Key Engineering Materials Vols. 348-349

Abstract: The effect of the preformed deflection angle in the lap zone on the failure mode and the stresses distribution in the single-lap joint made of the epoxy adhesive and the aluminium adherend was investigated using elasto-plastic finite element method (FEM) and the experimental method of the shear strength testing. The failed surfaces were studied with the diagrams of scanning electron microscopy (SEM). The results from the numerical simulation showed that all the peak stresses in adhesively bonded single lap joint were decreased first and increased again as the preformed deflection angle was increased from 00 to 150. The lowest values occurred when the angle was taken as 100 and its strength is 22.1% higher than that of the standard one as the peak stresses were decreased and the stress distributed more evenly. The SEM images also show that the failure mode of the joint was transmitted from the adhesion to the mixed one as the deflection angle increased.

Abstract: The inelastic behavior of a 17 level reinforced concrete building, for two structural models, is studied: reinforced concrete frames only (A case), and reinforced concrete frames, and K-type steel braces (B case). The building is designed to avoid that the drifts exceed 0.012 for the same seismic behavior factor level (Q= 3), in agreement with the Mexico City Code. The building is type B (offices). The soil is considered as soft and the foundation is a rigid box with point piles. With the design, A and B cases, the comparisons of the transversal sections, maximum lateral displacements, drifts, story shears, design mechanical elements and resultant reinforcement steel are made. With the step-by-step analysis, A and B cases, with representative accelerations records of the soft soil, the inelastic responses of the local and global ductility demands are determined and also are compared with the permissible levels of the Code. We pretend to observe the inelastic behavior (displacements, maxima demands of global and local ductility), for both types structural models, to decide which it is more convenient.

Abstract: The phase constitution, mechanical properties, and corrosion behavior of TiMoSn alloys were investigated by means of XRD, tensile test, electrochemical measurement and XPS techniques. The XRD analysis results showed that at room temperature TiMoSn alloys are mainly composed of β phase, with minor content of α" phase, in as-cast and solid solution treated conditions. The tensile test results indicate that the elastic moduli of the Ti-Mo-Sn alloys are in the range of 52~74GPa. The electrochemical measurement results indicate that TiMoSn alloys have excellent corrosion resistance in simulated body fluid. The XPS analysis results reveal that the passive films of TiMoSn alloys after polarization consist of TiO2, SnO2 and Mo2O5.

Abstract: Although the use of fracture mechanics based techniques in the evaluation of fatigue resistance in civil engineering has expanded steadily, it’s application in the field of orthotropic bridge decks remains very small. This is remarkable, since especially orthotropic decks have numerous fatigue prone details, which could benefit from a fracture mechanics based approach, or alternatively from the hot spot stress method. Currently, all international design guidelines for orthotropic plated bridge deck are based on a traditional nominal stress S-N approach and Miner’s Rule, and, although suggested by various in field designers and bridge owners, a fracture mechanics approach is currently limited to academic research. The paper gives an overview of current practice in the field, as well as an overview of attempts made academic researchers to apply fracture mechanics techniques. The available literature suggests that the lack of implementation of fracture mechanics results from a combination of factors including the lack of experience with these methods, conservatism, but also the complexity of geometry and loading conditions resulting in numerous variables. This paper may contribute to the implementation of modern fracture mechanics based techniques in this field by pointing out the opportunities and warning for the difficulties.

Abstract: Various asphalt pavement distresses, such as longitudinal cracking, thermal (transverse) cracking, and reflective cracking, are directly related to the fracture properties of the asphalt materials used in the pavement layers. The fracture resistance of asphalt materials significantly influences the service life of asphalt pavements and consequently affects the maintenance and rehabilitation costs of the pavement network. Therefore, there is significant interest in understanding the mechanism of fracture in asphalt pavements and in developing analysis tools that would lead to the selection of asphalt materials with increased fracture resistance. This paper reports a research effort of dynamic observation of the micro-fracture process of asphalt mixture under uniaxial compressed condition by using CT (computerized tomography) method. CT images were analyzed to investigate the process of micro-fracture of asphalt concrete. Research results show that the abnormal belt of density damage of asphalt concrete is the initiation belt of cracking and the turning point in the curve of stress and density distress increment is the point of cracking failure of asphalt concrete.

Abstract: In-situ SEM observations have revealed that fatigue crack propagation in aluminium alloys is caused by the shear band decohesion around the crack tip and the formation and cracking of the shear band is mainly caused by the plasticity generated in the loading part of the load cycle. This shear band decohesion process has been observed to occur in a continuous way over the time period during the load cycle. Based on this observation, in this study, the transient fatigue crack growth rate, da/dt, has been used to obtain the relationship between the conventional used parameter da/dN and the applied driving force. It is proven that two parameters are necessary in order to accurately describe fatigue crack propagation rate per stress cycle, da/dN. The well known stress ratio effects on fatigue crack propagation rate can be correlated by this model.

Abstract: The paper presents background information and experimental results regarding the assessment of fatigue damage in welded steel structures by thermographic investigations of thermomechanical coupling effects. The results confirm the high potential of specialized thermographic methods for the experimental characterization of all stages of fatigue damage in welded and un-welded components. The technique provides a new experimental mean to investigate early inhomogeneous fatigue damage as mesoplasticity and cracks in the weld toe. The method has been successfully applied during fatigue testing of welded components and allows detecting localized damage as early as 10% to 20% of the total fatigue lifetime of the tested specimens.

Abstract: The objective of robust design is to optimize the mean of a given target variable and to minimize the variability that results from uncertainty represented by “noise” factors. A recent strategy for robust design is based on stochastic processes, which has resulted in a new design technique called “stochastic design improvement” (SDI). In this work a home-made procedure is presented, which is based on the SDI technique and which is illustrated with reference to a case study which aims to increase the residual strength of a cracked stiffened aluminum panel.

Abstract: Cracking is one of the major distress types of asphalt pavements. Expected life of pavements, pavement condition and maintenance cost are directly related to pavement cracking. It is necessary to promote the understanding of cracking mechanism of hot mix asphalt (HMA) based on fracture mechanics. Simulation, surveying, observation, and measurement of cracking in pavement structures have been reported in literature in last three decades. However, cracking process in asphalt mixtures in a three dimensional scale is still a great challenge to road engineers. This paper reports a research effort of dynamic observation of the micro-fracture process of asphalt mixture under compressed condition using CT (computerized tomography) method. Through CT scanning, the clear CT images, which include the microcrack compressed, growth, bifurcation, development, the crack fracture, and the asphalt concrete sample failure were obtained. The CT numbers, CT images and the other data were analyzed. Analysis of the relationship between of CT number and volumetric strain suggests that damage value at zero volumetric strain can be used as threshold point in damage analysis. Corresponding stress value can be used as maximum allowable stress in design practice.

Abstract: The rock is assumed to be assembled by a great number of HMUs. The number of HMUs responding to external force is uncertain, which leads to the instability of hysteretic loops. Transition probability of HMUs to the external force and temperature varies with external loading frequency and temperature. The self-adaptive adjustment effects for rocks in frequency field are obtained by uniaxial and triaxial cyclic loading: when frequency increases, transition probability of the response of mesoscopic structure to applied loads reduces, the adjustment time of self-adaptive process becomes shorter, residual strain become smaller, and hysteretic loops tend to be easy to be stable. The self-adaptive adjustment effects in temperature field can also be obtained.