不同热处理态2024铝合金的腐蚀行为

分别在3.5%NaCl溶液、3.5%NaCl＋1.0%H2O2溶液和pH=12的3.5%NaCl溶液中进行动电位极化实验,研究2024 Al-Cu-Mg合金在不同热处理状态下的腐蚀行为。极化曲线表明,随着合金时效时间的延长,合金的腐蚀电位向负方向移动;向NaCl溶液中添加H2O2会使腐蚀电位正移;在pH=12的3.5%NaCl溶液中的极化曲线表现出明显的钝化现象。腐蚀试样表面表现为常见的腐蚀特征,但也有扩大的点蚀、晶间腐蚀现象出现。循环动电位极化曲线显示有宽的循环极化滞后环,不同的腐蚀模式表明合金的点蚀生长对合金的热处理状态敏感。通过显微组织分析,探讨了不同热处理状态下合金在不同NaCl溶液中的腐蚀机理。     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

模糊PID无速度传感器矢量控制系统的设计

以异步电动机矢量控制的基本方程式为基础,构建了一个无速度传感器矢量控制系统。系统的转速推算采用模型参考自适应控制(MRAC)理论。为了提高矢量控制系统的性能,提出并设计了3个模糊PID控制器,分别控制间接矢量控制系统中的速度、转矩和磁链。仿真结果表明基于该方法的系统能获得比较好的控制性能。     

Bimodal Heat Stability Curves of Fungal Pectolytic Enzymes and Their Implication for Softening of Canned Apricots

Pectolytic activity can be isolated from canned apricots showing severe softening induced by a single fungal lesion. The pectolytic activities of fungi important in causing progressive softening have bimodal heat stability curves with minimal stabilities at 70–80°C. The activities produced by fungi that do not cause appreciable softening do not show bimodal stability curves. A 6-min treatment at 75°C before carning eliminated detectable enzyme activity and significantly reduced subsequent softening in No. 303 cans of apricots to which single fruit halves with 5 mm fungal lesions had been added. These results suggest that a pretreatment at about 75°C could help alleviate this problem in canned apricots.     

On the fitting and extrapolation of J-resistance data derived through the linear normalization technique

In this paper, an assessment is made regarding the effects of J–R curve fitting and extrapolation methods in two J‐integral criteria – namely crack initiation, J_i, and tearing instability, J₅₀– which were obtained through the linear normalization technique. Power‐law, logarithmic and linear fits were concurrently applied to J–Δa data derived from sub‐sized compact tensile specimens machined from a nuclear grade steel and tested at 300 °C. Research results show that the logarithmic J–R fit is the most conservative approach within a broad range of elastic–plastic fracture resistance, compared to the conventional power‐law fit. On the other hand, the linear fitting method provided the most non‐conservative J‐predictions. The values of J_i and J₅₀ have been successfully correlated with the net energy absorbed during Charpy impact testing of the materials.     

A VARIABLE VOLTAGE AND FREQUENCY SCHEME TO OPTIMIZE THE EFFICIENCY OF A WIND-DRIVEN INDUCTION GENERATOR

ABSTRACT

The most recent approaches to harness energy from the wind utilizing induction generators has been made possible by advances in induction motor control using power electronic drives. The freedom in adjusting terminal voltage or current, and frequency al lows operation of the induction machine to fit any desired characteristic. The purpose of this paper is to propose a scheme using this technology to convert wind energy to electricity at maximum wind turbine power and induction generator efficiency. The Electromagnetic Transients Program (EMTP) is used to simulate the control of the induction generator. Simulation results show that this system operates as predicted in a stable fashion at the desired operating point.