# Experiment and Modeling on Macro Fiber Composite Stress-Induced Actuation Function Degradation
## 2 Experiment Set Up and Procedures
### Actuation Function Degradation Test of MFC Actuator Undergoing Cyclic Load  
In the present test, the MFC actuator is bonded to the middle surface of a steel beam to form an MFC piezoelectric beam test specimen. Cyclic loads are imposed on the MFC actuator through the bending resonant vibration of the beam excited by a shaker. Actuation function degradation data and curves of MFC actuation function index \(A\) versus \(N\)-cycles of cyclic load have been obtained for cyclic loads with four different stress amplitudes. The actuation function of an intact MFC actuator is characterized by the amplitude \(A_{0}\), which is the free end resonant displacement amplitude of the MFC piezoelectric beam actuated by the intact MFC actuator under certain input driving conditions for MFC. Then the MFC actuator is cyclically loaded by a shaker via the beam specimen. For every 50,000 cycles of cyclic load, there sonant amplitude \(A_{t}\)\((\sigma,\text{N})\) in the free end of the beam which is actuated by the attached MFC actuator is measured under the same input driving condition as that of \(A_{0}\). According to Equation (1) the actuation function index \(A\)\((\sigma,N)\) of the MFC actuator can be obtained, and it can be used to characterize the degenerate degree of the MFC piezoelectric actuation function undergoing \(N\) cycles of cyclic load with stress amplitude \(\sigma\). The test procedure of the MFC actuator actuation function degradation undergoing cyclic load is shown in Figure 1.  
According to the engineering properties of the MFC actuator, the linear-elastic tensile strain limit is 1000 \(\mu\epsilon\)[14], and the corresponding stress limit is 30 MPa. Considering this limit, cyclic loads with four different stress amplitudes are implemented for the actuation function degradation test of our MFC actuator. Four stress amplitudes of the cyclic loads are 12 MPa, 18 MPa, 24 MPa and 30 MPa. The corresponding dynamic strain amplitudes of the MFC actuator for these four stress amplitudes are:400 \(\mu\epsilon,600\)\(\mu\epsilon,800\)\(\mu\epsilon\) and 1000 \(\mu\epsilon\), respectively. The experimental results of the sensing function degradation test for the MFC sensor show that, when the MFC sensing function degradation degree is greater than 10%, which can be characterized by the reduction of its output voltage, it is considered that its function degradation is observable for the corresponding test case [12]. Hence, in the present test when the measured index \(A\) is greater than 90%, which indicates that the actuation function degradation degree is less than 10%, it will be considered that there is no actuation function degradation for the corresponding load cycles. The termination number of loading cycles for the present test is determined according to the variation of index \(A\). When the index \(A\) is basically stable, which indicates no observable further actuation function degradation can be confirmed, this function degradation test will be terminated. In the present study, an M2814-P1 MFC actuator is selected as our test object. Properties of this MFC actuator can be found in Table 1. Four of the same type of MFC actuators have been made into four MFC piezoelectric beam specimens to implement the actuation function degradation tests, which are shown in Figure 2. The four MFC piezoelectric beam specimens will be used for cyclic loading and actuation function degradation tests of cyclic loads with stress amplitudes 12 MPa, 18 MPa, 24 MPa and 30 MPa, respectively. The test layout of the cyclic loading experiment for the MFC actuator is shown in Figure 3. In the cyclic loading experiment of the MFC actuator, the MFC piezoelectric beam is fixed by one end, and the resonant bending vibration of the MFC piezoelectric beam is excited by a shaker, whose action point is near to the fixed end of the beam. The excitation signal input to the power amplifier of the shaker is a sinusoidal signal, and the frequency is equal to the first natural mode (first bending mode) frequency of the MFC piezoelectric beam specimen.