Papers by Keyword: Void

Abstract: Heat flux inside the HP LED chip is also increasing with the increasing drive current, integration and miniaturization of LED chips. The junction temperature of LED strongly depended on the heat transmission capacity of die attach layer, which provided the heat dissipation channel between the heat generating LED chips and the heat slug. The voids, interrmetallic compounds or a small delamination would lead to the increasing thermal resistance in the die attach layer. In this paper, the reliability of soldered-bonded interfaces was studied in high-brightness LEDs, which were prepared by Cree and currently available on the market. Results revealed that the higher drive currents would lead to the accelerated degradation or failure of treated LEDs. Additionally, the injection current had an important effect on void formation and growth at the solder-bonded interfaces. The larger drive current would induce the delamination between LED die and heat slug. This study provided some guidance for the end users and a theoretical basis for solder-bonded technologies.

Abstract: Void nucleation behavior of ferrite/martensite dual phase steels varying martensite fraction was investigated. As easily recognized, void fraction was increased with strain induced, and more voids were nucleated in the sample with higher martensite fraction. On the other hands, void fraction at ductile fracture was decreased with increasing martensite fraction. Void nucleation was observed to occur due to the fracture of martensite in DP steels. In the sample with high martensite fraction, many micro voids were nucleated at initial deformation with small strain and lead to ductile fracture. Inter-voids distance at the fracture was almost same among the DP steels with various martensite fractions. It is considered that the most effective factor on ductile fracture of DP steels was not 2^nd phase fraction but the distance between 2^nd phases which caused micro voids.

Abstract: It is possible to successfully propose the physical-metallurgical and structural conditions for the equilibrium between steel strength and toughness by designing structural parameters which have a positive effect on the relation between local and macroscopic fracture processes. In carbon steels and microalloyed steels for a wide range of technical uses, whose structure after heat treatment consists of a basic matrix of tempered lower bainite with precipitated carbides and sometimes other types of inclusions, toughness is dependent primarily on the size distribution of second phase particles, their volume ratio, and also the strength of the matrix/particle phase boundary and the mechanical properties of structural phases. By modelling and simulating the process of main crack formation during high-energy ductile fracture, it is possible to propose optimum physical-metallurgical and geometric parameters of steel structure in order to achieve the required relation between strength characteristics and toughness. This paper presents an analysis of results achieved in several tasks carried out to predict mechanical properties in ductile fracture, and it outlines potential future developments. The aims are to determine the limit characteristics of mechanical behaviour of structural steels which can be achieved with a view to the current structural situation and technological possibilities, and furthermore to propose future methods for determining relations between microstructure and toughness.

Abstract: Three-dimensional finite element analysis was made on the transverse joint stress state of the concrete pavement slab with void underneath using ANSYS. The transfer effect of dowel bar was discussed with aircraft loaded in the joints. The influence rule of load transfer effect under different dowel bar spacing and dowel bar cross section dimension was compared. Based on the results of finite element analysis, this paper had carried on the experimental study on stress-transferring effect on concrete pavement joints with different location of the dowel bar. The influencing factors of pavement slab transverse joint with void underneath and the resistance of pavement damage on the joint was analyzed. Theoretical analysis showed that in order to reinforce the resistance capacity of local cavity on concrete pavement joint, the largest spacing of transverse dowel bar set shall not be more than 45cm, and the main factors influencing the resistance void ability on the joint are top reaction modulus, coefficient of cavity, the concrete elastic modulus and coefficient of transverse reinforcement.

Abstract: Experiments have shown that initial voids may exist in the manufacturing processes of pure aluminum, which adversely affect its mechanical properties. In this study, the process of plastic deformation around voids in pure aluminum was examined at atomic scale through molecular dynamics (MD) simulation. The Modified Embedded Atom Method (MEAM) was employed to characterize the atomic interactions in the pure aluminum with two voids. The calculation results revealed that the interaction of two voids endures three phases when the interval of the voids is increased: void coalescence, void coactions followed by the formation of a stress shield zone, and interaction vanishing. The critical parameters of the interval for the three phases were defined as well in this work. It was observed that crack initiated and further propagated near the voids along the slip systems of FCC crystal, which eventually caused structural failure. Meanwhile, the evolution of micro structure in the crack propagation process was investigated by means of Common Neighbor Analysis (CNA). The results showed that the phase transformation occurred near the voids during loading process.

Abstract: For the development of the crystal pulling process for 450 mm-diameter defect-free Si crystals, the impact of thermal stress on intrinsic point defect behavior during crystal growth is studied using extensive density functional theory calculations. The impact of thermal stress on the so-called Voronkov criterion and on void formation is clarified and compared with published experimental results.

Abstract: The non-equilibrium statistical theory was used as a theoretical approach to modeling and predicting void evolution in metal materials. Fokker-Plank equation was introduced as the kinetic equation for the void evolution, from which the probability density distribution function of voids could be obtained. From the micro-mechanism of metal's irradiation damage, void growth rate equation was obtained using spherical Weilv model and control diffusion theory, and then was simplified based on appropriate assumptions. According to the probability density distribution function of void, a series of macro-mechanical characteristics caused by void growth can be calculated, such as: the critical radius of the void nucleation, the average radius of void. Thus the correlation between the void microstructure evolution and the macroscopic properties of metals can be achieved.

Abstract: Damage of metals subjected to large plastic deformations typical for forming processes is mainly governed by void nucleation, growth and coalescence. An opposite process may occur in deformation processes with negative stress triaxialities: the closure of strain-induced defects under large hydrostatic pressure. Understanding the mechanisms of damage growth and healing under plastic deformation of metals is still an urgent problem. In order to solve it a theoretical framework for anisotropic ductile damage based on a physically motivated concept for changes in the void volume and shape was recently developed [6]. Strain-induced damage was experimentally determined during uniaxial compression of cylindrical metallic specimens with artificial voids represented by fully-trough drilled holes. It was revealed that the governing physical mechanism of failure is a change in void shapes due to compressive stresses at low negative stress triaxialities in contrast to the growth of voids volume due to high positive stress triaxialities in the processes with dominating tensile stresses. The tensorial model presented in [6] proved to be able to describe kinetics of ductile damage, failure as the ultimate damage, and the closure of voids at negative stress triaxialities.

Abstract: The tendon ducts in post-tensioned bridge girders must be grouted fully so as to prevent water and oil stain from entering the ducts, which will decrease the durability and load-carrying capacity of post-tensioned bridges badly. This paper describes the motivations for and recent history of Impact-Echo (IE) scanning applications to internal grout condition evaluation of tendon ducts in post-tensioned girders. The tendon ducts’ internal injection quality of a railway bridge in Yichang city was evaluated using IE technology. During the in-situ testing, the IE signals were collected firstly, then the transformation from the time to the frequency domain was carried out using the principles of the fast Fourier transform, data interpretation was much simpler and quicker in the frequency domain. The grout condition of tendon ducts could be determined according to the frequency analysis, poorly grouted sections can be differentiated from the well-grouted sections within the tendon ducts. Results show that the IE method can be employed to detect voids in the grouted tendon ducts of many types of post-tensioned structures.

Abstract: The present work tries to make an attempt to improve previous work which offers a simple but effective way to construct satisfied predicating model. Indeed, recent work due to Peng et al. [13] and dealing with a two-step homogenization scheme is revisited here by introducing an iterative process which allows us to take into account differently the porosity of the elastic media. Several homogenization schemes (dilute, Mori-Tanaka, self-consistent ...) are presented and compared with experimental data. One can say that the current approach provides reasonably accurate predictions for the effective moduli of multiphase composites without using the n parameter as proposed by Peng et al. [13]