this arrangement, 3D shape and surface full-field deformation of Grade X65 steel specimens were measured, at different stages of axial test, until fracture. These data were then compared with the corresponding numerical FE results, making use of constitutive models calibrated on the same material by means of inverse methods in previous studies [20–22]. In this works a standard J2 and a more advanced J2–J3 plasticity models were adopted, and the calibration was achieved with tension, torsion and tension-torsion tests executed with the same equipment. A thorough additional validation of this calibration procedure is carried out, to grant the accuracy of the numerical simulation. The effectiveness of the full field measurement is assessed by comparing the DIC and numerical data. The results reported in this paper prove that the 360-deg DIC technique can be used to provide alternative data for calibration purposes, or be complementary to standard inverse material identification procedures. This is expected to be especially advantageous when local or directional experimental information is required (such for the case of anisotropic material characterization) and that it can reduce the number of the required experimental tests. 30.2 Experimental Setup and Test Methodology An isotropic Grade X65 grade steel for pipeline applications was used for the tests. A biaxial machine was used, capable of axial and torsional loads up to 100 kN and 1000 Nm respectively. The translational and rotational actuators can be controlled independently such that pure tension, pure torsion and mixed tension-torsion tests are possible. Load, torque, displacement and rotation are acquired by a biaxial load cell and digital encoders embedded in the actuators themselves. Additional details on the equipment can be found in [21]. Hollow cylindrical specimens are used, whose geometry is shown in Fig. 30.1a. They have threaded ends with machined parallel faces, to be held by custom-designed grips, thus allowing their correct constraining, as well as the application of tensional and torsional loads. The hollow shape is optimized by trading-off the need of homogenizing the stress and strain states in the radial direction, while avoiding geometrical instabilities. The biaxial machine was equipped with a Nikon D7000 SLR camera with a 60 mm Micro Nikkor lens, mounted on a slewing ring bearing coaxial with the specimen axis (Fig. 30.2). The frame allows a manual positioning of the camera around 14 fixed positions to cover the full 360 view of the specimen surface [23, 24]. In this paper an axial test was performed, measuring the deformation by 3D-DIC technique. Prior to testing, specimens were sprayed with a light gray primer paint as a base, and later with black acrylic paint to provide the speckle pattern needed for correlation. A calibration pattern was fixed on one specimen end (Fig. 30.1). The test was run applying a progressive and quasi-static displacement up to fracture. During the experiment, the actuator was put on hold at different stages to allow the acquisition of the image sequences [24]. The number of stages was selected to have a limited strain variation from one acquisition to another, to obtain accurate DIC matching. Each step corresponded to an estimated (by FE analysis) nominal strain increment (ΔL/L0) of about 0.01 mm/mm. At the end, 30 sequences of 4000 3000 pixel 2 images at different load levels were collected. For each stage, load and displacement were also recorded in synchronization with images timestamps. Optical acquisition was interrupted when the paint started to present appreciable detachments from the specimen surface, R 1,5 10 7 (a) (b) 2 2 0 40 20 SEZ A-A M16 A A 100 16 Fig. 30.1 (a) Specimen geometry. (b) Specimen setup and details of the calibration pattern 226 L. Cortese et al.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==