Variability, heterogeneity, and anisotropy in the quasi‐static response of laser sintered PA12 components

Abstract

Additive manufacturing (AM) receives an increasing industrial interest thanks to its advantages in the economic production of highly complex and small-series components. Especially laser sintering (LS) is in this context of particular interest for the production of plastic components, as it is generally deemed the most robust AM technology for polymer parts and therefore is expected to enable AM for functional components in the near future. However, to date, designers are often confronted with a severe lack of knowledge on the possible mechanical behavior of AM components. More specifically, the unit-to-unit variability, heterogeneity (within-part variation), and anisotropy of the mechanical properties very often prove to be substantial and therefore require more elaborated studies in order to take these effects into account in the engineering of reliable components. Moreover, typical experimental results that are used for the determination of the elastic stiffness tensor are subject to variability, caused by the influence of the difference in thermal history between produced parts. This work therefore focuses first on the identification and quantification of the variability and heterogeneity in the quasi-static response of laser sintering-polyamide 12 (LS-PA12) components. Second, also the anisotropy in this quasi-static response is studied. For the first part, uniaxial tensile tests are performed and the variability on the quasi-static properties is quantified by means of statistical analysis. Also, the elastic stiffness tensor is identified based on these tests. Next, the heterogeneity in the tested specimens is investigated by means of digital image correlation. Finally, in order to study the anisotropy in the quasi-static properties, the Virtual Fields Method is applied to determine the variability in the elastic stiffness tensor of the LS-PA12 material. A variability with a coefficient of variance of up to 6.5% on Young's modulus was measured. It was also found that the production planning has an important influence on the homogeneity of the mechanical properties of the produced parts. Finally, the Virtual Fields Method showed that, contrary to most literature on the topic, the elastic properties of LS-PA12 material is best described using an isotropic material model.

Material modelling and property mapping for structural FEA of thin-walled additively manufactured components

“…In contrast, for n 90 an indirect proportional relation to t is apparent. The magnitude as well as the finding that n 0 exceeds n 90 for bulk specimens, is supported by recent research (Lammens et al 2017;Faes et al 2017). While no explanation was stated in the previous studies, the differences in Poisson's ratio between orientations are likely related to the orientation of sintered layers that restrict transversal contraction to different extents.…”

“…Therefore, a stereo digital image correlation (DIC) system computed the strain fields in the gauge length of the specimens, by frame-wise comparison of an un-deformed reference image with the displacement in a speckle pattern that was applied to the coupon surface using an airbrush. This yields the strain in principal 1 1 and traversal coupon axes 1 2 , which based on n = −1 2 /1 1 allows a contactless evaluation of the Poisson's ratios ν for the different build angles (Faes et al 2017;Lammens et al 2017). The DIC setup consisted of two monochrome 5 MP cameras (Grasshopper3, FLIR Integrated Imaging Solutions, Inc., Wilsonsville, OR, USA), recording at a frame rate of 2 Hz in conjunction with the post-processing software Vic-3D 9 (Correlated Solutions Inc., Irmo, SC, USA).…”

“…an approach proposed by Huber (1923) that was shown to be an adequate estimate in additively manufactured PA12 (Faes et al 2017;Lindberg et al 2018). Ultimately, the tcl-application automatically generates independent material properties and maps them to the respective elements.…”

“…inter-layer). Young's modulus as well as Poisson's ratio can be directly determined in uniaxial tensile tests and thereof, a reasonable approximation of the shear modulus G ⊥ can be calculated (Faes et al 2017;Lindberg et al 2018) (see Section 2.4.2). Another characteristic of additively manufactured structures is the drastic dependency on local part thickness, which was reported not only for LS (Tasch et al 2018;Wörz and Drummer 2018;Sindinger et al 2020) but also for other polymer-based processes (Bell and Siegmund 2018) and common metal-based AM technologies including selective laser melting (Brown, Everhart, and Dinardo 2016;Barba et al 2020), electron beam melting (Algardh et al 2016;Wang et al 2020b) and laser powder bed fusion (Roach et al 2020).…”

Hybrid spatial uncertainty analysis for the estimation of imprecise failure probabilities in Laser Sintered PA-12 parts

“…Note that these phenomena are not mutually exclusive [3]. Even for PA-12, the most researched LS material, a considerable amount of non-determinism in the mechanical and dimensional properties was found in previous work (see e.g., [4,5,6]). At the micro-scale, for example, the mechanical material behavior is mainly affected by the local amount of pores and their topology, caused by insufficient sintering and limited powder flow due to short processing times [6,7].…”