The nucleation and growth of cavities in a superplastic quasi-single phase copper alloy

Experiments were conducted on a superplastic quasi-single phase copper alloy, Coronze CDA 638, to provide information on the nucleation and growth of internal cavities during deformation. It is shown that, at a temperature of 823 K, the cavities are generally associated with the presence of large Co-rich particles on the grain boundaries, with nucleation occurring at the particle/grain boundary interfaces. The cavities tend to form in stringers and these stringers are always oriented along the rolling direction regardless of the direction of the tensile axis. Thus, the cavity stringers are clearly associated with the Co-rich particles which also lie in stringers parallel to the rolling direction. A detailed series of tests at an initial strain rate of 1.3 × 10⁻⁵s⁻¹ shows that the cavities grow by a diffusion-controlled mechanism for cavity radii <15 μm and by the power-law mechanism for radii >20μm. It is demonstrated that this conclusion is consistent both with direct measurements of the cavity growth rates and with estimates of the increases in cavity size due to superplastic diffusion growth. The results therefore emphasize the importance of diffusion-controlled cavity growth at low strain rates.     

An experimental study on the alignment of cavities in a superplastic commercial copper alloy

A detailed study of cavitation was conducted in a commercial copper alloy deformed superplastically at a strain rate of 1.3 × 10^-3 s^-1. Cavities are observed to form in stringers parallel to the tensile axis due to cavity nucleation around aligned stringers of large cobalt-rich particles present in the as-received alloy. The increase in the number density of cavities and the range of cavity sizes observed indicates that cavities nucleate continuously during superplastic deformation. At large elongations, the interlinkage of cavities in a direction perpendicular to the tensile axis tends to mask the alignment of cavity stringers. It is demonstrated that the present results can rationalize satisfactorily the previous observations of cavity alignment in the superplastic copper alloy. Formerly with the Department of Materials Science, University of Southern California, Los Angeles, CA     

Fatigue Strength of Trolley Bridge Stringers Made of ASTM A7 Steel

Nineteen S15×42 (in U.S. units) stringers from a trolley bridge made of ASTM A7 carbon steel were tested in constant-amplitude fatigue. The bridge was built in 1900 and was abandoned in 1958. In 1985, the stringers were removed from the bridge and were tested in the laboratory. The test results showed that uniform corrosion and pitting reduced the fatigue strength of the stringers from Category A to Category D. The fatigue strength was reduced more by pitting than by uniform corrosion.     

THE PROPERTIES OF 18Ni 350 MARAGING STEEL PRODUCED FROM ELEMENTAL AND PREALLOYED POWDERS

Abstract

The strength of extruded and heat-treated bar of 18Ni 350 maraging steel produced from prealloyed powder has been found to be comparable to that of conventional, wrought 18Ni 350 maraging steel. Impact toughness was considerably higher. In contrast, properties of bar Obtained from an elemental powder mix were markedly inferior to wrought 18Ni 350 maraging steel. The high toughness of Charpy V-notch specimens produced from prealloyed powder was partly attributed to the presence of continuous, aluminium-rich stringers.This resulted in delamination of the specimens during testing and led to a fibrous fracture. Charpy V-notch specimens produced from the elemental powders also contained stringers. However, these were not continuous and therefore did not cause delamination. The resulting fracture surface was flat and relatively featureless. Electron-microprobe analysis has shown that the morphology of the stringers is largely determined by their composition and the composition at the stringer/matrix interface.     

Homogenization of Ti-Hastelloy-N

To homogenize Ti-Hastelloy-N, the segregated MC-type carbide stringers must be dissolved during solution annealing and enough time allowed for the carbide forming elements to diffuse uniformly throughout the matrix. A solubility product-l/Τplot and simple diffusion equation were used to estimate the parameters for a successful homogenization treatment. Compared to the material which contained stringers, the homogenized material had: 1) greater postirradiation ductility with significantly reduced scatter in values and 2) better resistance to intergranular cracking caused by tellurium vapor at 1033 K. The basic principles used to develop the homogenization treatment are applicable to other Ti or Nb stabilized alloys.     

Approximate Series Solution for Analysis of FRP Composite Highway Bridges

The design of a deck-and-stringer bridge system is usually reduced to the analysis of a T-beam section, loaded by concentrated loads corresponding to an equivalent fraction of the applied truck load. This equivalent load is defined by wheel load–distribution factors, which approximate the overall behavior of the bridge superstructure. In this paper, a one-term approximation of a macroflexibility series solution including deformations for fiber-reinforced polymer (FRP) deck-and-stringer orthotropic bridge systems, is used to develop explicit expressions for symmetric and asymmetric load distribution factors. It is significant that the equations presented herein include important parameters that represent, as accurately as possible, the response characteristics of the super structure, such as the geometry and material properties of the FRP deck and stringers, bridge aspect ratio, and number and spacing of stringers. As an illustration in actual design applications, the formulation presented in this paper is used to develop an analytical method for FRP deck-and-stringer bridge systems, and the method is verified by predicting the response of an all FRP model bridge in the lab and an FRP deck on steel stringers in the field. The results of the present formulation compare well with experimental lab and field results. The simplified analysis presented in this paper can be used with sufficient accuracy for the design of composite FRP deck on stringers bridges.     

Effectiveness of GFRP Sheets for Shear Strengthening of Timber

The objective of this study was to develop a cost-effective shear-strengthening technique for timber stringers that is environmentally friendly and leads to a durable structure. Testing was performed on creosote-treated Douglas fir beams, with dimensions of 100×400×3,650?mm, removed from a 40 year old bridge. Two strengthening schemes were investigated; incorporating vertical and diagonal glass fiber-reinforced polymer (GFRP) sheets applied to both shear spans. The diagonal scheme proved effective in increasing the average ultimate load, flexural stiffness, and deformability of the beams. Performance of the members reinforced using the vertical scheme, however, was poor compared to diagonally reinforced beams. The contribution of the diagonal sheets to the shear capacity of the stringers was around 12% at service loads and 40% at ultimate load. In conclusion, this study has shown that diagonal GFRP sheets are more effective than vertical sheets in shear-strengthening timber stringers with horizontal splits at their ends.     

Field Performance of the Fiber-Reinforced Polymer Deck of a Truss Bridge

The MD 24 Bridge over Deer Creek in Harford County, Md., was one of the projects chosen by the Federal Highway Administration’s Innovative Bridge Research and Construction Program for bridge deck replacement by fiber-reinforced polymer (FRP) composites. A thorough discussion is presented on Maryland State Highway Administration’s first bridge rehabilitation project utilizing a FRP deck. The discussion includes design details, installation procedure, construction methods and in situ load testing with a wireless monitoring system. The research team installed a monitoring system to record the effects of live loads on the bridge system, including truss members, steel stringers, and plate action of the FRP deck. Finite-element models were also used in this phase. Dynamic effects of the FRP system, composite action between steel stringers and the FRP deck as well as the effective width and distribution factors of stringers were obtained and compared with the AASHTO specifications. Recommendations are also offered on improving the design details based on this experience.     

The influence of the rolling direction on the mechanical behavior and cavity formation during superplastic deformation of 7075 A1 alloy

The superplastic 7075A1 alloy was tested over a range of strain rates 10⁻²−10⁻⁴s⁻¹ at a temperature range 430–510°C using specimens machined with the rolling direction parallel and perpendicular to the tensile axis. It is shown that the mechanical properties of the alloy, including the elongations to failure, are essentially identical. Microstructural observations show that the cavities tend to form in stringers and these stringers are always oriented along the tensile axis regardless of the rolling direction. The cavities are not nucleated primarily at large Fe-rich or Si-rich particles, nor do they grow from pre-existing microvoids which may be introduced during thermomechanical processing. The cavities are nucleated preferentially at small particles or some irregularities in the grain boundary during superplastic deformation.     

Repair of Full-Scale Timber Bridge Chord Members by Shear Spiking

The addition of vertically oriented shear spikes (fiberglass reinforced polymer rods) was shown to increase the effective stiffness of the stringers of a full-scale timber bridge chord specimen. Results found from the flexural load testing of a full-scale timber bridge chord laboratory specimen are presented. Reinforcement was provided with 19?mm diameter shear spikes bonded to the wood by an epoxy resin. The bridge chord specimen was intentionally damaged to simulate degradation. Shear spikes were then installed from the top of the member into predrilled holes to provide horizontal shear resistance and to improve the flexural effective stiffness. Results from the testing showed that with the insertion of five sets of shear spikes the average flexural effective stiffness recovered in the four stringers of the chord was 91.6%.     

Structural Modeling and Design Considerations for Double-Layer Shoring Systems

This paper presents structural modeling and design considerations for a double-layer shoring system made of wood posts based on analytical derivation, numerical computation, and the LeMessurier formula. Simplified two-dimensional models of double-layer shoring systems are developed. The analysis shows that system critical loads do not increase with the addition of individual pinned-ended shores, i.e., leaning columns, in the system during construction. If these individual pinned-ended shores are widely used, more cases of falsework failure may occur. The effects of lengths of horizontal stringers and vertical shores, stiffnesses of stringers, and positions of strong shores are investigated in the paper. The reinforcement by addition of bracing to the shoring systems can improve the system buckling strength. The study concludes that appropriately fastening a group of pinned-ended shores with horizontal bracings, i.e., strong shores, can increase the load-carrying capacity of a double-layer shoring system.     

High-strain-rate superplasticity in bulk cryomilled ultra-fine-grained 5083 Al

The ductility and creep of bulk ultra-fine-grained (UFG) 5083 Al (grain size ∼440 nm) processed by gas atomization, cryomilling, and consolidation were studied in the temperature range 523 to 648 K. Also, the creep microstructure developed in the alloy was examined by means of transmission electron microscopy (TEM). The ductility as a function of strain rate exhibits a maximum that shifts to higher strain rates with increasing temperature. An analysis of the experimental data indicates that the true stress exponent is about 2, and the true activation energy is close to that anticipated for boundary diffusion in 5083 Al. These creep characteristics along with the ductility behavior of 5083 Al are a reflection of its creep behavior as a superplastic alloy and not as a solid-solution alloy. In addition, the observation of elongations of more than 300 pct at strain rates higher than 0.1 s⁻¹ is indicative of the occurrence of high-strain-rate (HSR) superplasticity. Microstructural evidence for the occurrence of HSR superplasticity includes the retention of equiaxed grains after deformation, the observation of features associated with the occurrence of grain boundary sliding, and the formation of cavity stringers. Grain size stability during the superplastic deformation of the alloy is attributed to the presence of dispersion particles that are introduced during gas spraying and cryomilling. These particles also serve as obstacles for dislocation motion, which may account for the threshold stress estimated from the creep data of the alloy.     

Microstructural characteristics of 5083 Al alloys processed by reactive spray deposition for net-shape manufacturing

The superplastic forming of Al-Mg alloys is of considerable interest due to its potential for structural applications. In the present study, reactive spray-deposition processing is implemented to reduce grain size to the superplastic range and, thus, to obtain superplasticity for net-shape manufacturing. A typical Al-Mg alloy (5083) was selected for this study and was spray deposited using different N₂-O₂ atomization gas mixtures. The as-deposited microstructures reveal an equiaxed grain morphology, a limited dynamic recovery structure, and a distribution of intermetallic phases and oxide dispersoids. The grain size decreases with increasing atomization gas oxygen content and is affected by changes to the superheat temperature (ST) and by small additions of Zr. In some deposits, γ-Al₂O₃ crystallites less than 20 nm were distributed as stringers along grain boundaries; this observation was found to be strongly dependent on the oxygen content. The oxidation kinetics during reactive spray deposition were studied on the basis of the Mott-Cabrera theory of oxidation. A linear growth rate of dX/dt=2A ₀·exp (−Q/kT)·exp (k₀·P ^1/2/kT) was obtained, which suggests that the rate decreases rapidly with decreasing temperature or oxygen pressure and that, when the temperature (or oxygen pressure) is low, the rate decreases steadily with decreasing oxygen pressure (or temperature). Calculations of the width of oxide stringers as a function of oxygen content and ST show good agreement with the experimental observations. Finally, the volume fraction of oxide phases was estimated on the basis of kinetic considerations.     

A Model for the Growth of Hydrogen Attack Cavities in Carbon Steels

A detailed analysis of precision dilatometry data of McKimpson on plain carbon steel during the incubation stage of hydrogen attack has been carried out in the light of both the surface and grain boundary diffusion limited cavity growth models. The results are consistent with cavity growth by grain boundary vacancy diffusion provided the methane pressure in the cavity exceeds a threshold pressure. The threshold pressure observed by the analysis decreases with an increase in temperature from ~400 MPa at 658 K to ~200 MPa at 723 K. The magnitude of the threshold pressure is consistent with a constrained cavity growth model in which the strain due to atom plating is accommodated by local plastic deformation around the precipitates in the boundary. Quantitative agreement between model predictions and experimental data on expansion rates is good.     

Enhanced densification of cavitated dispersion-strengthened aluminum by thermal cycling

We report experimental data of creep cavity shrinkage for dispersion-strengthened-cast aluminum with about 23 vol pct submicron Al₂O₃ dispersoids, annealed isothermally or subjected to thermal cycling without applied stress. Thermal cycling is found to increase the rate of densification by a factor of 3 to 5.5 relative to isothermal annealing at the upper cycling temperature, allowing for recovery of full theoretical density in a shorter time. Isothermal densification is discussed in light of a diffusive cavity shrinkage mechanism, and a model considering thermal mismatch stresses is employed to rationalize the enhanced rate of densification observed during thermal cycling. Intermittent thermal-cycling densification is shown to improve creep life of dispersion-strengthened aluminum through the suppression of tertiary damage accumulation processes.     

Light distribution by linear diffusing sources for photodynamic therapy

The distribution of the light emitted by linear light diffusers commonly employed in photodynamic therapy (PDT) has been investigated. A device is presented which measures the angular distribution of the exiting light at each point of the diffuser. With these data the fluence rate in air or in a cavity at some distance from the diffuser can be predicted. The results show that the light is scattered from the diffuser predominantly in the forward direction. Experiments and calculations show that the fluence rate in air and in a cavity of scattering tissue at some distance from the diffuser has a maximum near the tip of the diffuser, instead of near the middle. However, the fluence rate resulting from an interstitial diffuser in a purely scattering tissue phantom shows a maximum in the bisecting plane of the diffuser as would be predicted when the diffuser emitted light isotropically. The scattering nature of the tissue is expected to cancel the anisotropy of the diffuser.     

Evaluation of Load Distribution Factor by Series Solution for Orthotropic Bridge Decks

In bridge engineering, the three-dimensional behavior of a bridge system is usually reduced to the analysis of a T-beam section, loaded by an equivalent fraction of the applied live load, which is called the live load distribution factor (LDF). The LDF is defined in the both the AASHTO Standard Specifications and the LRFD Specifications primarily for concrete slabs and has inherent applicable limitations. This paper provides explicit formulas using series solutions for LDF of orthotropic bridge decks, applicable to various materials but intended for fiber-reinforced polymer (FRP) decks. The present formulation considers important parameters that represent the response characteristics of the structure that are often omitted or limited in the AASHTO Specifications. A one-term series solution is proposed based on the macroflexibility approach, in which the bridge system is simplified into two major components, deck and stringers. The governing equations for the two components are obtained separately, and the deflections and interaction forces are solved by ensuring displacement compatibility at stringer lines. The LDF is calculated as the ratio of the single stringer interaction force to the summation of total stringer interaction forces. To verify this solution, a finite-element (FE) parametric study is conducted on 66 simply supported concrete slab-on-steel girder bridges. The results from the series solution correlates well with the FE results. It is also illustrated that the series solution can be applied to predict LDF for FRP deck-on-steel girder bridges, by favorable comparisons among the analytical, FE, and testing results for a one-third-scale bridge model. The scale test specimen consists of an FRP sandwich deck attached to steel stringers by a mechanical connector. The series solution is further used to obtain multiple regression functions for the LDF in terms of nondimensional variables, which can be used for simplified design purposes.     

黑龙江省逊克县新民金矿床流体包裹体研究

新民金矿床产出于小兴安岭一张广才岭多金属成矿带北缘。阐述了该金矿床的地质特征,指出金矿化包括石英-黄铁矿、石英-多金属硫化物和石英-方解石3个成矿阶段;并对前2个阶段硅化矿化石英细脉中的石英矿物的流体包裹体进行实验研究,提出该金矿矿石石英中的包裹体分为气液二相、含子矿物三相和富气相3种类型,以气液二相包裹体为主;流体包裹体的均一温度变化范围在234．8～420．5℃之间,盐度叫（NaCl）变化在2．6％～33．6％之间,可分为高温低盐度、中高温中低盐度和中温中高盐度流体等类型;流体包裹体气相成分主要为H20、CO2,液相成分也以H2O、CO2为主,含有Na^＋、K^＋、Ca^2＋、Mg^2＋离子,子矿物主要为石盐等;3类包裹体在同一矿物颗粒中同时共生发育,反映捕获流体为不均匀状态流体,指示成矿过程中存在一定程度的溶体不混溶作用。根据流体包裹体研究结果,确定矿床为浅成热液成因,而且矿床深部具有找寻斑岩型矿化的找矿潜力。     

Role of non-metallic inclusions in heat-treated steels for sour environment

Sulphide stress corrosion cracking (SSCC) studies on a set of three steels with different inclusion morphologies were conducted. Effect of heat treatment of the steels with and without rare-earth metal (RE) treatment upon SSCC resistance was studied. Threshold stress, corrosion rate and hydrogen content in the steels were determined. Quenched and tempered structure was found to improve the SSCC resistance only when the steel is RE treated. Corrosion rate and hydrogen content in the steels were found to decrease as the inclusion morphology is changed from elongated stringers to globular shape, which improves the SSCC resistance.     

易切削不锈钢1Cr17MoS的硫化物形态对力学性能的影响

易切削不锈钢1Cr17MoS中硫化物随冶炼控制方式的差异会呈现不同的形态。采用正常冶炼方式对1Cr17MoS钢进行锻制,硫化物呈细长条形状,长度一般为50～80 μm,长宽比为5～12;夹杂物尺寸为1～5 μm,大部分夹杂物小于3 μm。在冶炼中通过添加微量元素使1Cr17MoS钢经过锻制后的硫化物呈纺锤状、点球状,长度为10～30 μm,长宽比为2～5;夹杂物尺寸为3～20 μm,大部分夹杂物为10～20 μm。通过对不同硫化物形态的1Cr17MoS钢进行纵向和横向拉伸试验,结果表明,2种形态下1Cr17MoS钢的纵向拉伸断口均为韧窝断口,韧窝数量和大小受硫化物数量和宽度的影响,其形态与力学性能没有明显关系;1Cr17MoS的横向拉伸断口以准解理断口为主,还存在部分韧窝断口。传统冶炼方式中,冶炼过程未经过硫化物形态处理,细长条状硫化物破坏了试验钢基体的连续性,加速了横向拉伸的脆性断裂。而经过硫化物形态处理的冶炼方式中,纺锤状和点球状硫化物以韧窝形态出现,保留了试验钢的塑性特征,使其具备较好的横向拉伸性能。