Issue 51

J. M. Djoudaet alii, Frattura ed Integrità Strutturale, 51 (2020) 534-540; DOI: 10.3221/IGF-ESIS.51.40 535 size of 20 microns in diameter was coupled to digital image correlation (DIC). It has been applied to the case of a SENT structure with a notch made by FDM. The successive images recorded by a digital microscope allow a qualitative analysis of the evolutions of the local strains. The kinematic fields are obtained by DIC. The strain evolutions at the notch’s tip are highlighted. The deformation mechanisms at the local scale are confronted with the macroscopic behavior of the structure. K EYWORDS . Additive manufacturing; 3D printing; Digital Image Correlation; Micro speckle; Fracture mechanics. I NTRODUCTION dditive manufacturing (AM) is a promising principle able to shape complex structures through a layer by layer manufacturing. In recent years, AM has been the subject of several developments both by academic and/or industrial researchers. AM has great potential for the manufacture of several structures that can be integrated into the industrial sectors such as aeronautics, automotive and others [1]. To benefit from its advantages, it is important to characterize accurately the obtained structures to allow a better integration in industrial applications. In the specific case of fused deposition modeling (FDM) technologies, the mechanical behavior of the obtained structures suffers from anisotropy, which is introduced by the weld lines and the air gap in the printed part [2]. Many studies aiming to improve the mechanical properties of these structures have been conducted. Some of these studies focus on the modification of the shape of the printed patterns [3] and others are based on improving 3D printing algorithms to enhance the local fracture toughness of printed thermoplastics [4]. Besides of all these processes of pre-structured material depositions, accurate characterization of the mechanical behaviour of the obtained structure is determinant. The knowledge of their mechanical properties is important for use in the technological application. The mechanical characterization of a material goes through a knowledge of its properties both at macroscopic and local scales. Many technics have been developed to analyze strain evolutions at the local scale. These technics are based on the used periodic or random gratings and have been applied in most of the case in metal specimens [5]–[10]. The periodic gratings consist in arrays of cross lines or of nanoparticles and are deposited by lithographic technics (electron beam lithography, holographic or photolithography). The periodic gratings can be coupled with the technics like Moiré Interferometry [11] or with a specific approach like the one developed in our previous studies [8], [12]. The technics very often associated with the random gratings are Moiré interferometry and digital image correlation (DIC) [13]. In these technics, the roughness of the samples can be used in certain cases. In other cases, paint speckle pattern, deposition and annealing of thin metal layer in order to obtain metal nanoparticles, and lithography technics can be used to deposit gratings [6]. In a very recent study, DIC has been used to measure strains at the surface of 3D printed structures with a large dimension [14]. The DIC analyses the grey levels between two images, one of which is considered as "reference" and the other is "deformed". In this technic, the images used can be obtained by various imaging systems: optical or digital microscopes, scanning electron microscope, atomic force microscope, and others. In addition to the surface kinematic fields, DIC could allow to access the out-of-plane displacements. In the basic version of DIC, the roughness of the surface can be used like a speckle pattern but sometime the contrast of images is not enough. Simple surface preparation may be appropriate depending on material (polymers or alloys) and imaging technique used [5], [6]. The spatial resolution of strains measured by technics using gratings is strongly related to the grating parameters: the size of particles and pitch. With the progress in nanotechnology and nanofabrication, it is possible to deposit gratings with dozens of nanometre of pitch and diameter of nanoparticles. However, it is difficult to use lithographic techniques to deposit gratings on the surface of a polymer structure, because of the solutions used during the nanofabrication process. Moreover, it could be straightforward to image polymer materials by SEM. The key challenge for better understanding mechanisms leading to structure damages or failures remains the development of tools giving direct access to the local scale material behaviour. For a good characterization of polymer structures obtained by AM, it is necessary to develop new techniques to measure local deformations. A