This article presents the procedure to obtain constitutive equations to express the creep behavior of Araldite 2015 epoxy adhesive using the results of the uniaxial creep tests. The experimental data show that the examined adhesive has a nonlinear viscoelastic behavior. Three types of constitutive equations have been used: (a) Bailey-Norton equation, (b) generalized time-hardening model and (c) rheological model which is a series combination of springs and dampers known as Maxwell and Zener combination. The first two models have vast application in commercial finite elementbased software. It is shown that the generalized time-hardening model can simulate the creep behavior of the adhesive better than the Bailey-Norton model. However, this model is less accurate at the elevated temperatures. Therefore, an empirical equation based on Maxwell and Zener's was proposed which demonstrates a very good consistency with the results of the experimental data over the assumed range of stress and temperatures.
Fixed-cone (Howell-Bunger) valves have been in wide use for many years for flow control. These valves may face different types of damages and failures due to vibrational stresses during operation. In this study, a number of modal analyses of a Howell-Bunger (DN: 1000 mm) valve were conducted using the Finite Element Method (FEM), and its natural frequencies with vibrational mode shapes in five cases including fully opened, 20-, 40-, and 80%-opening conditions were determined; subsequently, the dimensionless coefficient ''Mercer'' was obtained for the valve. The result showed that the operating point of the valve is the flow rate of 16 m 3 /s and valve opening degree of 40%; in this case, due to reinforcement resulting from moving shell, the structure strength against vibration increases, and as a result, natural frequency increases as well. Keywords Failure Á Howell-Bunger valve Á Modal analysis Á Natural frequency Nomenclature c Mercer dimensionless coefficient D Nominal diameter of valve, m E Young's module, Pa f Natural frequency, Hz Q Passing flow rate of fluid, m 3 /s T V Thickness of the vane, m V Fluid velocity, m/s q Density, kg/m 3
In this paper, the effect of three main parameters: a) welding speed, b) cooling rate of fluid flow through the main pipe; and c) number of welding passes, have been studied to obtain an effective method to reduce the burn-through risk during the in-service welding of AISI-316 pipe branch connection to perform hot-tapping. In addition, important patents regarding the new methods of hot-tapping have been reviewed. To carry out numerical simulation, a 3D Finite Element (FE) based thermo-mechanical model has been developed. Using this model, thermo-mechanical stresses and temperature distribution along the main-pipe wall-thickness have been obtained with maximum and minimum allowable welding speeds; and with two high and low level of steam flow rate through the main pipe. The Von-Mises yield criterion using the temperature dependent yield stress has been used to check the main pipe failure during the welding process. The results show that current techniques, including API recommendations, which only rely on the observation of the main-pipe inner wall temperature, does not take into account the effect of mechanical or thermal stresses due to the inline pressure or other working parameters which have significant role in burn-through. In addition, the results show that the increase of welding speed reduces the risk of burn-through but it increases the risk of hot cracking. On the other hand, decreasing the steam flow rate has the opposite effect. It has also been shown that using smaller electrode size is the most effective way to decrease burn-through risk.
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