# Ultrasonic VMCF Tests on Very Large Specimens with Risk-Volume Up to 5000 mm+
## 2 Materials and Methods
### Ultrasonic Fatigue Tests  
Experimental tests were carried by using the ultrasonic fatigue testing machines developed at Politecnico di Torino. According to [7; 9], an ultrasonic generator (Branson DCX 4 kW) generates an electric sinusoidal signal with a frequency of 20 kHz, which is converted by a piezoelectric transducer in mechanical vibration. Two mechanical amplifiers (a booster, provided by Branson, and a horn, designed in Ti6 Al4V alloy by the Authors), rigidly connected through a threaded connection to the piezoelectric transducer, amplify the vibration amplitude in order to induce a fully reversed tension-compression stress within the specimen, which is rigidly connected with an adhesive butt-joint to the horn.  
Before the tests, the stress distribution within the risk-volume and the presence of spurious bending loads in Gaussian specimens were verified through a strain-gage calibration. Three strain gages were bonded to a Gaussian specimen: The location of the strain gages (Gage A and Gage B) used for the validation of the stress distribution is shown in Figure 2a; meanwhile, the location of the strain gages used to verify the presence of bending loads (Gage A and Gage C) is shown Figure 2b. It is worth noting that this gage calibration was not carried out on small hourglass specimens, since they have been widely employed and validated in the literature [10; 11; 12].  
Strain-gage rosettes in the half bridge configuration for the temperature compensation were used for the calibration: in particular, the HBM 1-XY91-1.5/350 strain-gage rosettes and the HBM XY31-1.5/350 strain-gage rosettes were used for the stress validation and for the assessment of bending loads, respectively. A strain-gage amplifier (Elyss SGA2-Box/7) was used for the amplification of the gage signal, which was acquired with multifunction data acquisition (National Instrument, NI 6363), at a sampling frequency of 300 kHz.  
After the calibration, the experimental tests were carried out at a constant stress amplitude up to failure, or up to \(10^{9}\) cycles (runout number of cycles). According to [9], the stress amplitude was kept constant with a closed-loop control based on the displacement at the specimen free end, \(U_{0}\), measured with a laser displacement sensor. The correlation between \(U_{0}\) and the stress amplitude within the risk-volume was verified through a second strain-gage calibration, according to [9], for both the hourglass and the Gaussian specimens. The temperature at the specimen center was monitored during the tests with an infrared sensor and intermittent tests [12] were carried out. In particular, a limited temperature increment of 2 \({}^{\circ}\)C (with respect to the specimen temperature at the beginning of the test, \(T_{0}\)) was conservatively set, to guarantee a uniform temperature distribution within the risk-volume of Gaussian specimens. When the maximum temperature was exceeded, the control system automatically stopped the test, to go back to the initial value \(T_{0}\). Three vortex tubes were also used to slow down the temperature increment when the specimen was loaded and to speed up the cooling phase.