Issue 41

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 41 (2017) 299-306; DOI: 10.3221/IGF-ESIS.41.40 301 the paper is as follows: to estimate the structural volume size R c , to present the crack growth experiments and temperature measurements and finally to use Q* as crack driving force. M ATERIAL AND TESTING METHODS pecimens were prepared from 4-mm-thick, hot-rolled AISI 304L stainless steel specimens. The mechanical properties and the chemical composition are reported in Tab. 1. R p0.2% (MPa) R m (MPa) A (%)  A-1 (MPa) HB C (%) Mn (%) Si (%) Cr (%) Ni (%) P (%) S (%) N (%) 279 620 57 202 170 0.026 1.470 0.370 18.100 8.200 0.034 0.001 0.058 Table 1 : Mechanical properties and chemical composition of the AISI 304L stainless steel. The specimens’ geometry is reported in Fig. 2a-b, which shows the crack starter, consisting of a sharp, 8-mm-deep V- notch. Both specimens’ surfaces were polished by means of emery papers starting from grade 80 up to grade 4000; afterwards, a black paint was applied to one specimen’s surface in order to increase the emissivity in view of the infrared thermal measurements. Push-pull (R=-1), constant amplitude, load controlled crack propagation tests were conducted on a servo-hydraulic Schenck Hydropuls PSA100 fatigue test machine equipped with a TrioSistemi RT3 digital controller (load capacity 100 kN). Load frequencies ranging between 4 and 40 Hz were adopted, depending on the applied stress level. After a fatigue test started, a crack initiated at the notch tip and it was propagated to a total initial crack length of approximately a=9 mm (according to Fig. 1a). Afterwards, crack growth experiments started. The temperature field was measured by means of a FLIRSC7600 infrared camera, having a 50-mm focal lens and equipped with an analog input interface, which was used to synchronize the force signal coming from the load cell with the temperature signal measured by the infrared camera. The infrared camera had a spectral response range from 1.5 to 5.1  m, a noise equivalent temperature difference (NETD) < 25 mK, and operated at a frame rate f acq equal to 200 Hz. A 30-mm spacer ring was adopted to improve the spatial resolution up to 20 or 23  m/pixel. As a result, the Field of View (FoV) was reduced to 320x256 pixels, which corresponds to a minimum of 6.4 mm x 5.1 mm and a maximum of 7.4 mm x 5.9 mm. Figure 2 : specimen’s geometry (r n =0.1 mm for 2  =45°, r n =0.15 mm for 2  =90°; thickness is 4 mm; dimensions in mm) (a,b) and AISI 304L austenitic microstructure (c) . In order to evaluate Q* at a given crack length during the fracture mechanics experiments, a trigger signal was manually sent to the infrared camera to start the acquisition of the thermal images at a frame rate f acq =200Hz for a duration of 5s, translating into 1000 acquired images. Such temperature maps were first processed by using the FLIR MotionByInterpolation tool to allow for the relative motion compensation between the fixed camera lens and the moving specimen subject to cyclic loads (the displacements to compensate ranged from 6 to 20 pixels within the FoV, depending on the stiffness of the specimen). To perform successfully the motion compensation, the force signal coming from the load cell must be sampled synchronously with the thermal images. After that, the spatial distribution of the mean temperature T m (r,  ) was calculated by averaging the available 1000 temperature maps according to Eq. (3) and by means of the ALTAIR 5.90.002 commercial software; finally, the Q* parameter was evaluated by applying Eq.(2). The crack length was measured by means of a AM4115ZT Dino-lite digital microscope operating with a magnification ranging from 20x to 220x. The microscope and the infrared camera monitored the opposite surfaces of the specimens; in S 2  150 2L=90 Machine grip Machine grip 8 w=46 a (a) r n 2  + (b) 50  m (c)