17 Characterizing Dynamics of Additively Manufactured Parts 173 17.1.2 Digital Image Correlation Background Digital Image Correlation (DIC) employs cameras and unique speckle patterns to quantify slight displacements of specimens during testing, resulting in a highly accurate record of a part’s displacement and strain fields [25]. This record is then used to extract mode shapes and relevant frequencies. Extraction of full field strain measurements and modal responses using the DIC algorithm proceeds as follows [26]. A visible, usually planar, surface of the specimen is covered in a speckle pattern, which must be highly contrasting, detailed and aperiodic. After images are taken, the software divides the speckle area into subsets, each of which has a unique intensity signature. A shape function is used as an approximate spatial filter, which is moved over the whole specimen area in search of the best match. That best match provides the displacement estimate. Smaller subsets (requiring more and/or smaller speckles) permit the computation of displacements at higher spatial resolution. Secondary quantities like strain, velocity and acceleration can be calculated from displacement, and time histories from video support the use of Fourier methods. DIC with one camera allows for 2D imaging and the addition of a second stereo camera allows for 3D imaging. 2D imaging is generally cheaper and easier to set up, but limits analysis to in-plane motion. For more general displacements across multiple degrees of freedom the 3D option is preferable. DIC can be used to perform non-destructive testing of lattice structures in AM parts [20]. This technique has proven to be particularly useful for the early detection of points of failure in AM parts, an important step forward in quality control [27]. 17.1.3 Objectives The wide range of processes included in AM along with a variety of machines for each process, a multitude of parameters that can be varied on each machine when creating nominally identical parts, and a virtually infinite selection of internal part geometries lead to high levels of uncertainty in material properties. While efforts have been made to quantify AM mechanical properties by static testing, much uncertainty remains regarding effects on dynamic response. Full-field measurements from DIC have been proven useful for characterizing mechanical properties from dynamic tests [28] for both isotropic and orthotropic materials [29]. This paper seeks to advance the knowledge of anisotropic build effects on dynamic behavior of AM parts, employing DIC and modal analysis along with systematic comparison to numerical models of part dynamics. Selection of print parameters, including build orientation and internal lattice is first discussed followed by details of the DIC set up and experimental methodology. Development of the accompanying finite element model is presented next. Effective bulk properties of the AM parts are determined through calibration of the model to experimentally measure modal frequencies. Finally, a discussion of the print parameter effects is provided with accompanying recommendations for further investigation. 17.2 Methodology 17.2.1 Design of Test Specimens Test specimens used for the series of experiments herein are built with ABSplus-P340 polymer using a uPrint SE Plus [30], or with 316L stainless steel using a Concept Laser M2, which is a direct metal laser sintering machine [31]. The parts are designed to be rectangular cantilevers with high aspect ratio cross-sections to obtain relatively large displacements normal to the largest face and provide a wide, flat surface convenient for DIC speckling. Three solid ABS parts are printed with the dimensions shown in Fig. 17.1. Each part is built in a different orientation, as shown in Fig. 17.2. That is, all three parts in this set have the same outer geometry, but have their build layers oriented in different Cartesian directions. A second set of three ABS parts is printed with internal lattices. Each lattice is a 2D cell pattern printed along the build orientation, as shown in Fig. 17.3. Thus the second set of parts corresponds to the same build orientations as in Fig. 17.2, with lattices oriented vertically with respect to the base plate. The lattices are only built in the cantilever section of each part—the base is left solid. The 2D cell pattern consists of 1.5 mm square holes separated by 1 mm solid walls, and surrounded by a 1 mm thick external wall. Figure 17.4 shows the outer dimensions of the lattice ABS set, which are increased from that of the solid ABS parts to allow for the internal lattice structures.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==