Effects of Anisotropy in Drawing and Extrusion Processes of Bulk Metal Forming

The effects of anisotropy of axisymmetric materials (round bars, tubes) on metal forming processes are discussed. These effects are strongest for thin‐walled hollow materials in metal forming processes when the wall thickness is not predetermined by the die (tube drawing without mandrel, free extrusion of hollow components). Similarly to the normal anisotropy of sheet metal, a high radial anisotropy increases the resistance against a variation of wall thickness in tube drawing. There are also effects in forming solid materials such as forward extrusion of bars whereby the buckling of cross sections is influenced through the variation of radial anisotropy with the distance from the axis. The favourable anisotropy properties depend on the actual priorities. If, for example, for a metal forming process the material anisotropy results in high compressive stresses this may be favourable for increasing the ductility of the material whereas the increase of the load acting on the tool reduces tool life.     

钛合金棒材轧制的孔型设计与应用

设计了钛合金棒材轧制用椭圆-圆孔型,并在φ250横列式轧机进行了椭圆-圆孔型系统和椭圆-方孔型系统轧制钛合金棒材的对比试验.其结果表明,两种孔型系统轧制的棒材力学性能基本一致,组织均为两相区加工组织,无原始β晶界,但椭圆-圆孔型系统轧出的棒材晶粒较细小,等轴晶较多,外表面质量好,可提高钛合金棒材的成品率.轧后检查辊槽表面,发现椭圆-圆孔型系统的辊槽无明显变化,可减少换辊、修辊的次数,降低轧辊消耗.     

THE ISOSTATIC PRESSING,VACUUM SINTERING,AND SWAGING OF THORIUM POWDER

Abstract

The isostatic pressing of calcium-reduced thorium powder into round bars and tubes, prior to vacuum sintering and swaging, is described. The simple equipment allows powder filling to be carried out in an enclosed system, thus avoiding fire and health hazards at this stage. A number of bars or tubes, of varying length and diameter, can be pressed simultaneously. The technique has certain advantages over conventional die compacting.

The effect of pressure on density and hardness through the bar section before and after sintering at various temperatures has been studied, and the optimum conditions determined for the manufacture of dense bars with high metal efficiencies (98%). Details are given of the microstructure and mechanical properties of thorium bars in the sintered, swaged, and annealed conditions.     

Yield Locus of a Cold Rolled and Solution Treated Nimonic-263 Alloy Sheet,Determined Using Asymmetrical Knoop Micro Hardness Indenter

The Von Mises or perhaps the Tresca criterion is adequate for predicting the onset of yielding under combined stress loading for isotropic materials. However, this prediction is not so straight forward for anisotropic materials in which the magnitude of tensile and compressive yield stresses are different along the different orientations. Such directionality in yield behavior of anisotropic materials may strongly depend on the nature and degree of crystallographic texture. Texture driven yield surfaces are the representatives of the states of plane stress (tension–tension, tension–compression and compression–compression) in thin wall tube by externally applied forces and internal pressure. The applications of yield surfaces are far too wide and particularly have direct relevance to the metal forming, such as cold rolling. A simple procedure based on Knoop microhardness measurements has been proposed in the literature for determining the plane stress yield surface of the sheet materials. In the present study, an attempt has been made to describe this technique and the procedure to determine the yield locus using Knoop microhardness indenter and compare the yield surface anisotropy determined with the experimentally observed tensile properties in the in-plane directions and thus establish the usefulness of this technique in case of a cold rolled and solution treated Nimonic-263 alloy sheet of 1.0 mm thickness in solution treated condition.     

On composite-structure weaknesses: Part II. Computer experiments,identification, and correlation

Part II of this article concerns correlating the composite-structure weaknesses (CSWs) with the local microstructure of simulated short-fiber-reinforced metal-matrix composite (MMC) samples. The proposed analytically-numerically-based approach was employed for numerical computation of the mesoscopic stress distribution in short fibers. Nine local microstructure-related parameters are defined, which incorporate local elastic and thermal anisotropies into presentation of a local microstructure. It is found that the nine defined parameters are adequate physical parameters, which also account for synergetic interactions due to orientation-induced local anisotropies, either elastic anisotropy or thermal expansion anisotropy. By using the defined physical parameters, CSWs can be correlated with the local microstructure of simulated MMC samples. A database which collects numerical correspondences between the orientation and geometry of composite constituents and CSWs has been established for a number of simulated MMC samples. Computer experiments confirm that not only the stiffness mismatch between fiber and matrix materials, but also orientation-induced local anisotropies, exert decisive influence on the distribution of CSWs in short-fiber composites. The mesoscopic stress and strain distributions within a short fiber are critically dependent on the local evolution of elastic and thermal anisotropies of surrounding grains. Whether a CSW may become a failure origin must be evaluated in conjunction with a local damage mechanism.     

Effects of Different Modes of Hot Cross-Rolling in 7010 Aluminum Alloy: Part II. Mechanical Properties Anisotropy

The influence of microstructure and texture developed by different modes of hot cross-rolling on in-plane anisotropy (A _IP) of yield strength, work hardening behavior, and anisotropy of Knoop hardness (KHN) yield locus has been investigated. The A _IP and work hardening behavior are evaluated by tensile testing at 0 deg, 45 deg, and 90 deg to the rolling direction, while yield loci have been generated by directional KHN measurements. It has been observed that specimens especially in the peak-aged temper, in spite of having a strong, rotated Brass texture, show low A _IP. The results are discussed on the basis of Schmid factor analyses in conjunction with microstructural features, namely grain morphology and precipitation effects. For the specimen having a single-component texture, the yield strength variation as a function of orientation can be rationalized by the Schmid factor analysis of a perfectly textured material behaving as a quasi-single crystal. The work hardening behavior is significantly affected by the presence of solute in the matrix and the state of precipitation rather than texture, while yield loci derived from KHN measurements reiterate the low anisotropy of the materials. Theoretic yield loci calculated from the texture data using the visco-plastic self-consistent model and Hill’s anisotropic equation are compared with that obtained experimentally.     

Technology innovations on high quality special steel products

Because special steels are the basic materials for industrialization,a national R&D program on special steel technology for high quality products have been granted to promote the special steel production and applications since 2008.Since then,great progresses have been achieved on ferritic stainless steel sheets,heat resistance steel pipes and tubes,microalloyed forging steel bars,and mould steel forgings for hot working. The ratio of ferritic stainless steel sheets produced by TISCO and Baosteel has been promoted through technology innovations and increased 42%and 48 in 2009 respectively.439M steel sheets have been used to exhaust system,and 444 steel sheets have been applied to kitchen and architecture.The production technologies for heat resistance steel pipes and tubes(T/P92,S3043,S31042 steels) have been developed in Baosteel and Pansteel to meet the market requirements.Meanwhile,the round bloom technology has been initiated in Xingcheng Steel.The new technologies for microalloyed forging steels,low cost steel bar,and tailored steel and components,were proposed by CISRI based on the understanding of precise chemical composition and segregation control.And the applications of microalloyed forging steel bars have been promoted remarkably.The researchers have developed high quality mould steel technologies,heavy section forgings of 718 steel,high premium H13 steel blocks and large diameter mandrels used for seamless pipe rolling.The technology innovations of the program could lead to the progress of production technology in special steel sheets,pipes,bars and forgings,and meet the demands from market.For the moment,it is actually believed that there still strong requirements for the technology innovations on special steels.     

Encoding of anisotropic diffusion with tetrahedral gradients: a general mathematical diffusion formalism and experimental results

A diffusion imaging method with a tetrahedral sampling pattern has been developed for high-sensitivity diffusion analysis. The tetrahedral gradient pattern consists of four different combinations of x, y, and z gradients applied simultaneously at full strength to uniformly measure diffusion in four different directions. Signal-to-noise can be increased by up to a factor of about three using this approach, compared with diffusion measurements made using separately applied x, y, and z gradients. A mathematical formalism is presented describing six fundamental parameters: the directionally averaged diffusion coefficient D and diffusion element anisotropies eta and epsilon which are rotationally invariant, and diffusion ellipsoid orientation angles theta, phi, and omega which are rotationally variant. These six parameters contain all the information in the symmetric diffusion tensor D. Principal diffusion coefficients, reduced anisotropies, and other rotational invariants are further defined. It is shown that measurement of off-diagonal tensor elements is essential to assess anisotropy and orientation, and that the only parameter which can be measured with the orthogonal method is D. In cases of axial diffusion symmetry (e.g., fibers), the four tetrahedral diffusion measurements efficiently enable determination of D, eta, theta, and phi which contain all the diffusion information. From these four parameters, the diffusion parallel and perpendicular to the symmetry axis (D and D) and the axial anisotropy A can be determined. In more general cases, the six fundamental parameters can be determined with two additional diffusion measurements. Tetrahedral diffusion sequences were implemented on a clinical MR system. A muscle phantom demonstrates orientation independence of D, D, D, and A for large changes in orientation angles. Sample background gradients and diffusion gradient imbalances were directly measured and found to be insignificant in most cases.     

Influence of limit stress states and yield criteria on the prediction of forming limit strains in sheet metals

Several researchers have proposed analytical methods for predicting the forming limit curve (FLC), which has been successfully used as a diagnostic tool in sheetmetal forming. However, these approaches lack ease of adaptability to various situations and also involve considerable complexity. Sing and Rao proposed a new FLC modeling approach based on limit stress states derived from yield criterion and material properties from a simple tensile test. The first aspect of this study addresses the influence of the shape of the forming limit stress curve (FLSC) upon the FLC. The FLC modeled from a singly linear FLSC exhibits good agreement with the experimental curve, unlike those modeled from an elliptical or a piecewise linear FLSC. It is, thus, established that a linearized limit stress locus describes adequately the actural localized neck condition for the materials chosen in this study. Second, the study focuses on the suitability of the different cases of Hill’s yield criterion for satisfactory prediction of FLCs. The FLCs predicted using different cases of Hill’s criterion are compared with experimental FLCs in the case of steel and copper. Different cases of Hill’s criterion provide a wider choice for FLC modeling for different classes of materials. The sensitivity of Hill’s stress exponent is also thoroughly explored for achieving a close correspondence between the predicted and experimental FLCs.     

Nonlinear Unified Strength Criterion for Concrete under Three-Dimensional Stress States

This paper shows that the mechanical behavior of concrete materials can be described by a nonlinear unified strength criterion which requires four independent parameters for model calibration. The criterion covers all the area from spatially mobilized plane curve in the lower bound to Mises circle in the upper bound on the deviatoric plane, with an exponential curve on the meridian plane in principal stress space. Four material parameters in the strength criterion have their clear physical meanings and their determination approaches are also given. Four sets of true triaxial experimental results are compared with the modeling results of the proposed criterion. It shows that the proposed criterion is capable to capture the characteristics of yielding and failure and reasonably calculate the multiaxis strength of concrete materials.     

热轧小圆钢耳子缺陷磨修工艺改进与实践

小圆钢表面耳子缺陷有两种解决方法:φ≥16 mm时,用手提砂轮机磨去耳子;φ＜16 mm时,用砂轮无芯磨床研磨去除耳子.两种方法在实施前必须对待磨修产品进行一次机械矫直,以保证待修品的平直度.该方法产量低,能耗大,且带来一系列问题.经改进,设计了"三步法"工艺即二次矫直加一次磨修工艺.该方法妥善解决了中低碳和低合金结构钢表面耳子缺陷处理时间长、合格率低的生产难题,使清除小圆钢表面耳子缺陷简便快捷,生产效率高、成本低;同时,圆钢的不圆度、平直度明显改观,产品合格率达100%.     

An improved method for detection of apoptosis in tissue sections and cell culture, using the TUNEL technique combined with Hoechst stain

The use of fatty materials in cereal bars gives to them a good energetic value; however they are exposed to oxidative rancidity which can affect their acceptability and nutritional value. So, the purpose of this research was to determine the stability in storage and the effect of antioxidants on three tipes of cereal bars with peanuts. Cereal bars with 18% of peanuts were prepared, with and without antioxidants (BHA + BHT; 100 ppm). Bars were packed in polyprolpilene-aluminium-polythilene bags, and were stored at room temperature (18-20 degrees C) for 90 days. Each 30 days, analysis of water activity (Aw); moisture content, peroxides index, sensory quality (flavor, aroma and appearance) and acceptability, were carried out. Moisture content was similar in all bars (7.6-9.6%) and Aw was higher in the bar which contained expanded amaranthus and antioxidant. At the 60th day of storage, the peroxide values were lower in the bars with antioxidants; only the bar which included expanded amaranthus showed significant differences (16.4 meq/kg in the bar with antioxidant and 25.7 meq/kg for the control bar). The sensory parameters were kept within normal status without differences between the bars with antioxidants and the control ones, along all the storage period. Shelf life of bars CM1 and CM2 was at least of 60 days when they are kept at 18-20 degrees C.     

Anisotropy of yielding in a Zr-2.5Nb pressure tube material

The anisotropy of yielding, as measured by the ratio of yield stress in the axial and transverse directions of Zr-2.5Nb pressure tubes used in Canada Deuterium Uranium (CANDU) nuclear reactors, was determined experimentally by testing samples in uniaxial tension. The yield anisotropy was measured in uniaxial tension in samples obtained from the three directions of a Zr-2.5Nb plate and in shear, by testing in torsion “mini” pressure tubes from the same material. From these experiments, the temperature and strain-rate dependence of the yield stress and the dependence of the anisotropy of yielding on temperature were also determined. It is shown that the yield anisotropy of pressure tube material is constant for temperatures up to about 800 K and that the strain-rate sensitivity is also constant up to about 700 K and is equal to ∼0.02. In addition, the activation energy (Q) of this material was estimated by using the temperature dependence and rate sensitivity of the yield stress. It was found to be of the same order of magnitude as that determined earlier by other investigators. A polycrystalline, nonlinear self-consistent model that takes into account the crystallographic texture of the material was used to derive the values of the critical resolved shear stress (CRSS) which are consistent with prismatic, basal, and pyramidal glide and the values of the Hill’s plastic anisotropy coefficients which are consistent with the observed anisotropy of yielding. The model provided an estimate of the complete stress tensor, describing yielding of a Zr-2.5Nb pressure tube material.     

Modeling Cross Anisotropy in Granular Materials

A constitutive model has been developed to capture the behavior of cross-anisotropic frictional materials. The elastoplastic, single hardening model for isotropic materials serves as the basic framework. Based on the experimental results of cross-anisotropic sands in isotropic compression tests, the principal stress coordinate system is rotated such that the model operates isotropically within the rotated framework. Experimental plastic work contours on the octahedral plane are plotted for a series of true triaxial tests on dense Santa Monica Beach sand to study the effects of cross anisotropy on the evolution of yield surfaces. The amount of rotation of the yield and plastic potential surfaces decreases to zero (isotropic state) with loading. The model is constructed for cases where the principal stress and material symmetry axes are collinear and no significant rotation of principal stresses occur. The model incorporates fourteen parameters that can be determined from simple experiments, such as isotropic compression, drained triaxial compression, and triaxial extension tests. A series of true triaxial and isotropic compression tests on dense Santa Monica Beach sand are used as a basis for verification of the capabilities of the proposed model.     

A local criterion for cleavage fracture of a nuclear pressure vessel steel

Experiments were performed on three heats of A508 class 3 steel in order to determine the mechanical conditions for cleavage fracture. These tests were carried out on various geometries including 4-point bend specimens and axisymmetric notched tensile bars with different notch radii which have been modelized using the finite element method. In one heat, the temperature range investigated was from 77 K to 233 K. It is shown that the cleavage resistance is increased by tensile straining. Moreover, the probability of fracture obeys the Weibull statistical distribution. All the results can be accounted for in terms of a local criterion based on Weibull theory and which takes into account the effect of plastic strain. In this criterion, the parameters which were experimentally determined are found to be temperature independent over the range 77 K to 170 K. The applicability of the approach proposed for cleavage fracture at the crack tip is also examined. It is shown that the experimental results published in the literature giving the variation of fracture toughness with temperature can be explained by the proposed criterion which predicts reasonably well both the scatter in the experimental results and theK _ICtemperature dependence.     

Testing mixture models of transitive preference: Comment on Regenwetter, Dana, and Davis-Stober (2011).

This article contrasts 2 approaches to analyzing transitivity of preference and other behavioral properties in choice data. The approach of Regenwetter, Dana, and Davis-Stober (see record 2011-00732-003) assumes that on each choice, a decision maker samples randomly from a mixture of preference orders to determine whether A is preferred to B. In contrast, Birnbaum and Gutierrez (2007) assumed that within each block of trials, the decision maker has a true set of preferences and that random errors generate variability of response. In this latter approach, preferences are allowed to differ between people; within-person, they might differ between repetition blocks. Both approaches allow mixtures of preferences, both assume a type of independence, and both yield statistical tests. They differ with respect to the locus of independence in the data. The approaches also differ in the criterion for assessing the success of the models. Regenwetter et al. fitted only marginal choice proportions and assumed that choices are independent, which means that a mixture cannot be identified from the data. Birnbaum and Gutierrez fitted choice combinations with replications; their approach allows estimation of the probabilities in the mixture. It is suggested that researchers should separate tests of the stochastic model from the test of transitivity. Evidence testing independence and stationarity assumptions is presented. Available data appear to fit the assumption that errors are independent better than they fit the assumption that choices are independent. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Bauschinger effect and multiaxial yield behavior of stress-reversed mild steel

Thin-walled cylindrical specimens subjected to torsional prestraining are stress-reversed along the Bauschinger curve. The Bauschinger effect (BE), yield behavior, and flow behavior of the stress-reversed mild steel were examined by using combined loadings of axial load, internal pressure, and torsion. The results indicate that the stress-reversed steel has the same yield stress at 0.2 pct offset strain in reloading tests of forward and reverse torsion, when the reverse strain is =0.77 pct. Furthermore, it is possible to cause the yield stresses in forward and reverse torsion to coincide in any offset strain. The yield locus of the stress-reversed steel is symmetric with respect to the tensile stress axis in a tension-torsion stress field. However, it has been found to be an anisotropy in the stress-reversed steel, and the magnitude of anisotropy is related to the offset strain. For example, there is a stronger anisotropy at 0.2 pct offset strain than at 2 pct offset strain, even though the BE is eliminated for the former. It is shown that the reduction in the BE by stress reversal is concerned with the relief of the long-range back-stress generated by prestraining. Besides, the roles of aging in the stress-aging process lie in the contributions to age hardening and development of directional back-stress.     

Statistical Damage Constitutive Model of Quasi-Brittle Materials

Recent studies have shown that statistical damage mechanics is one effective method to study the failure process of quasi-brittle materials. There are two key problems in setting up the statistical damage constitutive model of quasi-brittle materials, namely, determining the microunit strength and the parameters of statistical distribution that the microunit strength obeys. The four-parameter criterion is a failure criterion consisting of four unknown parameters. When the four parameters equal appropriate values, it may become the Drucker–Prager criterion (for rock), Mohr–Coulomb criterion (for rock), and Hsieh–Ting–Chen criterion (for concrete), so the four-parameter criterion may be used to simulate the elastoplastic behavior of rock and concrete quasi-brittle materials. In the paper, microunit strength is determined with the four-parameter criterion, thus the statistical damage constitutive model suits rock and concrete. The deficiencies of existing methods in determining the distribution parameters are investigated, and a new method for determining the distribution parameters is proposed. First, the theoretical relationships between the parameters and the strain and stress at the peak point of material failure curve are derived; second, the approximate relations between the strain and stress at the peak point of material failure curve and confining pressure are established through the curve fitting method; finally, the relations between the parameters and confining pressure are established. The proposed statistical damage softening constitutive model of quasi-brittle materials has universal meaning, the determination of distribution parameters has strict theoretical basis, and the distribution parameters can be conveniently obtained with general triaxial tests. Numerical examples are also presented to validate the model.