Dirk F. de Lange

A complete structural performance analysis and modelling of hydroxyapatite scaffolds with variable porosity

The use of hydroxyapatite (HA) scaffolds for bone regeneration is an alternative procedure to treat bone defects due to cancer, other diseases or traumas. Although the use of HA has been widely studied in the literature, there are still some disparities regarding its mechanical performance. This paper presents a complete analysis of the structural performance of porous HA scaffolds based on experimental tests, numerical simulations and theoretical studies. HA scaffolds with variable porosity were considered and fabricated by the water-soluble polymer method, using poly vinyl alcohol as pore former. These scaffolds were then characterised by scanning electron microscopy, stereo microscopy, X-ray diffraction, porosity analysis and mechanical tests. Different scaffold models were proposed and analysed by the finite element method to obtain numerical predictions of the mechanical properties. Also theoretical predictions based on the (Gibson LJ, Ashby MF. 1988. Cellular solids: structure and properties. Oxford: Pergamon Press) model were obtained. Finally the experimental, numerical and theoretical results were compared. From this comparison, it was observed that the proposed numerical and theoretical models can be used to predict, with adequate accuracy, the mechanical performance of HA scaffolds for different porosity values.

Influence of Modeling Assumptions on the Simulated EDM Performance

Electrical Discharge Machining (EDM) is a non-conventional machining process widely used to manufacture hard material components which are not easily machined by conventional machining processes. Several modeling approaches have since long been proposed to characterize the EDM process, based on the electro-thermal phenomena that occur. Several of these early models are analytical models that have the major advantage of providing predictions of process performance based on analytical solutions. Unfortunately, their derivation often demands a number of assumptions and simplifications, which can limit the scope and precision of the predictions obtained. Two of the most known analytical models are based on different heat sources: a point heat source [1], and a uniform disk heat source [2].In this paper, these two analytical EDM models are analyzed and compared with more elaborated numerical models in which specific modeling assumptions are lifted; the objective is to quantify the influence of each of these assumptions on the final result. Numerical simulations are based on the finite element method, which enables to study the influence of different temporal pulse shapes, the spatial intensity distribution, the temperature dependency of the thermal coefficients, and in particular the influence of the way latent heat is taken into account. The results are analyzed in terms of the predicted material removal rate (MRR), depth and radius of the crater. A comparison is presented between the two theoretical results [1], [2], and those obtained by the more elaborated numerical models. At the same time, a comparison is also made with some experimental data from literature.Copyright

Comparative Study of Analytical Expressions to Estimate the Deep Drawing Force of Cylindrical and Rectangular Parts

The deep drawing process has been widely used in the industry because it eliminates costly operations such as welding and machining. However, there are many parameters involved that affect the quality of the final products. One of the main parameters of the deep drawing process is the maximum deep drawing force (DDF) or drawing load, which is the maximum force required to perform a particular deep drawing operation. This maximum DDF is needed to define the required capacity of the press, and to calculate the deep drawing work and the process efficiency. Several analytical expressions to estimate the maximum DDF have been proposed in the literature, particularly for cylindrical parts. However, few research works have focused on analyzing the prediction performance of these expressions.In this paper, the performance of different analytical expressions to estimate the maximum DDF of cylindrical and rectangular parts, is evaluated and compared. Initially, several expressions proposed by different researches for the maximum DDF of cylindrical parts are presented. Then, these expressions are transformed into new expressions for the maximum DDF of rectangular parts by using different concepts of equivalency, such as the equivalent diameter concept. Finally, the prediction performance of all the expressions for both cylindrical and rectangular deep drawing is analysed and compared using experimental data from the literature. The performance is evaluated in terms of the prediction error. The results have suggested that the analytical expressions involving the largest number of parameters have a superior prediction performance than the analytical expressions involving less parameters.Copyright

Comparative study of numerical models of the laser forming process

Even though the laser forming process is not used at a large scale, it has a potential value for small product series. Its advantage is in its capacity to deform a sheet into arbitrary shapes by a contact-free laser irradiation, avoiding the need for costly tools in which materials need to be formed in arbitrary shapes. One of the reasons that the process has not become very popular is by the difficulty to predict and control the process and determine the right processing plan to obtain the shape needed. The simulation of the laser forming process is not easy to carry out, because the nature of the problem is three-dimensional and the process is transient. In addition, the description of the material behavior, which includes thermo-elastoplastic behavior, is complex and results in strongly nonlinear problems. Moreover, the temperature dependent material behavior, including the microstructural evolution of the material is often not known to a sufficient degree of precision, which leads to approximate descriptions. For that reason, in the current study a simple application, bending of a flat plate by irradiation over a straight line, is studied by a range of models with a varying degree of complexity. The models are compared in order to evaluate if a simplified model can be used to obtain adequate numerical results under particular conditions. Simplifications can be the reduction of the moving point heat source to a fixed but transient line heat source over the complete trajectory, or reducing the 3D model to a 2D model. From the analysis, it becomes clear that the effects of all three dimensions and the heat source movement are relevant for the ultimate precision of the simulation results, and to obtain the correct tendencies of the effect of changes of some of the parameters on the process results. Nevertheless, the results also demonstrate that some relevant thermal and stress data can be obtained using simplified models at a considerably lower computational cost. In particular, the thermal data are less affected by model simplifications, whereas the stress and deformation fields are more sensitive to the model approximations. An improved 2D model is proposed and evaluated, which is able to take into account the effect of the moving point heat source, while ignoring the longitudinal dimension.Even though the laser forming process is not used at a large scale, it has a potential value for small product series. Its advantage is in its capacity to deform a sheet into arbitrary shapes by a contact-free laser irradiation, avoiding the need for costly tools in which materials need to be formed in arbitrary shapes. One of the reasons that the process has not become very popular is by the difficulty to predict and control the process and determine the right processing plan to obtain the shape needed. The simulation of the laser forming process is not easy to carry out, because the nature of the problem is three-dimensional and the process is transient. In addition, the description of the material behavior, which includes thermo-elastoplastic behavior, is complex and results in strongly nonlinear problems. Moreover, the temperature dependent material behavior, including the microstructural evolution of the material is often not known to a sufficient degree of precision, which leads to approximate descr...

Study of the effect introduced by an integrating sphere on the temporal profile characterization of short laser pulses propagating through a turbid medium

When a nanosecond laser pulse is transmitted through a highly scattering material, its irradiance decreases as it propagates; this is because of the spatial and temporal pulse profile stretching owing to multiple scattering events. Although the effect of temporal distortion is much less significant than that of the spatial distortion for applications where the laser beam is focused on a subsurface target (writing of waveguides, for example), it becomes significant for applications where the laser pulse must attain certain temporal width after the beam propagated is collimated through a turbid medium (photoacoustic tomography, for example). The objective of this work is to determine the transfer function associated to an integrating sphere measurement of the temporal intensity profile involving turbid media samples. The transfer function is found to be related to the geometrical characteristics of the integrating sphere and the optical properties of the turbid media. This procedure opens a new possibility for optical property characterization and enables the use of an integrating sphere for time-dependent intensity measurements.

Development of a Virtual Platform to Evaluate the Performance of an Electrical Vehicle

Virtual simulations of electrical vehicle performance help to optimize vehicle design, by studying and predicting the effects of parameter variations on the vehicle performance, in order to find an optimum balance between the cost and benefit of design decisions. In this work, the development of a virtual platform to evaluate the performance of an electrical vehicle is presented and applied to the study of public urban transportation. The aim is to analyze the requirements and optimize specifications for a light weight, energy efficient, autonomous vehicle without energy supply along the trajectory, except in the stations.Virtual platforms for vehicle performance have been developed before, and in many cases characteristic velocity profiles are used as a reference, according to the traffic environment in which the vehicle will operate. Vehicle analysis and design is focused on feasibility of the vehicle to be able to follow the prescribed velocity profile.In the present study, the evaluation is instead based on the cost/benefit relationship for an urban transport vehicle on traffic-free trajectories, enabling to adjust and optimize the velocity profiles in order to optimize the energy use while minimizing travel time. Therefore, the virtual platform is focused on the calculation of the net energy usage, the travel time and the system cost corresponding to an electrical vehicle with different battery and ultra-capacitor energy storage capacities, regeneration and storage of brake energy and an automatic governor for autonomous vehicle control.The influence of design parameters, such as the installed motor power, energy storage capacity, vehicle weight, passenger load and vehicle control strategy on the time schedule and energy efficiency is studied. However, the effort does not aim for a straight forward optimization of efficiency or minimization of travel time. In fact, energy optimization often conflicts with the travel time optimization. Therefore, both are analyzed simultaneously in order to assist in the search for an optimum compromise. In addition, the results are interpreted in terms of the overall obtained benefits of travel time reduction or optimization of the energy use, in contrast with the corresponding increment of the investment cost of the vehicle related to the implementation of the studied parameter variation. Specific trajectory profiles, including height profiles can be defined for optimization of the vehicle system for application in specific locations with specific geographic conditions.Copyright

Influence of Geometrical Parameters on the Maximum Deep Drawing Height of Rectangular Parts

The deep drawing process is commonly used in the industry because its ability to produce parts with reduced weight and good mechanical properties at a high production rates. However, the elasto-plastic deformation mechanism of deep drawing is complex and difficult to analyse; this because there are many process parameters and variables involved that affect the quality of final products. Among these variables are the geometric parameters, which have been proved to have a great influence on the process. Theoretical and experimental analyses reported in the literature have been mainly focused on conventional cylindrical cup deep drawing. Few research works have dealt with the deep drawing analysis of non-cylindrical parts, particularly the influence of geometrical parameters on the deep drawing performance.This paper presents an analysis of the effect of geometrical parameters on the allowable deep drawing height (DDH) of rectangular parts before fracture. The aim is to identify the influence of the main geometrical parameters on the DDH, Numerical analyses based on the Finite Element Method (FEM) were used to investigate the influence of geometrical parameters, such as the radii, the metal sheet thickness, and the aspect ratio, among others, on the DDH.Copyright

Elastoplastic Analysis of the Erichsen Cupping Test Using Comsol Multiphysics FEM Code

One of the standard procedures to test the formability of sheet material is the Erichsen cupping test, in which the metal sheet blank is held in its place over a circular space and depressed by a semi-spherical punch. The depth of depression that can be reached is the measure of the formability. In this work the elastoplastic deformation of the sheet is analyzed by multipurpose Finite Element Method software Comsol Multiphysics. The Comsol package is not specifically developed or focused on the analysis of solid mechanical problems with elastoplastic model behavior and contact problems, and still limited literature is available in which sheet forming processes are analyzed with Comsol. In this work, the development and testing of a simulation model in Comsol is reported and comparison is made with results reported with other FEM software.The development and testing is realized in successive steps of increasing complexity. First a uniaxial stretching is simulated in order to evaluate the implementation of the elastoplastic material behavior. Next, the bending of a plate over a straight line is analyzed, adding the contact boundary condition between tool and sheet surface into the model. Finally, the axisymmetric model of the Erichsen cupping test is implemented.It is found that the default Von Mises yield function results in incorrect stresses, and needs to be replaced by a yield function in which the Von Mises stress is calculated based on the Cauchy tensor. The non-linear contact condition is a source of oscillations in the local stresses near the zone where contact is established. The simulation results obtained with the final model are compared with punch forces, stresses and strains obtained in literature, showing an adequate comparison.Copyright

Analysis of Effective Mechanical Properties and Anisotropy of Structured Porous Materials

Many natural and modern man-made materials are porous materials. In many cases, for practical applications and for the proper design of products it is necessary to understand the effective mechanical properties of the porous structure. Even though some general applicable theories exist to estimate the mechanical properties in function of the porosity fraction, or apparent density, the relation between the pore structure and size distribution is not clear. In particular, in case of structured porosity the mechanical properties demonstrate anisotropic behavior. For specialized applications there is an interest to be able to design materials with specified anisotropic properties or with properties varying in space.In order to design and optimize porous structures with specific anisotropic properties, a flexible simulation model is developed and tested to predict and understand the mechanical anisotropic properties of a large variety of porous structures. In order to be able to change the geometric pore structure without recreating a new model, the pore structure is implemented into a space dependent smoothed binary function.A comparison is made between results obtained with FEM models with a fully defined geometry and the FEM model with the porosity function to describe the pore structure in order to evaluate and quantify the error that is introduced by the latter implementation and investigate the possibility to mitigate the error.The model results are also compared with experimental en numerical results reported in literature.© 2014 ASME

Sheet Metal Blank Development of a Deep Drawing Fan Support Using Theoretical Rules and FEM

Deep drawing is a cost effective sheet metal forming process to produce many industrial components. However, complex geometrical drawn parts are difficult to form due to several modes and conditions of the material flow. Commonly problems associated to the forming operation are wrinkling and tearing defects, which affect the cost and quality of the parts. Actually, there are not theoretical methods developed in the literature yet, so the trial and error method are used to reduce or eliminate the deep drawing defects or inclusive is utilized in the earlier production stages, resulting in higher costs and longer production times.This paper describe a proposed solution to reduce or eliminate the wrinkles defects on the flanges of an industrial fan support that result from applying the forming process. An analysis procedure based on the development of the correct sheet metal blank considering three different blank geometries was proposed. The analysis include the analytical methods available in the literature, the simulation with a computer program based on the Finite Element Method (FEM) and experimentation. FEM model, simulation and results, these were validated by measuring the thickness profile on the flanges of a deep drawing part, before and after the procedure implementation. The results have shown that combining both the analytical and FEM methods, were possible to know the influence degree of the sheet metal blank geometry to reduce or eliminate the wrinkle defect and these can be used as an effective design tool.Copyright