Three-dimensional lattices have applications across a range of fields including structural lightweighting, impact absorption and biomedicine. In this work, lattices based on triply periodic minimal ...surfaces were produced by polymer additive manufacturing and examined with a combination of experimental and computational methods. This investigation elucidates their deformation mechanisms and provides numerical parameters crucial in establishing relationships between their geometries and mechanical performance. Three types of lattice were examined, with one, known as the primitive lattice, being found to have a relative elastic modulus over twice as large as those of the other two. The deformation process of the primitive lattice was also considerably different from those of the other two, exhibiting strut stretching and buckling, while the gyroid and diamond lattices deformed in a bending dominated manner. Finite element predictions of the stress distributions in the lattices under compressive loading agreed with experimental observations. These results can be used to create better informed lattice designs for a range of mechanical and biomedical applications.
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•Manufactured and tested lattice structures based on triply periodic minimal surfaces.•Lattices with equivalent masses deform differently depending on their cell geometry.•High stiffness seen for the structure which showed buckling and low failure strain.•Determined Gibson-Ashby factors enabling the design of optimised latticed components.
Metal components with applications across a range of industrial sectors can be manufactured by selective laser melting (SLM). A particular strength of SLM is its ability to manufacture components ...incorporating periodic lattice structures not realisable by conventional manufacturing processes. This enables the production of advanced, functionally graded, components. However, for these designs to be successful, the relationships between lattice geometry and performance must be established. We do so here by examining the mechanical behaviour of uniform and graded density SLM Al-Si10-Mg lattices under quasistatic loading. As-built lattices underwent brittle collapse and non-ideal deformation behaviour. The application of a microstructure-altering thermal treatment drastically improved their behaviour and their capability for energy absorption. Heat-treated graded lattices exhibited progressive layer collapse and incremental strengthening. Graded and uniform structures absorbed almost the same amount of energy prior to densification, 6.3±0.2 MJ/m3 and 5.7±0.2 MJ/m3, respectively, but densification occurred at around 7% lower strain for the graded structures. Several characteristic properties of SLM aluminium lattices, including their effective elastic modulus and Gibson-Ashby coefficients, C1 and α, were determined; these can form the basis of new design methodologies for superior components in the future.
Selective laser melting (SLM) is an attractive technology, enabling the manufacture of customised, complex metallic designs, with minimal wastage. However, uptake by industry is currently impeded by ...several technical barriers, such as the control of residual stress, which have a detrimental effect on the manufacturability and integrity of a component. Indirectly, these impose severe design restrictions and reduce the reliability of components, driving up costs. This paper uses a thermo-mechanical model to better understand the effect of laser scan strategy on the generation of residual stress in SLM. A complex interaction between transient thermal history and the build-up of residual stress has been observed in the two laser scan strategies investigated. The temperature gradient mechanism was discovered for the creation of residual stress. The greatest stress component was found to develop parallel to the scan vectors, creating an anisotropic stress distribution in the part. The stress distribution varied between laser scan strategies and the cause has been determined by observing the thermal history during scanning. Using this, proposals are suggested for designing laser scan strategies used in SLM.
Identifying novel natural fibers/fabrics with proper properties as reinforcement material is a new challenge in the field of bio-composites. Hence, the aim of this paper is to study the possibility ...of using a natural fabric extracted from Manicaria saccifera palm as a novel reinforcement in composites. This fabric was extensively characterized by chemical composition analysis, infrared spectroscopy (FTIR) analysis, morphological studies (SEM), thermo-gravimetric analysis (TGA) and physical /mechanical properties studies. From SEM analysis it was identified globular protrusions spread uniformly over the fiber which could help the mechanical interlock with the resin. As well, Manicaria fabric showed good thermal stability, low density, low moisture content and good tensile properties. Further, their properties are comparable to most natural cellulose fabrics and some synthetic fabrics, such as fiber glass fabrics. Manciaria saccifera fabric showed to be a suitable candidate as natural reinforcement material for the development of bio- composite.
In this paper we present a numerical investigation into surface-based lattice structures with the aim of facilitating their design for additive manufacturing. We give the surface equations for these ...structures and show how they can be used to tailor their volume fractions. Finite element analysis is used to investigate the effect of cell type, orientation and volume fraction on the elastic moduli of the lattice structures, giving rise to a valuable set of numerical parameters which can be used to design a lattice to provide a specified stiffness. We find the I-WP lattice in the 001 orientation provides the highest stiffness along a single loading direction, but the diamond lattice may be more suitable for cases where lower mechanical anisotropy is important. Our stiffness models enable the construction of a powerful numerical tool for predicting the performance of graded structures. We highlight a particular problem which can arise when two lattice types are hybridised; an aberration leading to structural weakening and high stress concentrations. We put forward a novel solution to this problem and demonstrate its usage. The methods and results detailed in this paper enable the efficient design of lattice structures functionally graded by volume fraction and cell type.
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•Surface-based lattice structures were designed and examined with finite element analysis to determine their elastic moduli.•The moduli were found along three loading directions for each lattice, and these were correlated with their volume fractions.•A factor of three difference was found between the moduli of the least stiff and most stiff lattice types.•The determined modulus-volume fraction relationships accurately predict the moduli of graded lattice structures.•We demonstrate a design approach for hybrid lattices which identifies and corrects regions of low structural connectivity.
A reliable numerical damage model has been developed for adhesively bonded joints under fatigue loading that is only dependant on the adhesive system and not on joint configuration. A bi-linear ...traction–separation description of a cohesive zone model was employed to simulate progressive damage in the adhesively bonded joints. Furthermore, a strain-based fatigue damage model was integrated with the cohesive zone model to simulate the deleterious influence of the fatigue loading on the bonded joints. To obtain the damage model parameters and validate the methodology, carefully planned experimental tests on coupons cut from a bonded panel and separately manufactured single lap joints were undertaken. Various experimental techniques have been used to assess joint damage including the back-face strain technique and in situ video microscopy. It was found that the fatigue damage model was able to successfully predict the fatigue life and the evolving back-face strain and hence the evolving damage.
We used X-ray computed tomography (CT), microscopy and hardness measurements to study Al–Si10–Mg produced by selective laser melting (SLM). Specimens were subject to a series of heat treatments ...including annealing and precipitation hardening. The specimen interiors were imaged with X-ray CT, allowing the non-destructive quantification and characterisation of pores, including their spatial distribution. The specimens had porosities less than 0.1%, but included some pores with effective cross-sectional diameters up to 260μm. The largest pores were highly anisotropic, being flat and lying in the plane normal to the build direction. Annealing cycles caused significant coarsening of the microstructure and a reduction of the hardness from (114±3) HV, in the as-built state, to (45±1) HV, while precipitation hardening increased this to a final hardness of (59±1) HV. The pore size and shape distributions were unaffected by the heat treatments. We demonstrate the applicability of CT measurements and quantitative defect analysis for the purposes of SLM process monitoring and refinement.
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•We examined SLM Al–Si10–Mg with X-ray CT, microscopy and hardness measurements.•The size, shape and position distributions of the internal pores were determined.•Heat treatments alter the microstructure and hardness, but the pores are unaffected.•The largest pores are highly anisotropic, being at or disc-like in the xy plane.•CT and statistical analysis can play a role in improving the manufacturing process.
The development and thermo-mechanical characterization of a novel green composite lamina, made of PolyLactic Acid (PLA) reinforced with a natural fabric extracted from Manicaria Saccifera palm, are ...presented. The composite was characterized by thermal-analysis (TGA), tensile, flexural, and izod impact tests, and scanning electronic microscopy (SEM). TGA analysis showed that the degradation process of the composite started earlier than that of neat PLA due to the lower thermal stability of the fabric. The mechanical tests showed that PLA properties were improved. The tensile strength, elastic modulus and impact resistance were improved by 26%, 51% and 56% respectively. Good dispersion and mechanical interlocking of PLA into the fabric were seen by SEM explaining the improvements of the mechanical properties of the composite. In summary, the good tensile properties and the excellent energy absorption capabilities of the MF/PLA composite lamina show great potential of Manicaria fabric as reinforcement in green composites.
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.
A novel technique for the dynamic characterization of metals from a single Taylor impact test is proposed. This computational characterization procedure is based on the formulation and solution of a ...first class inverse problem, in which the silhouette of the Taylor specimen’s final shape is expressed as a vector of its geometrical moments and used as input parameter. The inverse characterization problem is reduced to an optimization problem where the optimum material parameters for the Cowper–Symonds material model are determined. The optimization process is performed by a range adaptation real-coded genetic algorithm. Numerical example for the characterisation of 1018 steel is implemented and presented to validate the methodology presented in this paper. The effectiveness and simplicity of the proposed characterization procedure makes it an appropriate tool for the characterization of metals at high strain rates.
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