# Feasibility Study of Real-Time Monitoring of Pin Connection Wear Using Acoustic Emission
## 2 Experimental Setup
### AE Test Equipment and Procedure  
Wear testing of low rotational speed pin connections were performed by using the experimental setup, as shown in Figures 2 and 3. The test stand included the pin, the U-shaped part, and the circular part, as shown in Figure 2. Their material was (HSS). The chemical compositions are summarized in Table 1. The U-shaped part and the circular part were connected by the pin. Obviously, the wear occurred on two different areas: The first area was the contact surface between the pin and the U-shaped part, and the second area was the contact surface between the pin and the circular part. Furthermore, there were five bolts around the test stand (as shown in Figure 3); four of them were used to fix the U-shaped part to prevent it from shaking violently, and another bolt was applied to avoid the pin sliding along the hole. The pin was driven by the motor, whose rotational speed was 30 rpm. This rotational speed was set to accelerate the experiment. There were four sets of pin-connected components, and each pin-connected component included a pin, a circular part, and a U-shaped part. Before a new test, the used pin-connected component was replaced by a new set of pin-connected components. When a serious wear occurred in the contact surface between the pin and the connected components, the rotation of the pin might be jammed, and the stranded wire might be pulled off. To avoid such a situation, experiments were stopped after serious wear occurred. If no serious wear occurred, the experiment duration was controlled to around 10 h.  
Figure 3: Photos of the experimental apparatus.  
Figure 2: Schematic of the experimental setup.  
A PCI-2 8-channel system (from Physical Acoustic Corporation, Princeton, NJ, USA) was used to collect the AE signals. AE signals generated by the wear processes between the pin and the contact parts were detected by the piezoceramic AE sensor mounted on the upper surface of the U-shaped part. The piezoceramic AE sensor used in the experiment was R6a, whose frequency is from 35 kHz to 100 kHz, and whose resonance frequency is 55 kHz. The AE sensor was attached to the host structure in a nondestructive way, and it is capable of real time monitoring. The couplant was Vaseline, which can improve the coupling quality, and the pencil lead breaks method was used to check the coupling condition of the sensor. The results of pencil lead breaks showed that the amplitudes in four specimens were both 99 dB, which means high coupling quality. The acoustic emission waves caused by wear spread in the pin and the U-shaped part were collected by the sensor. When acoustic emission waves passed the contact surfaces between the pin and the U-shaped part, the phenomena of reflection, refraction, and attenuation occurred, which caused a portion of energy loss between the incident acoustic emission waves and the transmitted acoustic emission waves. Since the materials of the pin-connected components were same, the attenuation ratio was constant during the wear process. Moreover, during the wear process, the contact surfaces between the pin and the connected parts were squeezed tightly; therefore, the attenuation of the AE signals was low. When severe wear caused high energy incident acoustic emission waves, the energy of the transmitted acoustic emission waves also became larger. Therefore, the AE signals collected by the sensor reflected the wear degree of the pin.  
Meanwhile, mode 2/4/6 pre-amplifiers (Figure 4) were chosen to amplify the AE signals collected by the R6a sensor, and its gain was set to 40 dB. To shield the signals from low-amplitude background noise, a threshold was set. Usually, the threshold was 45 dB, which meant that only these AE signals whose amplitude was over 45 dB could be collected by the PCI-2 system. A diagram to illustrate the instrument connection is shown in Figure 4.