对塑料排水板板芯质量的试验研究

塑料排水板板芯质量问题已成为排水固结加固软土地基的焦点问题之一,实践中经常只能通过感官来判断是否能满足工程需要。文章通过压屈试验、纵向通水量等室内试验手段反映出外观品质差的板芯可能存在的一些影响排水固结效果的现象。期望对塑料排水板今后的生产、使用规范化具有一定的促进作用。

锈蚀钢筋压屈应力-应变本构关系模型研究

采用电化学加速试验方法获得216根锈蚀钢筋试件,对锈蚀钢筋试件进行压屈试验,分析钢筋长细比(长度与直径之比)和锈蚀率对锈蚀钢筋压屈应力-应变关系曲线、名义压屈强度等的影响.结果表明:随着质量锈蚀率的增大,锈蚀钢筋名义压屈强度逐渐降低;当长细比≤15时,长细比对锈蚀钢筋名义压屈强度影响不明显;当长细比>15时,长细比对锈蚀钢筋名义压屈强度影响显著.锈蚀钢筋名义压屈应力-应变曲线表现出三种形态特征,根据三种曲线特征建立了锈蚀钢筋名义压屈应力-应变本构关系的统一模型,提出了考虑长细比与质量锈蚀率影响的锈蚀钢筋名义压屈强度计算公式,计算值与试验结果符合较好.

High strength magnesium alloy with α-Mg and W-phase processed by hot extrusion

Fine-grained Mg-6Zn-4Y alloy was prepared by an ingot metallurgy process with hot extrusion at 300 ℃.The microstructure was studied by XRD,OM,SEM and TEM,and the tensile properties were tested at room temperature.The results show that the alloy is composed of α-Mg and W-phase.The microstructure of the as-extruded alloy has a bimodal grain size distribution.The fine grains with the mean size of 1.2 μm are formed by dynamic recrystallization.The coarse grains（about 23% in area fraction） are unrecrystallized regions which are elongated along extrusion direction.The engineering stress—strain curve shows a pronounced yield point.The ultimate tensile strength,yield strength,and elongation are（371±10） MPa,（350±5） MPa and（7±2）%,respectively.The high strengths are attributed to the fine-grained matrix structure enhanced by W-phase particles,nano-scaled precipitates,and strong basal plane texture.

Mechanical properties of strengthened surface layer in Ti-6Al-4V alloy induced by wet peening treatment

A modified surface layer was formed on Ti-6Al-4V alloy by wet peening treatment. The variations of the residual stress,nano-hardness and microstructure of the modified layer with depth from surface were studied using X-ray diffraction analysis,nano-indentation analysis, scanning electron microscopy and transmission electron microscopy observations. The results show thatboth the compressive residual stress and hardness decrease with increasing depth, and the termination depths are 160 and 80 μm,respectively. The microstructure observation indicates that within 80 μm, the compressive residual stress and the hardness areenhanced by the co-action of the grain refinement strengthening and dislocation strengthening. Within 80–160 μm, the compressiveresidual stress mainly derives from the dislocation strengthening. The strengthened layer in Ti-6Al-4V alloy after wet peeningtreatment was quantitatively analyzed by a revised equation with respect to a relation between hardness and yield strength.

Relationship between mechanical properties and grain size of AZ80 at 350℃ under different strain rates

The dynamic recrystallization refinement of magnesium alloy AZ80 by compression tests was studied,and its effect on the mechanical properties was investigated.It is observed that the microstructure of the as-cast billet with grain size of 240μm becomes refined to about 120,110,94 and 50μm after upsetting at 350℃ under strain rates of 0.01,0.1,1 and 10 s -1 respectively.The changes in the mechanical properties according to grain size show that yield strength significantly decreases with grain size increasing,while strain hardening exponent and micro hardness increase very sharply.Further,the grain size vs strain rate and change in Vickers micro hardness according to the various strain rates show that grain size and micro hardness decrease with strain rate increasing.

Scratching by pad asperities in copper electrochemical-mechanical polishing

Low dielectric constant materials/Cu interconnects integration technology provides the direction as well as the challenges in the fabrication of integrated circuits(IC) wafers during copper electrochemical-mechanical polishing(ECMP). These challenges arise primarily from the mechanical fragility of such dielectrics, in which the undesirable scratches are prone to produce. To mitigate this problem, a new model is proposed to predict the initiation of scratching based on the mechanical properties of passive layer and copper substrate. In order to deduce the ratio of the passive layer yield strength to the substrate yield strength and the layer thickness, the limit analysis solution of surface scratch under Berkovich indenter is used to analyze the nano-scratch experimental measurements. The modulus of the passive layer can be calculated by the nano-indentation test combined with the FEM simulation. It is found that the film modulus is about 30% of the substrate modulus. Various regimes of scratching are delineated by FEM modeling and the results are verified by experimental data.

Microstructures and mechanical properties of extruded Mg-1Mn-3.5Y and Mg-1Mn-1Y-2.5Nd alloys

Both Mg-1Mn-3.5Y and Mg-1Mn-1Y-2.5Nd alloys（mass fraction,%）were extruded at 380？C.Most of the（10^-10） crystal planes in the Mg-1Mn-3.5Y alloy are parallel to the normal direction,while most of the（10^-11）crystal planes in the Mg-1Mn-1Y-2.5Nd alloy are parallel to the normal direction.The tensile tests at room temperature,100℃ and 200℃ show that the Mg-1Mn-3.5Y alloy exhibits higher yield strength,but lower elongation to failure as compared with the Mg-1Mn-1Y-2.5Nd alloy. These differences in the tensile mechanical properties between the two alloys are mainly attributed to their different texture types and amount and distribution of the Mg24Y5 precipitates.The serration flow behavior is observed in the Mg-1Mn-1Y-2.5Nd alloy at 200℃,but does not occur in the Mg-1Mn-3.5Y alloy.The Mg-1Mn-3.5Y alloy shows the cleavage fracture mode,while the Mg-1Mn-1Y-2.5Nd alloy exhibits the dimple fracture mode.

Yielding behavior of copper nanowire in the presence of vacancies

In the present research,the measurement fluctuations of mechanical properties in nanowires (NWs) are investigated by using the molecular dynamics simulation.The large numbers of simulations are performed to study the yield behaviors of the NWs.The results have shown that the yield behavior of the smaller diameter NW is more sensitive to the presence of vacancies,and the dispersion of the measured mechanical properties for the small scale NW is larger than that for the large scale NW.Present results have also shown that vacancies escape from the bulk to the free surfaces as a result of high stress applied at the small scale systems similar to the dislocation starvation phenomenon observed in the compression test of nano-pillars,and dislocation nucleation induced by surface defect occurs after the vacancy reaches free surface leading to lower yield strength.Moreover,the strong surface vacancy interactions at the nanoscale level are also investigated.

Design and biomechanical study of a modified pedicle screw

Objective： In pedicle screw fixation, the heads of monoaxial screws need to be directed in the same straight line to accommodate the rod placement by backing out during operation, which decreases the insertional torque and internal fixation strength. While polyaxial screws facilitate the assembly of the connecting rod, but its ball-in-cup locking mechanism reduces the static compressive bending yield strength as compared with monoaxial screws. Our study aimed to assess the mechanical performance of a modified pedicle screw. Methods： In this study, the tail of the screw body of the modified pedicle screw was designed to be a cylindershaped structure that well matched the inner wall of the screw head and the screw head only rotated around the cyclinder. Monoaxial screws, modified screws and polyaxial screws were respectively assembled into 3 groups ofvertebrectomy models simulated by ultra high molecular weight polyethylene （UHMWPE） blocks. This model was developed according to a standard for destructive mechanical testing published by the American Society for Testing Materials （ASTM F1717-04）. Each screw design had 6 subgroups, including 3 for static tension, load compression and torsion tests, and the rest for dynamic compression tests. In dynamic tests, the cyclic loads were 25%, 50%, and 75% of the compressive bending ultimate loads respectively. Yield load, yield ultimate load, yield stiffness, torsional stiffness, cycles to failure and modes of failure for the 3 types of screws were recorded. The results of modified screws were compared with those ofmonoaxial and polyaxial screws. Results： In static tests, results of bending stiffness, yield load, yield torque and torsional stiffness indicated no significant differences between the modified and monoaxial screws （P〉0.05）, but both differed significantly from those ofpolyaxial screws （P〈0.05）. In dynamic compression tests, both modified and monoaxial screws showed failures that occurred at the insertion point of screw body into the UHMWPE block, while the polyaxial screw group showed screw body swung up and down the screw head because of loosening of the ball-in-cup mechanism. Conclusions： The modified screw is well-designed and biomechanically improved. And it can provide sufficient stability for segment fixation as monoaxial screws.