Issue 33

M.-L. Zhu et alii, Frattura ed Integrità Strutturale, 33 (2015) 67-72; DOI: 10.3221/IGF-ESIS.33.09 68 EXPERIMENTAL METHODS he material studied is stainless steel 316L, which has yield strength of 280 MPa. An average grain size was measured as approximately 17 μm. A standard compact-tension specimen was used, with a width of 60 mm, a thickness of 7 mm and a machined notch size of 12 mm. Pre-cracking was carried out under load-control using a load shedding scheme. The maximum load was decreased manually step-by-step from 10 kN to 6 kN, and the load was maintained constant at each step. The load ratio and loading frequency were 0.1 and 10 Hz, respectively, during the entire pre-cracking process. Crack growth was monitored by both the direct current potential-drop (DCPD) technique and surface replicas. The latter readings were taken as the true surface crack lengths whilst crack lengths from DCPD readings are indicative of the average crack lengths which were used to calculate the stress intensity factor K . The pre-cracking was terminated when the crack length reached 15 mm, after which a crack growth test was conducted for work reported in [9]. Further crack growth was allowed to remove the influence of pre-history and the original crack length in this work was 24.15 mm ( a /W ≈ 0.4). A random speckle pattern was applied on to one of the specimen surfaces with graphite powder deposited on a white paint background. The random speckle pattern generated from the current study may be described by its grey level intensity profile, which was a bell-shaped distribution and was deemed appropriate for image correlation purposes. The imaging system (L A V ISION , G MB H) consists of a CCD camera (2456 × 2058 pixels) and a Schneider Kreuznach F2.8 50mm lens with 100mm extension tubes. A region of interest, a rectangle of 1.67  1.4 mm with the crack tip in the centre, was selected for imaging in order to capture the near-tip strain data ahead and behind the crack tip. A resolution of 0.68 µm/pixel was achieved. An increasing loading scheme was applied to grow the crack into a steady-state condition at a load ratio of 0.1, from P max = 6 kN to 8.8 kN at a step about 10%. The loading waveform is trapezoidal with a 10 second loading/unloading and a 2 second hold at minimum and maximum loads. During the loading cycle, 23 images were collected during loading/unloading at a frequency of one image per second. Optical Microscopy (OM) was used to monitor the crack in situ and verify the crack tip position and the crack growth morphology. D ETERMINATION OF THE CRACK TIP n accurate determination of the crack tip position during crack growth is important in the DIC analysis in order to capture accurately the strains near the crack tip. This is especially true when the resolution of the image is limited by the pixel size. In this work, we propose a method for locating the crack tip by a combination of information from OM and the displacement distribution from DIC analysis. Fig. 1 presents a flow chart of the method. Firstly, determine the horizontal position of the crack tip x 0 (in pixel) from the reference image collected by the DIC system and the image from the optical microscopy OM. The value x 0 may be determined from the reference image facilitated by the neighbouring speckles. Secondly, calculate the average displacement value ܸ ௬ ഥ from the full data set of the displacement component in the Y direction. The values of V y were obtained from image correlation between P max and P min . The value of y 0 for the crack tip may be obtained when V y (x 0 , y 0 ) is equal to ܸ ௬ ഥ . The crack tip location (x 0 , y 0 ) is thus determined. The rigid body displacement was removed prior to this operation. In the case of stationary cracks, the crack tip location thus determined is fixed when the same reference image is used for subsequent correlation of deformed images. R ESULTS AND DISCUSSION o investigate the evolution of strain fields near the fatigue crack tips, a region of interest, centred at the crack tip, was imaged and analysed using the LaVision software. The correlation procedure was carried out from images collected in situ during the fatigue crack growth tests. The ranges of deformation and strain for each cycle were determined by correlating the deformed image at maximum load relative to a reference image, which was taken at minimum load at the beginning of each test. In addition, DIC analysis of several images collected under zero load was also carried out to assess the baseline errors for both displacement and strain. A subset size of 49 pixels by 49 pixels, or 33 µm × 33 µm, was chosen with a step size of 12 pixels, or 8.16 µm, in the DIC analysis. T A T