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1. Additive manufacturing of stiff and strong structures by leveraging printing-induced strength anisotropy in topology optimization

   https://doi.org/10.1016/j.addma.2023.103730  

   Kundu, Rahul Dev; Zhang, Xiaojia Shelly (August 2023, Additive Manufacturing)

   Anisotropy in additive manufacturing (AM), particularly in the material extrusion process, plays a crucial role in determining the actual structural performance, including the stiffness and strength of the printed parts. Unless accounted for, anisotropy can compromise the objective performance of topology-optimized structures and allow premature failures for stress-sensitive design domains. This study harnesses process-induced anisotropy in material extrusion-based 3D printing to design and fabricate stiff, strong, and lightweight structures using a two-step framework. First, an AM-oriented anisotropic strength-based topology optimization formulation optimizes the structural geometry and infill orientations, while assuming both anisotropic (i.e., transversely isotropic) and isotropic infill types as candidate material phases. The dissimilar stiffness and strength interpolation schemes in the formulation allow for the optimized allocation of anisotropic and isotropic material phases in the design domain while satisfying their respective Tsai–Wu and von Mises stress constraints. Second, a suitable fabrication methodology realizes anisotropic and isotropic material phases with appropriate infill density, controlled print path (i.e., infill directions), and strong interfaces of dissimilar material phases. Experimental investigations show up to 37% improved stiffness and 100% improved strength per mass for the optimized and fabricated structures. The anisotropic strength-based optimization improves load-carrying capacity by simultaneous infill alignment along the stress paths and topological adaptation in response to high stress concentration. The adopted interface fabrication methodology strengthens comparatively weaker anisotropic joints with minimal additional material usage and multi-axial infill patterns. Furthermore, numerically predicted failure locations agree with experimental observations. The demonstrated framework is general and can potentially be adopted for other additive manufacturing processes that exhibit anisotropy, such as fiber composites. 

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2. Exploring the Effect of Specimen Size on Elastic Properties of Fused-Filament-Fabrication-Printed Polycarbonate and Thermoplastic Polyurethane

   https://doi.org/10.3390/ma17112677  

   Chadha, Charul; Olaivar, Gabriel; Mahrous, Mahmoud A; Patterson, Albert E; Jasiuk, Iwona (June 2024, Materials)

   Additive manufacturing (AM) is often used to create designs inspired by topology optimization and biological structures, yielding unique cross-sectional geometries spanning across scales. However, manufacturing defects intrinsic to AM can affect material properties, limiting the applicability of a uniform material model across diverse cross-sections. To examine this phenomenon, this paper explores the influence of specimen size and layer height on the compressive modulus of polycarbonate (PC) and thermoplastic polyurethane (TPU) specimens fabricated using fused filament fabrication (FFF). Micro-computed tomography imaging and compression testing were conducted on the printed samples. The results indicate that while variations in the modulus were statistically significant due to both layer height and size of the specimen in TPU, variations in PC were only statistically significant due to layer height. The highest elastic modulus was observed at a 0.2 mm layer height for both materials across different sizes. These findings offer valuable insights into design components for FFF, emphasizing the importance of considering mechanical property variations due to feature size, especially in TPU. Furthermore, locations with a higher probability of failure are recommended to be printed closer to the print bed, especially for TPU, because of the lower void volume fraction observed near the heated print bed. 

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3. High-k, Low-Loss Ceramic-Thermoplastic Composite Feedstock Filaments for Fused Deposition Modeling of Microwave and mm-Wave Devices

   https://doi.org/10.5772/intechopen.1008537  

   Kosamiya, Vishvajitsinh; Wang, Jing (January 2025, IntechOpen)

   Maturing of additive manufacturing (AM) techniques has increased their utilization for fabricating radio frequency (RF) and microwave devices. Solid composites used in material extrusion AM have experienced considerable expansion over the past decade, incorporating functional properties into 3D-printed objects. There are encouraging indications from AM material research that electrically efficient AM materials can be discovered. These materials would be useful for producing microwave components in the future. One of the enabling techniques for fabricating these materials is to incorporate nano/microparticles or fillers into thermoplastic material. Composite material 3D printing is a novel approach to managing materials’ microwave properties. While extrinsic qualities (effective permittivity) can be controlled by shape and porosity management, intrinsic attributes are tied to the composition of composites. Furthermore, combining various materials to increase the spectrum of available microwave characteristics is made possible by multi-material 3D printing. In this chapter, we explore different methodologies to fabricate ceramic/thermoplastic composites for fused deposition modeling (FDM) of RF and microwave devices. Analytical models for predicting effective permittivity of the composite are discussed and application examples of FDM printed RF, microwave and mm-wave devices employing composites are presented. 

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4. Surface Topology as Non-Destructive Proxy for Tensile Strength of Plastic Parts from Filament-based Material Extrusion

   Harbinson, B.; Yost, S.F.; Vogt, B.D. (January 2023, Progress in additive manufacturing)

   Non-destructive characterization of 3D printed parts is critical for quality control and adoption of additive manufacturing (AM). The low-cost driver for AM of thermoplastics, typically through material extrusion AM (MEAM), challenges the integration of real-time, operando characterization and control schemes that have been developed for metals. Here, we demonstrate that the surface topology determined from optical profilometry provides information about the mechanical response of the printed part using commercial ABS filaments through calibration based correlations. The influence of layer thickness was examined on the tensile properties of MEAM ABS. Surface topology was converted into amplitude spectra using fast Fourier transforms. The scatter in the tensile strength of the replicate samples was well represented by the differences in the amplitude of the two fundamental waves that describe the periodicity of the printed roads. These results suggest that information about previously printed layers is transferred to subsequent layers that can be resolved from optical profilometry and offers the potential of a rapid, nondestructive post-print characterization for improved quality control. 

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5. Investigation of the Homogenized Anisotropy for the Triply Periodic Minimal Surface Lattice under Varying Applied Stress Orientation

   Wei, Chongyi; Smith, DE (September 2024, 2024 Solid Freeform Fabrication Conference)

   The advance of additive manufacturing makes it possible to design spatially varying lattice structures with complex geometric configurations. The homogenized elastic properties of these periodic lattice structures are known to deviate significantly from isotropic behavior where orthotropic material symmetry is often assumed. This paper addresses the need for a robust homogenization method for evaluating anisotropy of periodic lattice structures including an understanding of how the elastic properties transform under rotation. Here, periodic boundary conditions are applied on two-material representative volume element (RVE) finite element models to evaluate the complete homogenized stiffness tensor. A constrained multi-output regression approach is proposed to evaluate the elasticity tensor components under any assumed material symmetry model. This approach is applied to various lattice structures including scaffold and surface-based Triply Periodic Minimal Surface (TPMS). Our approach is used to assess the accuracy of rotation for assumed anisotropic and orthotropic homogenized material models over a range of lattice structures. 

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