# Guided Wave-Based Monitoring of Evolution of Fatigue Damage in Glass Fiber/Epoxy Composites
## 2 Experimental Test  
To perform experimental study, two GF/EP laminate with [0\({}_{6}\)/90\({}_{6}\)]\({}_{8}\) and [0\({}_{4}\)/90\({}_{8}\)]\({}_{8}\) layups were fabricated, respectively. The G15000 unidirectional prepregs were purchased from Weihai Guangwei Composites Co. Ltd., with thickness of 0.12 mm and resin content of 30-40%. After stacking the layers with designed layups, the laminates are manufactured through hot-press with temperature of 120 \({}^{\circ}\)C for 90 min. The cured laminates have a thickness of 2.6 mm, and they were then cut into 200 mm \(\times\) 30 mm specimens for tensile and fatigue tests. The average maximum tensile load for three specimens of [0\({}_{6}\)/90\({}_{6}\)]\({}_{8}\) layup is 22.15 kN, corresponding to an average tensile strength of 283.97 MPa, and for three specimens of [0\({}_{4}\)/90\({}_{8}\)]\({}_{8}\) the layup is 16.19 kN, corresponding to an average tensile strength of 207.56 MPa. The other two specimens from [0\({}_{6}\)/90\({}_{6}\)]\({}_{8}\) and [0\({}_{4}\)/90\({}_{8}\)]\({}_{8}\) layups are labeled as S_A and S_B for fatigue test, respectively. As illustrated in Figure 1a, the two ends of the specimens are reinforced for gripping, and on the surface of the specimens, two P-51 piezoelectric wafers purchased from Wuxi Haiying Co. were mounted by epoxy with a distance of 80 mm. The diameter and thickness of the wafers are 10 mm and 1 mm, respectively. In the following experiments, one wafer acts as actuator to excite diagnostic guided waves into the specimens and the other acts as sensor to receive the response waves. A test setup consisting of a MTS 810.25 fatigue test system and a guided waves monitoring system is established. The monitoring system contains a KH-7600 wideband amplifier produced by Krohn-Hite Corporation, a PXI-5441 arbitrary function generator, a PXI-5105 digitizer, and an embedded controller, which were all produced by National Instrument Corporation. In addition, a noncontact laser extensometer LE-05 produced by Electronic Instruments Research is also used to provide reference stiffness information to the guided wave-based technique. The overall view of the test setup is shown in Figure 1b. For the fatigue test, tension-tension cyclic loads are applied to the specimens. The maximum tensile loads applied to specimens S_A and S_B are 11 kN (equals to nominal stress of 141.03 MPa) and 8 kN (equals to nominal stress of 102.56 MPa), respectively; the stress ratios and loading frequencies of both cyclic loads are 0.1 and 1 Hz, respectively. In order to follow the evolution of fatigue damage, the longitudinal stiffness of the specimens is first obtained before  
Figure 1: Experimental setup: (**a**) glass fiber/epoxy GF/EP specimen and (**b**) fatigue test and monitoring systems. fatigue tests (zero fatigue cycle) and then at periodically paused intervals of each 1000 cycles. A set of small loads were applied to the specimens and the corresponding displacements were measured by the laser extensometer. With the information of loads and displacements, the longitudinal stiffness can be calculated. Table 1 lists the longitudinal stiffness data for both specimens within 10,000 fatigue cycles, while Figure 2 shows the variations of longitudinal stiffness with increase of fatigue cycles. It can been observed from Figure 2 that, with the increase of fatigue cycles, the longitudinal stiffness of the two specimens degrades; at the end of 10,000 fatigue cycles, the stiffness of specimens S_A and S_B reduces by approximately 11.2% and 8.6%, respectively. The stiffness reduction reflects the fatigue damage accumulation in the composite specimens [24; 25]. Meanwhile, for monitoring the fatigue damage by using guided waves, the PXI-5441 arbitrary function generator sends a diagnostic wave signal which is amplified by KH-7600 amplifier and drives the actuator to generate guided waves, the response wave signals are then sensed by the sensor and acquired by the PXI-5105 digitizer, whose sampling rate is set at 10 MHz.