VOLCO: a predictive model for 3D printed microarchitecture

Extrusion-based 3D printing is widely used for porous scaffolds in which polymer filaments are extruded in the form of log-pile structures. These structures are typically designed with the assumption that filaments have a continuous cylindrical profile. However, as a filament is extruded, it interacts with previously printed filaments (e.g. on lower 3D printed layers) and its geometry varies from the cylindrical form. No models currently exist that can predict this critical variation, which impacts filament geometry, pore size and mechanical properties. Therefore, expensive time-consuming trial-and-error approaches to scaffold design are currently necessary. Multiphysics models for material extrusion are extremely computationally-demanding and not feasible for the size-scales involved in tissue engineering scaffolds. This paper presents a new computationally-efficient method, called the VOLume COnserving model for 3D printing (VOLCO). The VOLCO model simulates material extrusion during manufacturing and generates a voxelised 3D-geometry-model of the predicted microarchitecture. The extrusion-deposition process is simulated in 3D as a filament that elongates in the direction that the print-head travels. For each simulation step in the model, a set volume of new material is simulated at the end of the filament. When previously 3D printed filaments obstruct the deposition of this new material, it is deposited into the nearest neighbouring voxels according to a minimum distance criterion. This leads to filament spreading and widening, which is studied experimentally to validate the method. Experimental validation demonstrates the ability of the VOLCO to simulate the geometry of 3D printed filaments. In addition, finite element analysis (FEA) simulations utilising 3D-geometry-models generated by VOLCO demonstrate its value and applicability for predicting the mechanical properties of porous scaffolds. The presented method enables scaffold designs to be validated and optimised prior to manufacture. Potential future adaptations of the model and integration into 3D printing software are discussed

Optimal orientation of fibre composites for strength based on Hashin’s criteria optimality conditions

© 2020, The Author(s). The Hashin’s strength criteria are usually employed in first ply failure and damage-onset analysis of fibre-reinforced composites. This work presents optimality conditions of local material orientations for these criteria, in terms of principal stresses and material strength parameters. Each criterion (matrix tensile/compressive, fibre tensile/compressive modes) has its conditions separately derived, analytically, based on a fixed stress field assumption. The conditions found show that orientations which coincide and do not coincide with principal stress directions may minimise local failure indices. These solutions are employed in a proposed algorithm, named HA-OCM (Hashin Optimality Criteria Method), which selectively satisfies the matrix failure modes (either tensile or compressive), iteratively and finite element-wise in composites. It is demonstrated that the HA-OCM is able to design single-layer plane structures with improved failure loads in comparison with designs following only maximum (in absolute) principal stress orientations. Results show that the material orientations have a trend to end up either aligned or at 90° with maximum in absolute principal stress directions. Global optima for compliance are, however, not guaranteed. To give an idea of gains in terms of failure loads, some HA-OCM designs show improvements of 71% and 140%, for example, in comparison with principal stress design

Dynamic compressive response of additively manufactured AlSi10Mg alloy hierarchical honeycomb structures

Periodic honeycombs have been used for their high strength, low weight and multifunctionality. The quasi-static and dynamic compressive responses of three types of additively manufactured AlSi10Mg honeycomb structures, specifically a single-scale honeycomb and two hierarchical honeycombs with two and three levels of hierarchy, respectively, have been investigated using experimental measurement and finite element (FE) simulations. The validated FE simulation has been employed to investigate the effects of relative density of the honeycombs and the key experimental parameters. The following failure modes of the three types of honeycombs have been observed both under quasi-static and dynamic compression: (1) the single-scale honeycomb experienced a transition of failure mechanism from local plastic buckling of walls to local damage of the parent material without buckling with the increase of the relative density of the honeycomb; (2) the hierarchical honeycombs all failed with parent material damage without buckling at different relative densities. For both quasi-static and dynamic compression, the hierarchical honeycombs offer higher peak nominal wall stresses compared to the single-scale honeycomb at low relative density of ; the difference is diminished as relative density increases, i.e. the three types of honeycombs can achieve similar peak wall stresses when Numerical results have suggested the hierarchical honeycombs can offer better energy absorption capacity than the single-scale honeycomb. The two-scale and three-scale hierarchical honeycombs achieved similar peak nominal wall stresses for both quasi-static and dynamic compression, which may suggest that the structural performance under out-of-plane compression is not sensitive to the hierarchical architecture. This work indicates that the structural advantage of hierarchical honeycombs can be utilised to develop high performance lightweight structural components

Design optimization for an additively manufactured automotive component

The aim of this paper is to investigate the design optimization and additive manufacture of automotive components. A Titanium brake pedal processed through Selective Laser Melting (SLM) is considered as a test case. Different design optimisation techniques have been employed including topology optimization and lattice structure design. Rather than using a conventional topology optimization method, a recently developed topology optimization method called Iso-XFEM is used in this work. This method is capable of generating high resolution topology optimised solutions using isolines/isosurfaces of a structural performance criterion and eXtended Finite Element Method (XFEM). Lattice structure design is the other technique used in this work for the design of the brake pedal. The idea is to increase the stability of the brake pedal to random loads applied to the foot pad area of the pedal. The use of lattice structures can also significantly reduce the high residual stress induced during the SLM process. The results suggest that the integration of the design optimization techniques with a metal additive manufacturing process enables development of a promising tool for producing lightweight energy efficient automotive components

The thermo-mechanical degradation of ethylene vinyl acetate used as a solar panel adhesive and encapsulant

The thermal ageing of an Ethylene-vinyl Acetate (EVA) polymer used as an adhesive and encapsulant in a photovoltaic module has been investigated. The EVA is used to bond the silicon solar cells to the front glass and backing sheet and to protect the photovoltaic materials from the environment and mechanical damage. Using a range of experimental techniques, including Dynamic Mechanical Analysis, Differential Scanning Calorimetry and Thermo-Gravimetric analysis, it was possible to show a link between changes in mechanical properties with both the transient temperature and the degree of long-time thermal ageing. Importantly, it was possible to show that the ageing related property changes were likely due to long term structural changes rather than any modification of the chemistry of the material

Nanoindentation shows uniform local mechanical properties across melt pools and layers produced by selective laser melting of AlSi10Mg alloy

Single track and single layer AlSi10Mg has been produced by selective laser melting (SLM) of alloy powder on a AlSi12 cast substrate. The SLM technique produced a cellular-dendritic ultra-fined grained microstructure. Chemical composition mapping and nanoindentation showed higher hardness in the SLM material compared to its cast counterpart. Importantly, although there was some increase of grain size at the edge of melt pools, nanoindentation showed that the hardness (i.e. yield strength) of the material was uniform across overlapping tracks. This is attributed to the very fine grain size and homogeneous distribution of Si throughout the SLM material

Selective laser melting of aluminium alloys

Metal additive manufacturing (AM) processes, such as selective laser melting, enable powdered metals to be formed into arbitrary 3D shapes. For aluminium alloys, which are desirable in many high-value applications for their low density and good mechanical performance, selective laser melting is regarded as challenging due to the difficulties in laser melting aluminium powders. However, a number of studies in recent years have demonstrated successful aluminium processing, and have gone on to explore its potential for use in advanced, AM componentry. In addition to enabling the fabrication of highly complex structures, selective laser melting produces parts with characteristically fine microstructures that yield distinct mechanical properties. Research is rapidly progressing in this field, with promising results opening up a range of possible applications across scientific and industrial sectors. This paper reports on recent developments in this area of research as well as highlighting some key topics that require further attention

Fibre architecture design of 3D woven composite with genetic algorithms: a unit cell based optimisation framework and performance assessment

There are vast possibilities in fibre architecture design of 3D woven reinforcement. This paper considers the application of Genetic Algorithm (GA) in 3D woven composites optimisation. A set of real and integral variables, representing 3D fibre architecture, are formulated into a mixed integer Genetic Algorithm. The objective function is evaluated through automation of the unit cell based finite element analysis, by using the open source pre-processor TexGen and the commercial solver ABAQUS. The mixed integer Genetic Algorithm is adapted to a micro-population, aiming to improve computational efficiency. The study uses statistical tests to quantify the performance of the Genetic Algorithm schemes and the choice of parameters. The proposed approach was applied to the optimisation of 3D woven composites for maximum buckling resistance for the case of a landing gear brace. This study demonstrated that the optimisation converged to the optimum design within 20 iterations, considering 300 out of 7000 permissible solutions. In terms of buckling performance, the optimum design performed twice as well as cross-ply laminated composites and at least 50% better than known orthogonal 3D woven composites

Fibre architecture design of 3D woven composite with genetic algorithms: a unit cell based optimisation framework and performance assessment

There are vast possibilities in fibre architecture design of 3D woven reinforcement. This paper considers the application of Genetic Algorithm (GA) in 3D woven composites optimisation. A set of real and integral variables, representing 3D fibre architecture, are formulated into a mixed integer Genetic Algorithm. The objective function is evaluated through automation of the unit cell based finite element analysis, by using the open source pre-processor TexGen and the commercial solver ABAQUS. The mixed integer Genetic Algorithm is adapted to a micro-population, aiming to improve computational efficiency. The study uses statistical tests to quantify the performance of the Genetic Algorithm schemes and the choice of parameters. The proposed approach was applied to the optimisation of 3D woven composites for maximum buckling resistance for the case of a landing gear brace. This study demonstrated that the optimisation converged to the optimum design within 20 iterations, considering 300 out of 7000 permissible solutions. In terms of buckling performance, the optimum design performed twice as well as cross-ply laminated composites and at least 50% better than known orthogonal 3D woven composites