PSI - Issue 18

Mehdi Mokhtarishirazabad et al. / Procedia Structural Integrity 18 (2019) 457–471 M. Mokhtarishirazabad / Structural Integrity Procedia 00 (2019) 000–000

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The compliance method was used to measure the crack length during the tests and to compare with values observed by DIC on the surface of the samples. Since the total crack mouth opening displacements in non-standard specimens were higher than the maximum travel range of the available crack mouth opening displacement (CMOD) gauges, a video extensometer (iMetrum Ltd) was used. The load was applied by a hydraulic Instron 1342 testing frame with a 250 kN capacity equipped with a load cell of the same capacity. Samples at the extreme end of each condition were heat tinted at 450 ºC for 30 minutes and then broken open to measure the physical crack length (Δa p ) and study the fracture surface. Full-field displacement around the crack tip was measured using a 3D Digital Image Correlation (DIC) system (LaVision Ltd). Images with 2254 × 2054 pixels resolution were captured over the duration of the test. LaVision cameras with 5 MP CCD and 12 bit depth and Tokina 100mm F2.8 Macro lenses with a working distance of 800 mm were used. Field of view (FOV) 47 × 38 mm2 was obtained. In order to provide imaging contrast, a fine speckle pattern was applied to the surface of the specimens using spray paint. The setup of test equipment and an example of the obtained images are shown in Fig.4. Image post-processing was done by Davis version 8.4.0 to extract the full field displacement around the crack tip. The results of DIC analysis will be presented in a separate work.

Fig. 4. (a) the setup of the 3D DIC equipment and (b) the acquired image of the surface of the sample.

3. Results and Discussion 3.1. Tensile properties

Table 2 shows the results of the tensile tests on 316L stainless steel alloy used in this study. An extraordinary strain hardening capacity was observed. That is the ratio of σ uts over σ ys was about 2.5 while the elongation at maximum load was around 50%. In addition, the elongation at fracture was about 80%. An average of the tensile properties of these samples was used in this study.

Table 2. Tensile properties of the 316L stainless steel Sample σ ys (0.2% offset), MPa

σ UTS , MPa

Elongation after Fracture, % E, GPa

TT-1 TT-2 TT-3

249 249 245

611 609 609

82 81 83

-

207 202

3.2. Fracture of CT specimen Fig. 5 shows a CT sample after the standard loading sequence is completed. Visual inspection showed that the un cracked ligament of the specimen had plastically deformed. It was also observed that the pin-holes had visibly deformed. Both these conditions invalidate the evaluations of fracture toughness by ASTM, therefore testing the CT samples was not completed. The possibility of specimen failure by plastic collapse was considered in the analysis. Failure of plane-sided CT specimen by plastic collapse has been previously reported by Wasylyk et al. (Wasylyk and Sherry 2010).