Optimisation of blade type spreaders for powder bed preparation in additive manufacturing using DEM simulations

Powders used in the Particle Bed Fusion process are spread onto compact layers and then are fused to generate a layer of the final part. This process is repeated layer-upon-layer to form the final products. It has recently been demon- strated [Powder Technology, 306 (2017) 45–54] that spreading the particles with a counter-rotating roller produces a bed with a higher quality (i.e. a lower void fraction) compared to a blade type spreader. This is related to the geometry of the two spreaders which directly changes the bed-spreader contact dynamic and consequently affects the bed's quality. Based on this rationale, here, it is postulated that changing the blade profile at the blade bed contact region can significantly enhance the bed's quality and improve the effectiveness of a blade as a spreading device. A set of Discrete Element Method (DEM) simulations is performed at device-scale to optimise the geometry of blade spreaders to yield the lowest void fraction using simple rod-shaped grains to control the computational costs. The blade profile is parametrised using a super-ellipse with three geometrical parameters. Firstly, it is demonstrated that geometric optimisation of a blade profile is an effective alternative to using more complex spreading devices. Secondly, for the proposed parametrisation, the optimum values are found using computer simulations and it is shown that bed volume fractions close to critical values are achievable. Finally, a new technique for multi-sphere approximation (MSA) is developed and applied to 3D models of real powder grains to generate realistic particle shapes for the DEM simulations. Then using these grains it is shown that the proposed optimum blade profile is capable of producing a bed with qualities comparable (and even better) to a roller at the actual operating conditions and with realistic grain characteristics

Enhancement of the spreading process in additive manufacturing through the spreader optimisation

Powders used in Particle Bed Fusion (PBF) process are spread onto compact layers and then are sintered. This process is repeated layer by layer to form the final products. The author has recently characterised the process and it is found that spreading the particleswithacounter-rotatingrollerproducesabedwithhigherqualities, i.e. lowervoid fractions and surface roughness [Powder Technology, 306 (2017) 45–54]. This is related to a particle dragging effect caused by the small contact area between powder grains and the blade. Therefore, here, it is postulated that changing the blade profile at the bladebed contact point can significantly influence the contact dynamics and hence increase the blade's effectiveness as a spreading device for PBF. A set of computer simulations using Discrete Element Method (DEM) are performed at device scales to optimise the geometry of blade spreaders to yield the lowest void fraction and surface roughness. The blade profile is parametrised using a super-ellipse with three geometrical parameters. It is firstly demonstrated that geometric optimisation of a blade profile is an effective alternative to using more complex spreading devices. Secondly, for the proposed parametrisation, the optimum values are found using computer simulations which can generate very high quality powder beds with volume fractions close to the critical value

Predicting Hydraulic Fracturing in Hyttejuvet Dam

Hydraulic fracturing can occur in the clay core of earth and rockfill dams if the vertical effective stress in the core is reduced to the levels that are small enough to allow a tensile fracture to occur due to hydraulic pressure of the seeping water. This situation may arise if the total stress in the core is reduced by the “arching effect” where the core settles relative to the filter or rock-fill shell of the dam. Water pressure increase in the core which occurs on first impounding of water, may reduce effective stresses further, and if they reach low enough values, a fracture will occur. The design of earth dams (especially those with thin vertical central cores) to resist hydraulic fracture is therefore of great importance, as there have been several dam failures in the past that have been attributed to the hydraulic fracture. In this paper, the behavior of Hyttejuvet Dam, which was thought to have failed due to hydraulic fracturing, is studied. 2D coupled consolidation finite element analysis of the construction and first impounding of the rockfill dam was carried out with elasto-plastic model (Drucker-Preger/Cap model) using ABAQUS software. The result of the analysis with respect to the pore pressure and settlement in some parts of the dam are compared with the measured data from the instruments in the dam. According to the result of the comparison, the appropriate model for predicting the behavior of Hyttejuvet Dam is obtained. Also different criteria are used to predict the hydraulic fracturing of the dam. By comparing the results of the study using these criteria, one may be able to predict the hydraulic fracturing mechanism in the clay core of the studied dam

An Insight to the Effect of Initial Static Shear Stress on the Liquefaction of Sands

The effect of initial static shear stress on cyclic behavior of sands has been the concern of many researchers for more than five decades. This study includes the results of a set of cyclic simple shear tests carried out on a uniform sand with relative densities of 20%, 40%, and 60%, under three different initial normal stresses of 50, 150, and 250 kPa. All tests were performed under constant volume condition. Results show that the behavior of sands due to initial static shear stress, is controlled by two contradictive elements: first one relates to the increasing dynamic shear modulus due to the initial static shear stress that ends in greater liquefaction resistance, and the second relates to the amount of irreversible shear strains which increases with greater value of driving shear stress and consequently reduces the liquefaction resistance. These elements form alternations in the value of Kα; being increased in some zones and decreased in others. New trends observed in the variation of liquefaction resistance due to the initial static shear stress, leaded the authors to define new parameters which can interpret the failure conditions and complexities of the behavior

Airfoil noise reductions through leading edge serrations

This paper provides an experimental investigation into the use of leading edge (LE) serrations as a means of reducing the broadband noise generated due to the interaction between the aerofoil's LE and impinging turbulence. Experiments are performed on a flat plate in an open jet wind tunnel. Grids are used to generate isotropic homogeneous turbulence. The leading edge serrations are in the form of sinusoidal profiles of wavelengths, λ, and amplitudes, 2h. The frequency and amplitude characteristics are studied in detail in order to understand the effect of LE serrations on noise reduction characteristics and are compared with straight edge baseline flat plates. Noise reductions are found to be insignificant at low frequencies but significant in the mid frequency range (500 Hz-8 kHz) for all the cases studied. The flat plate results are also compared to the noise reductions obtained on a serrated NACA-65 aerofoil with the same serration profile. Noise reductions are found to be significantly higher for the flat plates with a maximum noise reduction of around 9 dB compared with about 7 dB for the aerofoil. In general, it is observed that the sound power reduction level (ΔPWL) is sensitive to the amplitude, 2h of the LE serrations but less sensitive to the serration wavelength, λ. Thus, this paper sufficiently demonstrates that the LE amplitude acts as a key parameter for enhancing the noise reduction levels in flat plates and aerofoils

Accumulation of Rainfall in the Permeable Fill Behind a Soil Nail Wall

TOOBA deep excavation project was conducted in a densely developed area in the North West of Tehran, capital of Iran, to provide space for 4 basement levels for multiple buildings around the already functional TOOBA tower. Besides excavating on 3 sides of TOOBA tower, this project involved excavation to the depths varying from 9 to 28 meters depending on the sloping ground condition. A 5 story school building and a 2 story residential building abut the excavation boundary. Previous experience of constructed soil nail walls in cemented soils of Tehran indicated relatively small wall deformations. Therefore, except for the retaining system of the TOOBA tower, a soil nail wall system was seen appropriate for supporting excavation faces. Wall and ground deformations were monitored during and after construction and the ground around the site was regularly checked for tension cracks. Geotechnical explorations indicated the presence of a disturbed fill about 5 meters thick overlaying the intact cemented soil layer. In the 29th of August 2011 following a three-day rainfall a tension crack suddenly occurred on the eastern side of the excavation site. The maximum width of the crack at the surface of the road was 3 cm. This paper summarizes the information on the forensic study which concluded that the rainfall was confined to the fill layer. Therefore drainage system which was located in the cemented soil layer with lower permeability could not function properly. Limit equilibrium analysis correctly predicted the location of the tension crack and the unstable block

Deep Excavation on 3 Sides of a 21 Story Building: Accounts of a Successful Deep Excavation Project

TOOBA deep excavation project was conducted in a densely developed area in the North West of Tehran, capital of Iran, to provide space for 4 basement levels for multiple buildings around the already functional TOOBA tower. TOOBA tower is located in the northern leg of the excavation boundary in a way that the northern side of the building abuts the excavation edge. Hence, this project involved excavation on 3 sides of the roughly rectangular plan of TOOBA tower to the depth of 16.5 meters below its foundation level. The necessity for constraining the deformations of the tower commended the construction of contiguous bored concrete piles around the building supported at 4 different levels with tieback and wailing system. A monitoring program for measuring the deformations of the tower and supporting system was also enforced during and after excavation. The three-dimensional nature of the retaining structure required careful design and construction procedure to avoid problems such as the intersection of the anchors. Each tieback was given a unique direction which was defined by 3 angles relative to the local x, y and z axes. Therefore, complicated forces were exerted on the wailing system and piles. The excavation procedure was ensued with no excessive deformations occurring in the building during or after the excavation. This paper considers some of the design and construction aspects of this successfully completed project