Mechanical characterisation of Duraform® Flex for FEA hyperelastic material modelling

Laser Sintering (LS) is widely accepted as a leading additive manufacturing process with a proven capability for manufacturing complex lattice structures using a group of specially developed powder based materials. However, to date, very little research has been directed towards achieving greater knowledge of the properties of the elastomeric materials that can be used to produce energy absorbent items such as personalised sports helmets and running shoes via the LS technique. This paper will contribute to addressing this knowledge gap by examining the material properties and characteristics of Duraform ® Flex, a commercially available elastomeric material used for such LS applications. A 3D Systems HiQ machine fitted with a closed loop thermal control system was employed, together with a number of the advanced processing options available in the operating software. In order to measure the mechanical properties of this material, sets of ISO standard tensile test specimens were fabricated, employing a range of different manufacturing processing parameters. The result shows that varying key LS processing parameters such as powder bed temperature, laser power and the number of scanning exposures has a significant impact on the mechanical properties of the resulting part, including its ultimate strength and elongation at break. As LS is a layer manufacturing process, part properties are found to vary considerably between the horizontal (X-Y) and vertical (Z) build orientations. The paper demonstrates how the measured tensile stress-strain curve can be transformed into appropriate hyperelastic material models employing the data curve fitting process in PTC Creo 2.0 Simulate software, and how these material models can be used practically to match user requirements for the laser sintered parts, leading to design optimisation for both bulky solid and lightweight lattice components. The paper concludes with a discussion examining the potential future direction of the research. © 2014 Elsevier Ltd. All rights reserved

Optimisation of an elastomeric pre-buckled honeycomb helmet liner for advanced impact mitigation

Advances in computational modelling now offer an efficient route to developing novel helmet liners that could exceed contemporary materials’ performance. Furthermore, the rise of accessible additive manufacturing presents a viable route to achieving otherwise unobtainable material structures. This study leverages an established finite element-based approach to the optimisation of cellular structures for the loading conditions of a typical helmet impact. A novel elastomeric pre-buckled honeycomb structure is adopted and optimised, the performance of which is baselined relative to vinyl nitrile foam under direct and oblique loading conditions. Results demonstrate that a simplified optimisation strategy is scalable to represent the behaviour of a full helmet. Under oblique impact conditions, the optimised pre-buckled honeycomb liner exceeds the contemporary material performance when considering computed kinematic metrics head and rotational injury criterion, by up to 49.9% and 56.6%. Furthermore, when considering tissue-based severity metrics via finite element simulations of a human brain model, maximum principal strain and cumulative strain density measures are reduced by 14.9% and 66.7% when comparing the new material, to baseline

On the AIC-based model reduction for the general Holzapfel–Ogden myocardial constitutive law

© 2019, The Author(s). Constitutive laws that describe the mechanical responses of cardiac tissue under loading hold the key to accurately model the biomechanical behaviour of the heart. There have been ample choices of phenomenological constitutive laws derived from experiments, some of which are quite sophisticated and include effects of microscopic fibre structures of the myocardium. A typical example is the strain-invariant-based Holzapfel–Ogden 2009 model that is excellently fitted to simple shear tests. It has been widely used and regarded as the state-of-the-art constitutive law for myocardium. However, there has been no analysis to show if it has both adequate descriptive and predictive capabilities for other tissue tests of myocardium. Indeed, such an analysis is important for any constitutive laws for clinically useful computational simulations. In this work, we perform such an analysis using combinations of tissue tests, uniaxial tension, biaxial tension and simple shear from three different sets of myocardial tissue studies. Starting from the general 14-parameter myocardial constitutive law developed by Holzapfel and Ogden, denoted as the general HO model, we show that this model has good descriptive and predictive capabilities for all the experimental tests. However, to reliably determine all 14 parameters of the model from experiments remains a great challenge. Our aim is to reduce the constitutive law using Akaike information criterion, to maintain its mechanical integrity whilst achieving minimal computational cost. A competent constitutive law should have descriptive and predictive capabilities for different tissue tests. By competent, we mean the model has least terms but is still able to describe and predict experimental data. We also investigate the optimal combinations of tissue testsfor a given constitutive model. For example, our results show thatusing one of the reduced HO models, one mayneed just one shear response (along normal-fibredirection) and one biaxial stretch (ratio of 1 mean fibre : 1 cross-fibre) to satisfactorily describe Sommer et al. human myocardial mechanical properties. Our studysuggests that single-state tests (i.e. simple shear or stretching only) are insufficient to determine the myocardium responses. We also foundit is important to consider transmural fibre rotations within eachmyocardial sampleof tests during the fitting process.This is done byexcluding un-stretched fibres usingan “effective fibre ratio”, which depends on the sample size, shape, local myofibre architecture and loading conditions. We conclude that a competent myocardium material model can be obtained from the general HO model using AIC analysis and a suitable combination of tissue tests

Mechanical behaviour of additively manufactured elastomeric pre-buckled honeycombs under quasi-static and impact loading

Selective laser sintering has been used to manufacture different structural variations of a pre-buckled circular honeycomb. The mechanical behaviour of these structures has been examined under both quasi-static and dynamic impact loading. Pre-buckled circular honeycombs with aspect ratios e = 0.8 and e = 0.6 were compared to a traditional, straight-walled honeycomb. It has been found that the mechanical behaviour of the honeycomb can be tailored to yield different mechanical responses. Principally, decreasing the aspect ratio reduced the stress at yield, as well as the total energy absorbed until densification, however, this alleviated the characteristic stress-softening response of traditional honeycombs under static and dynamic conditions. When subjected to multiple cycles of loading, a stabilised response was observed. The numerical response closely agreed with the experimental results. A simplified, periodic boundary condition model also closely agreed with the experimental results whilst alleviating computational run time by nominally 75%. The numerical full factorial parameter design sweep identified a broad range of mechanical behaviour. This represents a valuable tool to identify optimal design configurations for future impact mitigating applications

Investigating the dynamic compression response of elastomeric, additively manufactured fluid-filled structures via experimental and finite element analyses

This study evaluates a fluid-filled, closed-cell lattice as a novel route to reducing peak acceleration in impact environments. A conical structure was designed and built using fused filament fabrication. One structure was manufactured hollow (100% air), another 70% filled with water (50% by height) and a third 100% water-filled. Peak acceleration was evaluated by performing 4.1 kg impacts at 1, 2, 3 m/s. Impacts were then simulated in shell and solid finite element analysis models, employing the smooth particle hydrodynamic method for the water and a surface-based fluid-filled cavity method for air. The air-filled, conventional closed-cell structures achieved the lowest peak accelerations at lower impact energies, however, water infill improved impact performance at higher energies. For low to medium impact energies, shell and solid modelling accurately simulated experimental trends, although the latter is more computationally expensive. Solid modelling is the only viable solution for scenarios achieving structural densification, due to the inaccuracies in shell-based models caused by the inter-surface penetrations. This work has demonstrated that fluid-filled structures provide a promising approach to reduce acceleration and so achieving enhanced protection, whilst also presenting a computational pathway that will enable efficient design of new and novel structures

Re-articulating the role of process design to support mass customisation: The case of rapid manufactured custom-made fixtures

Whilst the fulfilment of customised production affects the whole product realisation chain involving product design, process design and supply chain design, our assessment of the literature observes comparatively little attention has been given to process design. Within this area, this paper considers the opportunity for custom fixture manufacture, combining the power of modularity with the technologies of Rapid Manufacturing. Several examples are presented illustrating significant improvements in quality, fixture cost and overall time to market can be achieved through this approach

An explorative study on the antimicrobial effects and mechanical properties of 3D printed PLA and TPU surfaces loaded with Ag and Cu against nosocomial and foodborne pathogens

Antimicrobial 3D printed surfaces made of PLA and TPU polymers loaded with copper (Cu), and silver (Ag) nanoparticles (NPs) were developed via fused deposition modeling (FDM). The potential antimicrobial effect of the 3D printed surfaces against Escherichia coli, Listeria monocytogenes, Salmonella Typhimurium, and Staphylococcus aureus was evaluated. Furthermore, the mechanical characteristics, including surface topology and morphology, tensile test of specimens manufactured in three different orientations (XY, XZ, and ZX), water absorption capacity, and surface wettability were also assessed. The results showed that both Cu and Ag-loaded 3D printed surfaces displayed a higher inhibitory effect against S. aureus and L. monocytogenes biofilms compared to S. Typhimurium and E. coli biofilms. The results of SEM analysis revealed a low void fraction for the TPU and no voids for the PLA samples achieved through optimization and the small height (0.1 mm) of the printed layers. The best performing specimen in terms of its tensile was XY, followed by ZX and XZ orientation, while it indicated that Cu and Ag-loaded material had a slightly stiffer response than plain PLA. Additionally, Cu and Ag-loaded 3D printed surfaces revealed the highest hydrophobicity compared to the plain polymers making them excellent candidates for biomedical and food production settings to prevent initial bacterial colonization. The approach taken in the current study offers new insights for developing antimicrobial 3D printed surfaces and equipment to enable their application towards the inhibition of the most common nosocomial and foodborne pathogens and reduce the risk of cross-contamination and disease outbreaks

Eco-case based reasoning (Eco-CBR) for supporting sustainable product design

A major challenge for any manufacturer is including aspects of sustainable development in product design. These are related to the social, environmental and economic impacts of the proposed product. This paper proposes the development of an eco- case based reasoning (Eco-CBR) method for supporting sustainable product design. This approach is intended to be used to help industrial decision-makers to propose solutions to new product design feature requirements by reusing solutions from similar cases and their past experience. Information related to the solutions contains details of product dimensions, life cycle assessment (LCA) concerning their impact on carbon footprint, water eutrophication, air acidification and the total energy consumed by the product and related processes throughout its entire life cycle. These solutions also contain estimations of the manufacturing, environmental, end-of-life (EOL) and economic costs. The method is demonstrated using a case study that considers the design of the set of medical forceps, based upon identifying and utilising information related to the similarities within the existing cases in the CBR library. The paper demonstrates how this method can help the designer to shorten the process of design and contribute information that can itself maintain and add to the knowledge contained within the case bases stored in the library

Quantifying the microstructural and biomechanical changes in the porcine ventricles during growth and remodelling

Cardiac tissue growth and remodelling (G & R) occur in response to the changing physiological demands of the heart after birth. The early shift to pulmonary circulation produces an immediate increase in ventricular workload, causing microstructural and biomechanical changes that serve to maintain overall physiological homoeostasis. Such cardiac G & R continues throughout life. Quantifying the tissue's mechanical and microstructural changes because of G & R is of increasing interest, dovetailing with the emerging fields of personalised and precision solutions. This study aimed to determine equibiaxial, and non-equibiaxial extension, stress-relaxation, and the underlying microstructure of the passive porcine ventricles tissue at four time points spanning from neonatal to adulthood. The three-dimensional microstructure was investigated via two-photon excited fluorescence and second-harmonic generation microscopy on optically cleared tissues, describing the 3D orientation, rotation and dispersion of the cardiomyocytes and collagen fibrils. The results revealed that during biomechanical testing, myocardial ventricular tissue possessed non-linear, anisotropic, and viscoelastic behaviour. An increase in stiffness and viscoelasticity was noted for the left and right ventricular free walls from neonatal to adulthood. Microstructural analyses revealed concomitant increases in cardiomyocyte rotation and dispersion. This study provides baseline data, describing the biomechanical and microstructural changes in the left and right ventricular myocardial tissue during G & R, which should prove valuable to researchers in developing age-specific, constitutive models for more accurate computational simulations