Conference Paper

Designing low frequency bandgaps in additively manufactured parts using internal resonators

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... [2][3][4]) and developing lattice structures for specific applications, such as biomedical implants [5][6][7], heat exchangers [8,9] and sandwich structures for lightweighting of structural engineering components [10,11]. The utility of lattice structures extends to precision engineering applications, where vibration isolating lattice structures have been designed for some low frequency bandwidths [12,13]. Such designs could be incorporated into machine frames in an attempt to reduce the noise in a measurement system. ...
... This work aims to provide a framework for efficient generation of tetrahedral meshes of lattice structures suitable for parametric studies of a wide range of manufacturing defects. Although the authors' main objective is to support the development of machine frames [12,13], this framework can be generally applied to FE studies of strut-based lattice structures. [15] (b) radius variation [15] (c) Left: original unit cell design, Right: texture bias in the manufactured part [16]. ...
Article
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Strut-based lattice structures produced by powder bed fusion are prone to characteristic manufacturing defects that alter both their form and surface texture. Most studies in the literature focus on a subset of commonly observed defects, typically radius variation and strut waviness; surface defects remain relatively unexplored. Furthermore, there remains a need for the development of a general finite element modelling framework that can implement a range of defects into any strut-based lattice design. This paper presents a modelling framework for implementing a range of both form and surface defects into finite element meshes of strut-based lattices. A signed distance function forms the foundation for this framework, upon which surface meshes can be modified and converted into tetrahedral meshes via open-source software. The paper demonstrates how radius variation, strut waviness, elliptical cross sections and localised surface defects can be modelled in lattice struts, for which intuitive mathematical definitions are provided. A parametric study is performed to assess the sensitivity of the compressive Young’s modulus of BCCZ and octet-truss lattices to upskin and downskin surface defects. The results showed higher sensitivity in the octet-truss than in BCCZ; both designs were more sensitive to downskin than to upskin defects.
... The results for WP1 can be seen in Chapter 5, results for WP2 are presented in Chapter 6, and the results of WP3 are available in Chapter 7 and Chapter 8. The methodology presented in this chapter formed part of these publications[54,56,[159][160][161][162][163]. ...
Thesis
Advancements in additive manufacturing technology have allowed the realisation of geometrically complex structures with enhanced capabilities in comparison to solid structures. One of these capabilities is vibration attenuation which is of paramount importance for the precision and accuracy of metrology and machining instruments. In this project, new additively manufactured lattice structures are proposed for achieving vibration attenuation. The ability of these lattices to provide vibration attenuation at frequencies greater than their natural frequency was studied first. This is referred to as vibration isolation. For the vibration isolation study, a combination of finite element modelling and an experimental setup comprising a dynamic shaker and laser vibrometer was used. The natural frequencies obtained from the experimental results were 93 % in agreement with the simulated results. However, vibration attenuation was demonstrated only along one dimension and vibration waves were allowed to propagate, meaning the transmissibility was allowed to be greater than 0 dB. To achieve lower transmissibility, the project demonstrated that lattice structures can develop Bragg-scattering and internal resonance bandgaps. The bandgaps were identified from the lattices' dispersion curves calculated using a finite element based wave propagation modelling technique. Triply periodic minimal surface lattices and strut-based lattices developed Bragg-scattering bandgaps with a normalised bandgap frequency (wavelength divided by cell size) of ~ 0.2. The bandgap of the tested lattices was demonstrated to be tunable with the volume fraction of the lattice unit cell, thus, providing a tool to design lattice structures with bandgaps at required frequencies. An internal resonance mechanism in the form of a solid cube or sphere with struts was designed into the inner core of the unit cell of strut-based lattices. These new internal resonance lattices can provide (a) lower frequency bandgaps than Bragg-scattering lattices within the same design volume, and/or (b) comparable bandgaps frequencies with reduced unit cell dimensions. In comparison to lattices of higher normalised bandgap frequencies, lattices with lower normalised bandgap frequencies have cell sizes that are more suitable for manufacturing with the current additive manufacturing technologies and have higher periodicity within a constrained design volume, resulting in higher attenuation within the bandgaps and more homogenous structures. Similar to the Bragg-scattering lattices, the bandgaps of the internal resonance lattices were demonstrated to be tunable through modification of the geometry of the lattice unit cell. The internal resonance lattice experimentally demonstrated a bandgap of normalised frequency between 0.039 to 0.067 and an attenuation of up to -77 dB. These results are essential for engineering vibration attenuation capabilities within the macro-scale of materials for complete elimination of all mechanical vibration waves at tailorable frequencies. Future work will include further reduction of the bandgap frequencies and increasing the bandgap width by exploring new unit cell designs and new materials for additive manufacturing.
... The user has less design control with TPMS unit cells, however they are currently studied for applications such as biomedical implants [12]. Additionally, there are custom designs outside of strut-based and surface-based forms, for example those produced using topology optimisation [31] and with internal resonators for vibration isolation [32]. ...
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Additively manufactured lattice structures are popular due to their desirable properties, such as high specific stiffness and high surface area, and are being explored for several applications including aerospace components, heat exchangers and biomedical implants. The complexity of lattices challenges the fabrication limits of additive manufacturing processes and thus, lattices are particularly prone to manufacturing defects. This paper presents a review of defects in lattice structures produced by powder bed fusion processes. The review focuses on the effects of lattice design on dimensional inaccuracies, surface texture and porosity. The design constraints on lattice structures are also reviewed, as these can help to discourage defect formation. Appropriate process parameters, post-processing techniques and measurement methods are also discussed. The information presented in this paper contributes towards a deeper understanding of defects in lattice structures, aiming to improve the quality and performance of future designs.
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Chapter
The classical distinction between a structure and a material is increasingly blurred in lattice materials made from beam and shell-like structures. This chapter is concerned with the elastodynamic response of 1D and 2D lattice materials. Of particular interest is to understand elastic wave propagation and related phenomena unique to periodic structures. Starting from 1D structures, the chapter ends with 2D lattice materials, establishing relevant connections between the solid-state physics and structural dynamics literature along the way. The Floquet–Bloch theory from solid-state physics is applied in combination with the finite-element (FE) formulations that are widely used in structural dynamics and other engineering disciplines. Emphasis is given to the synthesis of existing knowledge on wave transmission in periodic materials, from the classical discrete atomic lattices to the rapidly emerging multifunctional lattice materials, and avenues for future development are indicated.
Book
Mechanical Vibrations: Modeling and Measurement describes essential concepts in vibration analysis of mechanical systems. It incorporates the required mathematics, experimental techniques, fundamentals of model analysis, and beam theory into a unified framework that is written to be accessible to undergraduate students, researchers, and practicing engineers. To unify the various concepts, a single experimental platform is used throughout the text. Engineering drawings for the platform are included in an appendix. Additionally, MATLAB programming solutions are integrated into the content throughout the text. © 2012 Springer Science+Business Media, LLC. All rights reserved.
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