Lattice structures can add value to high-performance components manufactured by laser powder bed fusion due to their high specific strength and stiffness. A further use of lattice structures is in ...thermo-mechanical applications, where the high surface area of the lattice may aid heat transfer. However, little characterisation of lattices under thermal loading is currently available in the literature. In this study, a custom-built test rig was used to characterise the thermal conduction for three triply periodic minimal surface lattice types, namely: gyroid, diamond and Schwarz primitives, with unit cell size and volume fraction being varied.
Results show that thermal conductivity is primarily a function of the material properties and volume fraction of the sample. However, some effects of the geometry, such as surface area to volume ratio, can be used to explain slight differences in the measured conductivity. The Schwarz primitive unit cell consistently gave the highest conductivity, with diamond and gyroid unit cells being marginally lower. Larger cell sizes typically gave higher conductivity than smaller cells, which can be attributed to greater intra-cell convective heat transfer and better interface coupling with the testing apparatus.
The experimental results are used to derive equations that allow samples with a specified thermal conductivity to be designed, thus demonstrating how a component may be manufactured with a custom thermal profile by varying the volume fraction of the lattice.
Tissue engineered bone scaffolds are potential alternatives to bone allografts and autografts. Porous scaffolds based on triply periodic minimal surfaces (TPMS) are good candidates for tissue growth ...because they offer high surface-to-volume ratio, have tailorable stiffness, and can be easily fabricated by additive manufacturing. However, the range of TPMS scaffold types is extensive, and it is not yet clear which type provides the fastest cell or tissue growth while being sufficiently stiff to act as a bone graft. Nor is there currently an established methodology for TPMS bone scaffold design which can be quickly adopted by medical designers or biologists designing implants. In this study, we examine six TPMS scaffold types for use as tissue growth scaffolds and propose a general methodology to optimise their geometry. At the macro-scale, the optimisation routine ensures a scaffold stiffness within suitable limits for bone, while at the micro-scale it maximises the cell growth rate. The optimisation procedure also ensures the scaffold pores are of sufficient diameter to allow oxygen and nutrient delivery via capillaries. Of the examined TPMS structures, the Lidinoid and Split P cell types provide the greatest cell growth rates and are therefore the best candidates for bone scaffolds.
► Developed a methodology for predicting progressive fatigue damage in adhesive joints. ► Implemented in commercial FEA package using cohesive zone modelling. ► Constant amplitude fatigue damage ...model successfully extended to variable amplitude. ► Fatigue loading represented by maximum load and load ratio. ► Much improved correlation with experimental data than the other models considered.
In this paper, the fatigue response of adhesively bonded joints under variable amplitude (VA) cyclic loading was predicted using a numerical model. The adhesive layer was modelled using the cohesive zone model with a bi-linear traction-separation response. A damage model, incorporating fatigue load ratio effects, was utilised in conjunction with the cohesive zone model to simulate the detrimental influence of VA fatigue loading. This model was validated against published experimental results obtained from fatigue tests of adhesively bonded single lap joints subjected to various types of VA fatigue loading spectra. This model successfully predicted the damaging effect of VA fatigue loading on the adhesively bonded joints and was generally found to be a significant improvement on the other damage models considered.
Additive Manufacturing (AM) enables the production of geometrically complex parts that are difficult to manufacture by other means. However, conventional CAD systems are limited in the representation ...of such parts. This issue is exacerbated when lattice structures are combined or embedded within a complex geometry. This paper presents a computationally efficient, voxel-based method of generating lattices comprised of practically any cell type that can conform to an arbitrary external geometry. The method of conforming involves the tessellation and trimming of unit cells that can leave ‘hanging’ struts at the surface, which is a possible point of weakness in the structure. A method of joining these struts to form an external two dimensional lattice, termed a ‘net-skin’ is also described. Traditional methods of manufacturing lattice structures generally do not allow variation of cell properties within a structure; however, additive manufacturing enables graded lattices to be generated that are potentially more optimal. A method of functionally grading lattices is, therefore, also described to take advantage of this manufacturing capability.
Structural adhesives are being widely used in the aerospace and automobile industries. However, in many applications, aggressive environments damage the adhesive systems and degrade the structural ...performance of bonded members. Cohesive zone models are often employed in the numerical analysis of adhesively bonded structural joints. To accurately model these bonded joints, the characterisation of the cohesive zone properties for different environmental conditions is important. In this regard, an experimental–numerical approach was developed to characterise the environment-dependent cohesive zone properties based on a miniature cantilever peel test. As moisture is a commonly encountered aggressive environment, the current methodology was implemented to characterise the moisture-dependent cohesive zone properties for an Al 2024-T3 and FM73 system.
Experimental studies have been undertaken to characterise the diffusion, swelling and thermal characteristics of a rubber toughened epoxy adhesive. Gravimetric experiments were carried out for three ...different thicknesses of bulk adhesive. The bulk adhesives were exposed to 81.5%, 95.8% RH and water immersion, all at 50°C. Dual stage moisture uptake profiles were observed for thin specimens at all levels of exposure. It was found that a dual stage uptake model fits the anomalous uptake response excellently. The swelling of the adhesive was characterised using the same exposure environment. Swelling was slow at low levels of moisture uptake and increased gradually before becoming linear with increasing moisture uptake. The initial slow swelling of the adhesive was attributed to the diffusion of water into the existing free volume. A moisture-dependent swelling coefficient was determined using finite element modelling, where the dual stage uptake model and moisture-dependent mechanical properties of the adhesive were taken into account. The thermal expansion of the bulk adhesive was determined using a bi-material curved beam specimen. The curvature of the beam reduced when it was exposed to a wet environment. This behaviour was successfully modelled using coupled moisture diffusion and stress analysis incorporating the derived moisture-dependent swelling behaviour.
This research investigates the use of a meshless smoothed particle hydrodynamics (SPH) method for the prediction of failure in an adhesively bonded single lap joint. A number of issues concerning the ...SPH based finite element modelling of single lap joints are discussed. The predicted stresses of the SPH finite element model are compared with the results of a cohesive zone based finite element model. Crack initiation and crack propagation in the adhesive layer are also studied. The results show that the peel stresses predicted by the SPH finite element model are higher and the shear stresses are lower than those predicted by the cohesive zone finite element model. The crack initiation and propagation response of the two models is similar, however, the SPH finite element model predicted a lower failure load than the cohesive zone finite element model. It is concluded that the current implementation of SPH method is a promising method for modelling cohesive failure in bonded joins but requires further development to allow for interfacial crack growth and better stress prediction under tensile loading to compete with existing methods.
The main aim of this paper is to investigate the behaviour of adhesively bonded CFRP joints subjected to cyclic low-velocity impacts and to compare this with fracture in specimens tested in standard ...fatigue (i.e. non-impacting, constant amplitude, sinusoidal fatigue). It is seen that the accumulated energy associated with damage in impact-fatigue is significantly lower than that associated with similar damage in standard fatigue and that the mechanisms of failure are very different for the two loading regimes. For both types of loading, fracture initiates in the adhesive layer and then propagates into the 0° ply of the composite adjacent to the adhesive layer. However, the fracture surfaces after impact-fatigue are generally less uniform and exhibit more signs of high rate/brittle fracture than seen in the fracture surfaces after standard fatigue testing. Various parameters are proposed to characterise damage in standard and impact-fatigue and it is shown that crack velocity, accumulated absorbed energy and normalised maximum force are all useful parameters for characterising damage evolution.
Material properties such as hardness can be dependent on the size of the indentation load when that load is small, a phenomenon known as the indentation size effect (ISE). In this work an inverse ...finite element method (IFEM) is used to investigate the ISE, with reference to experiments with a Berkovich indenter and an aluminium test material. It was found that the yield stress is highly dependent on indentation depth and in order to simulate this, an elastoplastic constitutive relation in which yielding varies with indentation depth/load was developed. It is shown that whereas Young's modulus and Poisson's ratio are not influenced by the length scale over the range tested, the amplitude portion of yield stress, which is independent of hardening and corresponds to the initial stress for a bulk material, changes radically at small indentation depths. Using the proposed material model and material parameters extracted using IFEM, the indentation depth-time and load-depth plots can be predicted at different loads with excellent agreement to experiment; the relative residual achieved between FE modelling displacement and experiment being less than 0.32%. An improved method of determining hardness from nanoindentation test data is also presented, which shows goof agreement with that determined using the IFEM.
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