Issue 43

P. Corigliano et alii, Frattura ed Integrità Strutturale, 43 (2018) 171-181; DOI: 10.3221/IGF-ESIS.43.13 174 Power [kW] Speed [mm/s] Entry angle [°] 6 80 20 Table 3 : Process parameters. Experimental setup Experimental measurements were carried out by the use of a servo-hydraulic Instron 8501 machine equipped with a load cell of 100 kN. Each test was led by imposing different values of the maximum load, Pmax, in the range between 8 kN and 40 kN, with a load ratio R = P min /P max = 0.1, considering the run-out at 10 7 cycles and adopting a variable load frequency. In particular, during each test two different frequencies were used, i.e. 0.5 and 5 Hz. The frequency of 0.5 Hz has been adopted to acquire the images to be analyzed using the Digital Image Correlation (DIC) in order to ensure an accurate analysis of the strain field of the specimen, while the frequency of 5 Hz was used to record the temperature increase during the load history by means of the Infrared Thermography (IRT), in order to be able to apply the Thermographic Method [21]. A diagram of the spectrum of the applied load is shown in Fig. 4. In particular, the first two cycles of each test were performed at 0.5 Hz in order to measure the initial state of the specimen, from a point of view of the strain, later cycles at higher frequency (5 Hz), necessary for the thermographic measurements and for the specimen damage, were alternated with low frequency cycles, until failure, as shown in Fig. 4. It is important to observe that, for each step, the effected measuring cycles at 0.5 Hz are only two. This was necessary in order to prevent sudden drops in temperature dictated by the reduction of the frequency of the load, thus compromising the measurement of the thermal history of the specimen by means of the IRT. Figure 4: Profile of the applied load to allow IRT and DIC measurements. In order to ensure the application of the stress in correspondence of only the weld bead, it was designed and created an appropriate loading system (Fig. 5). During each test, a surface of the specimens has been used to monitor the evolution of the temperature, in correspondence of the welded area, by the use of Infrared Thermography. The opposite side, instead, was used for monitoring the displacements and the relative strains using the optical technique of the Digital Image Correlation. For this purpose, one side of the specimens was black painted in order to ensure a good surface emissivity (ε = 0.92) and, during the test, a series of images at a frequency of 0.2 Hz were acquired in order to trace the evolution of the temperature during the cyclic application of loads. All images were acquired using a thermo-camera (FLIR, A40), characterized by a resolution of 320x256 pixels, and the results were analyzed by using a commercial software (Flir ThermaCAM ™ Researcher). On the opposite side of the specimen, however, it has been realized a surface pattern, characterized by an appropriate gray scale in order to allow the digital correlation of the images. The images used for the DIC analysis were