# Design and Experimental Study of a Down-Drive Piezoelectric High-Frequency Fatigue Testing Machine
## 2 Materials and Methods
### Working Principle  
Although fatigue testing machines have numerous successful applications, they can be traditionally categorized into three types based on the way energy is utilized when loading a specimen. The first type is the excitation energy recovery test system [25], where the specimen is loaded on a rotating eccentric wheel, where the released potential energy from the previous loading cycle is transferred as kinetic energy for the next cycle. The second type is the unincentivized energy recovery test system [26], with constant pressure and rate test loads induced by an electrohydraulic servo control system. Therefore, whether the test system is loaded or not, it still consumes a fixed amount of excitation energy. In contrast to a recycled test system, there is no energy to reuse here, and in addition, continuous testing generates waste heat, which requires additional energy for cooling. The last type is the resonant test system [27], where the driving frequency of the tester is designed to be close to the intrinsic frequency of the test system, which is the intrinsic vibration frequency of the system itself, and whose value is independent of the initial conditions and the magnitude of the external excitation to which it is subjected. In this design, a frequency is applied to the system to excite the resonance of a linear system or the subharmonic oscillations of a nonlinear system, and by applying a small excitation force, the amplitude can be increased so that the energy consumption of the tester is mainly focused on the resonance active damping. This fatigue tester consumes less energy than the first or second previously mentioned test systems.  
In this paper, a small-load fatigue tester based on a Type III resonance test system is designed to use a piezoelectric vibrator as the driving force source of the system, which generates vibrations under the action of an applied alternating electric voltage. Adjust the frequency of the alternating voltage applied to the piezoelectric vibrator, prompting the vibration system consisting of the base, guide rod, preset load adjustment mechanism, force detection mechanism, clamping mechanism, specimen, dynamic load adjustment mechanism, and fixing mechanism to resonate. At this time, the inertial force generated by the counterweight mass block in the resonance state reciprocates on the specimen. In addition to this, the inertia force generated by the mass of the counterweight disk, the piezoelectric vibrator, and the lower disk in the status of resonance also acts on the specimen at the same time, thus realizing fatigue testing.  
The fatigue testing machine needs to determine the force applied to the specimen in real time during fatigue testing, so it is necessary to build a force testing system as shown in Figure 2, which consists of transducers, boosters, 24 VDC power supply, acquisition card, and acquisition software (V190521).  
The transducer works on the principle of resistance strain-type, when subjected to external forces, the deformation of its internal elastomer, causing changes in the resistance value of the resistance strain gauges, the transducer's built-in circuitry will be converted into a change in resistance to voltage changes and output to the booster, which will be converted into a standard voltage signal. The acquisition card is powered by USB and acquires the voltage signal output from the booster and transfers it to the computer via the USB 2.0 bus. The acquisition software displays and records the loads acting on the sensors in real time, as shown in Figure 3 for the interface diagram of the acquisition software.