Three-Dimensional Finite Element Thermal Analysis in Selective Laser Melting of Al-Al2O3 Powder

The selective laser melting (SLM) of aluminium-based composites continues to be a challenge due to the high reflectivity, high thermal conductivity and oxidation of aluminium, all of which directly influence the thermal performance of each layer during SLM. Due to the extremely rapid melting and cooling rate of aluminium, however, it is difficult to measure thermal performance within practical SLM applications. A three-dimensional finite element simulation model is thus developed in this study to simulate the transient temperature distribution and molten pool dimensions of the premier layer during the SLM of Al-Al2O3 composite powder. In order to produce high-quality parts with minimum defects in a highly efficient manner, the predicted optimum volumetric energy density is found to be 40 J/mm3 , with laser power 300 W, scanning speed 1000 mm/s, hatch spacing 150 μm and layer thickness 50 μm; the molten pool size that is produced is 165 μm in length, 160 μm in width and 77 μm in depth, with a predicted maximum temperature of around 3400°C. All of these factors may contribute to the creation of good metallurgic bonding.Mechanical Engineerin

A transverse isotropic viscoelastic constitutive model for aortic valve tissue

A new anisotropic viscoelastic model is developed for application to the aortic valve (AV). The directional dependency in the mechanical properties of the valve, arising from the predominantly circumferential alignment of collagen fibres, is accounted for in the form of transverse isotropy. The rate dependency of the valve's mechanical behaviour is considered to stem from the viscous (η) dissipative effects of the AV matrix, and is incorporated as an explicit function of the deformation rate (λ˙). Model (material) parameters were determined from uniaxial tensile deformation tests of porcine AV specimens at various deformation rates, by fitting the model to each experimental dataset. It is shown that the model provides an excellent fit to the experimental data across all different rates and satisfies the condition of strict local convexity. Based on the fitting results, a nonlinear relationship between η and λ˙ is established, highlighting a ‘shear-thinning’ behaviour for the AV with increase in the deformation rate. Using the model and these outcomes, the stress–deformation curves of the AV tissue under physiological deformation rates in both the circumferential and radial directions are predicted and presented. To verify the predictive capabilities of the model, the stress–deformation curves of AV specimens at an intermediate deformation rate were estimated and validated against the experimental data at that rate, showing an excellent agreement. While the model is primarily developed for application to the AV, it may be applied without the loss of generality to other collagenous soft tissues possessing a similar structure, with a single preferred direction of embedded collagen fibres

Progressive dehydration in decomposing bone: a potential tool for forensic anthropology

The aim of this pilot study was to determine whether collagen and/or water content of bone vary during soft tissue putrefaction by thermogravimetric analysis with a view to eventually developing a possible forensic application to determine post-mortem interval. Porcine bone decomposed in a shallow burial showed an approximate difference in average mass loss of 15  ± 8% when heated between 22 and 100 °C, compared to 14 ± 3% for porcine bone decomposed in a surface deposition, equating to water loss. Mass loss showed peaks at 0, 250–500 and 1200–1500 cumulative cooling degree days’ (CCDD) deposition for the experimental porcine bone. Should these measurements prove consistent in future studies on a wider variety of porcine and eventually human skeletal elements, they may have potential to be corroborated with other data when determining post-mortem interval, especially with disarticulated bones. A downward trend in mass loss was apparent within shallow burial and surface deposition scenarios (inclusive of freeze-dried controls) for the thermolysis of collagen (and other proteins) between 220 and 650 °C during thermogravimetric analysis. This was inconsistent within the time frame examined (0–1450 cumulative cooling degree days), and so demonstrates less potential as an indicator of post-mortem interval during soft tissue putrefaction

Macro and nanoscale wear behaviour of Al-Al 2 O 3 nanocomposites fabricated by selective laser melting

Aluminium-based composites are increasingly applied within the aerospace and automotive industries. Tribological phenomena such as friction and wear, however, negatively affect the reliability of devices that include moving parts; the mechanisms of friction and wear are particularly unclear at the nanoscale. In the present work, pin-on-disc wear testing and atomic force microscopy nanoscratching were performed to investigate the macro and nanoscale wear behaviour of an Al-Al2O3 nanocomposite fabricated using selective laser melting. The experimental results indicate that the Al2O3 reinforcement contributed to the macroscale wear-behaviour enhancement for composites with smaller wear rates compared to pure Al. Irregular pore surfaces were found to result in dramatic fluctuations in the frictional coefficient at the pore position within the nanoscratching. Both the size effect and the working-principle difference contributed to the difference in frictional coefficients at both the macroscale and the nanoscale

Comparison between torsional spring constants of rectangular and V-shaped AFM cantilevers

The properties of force-sensing micro-cantilevers are of fundamental importance for measurements employing atomic force microscopy (AFM) techniques. Due to the well-known arguments of Sader, it is generally accepted that V-shaped cantilevers are more sensitive to lateral forces than rectangular ones. We present results of numerical (finite element modelling) and experimental comparison between torsional spring constants of rectangular and V-shaped commercial AFM cantilevers. As representative example of such beams, we considered AFM probes available commercially. In particular, we tested scaled-up models of V-shaped cantilevers which had the same geometrical shapes as commercial AFM cantilevers. Both the rectangular and the Vshaped larger scale models were made of the same material; they had the same length, thickness, normal spring constant, as well as the same location and shape of the tip base. In the experiments and the simulations, an external lateral load was applied to the free end of the tip. A good agreement between the experimental work and finite element method (FEM) simulations was observed. The results show that the torsional spring constant of the V-shape cantilevers considered here was greater than that of the equivalent rectangular beams by up to 45%. The discrepancy with the results from Sader should be caused by differences in both the load transfer scheme and the geometrical shapes of the V-shaped beams

A Re-Examination of the Taxonomic Boundaries of \u3cem\u3eSymphysia\u3c/em\u3e (Ericaceae)

DNA sequence data were generated for the nuclear ITS region for Symphysia racemosa and for 26 additional Vaccinieae representing 12 sections in the genus Vaccinium plus one species from each of five additional segregate genera. Our focus is on the placement of S. racemosa relative to Vaccinium sensu scricto and Vaccinium sect. Oreades (represented by V. poasanum). Maximum parsimony analysis of 608 bp of nrITS region suggests that S. racemosa and V. poasanum form a well-supported clade in spite of substantial morphological divergence. Futhermore, this clade is a sister group to a clade consisting of all segregate genera examined. These molecular results led us to undertake a morphological cladistic analysis of all of the other Central American green-flowered taxa. We suggest that the genus Symphysia should be expanded to encompass these 15 taxa, despite the lack of phylogenetic resolution within this group. This will necessitate eight new combinations, via. Symphysia almedae (= V. almedae), Symphysia costaricensis (= V. costaricense), Symphysia jefensis (= V. jefense), Symphysia orosiensis (= V. orosiense), Symphysia ovata (= Lateropora ovata), Symphysia perardua (= V. santafeënsis), Symphysia poasana (= Vaccinium poasanum), Symphysia santafeënsis (= L. santafeënsis), and Symphysia tubulifera (= L. tubulifera)

Mechanical and magnetic properties of spark plasma sintered soft magnetic FeCo alloy reinforced by carbon nanotubes

Different volume fractions (0.5 vol. % to 4.5 vol. %) of CNTs were used to reinforce a binary Fe50Co soft magnetic alloy. The first method for dispersion was involved dry mixing and ball milling of the powder, while the second was included wet mixing in dimethylformamide under ultrasonic agitation, drying and then dry ball milling. The powders were consolidated using spark plasma sintering. Tensile test and SEM analyses were performed to characterize the mechanical properties and the fracture surface of the sintered materials. The best magnetic and mechanical properties were achieved using the first method. A maximum enhancement in tensile strength of around 20% was observed in the 0.5 vol. % CNT composite with improved elongation compared to the monolithic Fe50Co alloy. In addition, the magnetic properties were enhanced by adding CNTs up to 1 vol. %, and an improvement in densification was observed in composites up to 1.5 vol. % CNT with respect to monolithic Fe50Co alloy

Quantifying microcracks on fractured bone surfaces - potential use in forensic anthropology

Bone fracture surface morphology (FSM) can provide valuable information on the cause of failure in forensic and archaeological applications and it depends primarily on three factors, the loading conditions (like strain rate), the ambient conditions (wet or dry bone material) and the quality of bone material itself. The quality of bone material evidently changes in taphonomy as a result of the decomposition process and that in turn is expected to affect FSM. Porcine bones were fractured by a standardised impact during the course of soft tissue decomposition, at 28-day intervals, over 140 days (equivalent to 638 cooling degree days). Measurements of the associated microcracks on the fractured cortical bone surfaces indicated a progressive increase in mean length during decomposition from around 180 µm to 375 µm. The morphology of these microcracks also altered, from multiple intersecting microcracks emanating from a central point at 0-28 cumulative cooling degree days, to longer linear cracks appearing to track lamellae as soft tissue decomposition progressed. The implications of these findings are that taphonomic changes of bone may offer the real possibility of distinguishing perimortem and taphonomic damage and also provide a new surrogate parameter for estimation of post-mortem interval (PMI) in forensics

In vivo measurement of human brain material properties under quasi-static loading

Computational modelling of the brain requires accurate representation of the tissues concerned. Mechanical testing has numerous challenges, in particular for low strain rates, like neurosurgery, where redistribution of fluid is biomechanically important. A finite-element (FE) model was generated in FEBio, incorporating a spring element/fluid–structure interaction representation of the pia–arachnoid complex (PAC). The model was loaded to represent gravity in prone and supine positions. Material parameter identification and sensitivity analysis were performed using statistical software, comparing the FE results to human in vivo measurements. Results for the brain Ogden parameters µ, α and k yielded values of 670 Pa, −19 and 148 kPa, supporting values reported in the literature. Values of the order of 1.2 MPa and 7.7 kPa were obtained for stiffness of the pia mater and out-of-plane tensile stiffness of the PAC, respectively. Positional brain shift was found to be non-rigid and largely driven by redistribution of fluid within the tissue. To the best of our knowledge, this is the first study using in vivo human data and gravitational loading in order to estimate the material properties of intracranial tissues. This model could now be applied to reduce the impact of positional brain shift in stereotactic neurosurgery