PSI - Issue 51

Maja Dundović et al. / Procedia Structural Integrity 51 (2023) 192 – 198 M. Dundovi ć et al. / Structural Integrity Procedia 00 (2019) 000–000

193

2

1. Introduction The mobility of compliant mechanisms, unlike conventional rigid body mechanisms, is fully or partially due to the elastic deformation of their compliant members. Today they are in widespread use in precision engineering, Choi et al. (2008), robotics, Mutlu et al. (2016), (bio)medical applications, Thomas et al. (2021) and Martin and Robert (2011), household appliances, etc. Compliant mechanisms have a considerably reduced number of parts with regard to conventional mechanisms, their assemblies are in general simple, compact and reliable, and in special cases may be even monolithic. The majority of their advantages, such as high accuracy, precision and resolution of movement, absence of wear, friction and backlash, no need for lubrication, no maintenance requirements, applicability in extreme environmental conditions, etc. results from this reduced number of parts. The main drawback of the application of compliant mechanisms lies in complex manufacturing, combined with complicated and iterative design process and kinematics caused by different structural nonlinearities, Zhu et al. (2020) and Hao et al. (2016). The advent of additive manufacturing technologies has made some compliant mechanisms possible, notably complex monolithic and thin walled variants of compliant mechanisms, as shown in Merriam et al. (2013) and Cuellar et al. (2018). Therefore, research into the properties and behavior of innovative materials used in additive manufacturing (AM) technologies and their applicability in such designs is appropriate, Ligon et al. (2017) and Li et al. (2019). In order to enable the simulation of the behavior of compliant mechanisms manufactured by AM technologies, an efficient numerical model for their compliant members, with either concentrated (i.e. short elements or flexure hinges) or distributed (i.e. long or leaf spring elements) compliance, should be developed first, Yong et al. (2008) and Lobontiu et al. (2000). The selection of materials for compliant mechanism design, although application dependent, can be guided by some general principles. In contrast to most other applications, the materials are chosen to maximize flexibility rather than stiffness, and the material with the highest strength to modulus ratio will allow a larger deflection before failure. This ratio is one of the most important criteria in material selection for compliant mechanisms applications. Polymers have a high strength to modulus ratio (e.g. the ratio for polymers is roughly twenty times larger than that of steel) and therefore their usage in high volume compliant mechanisms is justified, Howell (2001). Polymeric materials also have several drawbacks for application in compliant mechanisms, such as low fatigue strength, dependency of material properties on the loading rate, susceptibility to creep, stress relaxation and material nonlinearities, Cazin et al. (2020) Baragetti et al. (2020) and Matvienko (2020). Digital Light Processing (DLP), more precisely the bottom-up process, has been selected for manufacture of the test specimens used in the research performed in this article. DLP is a form of vat polymerization AM technology, a faster alternative to both stereolithography and polyjet technology, in which an entire layer is cured at once by means of an image projected by a DLP device as the UV light source, Liska et al. (2007) and Patel et al. (2017). As with most AM technologies, the application of the bottom-up DLP process results in anisotropic material properties, Monzón et al. (2017). The anisotropy in this case is challenging when modeling deformation behavior, and it is caused by variations of light intensity between the center and the border of each pixel of projected images, and residual stress resulting from the forces induced by the separation of the part from the vat after the curing of each layer. However, the manufactured parts can have negligible anisotropy in some cases, depending on material and process parameters used. A schematic representation of the bottom-up DLP technology, a batch of printed corner-filleted specimens and a characteristic stair like surface roughness of the printed specimens are shown in Fig. 1. In order to enable photoelastic observations the samples had to be printed using the transparent material. Photoelasticity is an optical technique for experimental stress/strain analysis, which uses the temporary birefringence exhibited by most transparent materials when subjected to strain. The fringes in the interferogram produced on samples stressed under the circular polariscope, known as isochromatics, are contours representing points of the same difference in principal strains, Ramesh and Sasikumar (2020) and Ajovalasit et al. (2015). Photoelasticity is used for experimental validation of numerical stress models particularly in the study of residual stress, Aben et al. (2008) and Okioga et al. (2014), fracture dynamics, Monka et al. (2019), stress concentration, Kožar et al. (2020), contact, Franulović et al. (2017), etc. Nowadays, with advances in digital photoelasticity and AM techniques, the major drawback in time and effort required for fringe analysis and model preparation have been resolved, Hurley et al. (2014) and Ju et al. (2017). Although, the quantitative data can be obtained using numerous methods, this tends to be cumbersome, and in this paper only the qualitative dynamic photoelastic study of corner-filleted flexure hinges produced by DLP technology from the acrylic based photopolymer resin is presented.