Effect of texture and testing conditions on strain localization during cyclic deformation of copper polycrystals

Fatigue tests were performed on pure copper polycrystals with a crystallographic texture different from that produced by ‘standard’ thermomechanical treatments, which emphasize multi-slip 111–100 textures. The texture along the loading axis deviated by 10–15° from these two poles for the samples used here. The experiments were initiated by ramp loading as a mechanical pretreatment and the cyclic stress–strain curve (CSSC) was established by step tests using enough cycles at each step to insure saturation. Under these conditions, a plateau was observed in the CSSC at an appropriate stress level and in a reproducible fashion.     

高碳钢高速变形的模拟及再结晶规律的研究

高碳钢试样在Ｇｌｅｅｂｌｅ－２０００试验机上进行高温多道次高变形速度的变形试验,高应变速率下的变形屈服应力服从Ｚｅｎｅｒ－Ｈｏｌｌｍａｎ因子（Ｚ＝Ａｏ＾ｎ＝εｅｘｐ（Ｑ／ＲＴ）的变化。高线预精轧时,发生道次间静态再结晶;而精轧时由于道次时间短,有道次应变积累现象,应变积累达到临界变形量后,引起动态再结晶发生。     

电阻应变计在弯矩测量中的应用

利用金属电阻奕变计式传感器测量弯矩,用三位半液晶显示器显示弯矩值的大小,并依据这一原理,设计出一种新型工具－－数显力矩扳手。     

A multi-field approach to modeling the dynamic response of cellular materials

A multi-field approach is developed for simulating the continuum-scale mechanical response of cellular materials. This approach departs from traditional methods used to model cellular materials, which focus almost exclusively on the mechanical response of the cellular solid, while essentially ignoring the fluids permeating these material systems. In the present work, conservation equations are derived in multi-field form, producing a coupled set of governing equations with source terms depending on gradients in the cellular solid stress, but also on gradients in the permeating fluid pressure and momentum exchange resulting from relative motion between the cellular solid and permeating fluid fields. The multi-field equations of motion are implemented in a standard finite-volume computational test bed and used to study the dynamic response of cellular material systems. The influence of various permeating fluids, along with the effects of aperture size, loading rate, and boundary conditions, also are examined. By incorporating an advanced constitutive model for cellular solids into a multi-field response formulation, a promising new approach for simulating the finite-strain dynamic response of cellular materials is offered. Results demonstrate that the permeating fluid can play a major role in the general response of cellular material systems, contributing to the overall load-carrying capacity of the materials and affecting rate dependence and signal propagation speeds. Furthermore, the results point to the usefulness of the multi-field formulation and provide evidence to suggest that any modeling approach developed for cellular materials gives a proper accounting of the pressure evolution and flow behavior of the fluids present in these material systems.     

Assessment of strain gradients affecting the performance of 3-D strain rosettes – relevant to the analysis of cement mantle strains in total hip replacements

A large scale model analysis, using embedded strain gauges, of the strain distribution in the cement mantle surrounding a femoral prosthesis is underway. In order to predict, and so avoid, positions of locally high strain gradients in this model, a finite element and experimental analysis of a similar problem was undertaken. For this purpose, a loose fitting rectangular steel insert inside a surrounding rectangular epoxy sheath was used to model an extreme case of the torsional and bending components of hip joint load. The axial component of joint load was modelled using an axisymmetric finite element model of a tapered shaft. The finite element results were used to determine suitable positions for embedding gauges in the experimental model. Results showed that the finite element analysis failed to adequately model the close sliding fit between the steel insert and epoxy. Altering the experimental model to artificially replicate the finite element contact conditions produced good correlation in bending, with experimental strains agreeing with simple bending theory to within 6%. Satisfactory correlation under torsional loading was not obtained, but strain magnitudes were low. Predicted positions for embedding gauges give conservative results, lessening the possibility of strain gradient induced error in the large scale model test of the cement mantle and prosthesis.     

New Transient Hot-Bridge Sensor to Measure Thermal Conductivity, Thermal Diffusivity, and Volumetric Specific Heat

A high sensitivity thermoelectric sensor to measure all relevant thermal transport properties has been developed. This so-called transient hot bridge (THB) decidedly improves the state of the art for transient measurements of the thermal conductivity, thermal diffusivity, and volumetric specific heat. The new sensor is realized as a printed circuit foil of nickel between two polyimide sheets. Its layout consists of four identical strips arranged in parallel and connected for an equal-ratio Wheatstone bridge. At uniform temperature, the bridge is inherently balanced, i.e., no nulling is required prior to a run. An electric current makes the unequally spaced strips establish an inhomogeneous temperature profile that turns the bridge into an unbalanced condition. From then on, the THB produces an offset-free output signal of high sensitivity as a measure of the properties mentioned of the surrounding specimen. The signal is virtually free of thermal emf’s because no external bridge resistors are needed. Each single strip is meander-shaped to give it a higher resistivity and, additionally, segmented into a long and short part to compensate for the end effect. The THB closely meets the specific requirements of industry and research institutes for an easy to handle and accurate low cost sensor. As the key component of an instrument, it allows rapid thermal-conductivity measurements on solid and fluid specimens from 0.02 to 100 W· m⁻¹·K⁻¹ at temperatures up to 250°C. Measurements on some reference materials and thermal insulations are presented. These verify the preliminary estimated uncertainty of 2% in thermal conductivity.     

Photoluminescence characterization of AlGaN-GaN pseudomorphic quantum wells and calculation of strain induced bandgap shifts

The low temperature (77 K) photoluminescence characteristics of Al _x Ga_1-x N-GaN strained layer quantum wells with differentx values grown by metalorganic chemical vapor deposition (MOCVD) were investigated. The photoluminescence spectra were useful in analyzing both quantum confinement effects and strain induced energy shifts. The strain induced shifts were found to be a strong function of aluminum compositionx. A model was developed to calculate the strain induced bandgap shifts atk = 0. The values predicted by this model which took into account the wurtzite crystal structure of the material system, were in good agreement with (i.e. within 2 meV of) the experimentally measured shifts.     

Mechanical and AC loss properties in mono-and multi-filamentary Bi-223 Ag-sheathed tapes

The evaluation of the bending strain tolerance and AC loss properties for monoand multi-filamentary Bi-2223 Ag-sheathed tape were carried out at liquid nitrogen temperature. For tapes with a filament number of over 19, the critical current (I_c) was maintained at the same values up to the bending strain of 0.3%, although the I_c of the mono-filamentary tape at the condition of 0.2% strain degraded to 90% of the value for the no-strain condition. The AC loss of the monofilamentary tape was the hysteresis type. On the contrary, the AC loss of the multi-filamentary tape was substantially dominated by the eddy current loss in the Ag matrix.     

Crack expanding with constant velocity in an anisotropic solid under anti-plane strain

An analytic solution is given for a crack expanding with constant velocity from zero length in an anisotropic material under anti-plane strain. Not all anisotropic materials can support anti-plane strain, and the study is therefore by necessity limited to a certain class of materials, including monoclinic materials. A double Laplace transform is used and the inversion technique is based on the self-similarity of the problem. The result shows that the crack shape is elliptic, as in the corresponding isotropic case. The displacement on the crack plane outside the crack is found to be zero. Expressions are given for the stresses, the stress intensity factor and the energy flux into the crack edge. In contrast to the isotropic case a transverse normal stress may appear, singular at the crack edge. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fabrication and Electromechanical Characterization of Silver-Clad BSCCO Tapes

The powder-in-tube technique consisting of industrial processes such as wire drawing and rolling has been widely used to fabricate superconducting tapes. In the present investigation a novel technique was adopted to fabricate BSCCO 2223 tapes. Instead of wire drawing, the silver billet was reduced in size by groove rolling. Stress conditions during groove rolling were analyzed and appropriate changes were incorporated to optimize the deformation process. Subsequent thermomechanical treatment resulted in tapes with average critical current densities of 18,000 A/cm². Phase development and microstructural evolution during the thermomechanical treatment were studied using XRD, SEM, and TEM. The electromechanical properties of monofilament and composite BSCCO tapes were evaluated by subjecting them to in situ tensile tests. The strain tolerance of the composite was found to be better than that of the monofilament BSCCO tape.