نشریه علوم کاربردی و محاسباتی در مکانیک
سال بیست و سوم شماره 2 (بهار و تابستان 1391)

The forming limit diagram (FLD) is a useful method for characterizing the formability of sheet metals. In this paper, different numerical models were used to investigate the FLD of Tailor Welded Blank (TWB). TWBs were Co2 laser-welded samples of interstitial-free (IF) steel sheets with different thicknesses. The results of the numerical models were compared with the experimental FLD as well as with the empirical model proposed by the North American Deep Drawing Research Group (NADDRG). The emphasis of this investigation is to determine the performance of these different approaches in predicting the FLD. These numerical models for FLD are: second derivation of thinning (SDT), effective strain rate (ESR), major strain rate (MSR), thickness strain rate (TSR) and thickness gradient (TG). Results of this research show necking will happen, when the value of MSR, TSR, ESR criteria is maximum, TG ≤ 0.78 and SDT criterion has the first peak in forming process time. Weld line movement is one of parameters which influence formability of TWBs. Maximum of weld line movement of TWB base on different numerical criteria was predicted and compared with experimental results. Results of this research show that SDT criterion has a good accuracy for prediction of forming limit diagram and also weld line movement in the forming of TWB.

Vibration cutting (VC), elliptical vibration cutting (EVC) and ordinary cutting (OC) of In738 work-pieces have been experimentally investigated in the present research. The experiments were carried out by using single crystal diamond tool and ultra-precision CNC lathe. Vibration frequency is 36.16KHz and the amplitude is. The influence of various cutting parameters including cutting speed, feed-rate, vibration amplitude and phase angle on the cutting force, surface roughness and tool life have been studied and the results have been compared. The results indicate that the cutting force in EVC is much less than the two other processes. The surface roughness in both the cutting and feed directions was also less than those in other processes.

In recent years, design of small aircrafts are studied and extensively developed. One of the most important of these aircrafts is micro aerial vehicles (MAVs). As part of aerodynamic design of MAVs, the wing design is important. In fact, the main part of this design is to find an appropriate shape for the wing. In this work, influence of the wing shape at low Reynolds number flow is investigated using several planforms. Results show that as aspect ratio decreases, the lift-curve slope decreases as predicted by aerodynamic theory. The performance of the inverse Zimmerman and rectangular planforms are higher than elliptic and Zimmerman planforms. Also, the inverse Zimmerman planform is less influenced with the side flow. Effects of the Reynolds number and the wing edges on aerodynamic coefficients are discussed. The CFD computations are performed, and verified with the experimental data and aerodynamic theory.

A neural network with feed forward topology and back propagation algorithm was used to investigate the effect of composition on mechanical properties in API X65 microalloyed steel (used in manufacturing of large diameter pipes). Experimental data was obtained by cutting 100 specimens from pipes manufactured in industrial scale (with similar heats and manufacturing processes). The chemical analysis and tensile tests were conducted according to the requirements specified by API 5L standard. Scatter diagrams and two statistical criteria: correlation coefficient and mean squared relative error were used to evaluate the prediction performance of developed model. With regard to the satisfactory performance of the developed neural network, it was used then to investigate the effect of Ni, Cu and microalloying elements (Nb + Ti + V + Al) on mechanical properties of test steel.

In previous researches, mechanical behavior of a microgripper with homogenous and the same material has been studied. In this paper considering the desired characteristics of FGMs, as a good thermal and wear resistance, the mechanical behavior of FGM microgripper under DC voltage and temperature variations is investigated. It is assumed that the two microbeams of the FGM microgripper are mounted symmetrically against each other and one side of the microbeams is made of a pure metal the other side is a mixture of metal and ceramic, and the percentage of ceramic varies exponentially through the microbeams thickness. The nonlinear equations governing the static deflection of microbeams due to the application of DC voltage and the temperature changes are derived and solved using the step by step linearization method and the Galerkin method. The response and the stability of the system against the variation of DC voltage and temperature are investigated. Also, the effect of geometrical dimensions, ceramic percentage and its variations through the microbeams thickness on the mechanical behavior and the system stability are studied.

In this paper, the dynamics of evaporation of water droplets on surfaces with different contact angle hysteresis is studied experimentally and theoretically. The evaporation of water droplets on surfaces usually occurs in three distinct stages, namely the constant contact area mode, followed by the constant contact angle mode, and the mixed mode in which both contact angle and contact area are decreased. Due to the unknown nature of the time dependency of both the dynamic contact angle and contact radius, solving the governing equations for the evaporation rate of a droplet is complicated. The experimental observations performed in this study for the evaporation of sessile droplets on different solid substrates under various conditions shows that the surfaces with a high contact angle hysteresis remain more in the constant contact area mode. On the other hand, the surfaces with a low contact angle hysteresis remain more in the constant contact angle mode. Based on these observations, it is assumed that the entire process of evaporation on surfaces with high and low contact angle hysteresis occur in a dominated mode. Next, based on this assumption, the governing equation for the droplet evaporation for both surfaces is solved separately using a theoretical model. A good agreement is observed between the results of the theoretical analysis with those of the experiments; this confirms the validity of the proposed theoretical model.

Machining operation is one of the most applicable processes for the manufacturing of industrial parts. Usually, the quality of finished part in turning operation is measured in terms of surface roughness. In turn, surface quality is determined by machining parameters and tool geometry specifications. The main objective of this study is to model and optimize machining parameters and tool geometry in order to improve the surface roughness in turning operation of AISI1045 steel. Simultaneous optimization of machining parameters and tool geometry was performed using experimental data and statistical methods. A Taguchi approach is employed in order to gather experimental data and then, based on signal-to-noise (S/N) ratio, the best sets of cutting parameters and tool geometry specifications have been determined. Experimental results are provided to illustrate the effectiveness of the proposed approach.