## 2 Ultrasonic Fatigue Device and Data Acquisition System Development  
Three major components of the test device can be summarized as follows [2]. 1. Control system: computer with National Instruments(r) (Austin, TX, USA) PCIe internal card and self-programming VC++ software keeping the initial loading parameters as required. 2. Actuation system: * Power generator: Branson(r) (St. Louis, MO, USA) power generator receives the control signal from the NI(r) PCIe internal card and generates 20 kHz sinusoidal electrical current. * Piezoelectric converter: Piezoelectric converter transforms the electrical energy into longitudinal mechanical vibration at the same frequency. The vibration amplitude is very small (several microns). * Booster/horn: The role of the booster is to provide a plane for arranging the whole assembly. Due to the small vibration amplitude from the piezoelectric ceramic, the horn (also called a sonotrode) is presented to amplify the value from several to a dozen microns, for obtaining the required vibration corresponding to the specific stress amplitude in the test section of the specimen. 3. Acquisition system: [PERSON](r) (Osaka, Japan) LK-G5001P laser sensor and Flir(r) (Wilsonville, OR, USA) A325 sc infrared camera were applied to measure and analyze the vibration mode,deformation, and temperature due to the self-heating phenomenon, as shown in Figure 1. NI(r) 6221 external data acquisition card (DAQ) driven by LabVIEW combined the signals during the test. Four main parameters were monitored during the test, as shown in Figure 2. Resonance frequency (in black) was converted by the feedback voltage from the power generator. Maximum temperature of the test section of the specimen (in red) was obtained from the infrared image, and it was chosen to be the representative value of the specimen. Amplitude and mean value were calculated from the vibration wave at the top (in green) and bottom (in blue) of the specimen. When the test began, the temperature of the specimen increased due to the self-heating of the material. Therefore, resonance frequency of the assembly (piezoelectric ceramic, booster, horn, and specimen) decreased at the same time. These two values would be steady, after the balance between energy generation and dissipation of the assembly was achieved. Self-heating caused by the internal friction and microscopic plasticity under high loading frequency is significant during the ultrasonic test. Temperature in the test section of the specimen is up to 300 \({}^{\circ}\)C in some cases [31; 32]. The emissivity also changes as the specimen surface temperature changes. Conventional optic displacement sensor, which calculates the displacement of reflected light intensity, is not suitable for this kind of test. Contacted strain gauge is also difficult to calibrate for the same reason. The noncontact laser sensor was chosen in this test. LK-G5001P series sensor with two LK-H022 probes was employed for the test. Sampling rate of 200 kHz was used, taking accuracy and memory consumption into account at the same time. Figure 1: Schema of ultrasonic fatigue test device: (**a**) test device with data acquisition system, (**b**) laser sensor probes’ arrangement. Figure 2: Data acquisition interface (\(R=-1\), \(f=20\) kHz, \(N_{f}=1\times 10^{8}\), no crack). For measured vibration of nearly 20 kHz, there were 10 points to describe one sinusoidal wave on average. Two probes of the laser sensor were used at the same time for revealing the vibration mode of the specimen. One was set at the bottom of the specimen and the other one was set at the bottom of the horn. The bottom surface of the horn and the top surface of the specimen were considered in the same vibrating situation. The upper probe could be rotated less than 30\({}^{\circ}\) due to the geometric obstacle, as shown in Figure 1b. The reading was adjusted through the experimental calibration.