Age-dependent changes of the periodontal ligament in rats

Even after the end of the natural tooth eruption, there is a continuous renewal of the periodontal collagenous fiber system, depending on functional demands. The aim of this study was to analyse the age-dependent changes and regional differences of the collagen renewal rate of the periodontal ligament in healthy rats. The study was performed by autoradiography of the molars of rats aged 1, 8, and 18 months, where collagen was labelled by intravenously applied 3H-proline. After an 8-hour incorporation period, the animals were killed. For comparative examinations, molar roots were subdivided into cervical, middle, and apical thirds. Structural and quantitative analyses were performed by light microscopy and autoradiography, using an image-analysing computer-assisted operating unit that determined the 3H-proline-labelled collagen by photometry based on extinction measurement. With increasing age of the animals, the number of silver grains (3H-proline-blackened collagen) was reduced and the quantitative evaluation indicated a reduction of 3H-proline in the periodontal ligament. The lowest level of 3H-proline activities was observed in the middle, and the highest level in the apical root third, independent of age. All preparations revealed condensations of silver grains, which were located in the region of the periodontal ligament adjacent to the alveolar bone, but did not reveal any preferred position with regard to the dental topography. With progressive age, the uptake of 3H-proline in the periodontal ligament was reduced by about 20 to 30%, a result that corresponds to a decrease in collagenous fiber production. Collagen was mainly formed in the apical and cervical root third, starting from the alveolar bone side, presumably in response to functional strain.     

‘Effects of novel root repair materials on attachment and morphological behaviour of periodontal ligament fibroblasts: Scanning electron microscopy observation’

The aim of this study was to evaluate the adhesion of periodontal ligament fibroblasts (PDLs) on newly proposed root repair materials [Biodentine, MM‐MTA, polymethylmethacrylate (PMMA) bone cement, and SDR], in comparison with contemporary root repair materials [IRM, Dyract compomer, ProRoot MTA (PMTA), and Vitrebond]. Five discs from each material were fabricated in sterile Teflon molds, and the specimens were aged and prewetted in cell culture media for 96 hours. Three material discs were used for scanning electron microscopy (SEM) for the assessment of the attachment, density, and morphological changes in the PDLs, while two samples were used for energy dispersive x‐ray spectroscopy (SEM‐EDX) to determine the elemental composition of the materials. Human PDLs were plated onto the materials at a density of 10,000/well, and incubated for 3 days. The SEM micrographs were taken at different magnifications (500× and 5000×). In the SEM, the cells were attached and well spread‐out on the surfaces of the Biodentine, PMTA, and Dyract compomer, while varied cell densities and morphological alterations were observed in the Vitrebond, IRM, MM‐MTA, SDR, and PMMA bone cement groups. The SEM‐EDX analysis revealed a maximum calcium percentage in the PMTA specimens, as well a maximum silicon percentage in the Dyract compomer specimens. This in vitro study demonstrated that the Biodentine and Dyract compomer supported PDL cell adhesion and spreading. The PMTA presented a favorable scaffold for better attachment of the PDL cell aggregates. Therefore, the calcium and silicon content of a material may enhance the PDL cell attachment.     

Dynamic mechanical behaviour of AZ31 magnesium alloy

To investigate the dynamic mechanical behavior of AZ31 Mg alloy,dynamic compression was carried out using a split Hop kinson pressure bar(SHPB) apparatus at strain rates up to 2.0×103 s-1,and d ynamic hardness was tested employing a dynamic hardness device at room temperatu re.Microstructural characteristic was analysed by optical microscopy.The dynam ic compression results demonstrate that AZ31 Mg alloy exhibits obvious yield phe nomena and strain hardening behavior at high strain rates.The basically same cu rvature of stress-strain curves shows a similar strain hardening rate.The dyna mic yield strength changes little,and the peak stress increases with the strain rates.The dynamic hardness test results indicate that the dynamic mechanical p roperties of AZ31 alloy sheet are anisotropic.The dynamic hardness increases sl owly with average strain for the 0° and 45° oriented samples.For the 90° ori e nted sample,dynamic hardness with strain increases rapidly first and then decre ases when the strain is more than 0.14.An examination by optical microscopy aft er high strain rate deformation reveals the occurrence of twinning,and the twin area percentage escalates with the strain rate increasing.     

Neural model of the dynamic behaviour of a non-linear mechanical system

The availability of a model to prepare, supervise and analyse vibration tests of complex non-linear mechanical systems is vital. Analytical models, however, have some limitations when it comes to integrate the complex mathematical and physical parameters of non-linear mechanical systems. In this paper, an original approach based on multilayer perceptron neural network with a time regression input vector is proposed as an alternative solution. An experimental set is designed for this purpose: a non-linear mechanical system is subjected to random, shock and swept sine excitations on a vibrating platform. Its mechanical response is measured with accelerometers. The model is trained and validated based on the raw experimental data. Finally, simulation results from the analysis are presented and discussed.     

Effect of mechanical properties on frictional behaviour of metals

The coefficient of friction was experimentally determined for some common metals and is correlated with their mechanical properties. Strong metals are shown to give a lower coefficient of friction and are characterised by high hardness, low shear to hardness ratio and low surface energy to hardness ratio     

Investigation of the mechanical behaviour of multi-phase TRIP steels using finite element methods

In recent years, the development of high-strength steels such as multi-phase TRIP (transformation-induced plasticity)-aided steels have shown great promise due to their excellent combination of high strength and ductility, which allows for the use of thinner car frame components and subsequently weight reduction. The TRIP effect, characterized by the phenomenon known as strain-induced martensitic transformation (SIMT), enhances the work hardenability of such steels as the austenite phase transforms to the much harder martensite phase during plastic straining. However, various factors exist which affect the mechanical behaviour of TRIP steels. This study will aim, through the use of finite element models, to investigate the role and influence of each of these factors on the TRIP effect in multi-phase TRIP steels. These factors include the rate at which the martensitic transformation proceeds, the state of stress to which the material is subjected to and the interaction between the surrounding matrix and embedded retained austenite islands in multi-phase TRIP steels. Investigation of these factors will provide further insight on each of their contributions to the TRIP effect in order to exploit the potential benefits offered by these steels.     

Effect of mechanical pre-treatments in the behaviour of nanostructured PVD-coated tools in turning

As a consequence of the lack of characterization of advanced physical vapour deposition (PVD) coatings in the scope of turning, a methodology is presented to evaluate the performance of nanostructured coated tools in the scope of difficult to machine materials turning in particular are austenitic stainless steels. A main aspect of this research is the evaluation of different mechanical pre-treatments before PVD coating in cutting tools. In a first stage, four advanced PVD coatings were analyzed prior to studying the effect of pre-treatments. This stage allowed to identify the nanostructured AlTiSiN coating, commercial denomination nACo, as the one with the best performance for turning austenitic stainless steel. Once the best coating is identified in a second stage, the influence of drag-grinding and microblasting mechanical substrate pre-treatments was analyzed with regard to the performance of coated tools. Several aspects were considered: geometric modification of cutting edge, coating adhesion, substrate roughness and machining performance. The performance of the mentioned coatings was evaluated through wear tests. The machined material was AISI 304L and machining operation was cylindrical turning. Each test started up using a new edge. Results showed that drag-grinding pre-treatment leads to improved coating adhesion and, therefore, in the performance of the tool. Therefore, combination of nACo coating with drag-grinding pre-treatment offers a good solution for difficult to machine metals.     

Surface fatigue behaviour mapping of PVD coatings for mechanical purposes

Most mechanical components used for transmission of movement are subjected to repeated impacts or cyclic stress. If these elements are well designed and the materials well chosen, their durability is linked to surface fatigue mechanisms. In order to improve the fatigue behaviour of these parts, hard coatings, such as physical vapour deposition (PVD) or plasma‐assisted chemical vapour deposition (PACVD) coatings, can be appropriate. Unfortunately, such hard coatings cannot be used for elements whose replacement is more difficult than, say, cutting tools. An understanding of failure mechanisms should make it possible to optimise the fatigue behaviour of hard coatings. In order to study the surface fatigue behaviour of thin, hard PVD coatings, a special apparatus has been developed to carry out repeated impacts over a broad range of speeds. The possibility of mapping the fatigue behaviour of different coatings is illustrated through the examples of TiN and TiN/CrN multilayer coatings of different thicknesses deposited on several substrates.     

Nanocomposite biomaterial mimicking aortic heart valve leaflet mechanical behaviour

The main problem with polymeric heart valves (which are already biocompatible) is that they usually fail in the long term owing to tearing and calcification of the leaflets under high dynamic tensile bending stress and oxidative reactions with blood. To overcome this shortcoming, it is hypothesized that synthetic valve leaflets which mimic native valve leaflet structure fabricated from fibre-reinforced composite material will optimize leaflet stresses and decrease tears and perforations. The objective of this study is to develop a PVA-BC (polyvinyl alcohol-bacterial cellulose)-based hydrogel that mimics not only the non-linear mechanical properties displayed by porcine heart valves, but also their anisotropic behaviour. By applying a controlled strain to the PVA samples, while undergoing low-temperature thermal cycling, it was possible to create oriented mechanical properties in PVA hydrogels. The oriented stress-strain properties of porcine aortic valves were matched simultaneously by a PVA hydrogel (15 per cent PVA, 0.5 BC cycle 4, 75 per cent initial tensile strain). This novel technique allows the control of anisotropy to PVA hydrogel, and gives a broad range of control of its mechanical properties, for specific medical device applications.     

Numerical study of mechanical behaviour of tubular structures under dynamic compression

Journal of Mechanical Science and Technology - This paper presents a novel point of view for the performance investigation and optimization of energy absorber devices, which is numerically...     

Influence of friction on the local mechanical and electrochemical behaviour of duplex stainless steels

The electrochemical behaviour of ferritic and austenitic phases in duplex stainless steel (UNS S32304) and the modifications induced by straining during sliding were studied by potentiodynamic polarisation curves determined at the microscale in a 1 M NaCl (pH 3) solution, using an electrochemical microcell. The mechanical properties and stress state of each phase were determined by microhardness and X-ray microdiffraction measurements, before and after straining. The results show that sliding generates elastic straining of the ferrite and plastic deformation of the austenite. The electrochemical behaviour of these phases is dramatically altered, inducing a reduction of the corrosion resistance and of the passivation properties.     

Polarizing light microscopy of intestine and its relationship to mechanical behaviour

The polarizing optical microscope has been used to observe morphologically the effect of stress on rat and bovine intestine. Collagen fibres about 6 μm in diameter were found to be biaxially oriented at approximately +30° and ?30° to the longitudinal direction. The fibres are arranged in layers with the fibres in each layer densely packed in parallel undulating arrays. The undulations give rise to the extinction pattern observed in the polarizing optical microscope. The initial response to stress is straightening of the fibres. Gradual straightening of the fibres is related to the increasing stiffness of the tissue observed in the stress-strain relationship. Once the fibres are straightened, the biaxial orientation of the fibres produces higher strength in the longitudinal direction than in the transverse direction. This organization of intestinal collagen fibres has not been reported previously and is not observed in other biaxial tissues such as skin and aorta. Thus, intestine is a unique tissue for studying the relationships of mechanical behaviour to structure and organization of collagen.     

Experimental and mechanical model for predicting the behaviour of minor axis beam-to-column semi-rigid joints

This paper describes a series of experimental tests followed by finite element simulations produced to enable the prediction of moment resistance and rotation capacity of minor axis beam-to-column semi-rigid connections. These investigations motivated the development of a mechanical model to assess the connection's structural response. The mechanical model is based on the component method of design, in accordance with the Eurocode 3 specification. This philosophy implies that each joint component is represented by a spring possessing a non-linear force versus displacement (F-Δ) curve. The model was subsequently calibrated against experimental and finite element results previously performed.     

The influence of the mechanical behaviour of the middle ear ligaments: a finite element analysis

The interest in computer modelling of biomechanical systems, mainly by using the finite element method (FEM), has been increasing, in particular for analysis of the mechanical behaviour of the human ear. In this work, a finite element model of the middle ear was developed to study the dynamic structural response to harmonic vibrations for distinct sound pressure levels applied on the eardrum. The model includes different ligaments and muscle tendons with elastic and hyperelastic behaviour for these supportive structures. Additionally, the nonlinear behaviour of the ligaments and muscle tendons was investigated, as they are the connection between ossicles by contact formulation. Harmonic responses of the umbo and stapes footplate displacements, between 100 Hz and 10 kHz, were obtained and compared with previously published work. The stress state of ligaments (superior, lateral, and anterior of malleus and superior and posterior of incus) was analysed, with the focus on balance of the supportive structures of the middle ear, as ligaments make the link between the ossicular chain and the walls of the tympanic cavity. The results obtained in this work highlight the importance of using hyperelastic models to simulate the mechanical behaviour for the ligaments and tendons.     

Shock machine for the mechanical behaviour of hip prostheses: a description of performance capabilities

The aim of this study was to describe the behaviour of a shock machine designed for testing hip prostheses. A microseparation between head and cup occurs inducing a shock of several times the body weight, leading to fracture of ceramic femoral components. Femoral heads and cups of diameter 32 mm manufactured from alumina were tested in dry and wet conditions. Implants were subjected to shocks with a load profile of 9 kN load at 2 Hz and various microseparations. Position is monitored and force is measured with two acquisition systems. The working range and the device capabilities were investigated. Only cups tested in dry conditions failed. Observations by scanning electron microscopy revealed intergranular and transgranular fractures. Two wear stripes were observed on the heads. Three‐dimensional roughness of wear stripes was measured. Since experimental results are in good agreement with retrieved femoral heads, the shocks machine reproduces the in vivo degradations. Copyright © 2011 John Wiley & Sons, Ltd.     

Development of a high speed system to control dynamic behaviour of mechanical structures

Techniques for using real-time system control to modify the dynamics of mechanical structures have been developed and demonstrated on a two-stage mechanical positioning system. Methods for designing control algorithms for fast sampling rates were investigated, and simulations of the system which accounted for many of the nonlinearities of digital control were developed. The results were implemented on an experimental system which showed a dramatic improvement in performance over the uncontrolled case. A unique method of handling known decaying periodic disturbances to the system was also developed and tested. This method resulted in more than 70% reduction of the error due to the disturbance. A computer control system that uses a high speed Digital Signal Processor (DSP) chip to implement control algorithms with fast sampling rates was developed. There still remain, however, several approaches to be investigated before the immense capabilities of DSPs as compensators in digitally controlled mechanical systems can be realised.     

Correlation of some mechanical properties of embedding resins with their behaviour in microtomy

The technology of ultramicrotomy is now well established, but the properties of the resin that determine the different forces needed to generate a section have been neglected, although this process could introduce artefacts in the thin sections. We have investigated the principal resin dependent factors involved in the sectioning process and determined the related mechanical properties. Tensile experiments have given the best correlation with the sectioning quality of the resin: the elastic (Young's) modulus value (depending on polymer structure or hardening mode), the presence of a short plastic flow for a controlled fracture and enough flexibility to minimize shearing, and internal cracks, appear to be the main characteristic parameters. The ultrathin section seems to be generated by a process close to cleavage, favoured by the relative hardness of the embedding media, while machining and “true” sectioning requires softer resins. Consequently, the rupture follows the path of least resistance in the specimen-resin composite, providing sections with a surface relief. Embedded biological material copolymerizes with polycondensed matrix (epoxy resins), and, by reducing the heterogeneity, gives smoother sections. Embedments hardened by radical polymerization provide a rougher relief, since almost no copolymerization occurs, offering to the microtome a heterogeneous block with two constituents of very different mechanical properties. The surface relief seems to be an important factor in labelling, staining, and imaging, and more attention has to be paid for some improvements of the quality of the information provided by electron microscopy.     

Time-dependent characteristics of the nonequilibrium condensation in subsonic flows

High-speed moist air or steam flow has long been of important subject in engineering and industrial applications. Of many complicated gas dynamics problems involved in moist air flows, the most challenging task is to understand the nonequilibrium condensation phenomenon when the moist air rapidly expands through a flow device. Many theoretical and experimental studies using supersonic wind tunnels have devoted to the understanding of the nonequilibrium condensation flow physics so far. However, the nonequilibrium condensation can be also generated in the subsonic flows induced by the unsteady expansion waves in shock tube. The major flow physics of the nonequilibrium condensation in this application may be different from those obtained in the supersonic wind tunnels. In the current study, the nonequilibrium condensation phenomenon caused by the unsteady expansion waves in a shock tube is analyzed by using the two-dimensional, unsteady, Navier-Stokes equations, which are fully coupled with a droplet growth equation. The third-order TVD MUSCL scheme is applied to solve the governing equation systems. The computational results are compared with the previous experimental data. The time-dependent behavior of nonequilibrium condensation of moist air in shock tube is investigated in details. The results show that the major characteristics of the nonequilibrium condensation phenomenon in shock tube are very different from those in the supersonic wind tunnels.