PSI - Issue 5

Behzad V. Farahani et al. / Procedia Structural Integrity 5 (2017) 981–988 Behzad V. Farahani et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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within a digital image processing approach, a novel methodology has been carried out on thinning fringes in photomechanics by (Ramesh 1992). They properly developed an algorithm applicable to any orientations of the fringe. In 2001, (Gonzalez & Woods 2001) published a book described DIC fundamentals and regulation, usage in distinct domain such as the spatial and frequency domain leading to image enhancement, lighting and camera considerations. Currently, DIC is a widely used method in experimental procedures to evaluate the mechanical behaviour of the material, particularly the fracture properties, fatigue and failure modes in a variety of structural components. As an illustration, the stress intensity factor (SIF) can be evaluated by DIC analysis due to its simplicity (Cintrón & Saouma 2008; Hamam et al. 2007; Chen et al. 2015; McCormick & Lord 2010; Pan et al. 2009). In addition, DIC was applied on a hybrid experimental/numerical methodology which a numerical SIF calculation hinges upon a stress field obtained from DIC (Tavares et al. 2015). In the framework of the current study, a proper investigation to access the failure criteria performance using uncoupled or sudden failure criteria has been carried out by (Driemeier et al. 2015). They obtained the experimental data on a bi-failure specimen using a DIC approach in which the numerical models were therefore validated. Nevertheless, the objective of the present study is to experimentally measure deformation field using DIC and to numerically address the performance of a well-known coupled failure GTN criteria. The analyses will be made using an Aluminium alloy AA6061-T6 specimen, designed to generate two distinct failure modes, hence the name bi-failure. The first failure occurs at high stress triaxiality, and the second occurs in a state of almost pure shear, e.g., at low stress triaxiality. These failure modes occur in a simple tensile loading and they generate a complex loading history. The ability of the here studied failure criteria of predicting the load-displacement pattern is seen as a limitation or as a strength of the given studied model. 2. Experimental aspects A bi-failure specimen was tested using a standard servo-hydraulic system, 10 ( ) MTS-812 tensile machine, with a crosshead velocity of 1 ( / ) and a frequency of = 5 ( ) , the sample geometry with its dimension is shown in Fig. 1-a . The specimen thickness is = 1 ( ) . The notation is that the standard bi-failure specimen was machined between the central full holes ( 10) . So, the corresponding distance accounted for = 1.5 ( ) . The torsion issue along the loading direction was thereby avoided. The specimen was painted in white colour and sprinkled with black dots to generate the random pattern for DIC analysis, see Fig. 1-b. The specimen is connected to the tensile machine via specific rods passing through holes positioned in the extremes of the specimen. Any cross motion of the specimen is accommodated by a fixture. Such a motion may take place if one of the notched sides breaks first than the other, if different notch radii were machined. A computer station received the images from two synchronized DIC cameras and the load cell. Then, the cameras were calibrated using a 12-by-9 calibration pattern, 2.5 (mm). Since failure phenomenon occurs in a plane perpendicular to the camera, the failure proceeds in a local fashion with negligible, if some, out of plane deformation. Therefore, it is adequate to deliberate 2D DIC acquisition. The material properties for Aluminum alloy AA6061-T6 are presented in Table 1.

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Fig. 1. Bi-failure specimen, (a) a geometric view of the bi-failure test specimen, dimensions are in millimetres and (b) speckled pattern.