A. Gonzalez-Herrera

Plastic Zone Study by Means of Tri-Dimensional Finite Element Models on Fatigue Crack Closure

This paper focuses on the study of the plastic zone in fatigue crack closure based on the results obtained by means of 3D Finite Element Analysis (FEA). These results show the crack behavior through the thickness. The plastic zone is visualized and quantified. It does not correspond to the classical shape. The plastic zone in the interior surface is similar to those obtained in 2D plane strain conditions and a reduced effect of closure is observed. However, close to the external surface, 2D plane stress results are not reproduced, the plastic zone size is smaller and an important change is observed. This transition is developed in a thin external portion of the specimen and it can only be captured if a fine mesh of the thickness is done.

Tympanic-ossicular prostheses and MEMS technology : whats and whys

Conclusions. Microelectromechanical systems (MEMS) technology fulfils the requirements of implantable middle ear devices and consequently it becomes an excellent option to design and develop the related transducers. Objectives. To present a summarized overview of the fundamentals of mechanical technologies in relation to middle ear implants research. Materials and methods. Analysis of the possibilities, limitations and practical applications of MEMS as regards the research, development, transference and fabrication processes. Results. MEMS is a new technology with the potential to develop small integrated mechanical and electronic systems that share many processes of integrated circuits technology and its wide application potential. Middle ear prostheses are essentially special implantable transducers that mimic the properties of the tympano-ossicular system: electromechanical systems that deliver low energy pulses safely and efficiently into the labyrinth fluids. They primarily require: active mechanisms to preclude potential damage levels; minimum energy consumption; adequate dimensions for the middle ear; and biotolerable materials. Additionally, development and translational aspects of the selected technology are of utmost importance in this field.

Numerical and Experimental Analysis of Crack Closure

The fatigue life of metallic materials is strongly influenced by crack closure effects. Finite element (FE) methods allow the study of crack closure with great detail and can provide valuable information about phenomena occurring in the bulk of the material. In this work the distribution of stresses through the thickness of a cracked specimen has been studied using 3D FE simulations. It was found that the transition between the interior of the specimen (plane strain) and the surface (plane stress) differs from that predicted by 2D plane stress models. In addition, an attempt is presented to experimentally validate the results at the surface level. For this purpose full-field image correlation technique was utilized. This allowed direct comparison between the displacement field predicted by the numerical simulations and the experimental results measured by digital image correlation.

A study of sound transmission in an abstract middle ear using physical and finite element models

The classical picture of middle ear (ME) transmission has the tympanic membrane (TM) as a piston and the ME cavity as a vacuum. In reality, the TM moves in a complex multiphasic pattern and substantial pressure is radiated into the ME cavity by the motion of the TM. This study explores ME transmission with a simple model, using a tube terminated with a plastic membrane. Membrane motion was measured with a laser interferometer and pressure on both sides of the membrane with micro-sensors that could be positioned close to the membrane without disturbance. A finite element model of the system explored the experimental results. Both experimental and theoretical results show resonances that are in some cases primarily acoustical or mechanical and sometimes produced by coupled acousto-mechanics. The largest membrane motions were a result of the membranes mechanical resonances. At these resonant frequencies, sound transmission through the system was larger with the membrane in place than it was when the membrane was absent.

Effect of the middle ear cavity on the response of the human auditory system

The effect of the acoustic cavities on the response of the auditory system has been usually focused on the influence of the external ear canal (EEC). The presence of the middle ear cavity (MEC) has been ignored. Experimental difficulties to obtain information inside this cavity without altering the whole system make difficult its study. In order to explore the influence of this cavity, a numerical study is made. This is made by means of a complete finite element (FE) model including the tympanic membrane, ossicular chain, and acoustic cavities. Different FE models are used to analyze the influence of each component. By means of different calculations removing these components from the model, their relative effects can be distinguished. At low frequencies (below 2 kHz) the influence of the MEC is negligible. Piston-like motion is dominant. Nevertheless, at higher frequencies a new resonant peak appears at a frequency of 4 kHz. This is due to the presence of the MEC. It combine with the pressure gain due to...

Numerical Analysis of the Influence of the Auditory External Canal Geometry on the Human Hearing Response

This paper presents the analysis and discussion about different effects of the external auditory canal (EAC) geometry on the response of the human hearing system. Simulation has been made by means of 3D finite element models which included EAC and a model of the ossicular‐eardrum system. Different EAC geometries were constructed, coupled to a middle ear model validated in previous works. The EAC geometry is based on anatomical measurements taken from the literature. The relative position and orientation of the tympanic membrane and section reduction of the canal at the isthmus were studied and analyzed with a harmonic analysis. A sound pressure level of 90 dB was applied at the canal entrance and through fluid‐structure coupling, the pressures in the umbo and the displacements of umbo and stapes footplate were measured in a frequency range from 100 Hz to 20000 Hz.

INFLUENCE OF THE EARDRUM STIFFNESS ON THE MIDDLE EAR SOUND FUNCTION TRANSFER

The study has been developed by means of a FEM of the middle ear. This model was previously validated with experimental data. It is composed by the tympanic membrane, osicular chain, ligaments and tendons support. The cochlear load is simulated with a mechanical equivalent system (damper-mass-damper), and the properties values of materials are taken from the literature according to commonly accepted values [Caminos, 2011]. A sound presure level of 80 dB is applied on the tympanic membrane and the stape footplate velocity is calculated using a harmonic analisys in a frecuency range from 100 to 10 KHz. SVFT is the ratio between the stape footplate velocity and pressure over the tympanic membrane. This parameter is directly proportional to the middle ear acoustic admittance. It is calculated with different values of eardrum Young´s modulus (YM), in the frequency range studied.

Key Aspects in 3D Fatigue Crack Closure Numerical Modelling

Since long time, fatigue crack closure has been studied by means of finite element models. Initially by bi-dimensional models and recently, due to the higher computational capabilities, the use of three-dimensional models has been extended, providing a wider comprehension of the problem. Starting with the methodology used for 2D cases, a specific methodology for 3D models has been developed. Key parameters affecting the model have been analyzed and recommendations have been established. The numerical accuracy is evaluated in terms of crack closure and opening values. They main issues studied are the material behaviour, the loading cycles and crack growth scheme, the contact simulation, the meshing and the element size at the crack tip and along the thickness, the plastic wake computed and the opening and closure definition considered. This paper summarises the main learning and recommendations from the latest numerical modelling experience of the authors.

Numerical Analysis of the Pivot Node in Fracture Problems

Recent studies have allowed us to identify a narrow region of the thickness of the crack front in fracture problems that presents interesting characteristics for the numerical-experimental correlation. Taking the three-dimensional distribution of the stress intensity factor (K) as a reference, we observe how it remains invariant and independent of the main factors influencing this type of analysis. This article presents a summary of how to identify this point through the numerical simulation of the problem and its relationship with parameters such as thickness, load level or angle of curvature. The simulations are carried out with the ANSYS software in an aluminium CT specimen subjected to a fracture loading process in mode I.

Analysis of the Mechanical Properties of the Human Tympanic Membrane and Its Influence on the Dynamic Behaviour of the Human Hearing System

The difficulty to estimate the mechanical properties of the tympanic membrane (TM) is a limitation to understand the sound transmission mechanism. In this paper, based on finite element calculations, the sensitivity of the human hearing system to these properties is evaluated. The parameters that define the bending stiffness properties of the membrane have been studied, specifically two key parameters: Youngs modulus of the tympanic membrane and the thickness of the eardrum. Additionally, it has been completed with the evaluation of the presence of an initial prestrain inside the TM. Modal analysis is used to study the qualitative characteristics of the TM comparing with vibration patterns obtained by holography. Higher-order modes are shown as a tool to identify these properties. The results show that different combinations of elastic properties and prestrain provide similar responses. The presence of prestrain at the membrane adds more uncertainty, and it is pointed out as a source for the lack of agreement of some previous TM elastic modulus estimations.