## 3 SIF-Controlled In-Situ Fatigue Experiment
### Experimental Setup  
An experimental specimen was designed to collect AE signals during fatigue crack growth in thin metallic plates. A fatigue test coupon of a 1-mm thick aluminum 2024-T3 was cut to a width of 103 mm and a length of 305 mm. The width of the specimen was sufficient to allow a long crack to form. The specimen was fatigue loaded between load limits of 1.38 kN lower bound to 13.85 kN upper bound at a loading frequency of 10 Hz. This loading generated a 4-mm crack along the centerline of the specimen after 322,000 cycles.  
After the crack was initiated and grown to 4 mm, the specimen was removed from the MTS machine and AE instrumentation was applied. Two PWAS sensors, two S9225 sensors, and a clay non-reflective boundary were applied to the specimen. The sensors were bonded in a linear configuration at 5 mm and 25 mm from the crack on both sides. The non-reflective boundary was applied to the edges of the specimen to damp out reflecting waves from plate boundaries. The PWAS sensors were bonded using M-Bond AE-15, a two-part epoxy system with a resin and curing agent. This adhesive system was chosen because of its resilience to debonding during excessive fatigue loading. The electromechanical impedance spectrum (EMIS) of the PWAS sensors was measured periodically to ensure no defects or bonding deterioration [37]. The S9225 sensors were bonded using hot glue. Figure 6 shows the setup of this AE test specimen loaded into the MTS machine and ready for AE monitoring.  
After AE instrumentation was applied, the test specimen with a 4-mm crack was loaded back into the MTS machine (shown in Figure 6) for further fatigue loading with simultaneous AE monitoring. The system of hardware used in the data acquisition is shown in Figure 7. The Omicron Lab Bode 100 Vector Network Analyzer was used to  
Figure 6: A 1-mm aluminum 2024 test specimen with AE instrumentation applied: Two PWAS, two S9225, and clay non-reflective boundary.  
analyze the EMIS of the bonded PWAS. The PWAS sensors were connected to the MISTRAS AEWinTM computer through an acoustic preamplifier, which is a bandpass filter that filters signals between 30 kHz and 700 kHz. In this experiment, 40 dB of gain was selected on the preamplifier from options of 20 dB, 40 dB, or 60 dB. In the data acquisition software in the AEWinTM system, a sampling frequency of 10 MHz was selected to capture the AE signals from the PWAS. The timing parameters set for the MISTRAS system were peak definition time (PDT) = 200 \(\upmu\)s, hit definition time (HDT) = 800 \(\upmu\)s, and hit lockout time (HLT) = 1000 \(\upmu\)s. Figure 8 also shows the experimental setup and other auxiliary experimental equipment, including the camera and eddy current equipment, which were used to track crack growth continuously. The camera was continuously capturing video of the crack tips with adjacent measurement tape adhered to the test specimen, while an eddy current probe from Eddyfi TechnologiesR was used periodically to track the growth of the crack.  
Figure 8: Experimental setup of fatigue specimen loaded into MTS machine with auxiliary experimental equipment.  
Figure 7: Experimental setup of the test specimen and data acquisition hardware. (MTS: material testing system; EMIS: electromechanical impedance spectroscopy).