Pasquale Franciosa

Mechanical behavior of post-restored upper canine teeth: A 3D FE analysis

OBJECTIVES The aim was to evaluate the stress distribution, comparing an anterior sound tooth with post-endodontic restored teeth under mechanical loading. METHODS A three-dimensional finite element analysis was performed based on micro-CT scan images of a maxillary canine. Twelve models with different crown properties and post-configurations were simulated. The model of the maxillary sound canine was also created and investigated. A load of 50N was applied at a 63° angle with respect to the longitudinal axis of the tooth on the palatal surface of the crown. Principal stresses were registered. Numerical FEA results were statistically analyzed to show the influence of post shape and crown materials. RESULTS All analyzed models (M1-M12) exhibited a high stress gradient, due to different material stiffnesses present at the various interfaces. The most uniform mechanical behavior of the investigated models, very similar to sound tooth, was the combination of a composite crown and a cylindrical or conical fiber-glass post. SIGNIFICANCE The results of this study facilitate informed clinical choice between possible material combinations in restorative procedures of endodontically treated anterior teeth.

Numerical fatigue 3D-FE modeling of indirect composite-restored posterior teeth

OBJECTIVE In restored teeth, stresses at the tooth-restoration interface during masticatory processes may fracture the teeth or the restoration and cracks may grow and propagate. The aim was to apply numerical methodologies to simulate the behavior of a restored tooth and to evaluate fatigue lifetimes before crack failure. MATERIALS AND METHODS Using a CAD-FEM procedure and fatigue mechanic laws, the fatigue damage of a restored molar was numerically estimated. Tessellated surfaces of enamel and dentin were extracted by applying segmentation and classification algorithms, to sets of 2D image data. A user-friendly GUI, which enables selection and visualization of 3D tessellated surfaces, was developed in a MatLab(®) environment. The tooth-boundary surfaces of enamel and dentin were then created by sweeping operations through cross-sections. A class II MOD cavity preparation was then added into the 3D model and tetrahedral mesh elements were generated. Fatigue simulation was performed by combining a preliminary static FEA simulation with classical fatigue mechanical laws. RESULTS Regions with the shortest fatigue-life were located around the fillets of the class II MOD cavity, where the static stress was highest. SIGNIFICANCE The described method can be successfully adopted to generate detailed 3D-FE models of molar teeth, with different cavities and restorative materials. This method could be quickly implemented for other dental or biomechanical applications.

Effects of thread features in osseo-integrated titanium implants using a statistics-based finite element method

OBJECTIVE To investigate the influence of implant design factors in terms of bone integrity and implant stability. MATERIALS AND METHODS A 3D parametric CAD model was developed. Then, once domain settings and boundary conditions were defined, a 3D FEM model was created. To simulate the physical interaction at the bone-implant interface, identity pairs were introduced. After generating different design scenarios with a DOE approach, the most significant design factors were obtained. RESULTS This study showed that the geometry of the screw thread highly influenced the implant stability. In particular the degree of bone damage became minimal when adopting 0.40 mm for the thread width and 0.05 mm for the thickness. SIGNIFICANCE Thread width and thickness play a crucial role to reduce induced stresses and damage in bone. Considering these preliminary results, future improvements should focus on investigating also two-factor and higher interactions to better understand the implant loading mechanism.

Improving comfort of shoe sole through experiments based on CAD-FEM modeling

It was reported that next to style, comfort is the second key aspect in purchasing footwear. One of the most important components of footwear is the shoe sole, whose design is based on many factors such as foot shape/size, perceived comfort and materials. The present paper focuses on the parametric analysis of a shoe sole to improve the perceived comfort. The sensitivity of geometric and material design factors on comfort degree was investigated by combining real experimental tests and CAD-FEM simulations. The correlation between perceived comfort and physical responses, such as plantar pressures, was estimated by conducting real tests. Four different conditions were analyzed: subjects wearing three commercially available shoes and in a barefoot condition. For each condition, subjects expressed their perceived comfort score. By adopting plantar sensors, the plantar pressures were also monitored. Once given such a correlation, a parametric FEM model of the footwear was developed. In order to better simulate contact at the plantar surface, a detailed FEM model of the foot was also generated from CT scan images. Lastly, a fractional factorial design array was applied to study the sensitivity of different sets of design factors on comfort degree. The findings of this research showed that the sole thickness and its material highly influence perceived comfort. In particular, softer materials and thicker soles contribute to increasing the degree of comfort.

Fault Diagnosis and Fault-Tolerant Control of Uncertain Robot Manipulators Using High-Order Sliding Mode

A robust fault diagnosis and fault-tolerant control (FTC) system for uncertain robot manipulators without joint velocity measurement is presented. The actuator faults and robot manipulator component faults are considered. The proposed scheme is designed via an active fault-tolerant control strategy by combining a fault diagnosis scheme based on a super-twisting third-order sliding mode (STW-TOSM) observer with a robust super-twisting second-order sliding mode (STW-SOSM) controller. Compared to the existing FTC methods, the proposed FTC method can accommodate not only faults but also uncertainties, and it does not require a velocity measurement. In addition, because the proposed scheme is designed based on the high-order sliding mode (HOSM) observer/controller strategy, it exhibits fast convergence, high accuracy, and less chattering. Finally, computer simulation results for a PUMA560 robot are obtained to verify the effectiveness of the proposed strategy.

Passive diastolic modelling of human ventricles: Effects of base movement and geometrical heterogeneity

Left-ventricular (LV) remodelling, associated with diastolic heart failure, is driven by an increase in myocardial stress. Therefore, normalisation of LV wall stress is the cornerstone of many therapeutic treatments. However, information regarding such regional stress-strain for human LV is still limited. Thus, the objectives of our study were to determine local diastolic stress-strain field in healthy LVs, and consequently, to identify the regional variations amongst them due to geometric heterogeneity. Effects of LV base movement on diastolic model predictions, which were ignored in the literature, were further explored. Personalised finite-element modelling of five normal human bi-ventricles was carried out using subject-specific myocardium properties. Model prediction was validated individually through comparison with end-diastolic volume and a new shape-volume based measurement of LV cavity, extracted from magnetic resonance imaging. Results indicated that incorporation of LV base movement improved the model predictions (shape-volume relevancy of LV cavity), and therefore, it should be considered in future studies. The LV endocardium always experienced higher fibre stress compared to the epicardium for all five subjects. The LV wall near base experienced higher stress compared to equatorial and apical locations. The lateral LV wall underwent greater stress distribution (fibre and sheet stress) compared to other three regions. In addition, normal ranges of different stress-strain components in different regions of LV wall were reported for five healthy ventricles. This information could be used as targets for future computational studies to optimise diastolic heart failure treatments or design new therapeutic interventions/devices.

A computer-aided tool to quickly analyse variabilities in flexible assemblies in different design scenarios

In automotive and aeronautic fields, controlling the final shape of flexible assemblies (with sheet metal parts) is a key issue. Even assuming as known the shape errors in single manufactured parts, the assembly process can cause wide variability due to their flexibility and to the choice of fixtures and clamps as well as to the fastening technique adopted. Here it is strategic to analyse different assembly configurations at the beginning of the design phase and chose the one that assures less variability on the key characteristics to be achieved. This paper presents a FEM-based computer tool able to statistically analyse variations occurring in assembly processes of flexible parts. After assigning fixtures, clamping points, fastening joints and assembly sequence, the tool quickly outputs the statistical variability of the key characteristics. It is so possible to span a variety of design solutions predicting failures and controlling final variations. A case study will show how it works.

FEM and BEM Stress Analysis of Mandibular Bone Surrounding a Dental Implant

In the present work the structural behaviour of a mandible with a dental implant, considering a unilateral occlusion, is numerically analysed by means of the Finite Element Method (FEM) and the Boundary Element Method (BEM). The mandible, whose CAD model was obtained by computer tomography scans, is considered as completely edentulous and only modelled in the zone surrounding the implant. The material behaviour of bone is assumed as isotropic linear elastic or, alternatively, as orthotropic linear elastic. With reference to the degree of osteo-integration between the implant and the mandibular bone, a partial osteo-integration is considered; consequently a nonlinear contact analysis is performed, with allowance for friction at the interface between implant and bone. A model of a commercial dental implant is digitised by means of optical 3D scanning process and fully reconstructed in all its geometrical features. Special attention is drawn to the mathematical reconstruction of the CAD model in order to facilitate the meshing process in the BEM environment and reduce the geometrical imperfections generated during the CAD to CAE translation process. The results of FEM and BEM analyses in terms of stress distribution on the mandible are compared in order to benchmark the two methodologies against accuracy and pre-processing efforts.

A CAD-Based Methodology for Motion and Constraint Analysis According to Screw Theory

The need for a designer to have a tool able to do motion and constraint analysis, to check for the under-constrained and/or over-constrained status of an assembly, is strategic in a design contest where several changes are made during the design process by using CAD. Traditional kinematic tools provide little information on over-constraints at 3D level. Screw theory has been already used in mechanical assemblies, in a top-down design, to do motion and constraint analysis. This theory is here used to analyze mechanical assemblies in the contest of a feature-based CAD system. The structure of the CAD assembly is captured and described as assembly graph, similar to Datum Flow Chain, through which the motion or constraint status of any part (in terms of twist and wrench matrices), can be obtained. The underlying algorithm is based on the Kirchoff’s rules successfully applied by Davies to mechanisms. How to automatically create the assembly graph, detect the useful loops and then write the loop kinematic equations is described. Three case studies are presented related to CAD assemblies of mechanisms built up in SolidWorks® CAD system by Dassault Systemes from which assembly constraints have been acquired.Copyright

Rolling element bearing fault diagnosis using integrated nonlocal means denoising with modified morphology filter operators

The impulses in vibration signals are used to identify faults in the bearings of rotating machinery. However, vibration signals are usually contaminated by noise that makes the process of extracting impulse characteristic of localized defect very challenging. In order to effectively diagnose bearing with noise masking vibration signal, a new methodology is proposed using integrated (i) nonlocal means- (NLM-) based denoising and (ii) improved morphological filter operators. NLM based denoising is first employed to eliminate or reduce the background noise with minimal signal distortion. This denoised signal is then analysed by a proposed modified morphological analysis (MMA). The MMA analysis introduces a new morphological operator which is based on Modified-Different (DIF) filter to include only fault relevant impulsive characteristics of the vibration signal. To improve further performance of the methodology the length of the structure element (SE) used in MMA is optimized using a particle swarm optimization- (PSO-) based kurtosis criterion. The results of simulated and real vibration signal show that the integrated NLM with MMA method as well as the MMA method alone yields superior performance in extracting impulsive characteristics of vibrations signals, especially for signal with high level of noise or presence of other sources masking the fault.