Issue 41

S. Beretta et alii, Frattura ed Integrità Strutturale, 41 (2017) 269-276; DOI: 10.3221/IGF-ESIS.41.36 274 Similar trends can be observed for the analyses performed for the SENT geometry, Fig. 4. For field-of-view dimensions lower than d<0.2mm the most accurate SIF estimations are given by the single and 2-terms regressions. Again, the regressions presented refer to an idealized linear elastic continuum and do not consider the local plasticity arising from the crack tip. The 3-terms regression is providing also for the SENT geometry the most accurate results for field-of-views larger than d>0.2mm . The adoption of larger number of terms to fit the Williams expansion do not improve further the SIF accuracy as already shown by M. Mokhtari and coauthors [1]. E XPERIMENTS he experimental set-up adopted in the present work is shown in Fig. 5a. A Leica DFC290HD camera was used in conjunction with Optem lens in order to capture real-time in-situ DIC images. The specimens were machined by electrical discharge machining (EDM) to a dog-bone geometry with gauge section of 6mm x 3mm and a lateral notch with 1mm depth. The specimen surfaces in proximity of the notch tip were prepared for DIC measurements. Initially, the target surface was polished with abrasive paper up to a grit of P2500. Successively, a fine speckle pattern was produced using an IWATA airbrush and black paint. A very refined pattern is required for the present analysis, and this was accomplished by using the airbrush with a 0.18mm wide needle. The image resolutions available with this speckle pattern range from 2.13µm/px to 0.53µm/px. The tested alloy is a quenched and tempered high strength 30NiCrMoV12 steel with Young’s modulus equal to 207 GPa, yield stress σ Y =945MPa and ultimate tensile stress σ u =1023MPa. Figure 5 : Experimental set-up for DIC measurements: a) The optical microscope, the hydraulic load frame, and the specimen geometry adopted for the present study; b) the crack propagation is affected by the crack closure, the analysis is based on the effective SIF range ΔK eff . The specimens were cyclically loaded by means of a servo-hydraulic MTS Landmark 810 load frame. The load ratio R was set to 0.05. Initially, the specimens were pre-cracked to a crack length of 1.7mm. Successively, the crack was grown from 1.7mm to a final crack length of 2.58mm and different DIC measurements were performed at different crack advancements (Tab. 1). a [mm] K theo [MPa√m] K open [MPa√m] (2-terms) K open /K theo (2-terms) K open [MPa√m] (3-terms) K open /K theo (3-terms) 1.70 18.65 7.46 0.4 8.01 0.43 1.83 20.25 7.49 0.37 8.50 0.42 1.95 21.83 7.86 0.36 8.30 0.38 2.17 25.09 9.78 0.39 9.53 0.38 2.58 33.77 13.85 0.41 14.18 0.42 Table 1 : SIF measurements by DIC displacement field measurements during crack advance. Fig. 5b depicts the DIC acquisition strategy performed at different crack lengths. The images were acquired during two load cycles. The reference image for correlation is selected as the image captured at minimum load. The images captured T