Titanium dental implants coated with Bonelike®: Clinical case report

The aim of the study was to evaluate the direct bone bonding and osteointegration of the commercial pure (cp Ti) implants coated with Bonelike® synthetic bone graft by plasma spraying. The Bonelike® coated implant was placed in the mandible of a 40-year-old patient and it was removed after a healing period of 3 months with a trephine of 6 mm diameter. The structure of the coating and new bone/implant interface of retrieved samples were evaluated using scanning electron microscopy (SEM) and histological analysis using light microscopy. In vivo microstructure observations of Bonelike® coated retrieved implants showed excellent bone remnants on its surface without any tissue and inflammatory signs observed. The reported Bonelike® coated (cp Ti) implants improved primary stability, which may increase the lifetime of the implant. Bonelike® coated dental implants proved to be highly bioactive with extensive new bone formation and strongly bonded to Bonelike® coating.     

Experimental studies on mechanical properties of cellular structures using Nomex® honeycomb cores

An experimental method is presented to obtain the effective in-plane compliance matrices of cellular structures using Nomex® honeycomb cores without a priori assumptions such as orthotropy. In this method, firstly, uni-axial tension tests are carried out for different material orientations. The independent variables in these experiments are the material orientation and displacement of the actuator, while the main dependent variables are positions of the marker points and the force acting on the specimens. Marker tracking technique is used to determine the marker positions which are processed to get strain of the measuring domain, while the stress is estimated through external loading and core geometry. The analysis is confined to the measuring domain under near homogeneous stress and strain fields. The experiment results are processed with transformation and least squares functions to obtain all effective in-plane elastic parameters, which are compared with analytical solution based on deformation of idealized cell structure. Through this comparison, the effects of geometrical parameters of cell structure are discussed in detail. By means of the introduced method, the problem of lack of experimental studies on the effective in-plane compliances of cellular structures in the literature is expected to be solved.     

Determination of in-situ fibre, matrix and interface properties in a composite using tensile tests and an extended shear-lag model

The strength and modulus values of the matrix and of the fibres as incorporated in a ceramic matrix composite (CMC) may differ from those obtained on the matrix and the fibres separately for a number of reasons. The same applies for the interfacial properties obtained from tests on specimens designed for that purpose, and their values in a real CMC. This paper outlines a methodology which allows the determination of the in-situ values of these parameters in the composite. It is based on an evaluation of the results of tensile tests in the light of a newly developed extended shear-lag model. Calibration of the five model parameters leads to the identification of the fibre and matrix moduli, as well as of the interfacial friction coefficient and the radial interface strength. The axial stress profiles obtained from the shear-lag model subsequently allow the determination of the fibre and matrix strength. The methodology is illustrated by application to a 2D woven CMC. In-situ properties at the level of individual tows, and on the level of individual fibres within a longitudinal tow are obtained.     

Soil deposit stochastic settlement simulation using an improved autocorrelation model

Soil parameters are spatially random variables. Thus, the spatial correlation relationship, besides the mean and variance, of a specific soil site is needed for any realistic stochastic modeling. In this regard, an improved autocorrelation model involving a linear, an exponential and cosine terms, named linear-exponential-cosine (LNCS), is adopted here to capture the spatial properties of the soil deposits. Further, a random field of the soil deposit is simulated using a two-dimensional Karhunen–Loéve expansion based on the new autocorrelation model. Furthermore, two cases for the soil settlement are calculated with the random field of the soil deposits. One case is the stochastic settlement from a reference paper. Some comparisons are undertaken, and it is found that the mean value agrees well with the reference. The other case involves the differential settlement analysis of a real engineering project. The settlement is calculated with the random field, the uniform field respectively, and is compared with the on-site measured values. The results show that the random field model can capture the differential settlement better than the corresponding uniform field model.     

基于单自由度参数激励模型的车辆随机振动分析

包装件振动可靠性的分析和优化需要构建高效准确的车辆随机振动载荷分析模型.该研究将车辆建模为单自由度参数激励系统,分析车辆在平稳载荷下的加速度响应.基于Mathieu方程分析车辆动态响应特性,结果表明,车辆响应中出现少量的大幅振动响应的原因是参数激励系统中随机过程β(t)的波动引起了系统的失稳.分别构建车辆在稳态、失稳和...     

Biomechanical analyses of whiplash injuries using an experimental model

Neck pain and headaches are the two most common symptoms of whiplash. The working hypothesis is that pain originates from excessive motions in the upper and lower cervical segments. The research design used an intact human cadaver head-neck complex as an experimental model. The intact head-neck preparation was fixed at the thoracic end with the head unconstrained. Retroreflective targets were placed on the mastoid process, anterior regions of the vertebral bodies, and lateral masses at every spinal level. Whiplash loading was delivered using a mini-sled pendulum device. A six-axis load cell and an accelerometer were attached to the inferior fixation of the specimen. High-speed video cameras were used to obtain the kinematics. During the initial stages of loading, a transient decoupling of the head occurs with respect to the neck exhibiting a lag of the cranium. The upper cervical spine-head undergoes local flexion concomitant with a lag of the head while the lower column is in local extension. This establishes a reverse curvature to the head-neck complex. With continuing application of whiplash loading, the inertia of the head catches up with the neck. Later, the entire head-neck complex is under an extension mode with a single extension curvature. The lower cervical facet joint kinematics demonstrates varying local compression and sliding. While the anterior- and posterior-most regions of the facet joint slide, the posterior-most region of the joint compresses more than the anterior-most region. These varying kinematics at the two ends of the facet joint result in a pinching mechanism. Excessive flexion of the posterior upper cervical regions can be correlated to headaches. The pinching mechanism of the facet joints can be correlated to neck pain. The kinematics of the soft tissue-related structures explain the mechanism of these common whiplash associated disorders.     

基于黏弹性本构模型的热固性树脂基复合材料固化变形数值仿真模型

针对热固性树脂基复合材料热压罐成型工艺过程,采用广义Maxwell(麦克斯韦)黏弹性本构模型建立了残余应力和固化变形的三维模型。模型考虑了复合材料固化过程中的热-化学效应、材料的热胀冷缩效应、基体树脂黏弹性效应以及材料的各向异性。通过与文献中实验结果的比较,证明了所建立的模型具有较高的可靠性。对复合材料C型制件的固化过程进行了数值模拟和实验对比,比对结果表明该数值模型具有较高的准确性。     

Vibration analysis of multi-walled carbon nanotubes using a spring–mass based finite element model

We report a study of the vibrational characteristics of multi-walled carbon nanotubes modeled exclusively using springs and lumped masses. Based on the atomic microstructure of the nanotube, three-dimensional nanoscale spring elements are utilized to simulate the dynamic behavior of each layer of the multi-walled carbon nanotubes. Appropriate spring elements are also developed to model the interlayer interactions and describe the van der Waals potentials between carbon atoms on different layers. Direct application of the physical variables of molecular mechanics theory to the springs is used to simulate the relative translations and rotations between atoms as well as the masses of the carbon atoms. The stiffness and mass matrices of the problem are used to construct the dynamic equilibrium equation. The natural modes of vibration and the corresponding natural frequencies are derived by solving the eigenvalue problem for different support conditions. The present method suggests novel basic modes of vibration, beyond those reported in the literature pertaining to multi-walled carbon nanotubes. The effects on the basic modes and natural frequencies created by van der Waals interactions and geometric parameters such as number of layers and aspect ratio are investigated in the context of elastic support conditions. Comparisons with other theoretical studies reveal very good correlations in terms of fundamental modes and frequencies.     

Broadband model order reduction of polynomial matrix equations using single‐point well‐conditioned asymptotic waveform evaluation: derivations and theory

To effect a model order reduction (MORe) process on a system which has a polynomial matrix equation dependence on the MORe parameter, researchers generally take one of two approaches. The first is to linearize the system by introducing extra degrees of freedom and then to solve the resulting expanded, linear system with a method such as Lanczos or Arnoldi. The second approach is to work directly with the polynomial system and use a technique such as asymptotic waveform evaluation (AWE). Of course, each approach has advantages and disadvantages. In this paper, a new technique will be presented which has some desirable characteristics from both approaches and which is able to circumvent simultaneously some of their disadvantages. It can be shown that both the Arnoldi and the AWE methods are special cases of this new technique. Finally, numerical results will show the viability of the new method, which will be called the well‐conditioned asymptotic waveform evaluation (WCAWE) method. Copyright © 2003 John Wiley & Sons, Ltd.     

基于弹塑性损伤本构模型的复合材料层合板破坏荷载预测

在连续损伤力学和塑性力学框架内,建立一个同时考虑塑性效应和损伤累积导致材料属性退化的复合材料弹塑性损伤本构模型。基于最近点投影回映算法,开发本构模型的应变驱动隐式积分算法以更新应力及与解答相关的状态变量,并推导与所开发算法相应的数值一致性切线刚度矩阵,保证有限元分析采用NewtonRaphson迭代法解答非线性问题的计算效率。采用断裂带模型对已开发的本构模型软化段进行规则化,以减轻有限元分析结果的网格相关性问题。对损伤变量进行粘滞规则化,并推导出相应的粘滞规则化数值一致性切线刚度张量,解决了在有限元隐式计算程序中采用含应变软化段本构关系的数值分析由于计算困难而提前终止的问题。开发包含数值积分算法的用户材料子程序UMAT,并嵌于有限元程序Abaqus v6.14中。通过对力学行为展现显著塑性效应的AS4/3501-6V型开口复合材料层合板的渐进失效分析,验证本文提出的材料本构模型的有效性。结果显示,预测结果与已报道的试验结果吻合良好,并且预测精度高于其他已有弹性损伤模型。表明已建立的弹塑性损伤本构模型能够准确预测力学行为,展现显著塑性效应的复合材料层合板的破坏荷载,为其构件和结构设计提供一种有效的分析方法。     

Assessment of fatigue crack closure under in-phase and out-of-phase thermomechanical fatigue loading using a temperature dependent strip yield model

In this paper fatigue crack closure under in-phase and out-of-phase thermomechanical fatigue (TMF) loading is studied using a temperature dependent strip yield model. It is shown that fatigue crack closure is strongly influenced by the phase relation between mechanical loading and temperature, if the temperature difference goes along with a temperature dependence of the yield stress. In order to demonstrate the effect of the temperature dependent yield stress, the influence of in-phase and out-of-phase TMF loading is studied for a polycrystalline nickel-base superalloy. By using a mechanism based lifetime model, implications for fatigue lives are demonstrated.     

Oxidation kinetics of metallic inclusions in a composite with an Al2O3‐ZrO2 matrix

Many studies have been conducted to obtain toughness in ceramic‐ceramic and metal‐ceramic composites, using new processing techniques or by addition of metallic inclusions in the ceramic matrix. The oxidation behavior of metallic inclusions, niobium and nickel metallic particles embedded in an Al₂O₃‐ZrO₂ matrix, was measured through thermogravimetric analysis. In this work oxidation mechanisms were proposed and kinetic parameters defined using a software module for the kinetic analysis of thermal measurements by means of multiple linear or non‐linear regression. The morphology of the oxidized surfaces of the samples was examined by using scanning electron microscopy. Oxides were identified by X‐ray diffraction. The oxidation kinetics of metallic particles of niobium and nickel are highly complex, for they depend on various factors, i.e., metal’s characteristics, processing of the composite, oxygen diffusion through the matrix, grain boundaries, and heating rate applied to the material.     

Unstable matrix equations and their relationship to measuring the emittance of an electron beam using beam position monitors

In general, the spatial distributions of electron beams from photoinjectors are unknown and are not well approximated by a Gaussian. Therefore, when measuring the emittance, it is important to make no assumptions about the beam's spatial distribution. An emittance diagnostic that fulfills this requirement uses beam position monitors to measure the second moment of the electron beam's image charge. This information, coupled with the beam line's transfer matrix, forms a set of linear equations. The least squares solution to these equations estimates the necessary beam parameters. More often than not, however, this solution is unstable and amplifies the errors in the second moment measurement. For this diagnostic to be viable, a stable implementation has to be found. Described here is an approach that accomplishes this task.     

Estimation of time‐to‐failure distribution derived from a degradation model using fuzzy clustering

Some life tests are terminated with few or no failures. In such cases, a recent approach is to obtain degradation measurements of product performance that may contain some useful information about product reliability. Generally degradation paths of products are modeled by a nonlinear regression model with random coefficients. If we can obtain the estimates of parameters under the model, then the time‐to‐failure distribution can be estimated. In some cases, the patterns of a few degradation paths are different from those of most degradation paths in a test. Therefore, this study develops a weighted method based on fuzzy clustering procedure to robust estimation of the underlying parameters and time‐to‐failure distribution. The method will be studied on a real data set. Copyright © 2000 John Wiley & Sons, Ltd.     

Structural performance evaluation of an aged structure using a modified plasticity model in inverse solution method

In this research, the trends of degradation of stiffness and compressive strength of mass concrete of the Dez concrete arch dam, over time have been evaluated using a new and robust method. In previous studies of the corresponding author, he was successful to estimate the degradation of the stiffness of dam’s concrete at the end of a selected time interval, i.e. the Year 2007 from the interval 1965–2007 using a novel thermal inverse analysis. But, he was interested in knowing the trend of this degradation over that time interval. In previous studies, the author used isotropic and transversely isotropic elastic material models for evaluating the stiffness degradation. By these material models, the reduction of the compressive strength of the mass concrete could not be determined. In this new attempt, to overcome the above-mentioned shortcomings, it was decided that the time interval is limited to a maximum of one year from 1965 to 2007. In addition, a more sophisticated material model for concrete of the dam, named as, concrete damaged plasticity model, from the ABAQUS standard material library, was employed to simulate the long-term concrete behaviour. By this material model, the reduction of the compressive strength of the dam’s concrete could be measured. Obtained results demonstrated that the reduction of stiffness and strength of dam’s concrete have not a constant rate of change and they have been accelerated considerably after passing approximately 20 years from the commissioning of the dam.     

Deformation analysis of tunnel excavation in gravelly formation using the anisotropic degradation model

This study adopts a shear-induced anisotropic degradation model to analyze the deformation of excavation in gravelly formations. The adopted model is a variable moduli model with the following characteristics: the stress–strain relationship originates from degradation of the bulk modulus and shear modulus subjected to different loadings. Under hydrostatic loading, gravelly soil may behave as an isotropic material. However, when gravelly soil is subjected to shear loading, the material becomes anisotropic and degrades before ultimate strength is attained. Therefore, this study introduces an anisotropic factor to reflect the tendency to shear-induced volumetric deformation. To analyze the deformation of the Pakuashan tunnel, which passes through a gravelly formation in Taiwan, the model was first validated by comparing the drained triaxial test results of gravelly materials sampled from the tunnel. The proposed model is implemented with a finite element code to predict the tunnel deformation under construction. A comparison between the monitoring data and numerical analysis shows that the proposed model can reasonably simulate the behavior of a gravelly formation under excavation. Numerical analysis shows that the main deformation of the tunnel is the result of significant degradation of the moduli around the whole section, especially at the crown and invert of the tunnel.     

On the constitutive relation of materials with microstructure using a potential-based cohesive model for interface interaction

Macroscopic constitutive relationship is estimated by considering the microscopic particle/matrix interfacial debonding. For the interfacial debonding, the PPR potential-based cohesive model is utilized. The extended Mori-Tanaka model is employed for micromechanics, while a finite element-based cohesive zone model is used for the computational model. Both models (theoretical and computational) agree well each other in representing the macroscopic constitutive relationship on the basis of the PPR model. The microscopic interfacial cohesive parameters of the PPR model are estimated from macroscopic composite material behavior. In addition, different microscopic debonding processes are observed with respect to different macroscopic constitutive relationships (e.g. hardening, softening, and snap-back).