# Experiment and Modeling on Macro Fiber Composite Stress-Induced Actuation Function Degradation
## 1 Introduction  
The functional structural system of piezoelectric smart/intelligent structure was developed rapidly. A large amount of research works are devoted to the application study of piezoelectric elements, such as the piezoelectric actuator, sensor and energy transducer in structural vibration control, structure health monitoring and energy harvesting. Nevertheless, performance reliability of the three mentioned types of piezoelectric elements is very important for the practical application of the piezoelectric smart structure system. Experimental results of piezoelectric ceramic samples show remarkable degradation of piezoelectric properties after mechanical loading due to the stress depolarization effect [1; 2; 3; 4; 5; 6; 7; 8; 9]. In applications, the piezoelectric properties' degradation of piezoelectric ceramics in piezoelectric actuators is inevitable, due to the effect of stress depolarization when piezoelectric actuators and the main structure bear the external load simultaneously. Such an occurrence will degrade the actuation function of the piezoelectric actuator and finally lead to the function failure of the piezoelectric smart structure system. Recently, function degradation of piezoelectric transducers in an energy harvesting system was studied by a few researchers. [PERSON] et al. [10] studied the degradation of the energy conversion function of piezoelectric ceramics patch in the piezoelectric energy harvesting system. Bimorph piezoelectric beams were subjected to lifespan testing through electromagnetic tip actuation with a large number of cycles. Experimental results show that the degradation of the piezoelectric material will lead to the resonance frequency shift of the double crystal piezoelectric beam, which has a strong impact upon the efficiency of the energy collecting system utilizing resonance mode for energy harvesting. [PERSON] and [PERSON] [11] studied the functional reliability of Macro Fiber Composite (MFC) in energy harvesting system. Aimed to obtain the upper limit of dynamic strainon MFC, this strain limit is expected to be used as a failure limit in the design of piezoelectric energy harvesters (PEHs). Based on experimental results, 600 \(\mu\varepsilon\) was proposed to be the upper limit of safe strain amplitude for a reliable performance of MFC in the energy harvesting system. [PERSON] et al. [12] investigated the degradation of bimorph piezoelectric bending beams under symmetrical and asymmetrical sinusoidal loading. It is concluded that the degradation of the bimorph piezoelectric patch will bring a loss in output power and the shift in resonance frequencies, and micro-cracking was shown to occur predominantly in piezoelectric layers under tensile stress. [PERSON], [PERSON], and [PERSON] [13] proposed a prognostic approach which involves a training machine learning algorithm to model the degradation of the piezoelectric transducers through a health index, and a use of the learned model to estimate the health index of similar transducers, where electric signals of the piezoelectric transducer are used as the condition monitoring data. [PERSON] et al. [14] investigated the reliable performance of hard and soft piezoelectric single crystal-based fiber composites (SFC)-based piezoelectric energy harvesters (PEHs). Base acceleration of both PEHs was held at 7 m/s\({}^{2}\) and the frequency of excitation is tuned to their resonant frequency, and then the output power was monitored for 10\({}^{7}\) fatigue cycles. Experimental results indicate that fatigue-induced performance degradation in soft SFC-based PEH is more prominent than that in hard SFC-based PEH. The energy conversion efficiency of the piezoelectric transducer is the highest when it works in its resonance state. Above-mentioned researches have been mainly focused on the function degradation test of a piezoelectric transducer under this resonance condition. On the other hand, the piezoelectric actuator is the key component of a piezoelectric active control system for structural vibration control.