Issue 41

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 41 (2017) 299-306; DOI: 10.3221/IGF-ESIS.41.40 302 particular, the microscope images were used to single-out the crack tip position, which was subsequently reported in the infrared thermal images. A total number of 10 specimens was fatigue tested. THE STRUCTURAL VOLUME SIZE V C he experimental procedure to evaluate the structural volume size R c (see Fig. 1a) was a tricky point, which was tackled in [9] and is summarized here. The underlying concept to evaluate the structural volume size R c , thought of as a material property for a given applied nominal load ratio, is widely adopted in notch fatigue [6,10-14] and, if formalized in the present case, it states that the averaged energy Q* must be the same at the conventional fatigue limit, whatever the geometrical features involved. Alternatively stated, the characteristic energy Q* at the fatigue limit of a plain and a notched (either blunt or severe) specimen must be the same. Figure 3 : temperature field for a specimens having a 9.803-mm-long crack (see crack length definition ‘a’ in Fig. 1a), loaded at  K=32.7 MPa·m 0.5 , f L =35 Hz (a) , and microscope image of the fatigue crack taken on the opposite surface (b) . Figure 4 : Radial temperature profile (a) and specific heat flux distribution measured along the boundary of V c (b) , during tension- compression fatigue test with a=9.803 mm,  K=32.7 MPa·m 0.5 , f L =35 Hz. Unfortunately, at the fatigue limit of the present cracked specimens, the thermal rise close to the crack tip was vanishingly small in relation to the thermal accuracy of the available infrared camera. Therefore, calibration of R c was performed at a fixed finite-life, rather than at the fatigue limit. The chosen reference fatigue life was on the order of 10 5 cycles and the calibration procedure is explained in the following with reference to a single fatigue test. A specimen was pre-cracked to obtain a total crack length a=9.803 mm (according to Fig. 1a). Fig. 3a shows the temperature field calculated by averaging the 1000 available frames, after the motion compensation algorithm has been applied. The crack tip position (see Fig. 3b) could be determined from the digital microscope image captured on the opposite surface. The number of cycles to 308,8 309 309,2 309,4 309,6 309,8 310 0,000 0,001 0,002 0,003 0,004 T m [K] r [m] డ୘ డ୰ ቚ ୰ୀୖ ౙ ൌ -51.9 K/m r=R c  0 ° R c = 5.2·10 -4 m f L = 35 Hz   16 W/(m·K)  K = 32.7 MPa·m 0.5 (a) -90.0° -112.5° -135.0° -157.5° 180.0° 157.5° 135.0° 112.5° 90.0° 67.5° 45.0° 22.5° 0.0° -22.5° -67.5° -45.0° h [W/m 2 ] 10 2 10 10 3 10 4 10 5 T (a) (b) 1 mm  2R c (b) Crack tip