Materials Science Forum Vols. 514-516

Abstract: This paper reports the results of a numerical study concerning the influence of local brittle zones intersecting the crack front on the fracture behaviour of welded joints. This work was performed using the numerical simulation of the three point bending test of weld samples with different amount of brittle structures at the crack front. Using 3D finite element discretization it was possible to simulate welded samples with very small fractions of brittle zone at the crack front, such as 5 %. Comparing the results of samples with increasing proportion of brittle zone it was observed a significant decrease in the crack growth resistance with increasing amounts of brittle material. This decrease in crack growth resistance was obtained even for samples with very small amounts of brittle material at the crack front.

Abstract: Mixtures of clays are often used in the manufacture of bricks. In industrial practice, it is desirable to be able to predict, in an expeditious way, what the effects of a change in raw materials or the proportions thereof might be in the various processing steps towards the final product. When the property of interest is basically determined by the combination of raw materials, an optimisation methodology specific to the design of mixture experiments can be successfully used. In the present study, fired bending strength and linear firing shrinkage were selected as the properties to model. Ten formulations of three different clays were selected and used in the experiments design. Those formulations were wet processed, uniaxially pressed, and then fired at 1000 °C for 2 h.. From the experimental results, regression models were calculated, relating each technological property with the proportions of raw materials. The regression models were then used simultaneously to define which combination of those three raw materials was most adequate to produce, under constant processing conditions, a brick with specified properties.

Abstract: A 3D numerical code, based on the finite volume method, able to model the cooling stage of an extrusion line is presented and validated. For this purpose, an analytical solution of a simple heat transfer multi domain problem was developed, the result obtained being compared with the predictions given by the numerical code. A prior study performed with the above mentioned code showed that in general when a reduction of the profile average temperature is imparted, lower temperature homogeneity is also obtained, being the only exceptions the reduction of the extrusion velocity and splitting the calibrator into several units, separated by annealing zones. Therefore, the only way to improve the cooling efficiency without compromising the production rate is to divide the total cooling length into several independent units. In this work that investigation is further extended to study the influence of the individual cooling units and annealing zones lengths distributions on the global cooling efficiency.

Abstract: Hydroxyapatite (Ca5(PO4)3 OH) (HAp) is a crystalline structure and composition analogue to calcified tissues of vertebrates. The biomedical significance of HAp is its bioactivity – HAp ceramics leads to the formation of new bone on their surface. HAp properties are ascribed to the characteristic surface structure of HAp, while the detailed mechanism is still unknown. Modeling and computation of HAp molecular nanostructures, exploration of the possible mechanisms of its surface charging (polarization), based on proton transfer, and the discussion of the adhesion properties of HAp nanoparticles and ceramics are the aim of the work.

Abstract: The modeling of heat transfer in materials containing exothermic components must take into consideration the presence of heat sources in the Fourier–Kirchhoff equation. The aim of this investigation was the identification of real and effective thermophysical parameters of the insulating–exothermic materials used as riser sleeves containing these exothermic heat sources. The experiments of steel pouring into the mould, containing different insulating and exothermic sleeves were carried out, using thermocouples measurement systems (thermal analysis of casting–mould system). Then the thermophysical coefficients of these materials were calculated using inverse problem solution. The worked time–dependent formula of exothermic reaction heat (heating yield in W/m3) was called heat source function. The paper presents the basis and the practical expression of heat source by different functions, its justification and the results of simulations using these functions. The numerical system Calcosoft and its Inverse Solution procedure were applied.

Abstract: One of the most relevant technological parameters for the accurate numerical simulation of the deep drawing process is friction, since the contact between the blank sheet and tools develops friction forces that act as supplementary boundary conditions that determine the final part shape. Most applications reported in literature are still restricted to Coulomb’s law with a constant friction coefficient over the entire process. Although it is consensual that state conditions of contact surfaces and consequently the friction behaviour are influenced by a large number of parameters, there is no agreement about a wide-ranging law to accurately describe the friction evolution. One possibility is to use phenomenological laws that accurately fit experimental data. A Voce type law is used in this work to describe the evolution of the friction coefficient over the entire process as function of the contact pressure. This type of law guarantees a good correlation with experimental data and also numerical stability. The Voce type law was implemented in the static implicit code DD3IMP. The analysis of the relevance of considering the evolutional friction law in the numerical simulation is performed for a U-rail. This shape was selected for this study due to the simple deformation mechanisms that are involved, but also because it is a rail specially conceived to emphasize 2-D springback defects. The blank sheet material selected is a 6016-T4 aluminium alloy. The plastic behaviour is modelled using the 1948 Hill’s criterion with isotropic and kinematic hardening. The numerical results obtained considering the evolutional friction law are compared with two other results obtained with: (i) a constant friction value of 0.10, normally used for this material in industrial practice and (ii) three constant friction coefficients for each contact zone (flat, die radius and punch radius).

Abstract: When applied to product engineering, the Taguchi method allows the identification of the conditions that lead to the development of similar products with a small variation around a particular target value. In the present work, the Taguchi method was used to evaluate the effect of four processing parameters on the mechanical properties of a commercial dense cordierite in order to maximize the flexural strength of the resulting bodies. Using a L9 orthogonal array, four control factors (named A, B, C and D) at three levels, namely type (AS1, AS2 and AS3) and content (18, 20 and 22 wt%) of alumina added, cooling rate (1, 3 and 5 °C⋅min-1) and average particle size of each batch (5, 6 and 7 μm) were set. Hence, with a reduced number of experiments, it was possible to identify the factors and their levels, which have most contributed to an increase in flexural strength of the resulting cordierite bodies. The optimum conditions to obtain bodies having a flexural strength higher than 100 MPa were the addition of 22 wt% of AS2 alumina, an average mixture particle size of 5 μm and a cooling rate of 1 °C⋅min-1. The type of alumina had a negligible effect on flexural strength whereas the other factors affect considerably the strength.

Abstract: The application of nonparametric method of functional regression for the classification of different types of materials studied by thermo-gravimetric analysis is proposed in this work. The method is illustrated with two case studies for classifying different materials. Some simulation study shows the performance of the method when the correlation between groups parameters and the error variance change.

Abstract: In this paper, the design and characteristics of a novel ultrasonic motor which is applicable to optical zoom or auto focusing operation of lens system for mobile phone are investigated. Its design and simulation of performances are carried out by FEM (Finite Element Method) commercial software. The shape of the motor is like square without one side, which is able to insert optical lens. Two sheets of piezoelectric ceramic are fixed to both sides of the two legs of an elastic body, respectively. To drive the ultrasonic motor, the voltage is applied to the two sheets of piezoelectric ceramic bonded to one leg. The rotation direction can be easily changed by switching the applied voltage to the piezoelectric sheets bonded to the other legs. The proto type of motor is fabricated and its outer size is 10*10*2 mm3 including the camera lens of which the diameter is 7.5 mm. Its power consumption is about 0.3 W and the rotation speed is adjustable from 10 to 200 rpm according to the applied voltage.

Abstract: Compensation has been reported for the relaxation parameters: the activation energy W and the pre-exponential factor τ0, determined from the Thermal Sampling of Thermally Stimulated Depolarization Current technique. Below the glass transition it is assumed that the relaxation time follows an Arrhenius equation. In the vicinity of glass transition temperature an experimental thermogram may be analyzed using the Vogel-Fulcher-Tamman-Hesse (VFTH) or the Williams- Landel -Ferry equation. In this article we use the VFTH relationship to study the compensation effect in the range of glass transition. For an elementary peak obtained by TS there is a relationship between the activation energy W, the temperature of the maximum current Tm, the VFTH temperature, the compensation temperature Tc and the compensation time τ c. We employ this relationship for a basic analysis of the compensation effect in the temperature range around Tg. By numerical simulations, and assuming parameters similar to those measured experimentally, we show that it is possible to observe a compensation point in some well defined conditions