*10^-3) was employed thus the fatigue life can always be below 10^5 cycles.
It was found that except for the initial testing stage, the IF always increases
steadily with the increasing fatigue cycles. Before the fatigue failure, the IF
can reach 2.5 to 3.4 times of the initial value, which is thought to be caused
by the micro-cracks forming and growing. In comparison, the resonance frequency
of the specimen just drops less than 2% compared with the initial value.
Finally, a general fatigue criterion based on IF measurement is suggested for
all the metallic materials.
