Issue 43

P. Corigliano et alii, Frattura ed Integrità Strutturale, 43 (2018) 171-181; DOI: 10.3221/IGF-ESIS.43.13 175 acquired by a digital camera (Sony ICX 625- Prosilica GT 2450 model) with a resolution of 2448 pixels on 2050 and were focused through an optical system consisting of an objective Linos Photonics and a lens Rodagon f .80 mm that, under proper lighting, ensures a resolution of approximately 160 pixels/mm. The correlation of the digital images was performed using a dedicated commercial software (Vic-2D). The entire experimental setup, adopted during the test, is represented in Fig. 6. Figure 5: Gripping system of the testing machine. Figure 6: Experimental setup used for testing: a) light source, b) lens, c) camera, d) load cell, e) thermal-camera, f) specimen. R ESULTS AND DISCUSSION igs. 7-9 show the maps of the horizontal strain (ε x ), vertical (ε y ) and the maximum principal strains (ε 1 ), respectively, calculated at the maximum load (40 kN). Specifically, the strains were evaluated taking as a reference image (zero strain), the image acquired at the minimum load of the first cycle of the entire loading history, (Fig. 10). In this way it was possible to identify the process of damage accumulation during the cyclic application of loads. The results show that, by increasing the number of cycles of load applied, the area subject to the maximum strain in the vicinity of the radius of curvature of the weld toe, tends to increase, as index of the growing damage induced on the component. By thermographic examinations it was possible to investigate the trend of the surface temperature of the specimen, with particular attention to the welded area. As evidenced by thermal images, the modes of failure detected are different. In particular, it has been found that the fatigue cracks are generated in correspondence of the radius of curvature, along the horizontal plate, then propagate initially in the direction perpendicular to the plate itself, and thereafter change direction propagating longitudinally up to the complete break (as shown in Fig. 11, P max = 20 kN). In other cases the cracks are generated from the radius of curvature in the vertical plate, and propagate transversely in the upper joint (Fig. 12, P max = 30 kN). During the tests it was found that the cracks propagate in the surrounding area in which the maximum temperature increase was recorded, which coincides with the areas in which the maximum strain was observed (Fig. 9 and Fig. 12). The time profile of the temperature, obtained from the infrared images, shows the typical trend of the  T -N curve (Fig. 13) that, for higher levels of applied stress, with respect to the fatigue resistance, is characterized by three phases: an initial increase of the temperature (phase I), the maintenance of an approximately constant value of temperature increase ΔT AS (Phase II) and a sudden increase in temperature as soon as the plastic deformations become relevant, bringing the test specimen to fracture (phase III). F