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Energy densities investigated.
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Parts processed by Additive Manufacturing can now be found across a wide range of applications, such as those in the aerospace and automotive industry in which the mechanical response must be optimised. Many of these applications are subjected to high rate or impact loading, yet it is believed that there is no prior research on the strain rate depe...
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Porous structures from aluminum alloys have used in several prominent applications such as bulletproof clothing, fuel cells, automotive, aerospace and heat exchanger products. The low cost and short production time make Direct Metal Laser Sintering (DMLS) as a natural choice over the existing traditional techniques in a production of customized par...
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... Unlike metals or ceramics, polymers possess rather high sensitivity to strain rate as their strength can be double or tripled with the increasing strain rate by several orders in magnitude [29,30]. Despite the increasing number of studies on the mechanical behaviour of polymer PBF parts, research into their rate dependency is still limited [20,27,31,32]. Mehdipour et al. [20] assessed the rate dependency together with the anisotropy of L-PBF printed PA12 tensile dog-bone specimens manufactured in three build orientations; rate dependency of the tensile strength and elongation to break were observed within a low strain rate range on the order from 10 − 3 to 10 − 2 s − 1 . ...
... Mehdipour et al. [20] assessed the rate dependency together with the anisotropy of L-PBF printed PA12 tensile dog-bone specimens manufactured in three build orientations; rate dependency of the tensile strength and elongation to break were observed within a low strain rate range on the order from 10 − 3 to 10 − 2 s − 1 . Cook et al. [31] conducted mechanical testing of L-PBF processed polyamide in a wide range of strain rates (10 − 3 to 10 3 s − 1 ) to construct an Eyring plot of flow stress versus logarithm of strain rate and demonstrated a significant strain-rate dependence. This rate-dependence, often a bi-linear response at high rates, has been shown to be a material property and is due to the movement of the β transition to room temperature at these rates. ...
Laser powder bed fusion (L-PBF) of engineering thermoplastics is a well-established additive manufacturing technique. In this work, an experimental investigation of the tensile-shear behaviour of L-PBF processed polyamide-12 (PA12) is presented, with a particular focus on the concurrent effect of build orientation and rate dependency at strain rates ranging from 10-3 to 103 s-1. Results show that the deformation behaviour features significant anisotropy at both low and high strain rates, while quasi-static ultimate strength in either tension or shear shows no dependence on the build direction. As the strain rate increases, the tensile strength is characterised by apparent positive rate dependence but remains insensitive to the build orientation; however the shear strength displays significant build orientation-dependent anisotropy, highlighted by the decreasing shear strength of specimens printed in the loading direction. Post-mortem scanning electron microscopy (SEM) demonstrates torn dimples with stretched filaments in quasi-static fractography, while cleavage facets with flake structures retrieved from high-rate loading. A fibrillar structure is observed from high-rate torsional loading indicating adiabatic decohesion during rapid crack propagation. These findings draw attention to the significant anisotropy of L-PBF PA12 parts in a high-rate loading regime and provide directions to improve the current L-PBF techniques.
... The mechanical properties of SLS PA12 have been studied by several researchers, focusing on the effect of processing conditions like printing position and direction [3][4][5], and energy density [6,7] on the achieved mechanical properties, where some amount of anisotropy was detected in tensile testing. Other works looked at the response of sintered PA12 to fatigue loading to study creep and relaxation effects [8], crack growth [9], the effect of humidity on crack growth [10], and the strain rate dependency of PA12 as a function of energy density [11]. Despite all the published research, more efforts are needed to fully understand the polymer response obtained under the different process conditions that can exist during SLS. ...
... At even higher strain rates (160-900 s − 1 ), the samples tested with the Hopkinson bar are not in a state of stress equilibrium, and oscillations appear even in the elastic regime, preventing obtaining a reasonable elastic modulus [11,18]. In addition, some oscillations can also be found in the plastic regime due to electronic noise, and the final strain shown in the curves does not correspond to the strain to failure of the material but to the end of the first deformation pulse introduced by the input bar. ...
... From the curves of Fig. 5, it can be observed that the SLS-printed PA12 polymer shows a slight strain softening after yield, which has also been observed for extruded PA12 [11], which is characteristic of a glassy polymer deformed at a temperature below its glass transition temperature (≈53 • C). The degree of strain softening appears to increase at higher strain rates, which has been related to adiabatic heating within the sample [19]. ...
Selective laser sintering (SLS) of polymers has made possible the introduction of lattice-based cells as building blocks of polymer parts, allowing to obtain optimal specific properties. The actual mechanical performance of an SLS part is strongly dependent on the printing direction and part shape. Nevertheless, macroscopic testing is not usually possible due to size restrictions of the fabricated part, as it happens, for example, in lattice-based materials. In this case, microscopic tests performed directly on the lattice surface become fundamental. In this work, the mechanical behavior of SLS PA12 under a wide range of strain rates from 10⁻³ to 10³ s⁻¹ and different printing directions is characterized using macroscopic tests on bulk samples and compared with nanoindentation performed directly on the lattice material surface. An excellent correlation was found between the rate-dependent mechanical behavior obtained at the two scales, validating microscopic techniques for characterizing the mechanical response of SLS fabricated PA12 parts.
... Complex 3D printed porous scaffolds could also be suitable for creating biomimetic structures that could replicate or provide a realistic tumor cell microenvironment for 3D in vitro studies of cancer cells [15]. 2 of 9 Furthermore, new materials and technologies offer promising opportunities in designing better protection equipment. Strain rate sensitive materials suitable for additive manufacturing techniques [16] can be combined with intelligent designs to enhance impact energy absorption. Lightweight lattice structures can be designed to absorb substantial amounts of energy and outperform conventional materials [17][18][19]. ...
A computational method for generating porous materials and composite structures was developed and implemented. The method is based on using 3D Voronoi cells to partition a defined space into segments. The topology of the segments can be controlled by controlling the Voronoi cell set. The geometries can be realized by additive manufacturing methods, and materials can be assigned to each segment. The geometries are generated and processed virtually. The macroscopic mechanical properties of the resulting structures can be tuned by controlling microstructural features. The method is implemented in generating porous and composite structures using polymer filaments i.e., polylactic acid (PLA), thermoplastic polyurethane (TPU) and nylon. The geometries are realized using commercially available double nozzle fusion deposition modelling (FDM) equipment. The compressive properties of the generated porous and composite configurations are tested quasi statically. The structures are either porous of a single material or composites of two materials that are geometrically intertwined. The method is used to produce and explore promising material combinations that could otherwise be difficult to mix. It is potentially applicable with a variety of additive manufacturing methods, size scales, and materials for a range of potential applications.
... However, it is known that polymers are dependent on strain rates (e.g [2][3][4].). There are limited published studies on selectively sintered PA12 (e.g [5,6].). Therefore, this paper presents a solution to analyse the strain rate-dependent material and damage behaviour of selectively laser sintered PA12. ...
... It consists of a formulation of the accumulated plastic strain rate p and a signum function sign, Eqs. (4)- (5). The signum function is used for calculating the direction of extension of the yield surface. ...
This contribution presents an investigation of strain rate dependent behaviour for selective laser sintered polyamide 12. Two different cases are considered: a strain rate change within tensile loading and a relaxation test incorporating a strain rate change after each holding time section. For the simulation of material behaviour the material model of Bodner-Partom and Chaboche were used. The damage behaviour was considered by the Lemaitre crack model. Thereby it was determined which numerical model is more suitable to reproduce the strain rate-dependent behaviour. Tensile and relaxation tests at constant speeds served as the basis for these investigations.
Metamaterials produced using additive manufacturing represent advanced structures with tunable properties and deformation characteristics. However, the manufacturing process, imperfections in geometry, properties of the base material as well as the ambient and operating conditions often result in complex multiparametric dependence of the mechanical response. As the lattice structures are metamaterials that can be tailored for energy absorption applications and impact protection, the investigation of the coupled thermomechanical response and ambient temperature‐dependent properties is particularly important. Herein, the 2D re‐entrant honeycomb auxetic lattice structures additively manufactured from powdered stainless steel are subjected to high strain rate uniaxial compression using split Hopkinson pressure bar (SHPB) at two different strain rates and three different temperatures. An in‐house developed cooling and heating stages are used to control the temperature of the specimen subjected to high strain rate impact loading. Thermal imaging and high‐speed cameras are used to inspect the specimens during the impact. It is shown that the stress–strain response as well as the crushing behavior of the investigated lattice structures are strongly dependent on both initial temperature and strain rate.
