An improved methodology for determining tensile design strengths of Alloy 617

This paper presents an improved methodology for determining high-temperature tensile design strengths of Alloy 617, which is regarded as one of main structural materials for very high temperature reactor (VHTR) system. In establishing time-independent allowable stress values, an existing ASME standard procedure is preliminarily analyzed and their limitations are pointed out. Then, an improved methodology, which has a consistent and quantifiable design margin at low and high temperatures for tensile strengths, is proposed and compared with the ASME method. To find suitable curves of temperature trend to the tensile strength data, three fitting methods are demonstrated, and a statistical technique is adopted for design use. The results will be utilized to reasonably determine the tensile design strengths of Alloy 617 for application in the VHTR system.     

Measurement of the hysteresis characteristics of pipe steels under elastic and plastic tensile strain

The changes in the shape and characteristics of the major and asymmetric hysteresis loops of low-carbon pipe steels under the action of elastic and plastic tensile strains were studied. The hysteresis loop segments that experienced the most considerable transformations under the tension of a material were determined. The possibilities of the inspection of tensile strains by the coercive return magnetization and the magnetic field corresponding to a fixed magnetization on the descending branch of a hysteresis loop were shown.     

A methodology for determining the true stress-strain curve of SA-508 low alloy steel from a tensile test with finite element analysis

The true stress-strain curve of a material should be determined for plastic property input to numerical analysis. This study proposes a simple methodology for determining the true stress-strain curve of SA-508 Grade 3 Class 1 low alloy steel using limited information from a general tensile test with finite element analysis. Measured engineering stresses and strains can be reasonably converted to true stresses and strains under uniform deformation before necking. True stress-strains are difficult to determine after necking because of nonuniform deformation without specialized measurement techniques. Five post-necking strain hardening models are considered, namely, linear, swift, Ludwick, Hollomon-linear (HL) and Hollomon-linear-constant (HLC) models. The equations for each model can be determined using the results of the tensile test, which include the true stress-strain value at the maximum load point and the corrected true stress-strain value at the fracture point plus the Considere instability criterion. The HL and HLC models suggested that the engineering stress-strains from the finite element analysis are consistent with the experimental results.     

Prediction of subsurface stress in elastic perfectly plastic rough components

Understanding and anticipating the effects of surface roughness on subsurface stress in the design phase can help ensure that performance and life requirements are satisfied. One approach used to address this problem is to simulate contact between digitized real, machined surfaces, and then analyze the predicted subsurface stress field. Often, elastic-perfectly plastic contact models are used in these simulations because of their relative computational efficiency. Reported here is an analysis of the magnitude and location of maximum stress predicted using an elastic-perfectly plastic model. Trends are identified which then enable estimation of the upper bound of the simulation results based on surface discretization, operating conditions, and material properties. These estimations can be used as an effective and efficient tool for rapid prediction of maximum subsurface stress in real surface contact.     

Improved performance of grinding wheels in machining plastic steel and alloy blanks

Formulas are proposed for the forces required to rupture the bonds between metal buildup and the abrasive grains of a grinding wheel. The most effective methods of reducing wheel deterioration due to metal buildup are established.     

A new methodology for determining the stress state of the plastic region in machining with restricted contact tools

In rigid-plastic slip-line theory, once the geometry of the slip-line field is established, the stress state of the plastic region (including the primary and secondary deformation zones) in restricted contact machining is governed by the hydrostatic pressure P_A (at a point on the intersection line of the shear plane and the work surface to be machined) and the frictional shear stress τ on the tool rake face. Based on the recently established universal slip-line model and a detailed study of six representative machining cases, a new methodology for determining the stress state of the plastic region, i.e. maximum value principle, is presented in this paper. According to this principle, the stress state of the plastic region can be determined by giving both P_A and τ their theoretical maximum permissible values. The theoretical maximum permissible values of P_A and τ can be found by satisfying four mechanical and geometrical constraint conditions under which the universal slip-line model applies. A comprehensive assessment factor is introduced in this paper. It is shown that the three machining parameters investigated in this present study, i.e. cutting force ratio, chip thickness ratio, and chip back-flow angle can be simultaneously considered to form a comprehensive criterion to compare predicted and experimental results. The applicable range of the maximum value principle is also discussed.     

用于微反应器塑件整平的等温热压工艺

以一种微流控微反应器塑件为对象,提出了用于微塑件整平的等温热压工艺,研究了等温热压工艺对平板微塑件的整平机理。建立了描述该工艺的弹塑性数学模型,计算分析了外加压力和温度对塑件表面形貌的影响。综合考虑塑件整平效果和微结构保形,开展等温热压整平工艺实验,分析了关键工艺参数对器件整平精度的影响。研究结果表明:与压力载荷相比,热载荷对不平度的改善效果更明显;由于塑件端部区域受力面积大,其两端变形量均大于中间反应腔的变形量。在相同压力条件下,70℃时平面度和不平度的变化率均为最高。通过工艺优化,微反应器塑件平面度提高到了10μm内,最大变化率可达72.7%;而不同区域的不平度变化率为3.50%~53.50%,微结构尺寸变化可控制在5μm以下。本文研究成果对提高平板微塑件平整精度有借鉴作用。     

Magnetic method for estimating uniaxial compressive and tensile elastic stresses

It is shown that uniaxial compressive and tensile elastic stresses can be controlled based on the coercive force, relaxation magnetization, and magnetic susceptibility in a multifactor model.     

An exact solution for the elastic/plastic bending of anisotropic sheet metal under conditions of plane strain

A theoretical analysis for the elastic/plastic bending of sheet metal exhibiting a state of normal anisotropy is considered in this paper, assuming a plane strain condition to exist in the deformation process. The material is supposed to yield according to Hill's quadratic yield criterion and its associated normality rule of plastic flow. The relationship between the bending couple and the curvature of the bent sheet is presented in a graphical form that reveals the influence of anisotropy and strain-hardening on the bending characteristic of the sheet metal. The results indicate that the elementary bending theory significantly overestimates the magnitude of the bending couple to produce a given elastic/plastic curvature of the bent sheet.     

Process capability study of polyjet printing for plastic components

The purpose of the present study is to investigate process capability of polyjet printing (PP) for plastic components. Starting from the identification of component, prototypes with three different type of plastic material were prepared at different orientations. Measurements on the coordinate measuring machine helped in calculating the dimensional tolerances of the plastic components produced. Some important mechanical properties were also compared to verify the suitability of the components. Final components produced are acceptable as per ISO standard UNI EN 20286-I (1995) and DIN16901. The results of study suggest that PP process lies in ±4.5sigma (σ) limit as regard to dimensional accuracy of plastic component is concerned. This process ensures rapid production of pre-series technological prototypes and proof of concept at less production cost and time.     

Preliminary application of the draft code case for alloy 617 for a high temperature component

The ASME draft Code Case for Alloy 617 was developed in the late 1980s for the design of very-high-temperature gas cooled reactors. The draft Code Case was patterned after the ASME Code Section III Subsection NH and was intended to cover Ni-Cr-Co-Mo Alloy 617 to 982°C (1800°F). But the draft Code Case is still in an incomplete status, lacking necessary material properties and design data. In this study, a preliminary evaluation on the creep-fatigue damage for a high temperature hot duct pipe structure has been carried out according to the draft Code Case. The evaluation procedures and results according to the draft Code Case for Alloy 617 material were compared with those of the ASME Subsection NH and RCC-MR for Alloy 800H material. It was shown that many data including material properties, fatigue and creep data should be supplemented for the draft Code Case. However, when the evaluation results on the creep-fatigue damage according to the draft Code Case, ASME-NH and RCC-MR were compared based on the preliminary evaluation, it was shown that the Alloy 617 results from the draft Code Case tended to be more resistant to the creep damage while less resistant to the fatigue damage than those from the ASME-NH and RCC-MR.     

Comparative study on the high-temperature tensile and creep properties of Alloy 617 base and weld metals

This paper presents a comparative investigation on the high-temperature tensile and creep properties of Alloy 617 base metal (BM) and weld metal (WM) fabricated by a gas tungsten arc weld process. The WM had higher yield strength and lower ultimate tensile strength than the BM does; however, its elongation was significantly lower than that of the BM. The creep curve of the BM and WM was somewhat different from that of typical heat-resistance steel, and did not show a textbook creep. The WM exhibited a longer creep rupture life, lower creep rate, and lower rupture ductility than the BM. However, as the creep rupture time reached approximately 36,800 h, the creep life of the WM was expected to be almost similar to that of the BM; and after 36,800 h, its creep life was expected to be worse than the BM. Loner creep tests is needed to investigate the long-term creep life of the WM. The creep failure mode of the BM and WM was obviously an intergranular cracking of the cavity formation and growth mechanisms, although it was more evident in the WM. The BM had a more ductile fracture surface than the WM.     

An elastic–plastic asperity interaction model for sliding friction

A finite-element model of the interaction of an elastic–plastic asperity junction based on cylindrical or spherical asperities is used to predict sliding friction coefficients. The modelling differs from previous work by permitting greater asperity overlaps, enforcing an interface adhesional shear strength, and allowing material failure. The results of the modelling were also used to predict friction coefficients for a stochastic rough surface. The asperities were based on the titanium alloy Ti-6Al-4V, and the magnitudes of the predicted friction coefficients were generally representative of experimental measurements of sliding friction. The results suggest that friction arises from both plasticity and tangential interface adhesion.     

Minimizing manufacturing costs for thin injection molded plastic components

Minimizing the cost of manufacturing a plastic component is very important in the highly competitive plastic injection molding industry. The current approach of R&D work focuses on optimizing the dimensions of the plastic component, particularly in reducing the thickness of the component during product design, the first phase of manufacturing, in order to minimize the manufacturing cost. This approach treats the component dimensions established in the product design phase as the given input, and uses optimization techniques to reduce the manufacturing cost of mold design and molding for producing the component. In most cases, the current approach provides the correct solution for minimizing the manufacturing cost. However, when the approach is applied to a thin component, typically when miniaturizing products, it has problems finding the true minimum manufacturing cost. This paper analyses the shortcomings of the current approach for handling thin plastic components and proposes a method to overcome them. A worked example is used to illustrate the problems and compare the differences when using the current approach and the new method proposed in the paper.     

Minimizing manufacturing costs for thin injection molded plastic components

Minimizing the cost of manufacturing a plastic component is very important in the highly competitive plastic injection molding industry. The current approach of R&D work focuses on optimizing the dimensions of the plastic component, particularly in reducing the thickness of the component during product design, the first phase of manufacturing, in order to minimize the manufacturing cost. This approach treats the component dimensions established in the product design phase as the given input, and uses optimization techniques to reduce the manufacturing cost of mold design and molding for producing the component. In most cases, the current approach provides the correct solution for minimizing the manufacturing cost. However, when the approach is applied to a thin component, typically when miniaturizing products, it has problems finding the true minimum manufacturing cost. This paper analyses the shortcomings of the current approach for handling thin plastic components and proposes a method to overcome them. A worked example is used to illustrate the problems and compare the differences when using the current approach and the new method proposed in the paper.     

应变式称重传感器弹性体线性度的有限元分析

弹性体是应变式称重传感器的关键元件。采用有限元法对应变式称重传感器弹性体进行分析。用此方法首先要得到弹性体的应变分布特性,确定考察应力线。然后分级加载,得到载荷与考察应力线之间的一元线性回归方程,分析得出弹性体的线性度。分析结果表明,此弹性体的线性度很好,能满足使用要求。     

Method for determining the true stress of cross-shaped specimens subjected to biaxial tensile loads

For typical cross-shaped specimens subjected to simultaneous biaxial tensile loads, the specimens’ central section would elongate along different directions under plane stress condition. Consequently, the width of the specimens’ gauge length section would gradually decrease along the tensile direction, and true stress, which is based on the instantaneous width, is different from the values obtained from uniaxial tensile tests. To calculate the true stress of cross-shaped specimens, a horizontal biaxial tensile device was developed to apply the equi-biaxial loads. An optical observation method was adopted to measure the specimen’s true deformation. Then, an exponential fitting method was proposed on basis of the true deformation to describe the nonlinear deformation trajectory of the specimen’s gauge length section. Based on a theoretical model, the calculation method of the true stress was established, and the relationships between the true and engineering stress-strain curves were also discussed.     

Coercive-force hysteresis of carbon steels during elastic cyclic tensile deformation

Dependences of the coercive force and mechanical stresses on cyclic tensile in the region of elastic deformation of steel samples with various carbon contents (Armco iron, 3, 8) are investigated. It is shown that the dependence of the coercive force on cyclic elastic tensile deformation is reversible for well-annealed samples. The dependence for plastically deformed steels has a hysteresis loop that is significantly determined by the carbon content and increases with an increasing degree of plastic deformation. It is assumed that the hysteresis of the coercive force as a function of elastic cyclic tensile deformations in plastically deformed carbon steels is caused generally by the appearance “free” carbon atoms, i.e., those not bound in carbide phases, which act as interstitial impurity atoms in the lattice of α-iron.     

Test methodology for the formulation and selection of components of thread moulding oils

In the course of a Kplus research project which focused on the internal thread moulding process, a test methodology for evaluating the efficiency of forming oils was developed. The various tests should provide tribological characteristic values, such as coefficient of friction, with respect to the real production environment. To guarantee the transferability of results, the complex of loads acting on the components in the model tests must correspond to the real forming process. Especially parameters such as contact pressure, relative speed in the forming zone and the increase of surface of the workpiece during the forming process must be considered in tribological testing. The tribological data provided information to select components of thread moulding oils and to optimise the formulation. Best practice moulding oils must satisfy different demands in respect of physical, physical–chemical, chemical and additional properties. In addition to the cooling function and the reduction of friction, there is also a significant influence on the quality of the workpiece and the moulding process itself. Copyright © 2008 John Wiley & Sons, Ltd.