热处理缺陷裂纹产生原因的分析

主要分析各种热处理方法及其他因素使金属零件产生裂纹的原因。     

浅析给水排水构筑物裂缝的成因及控制

本文简要分析给水排水构筑物裂缝的成因,重点介绍从设计、施工、材料等方面进行裂缝控制的综合技术措施。     

Effect of Trace Element on Microstructure and Fracture Toughness of Weld Metal

The microstructure and fracture toughness of weld metal under Ti-free and Ti-containing different fluxes were investigated in this study.It was found that the trace element Ti of flux in submerged arc welding produced significant influence on the fracture toughness.The addition of 60 ppm Ti induced the sharp increase in J_(0.2) value from 232.78 to 364.08 kJ/m~2.Microstructure characterization revealed that a large number of oxide inclusions prompted the nucleation of acicular ferrites and refined grains,which improved the fracture toughness of Ti-containing weld metal greatly.Moreover,the crack propagation path was more tortuous and bifurcated due to the small amount of carbide precipitations along grain boundaries and blocky martensite-austenite islands for Ti-containing weld metal.Meanwhile,the large-angle grain boundaries caused crack deflection and increased the resistance of crack propagation compared to Ti-free weld metal.     

基于神经网络的铝合金裂纹缺陷识别

将人工神经网络方法应用于铝合金工件裂纹缺陷识别,以克服传统人工识别的局限性,从而提高裂纹缺陷识别的准确率。通过设计并搭建水浸超声检测系统,获得超声检测缺陷的波形数据,并对收集到的缺陷波形数据进行特征提取,从中筛选出有用的特征信息,经过小波去噪处理后作为特征信号输入概率神经网络,并进行网络训练,实现对不同裂纹尺寸的智能识别。实验结果表明:该方法可提高对裂纹缺陷尺寸识别的准确率和检测效率,具有较好的应用前景。     

Finite element analysis of laser-generated ultrasound for characterizing surface-breaking cracks

A finite element method was used to simulate the wave propagation of laser-generated ultrasound and its interaction with surface breaking cracks in an elastic material. Thermoelastic laser line source on the material surface was approximated as a shear dipole and loaded as nodal forces in the plane-strain finite element (FE) model. The shear dipole-FE model was tested for the generation of ultrasound on the surface with no defect. The model was found to generate the Rayleigh surface wave. The model was then extended to examine the interaction of laser generated ultrasound with surface-breaking cracks of various depths. The crack-scattered waves were monitored to size the crack depth. The proposed model clearly reproduced the experimentally observed features that can be used to characterize the presence of surface-breaking cracks.     

Ratchetting limits for cracked bodies subjected to cyclic loads and temperatures

This paper describes a methodology capable of directly evaluating the ratchetting limits in cracked bodies, subjected to cyclic loads and temperatures, for an elastic-perfectly plastic material. The foundation behind the adopted numerical procedures, the Linear Matching Method (LMM), is described. This is then followed by the application of the method to the cracked Bree problem, under variations in cyclic loading histories and structural geometries. The results from the ensuing analyses revealed the method’s ability in coping with the effect of the elastic singularity at the crack tip. The applicability of such procedures is further examined, with additional solutions to an industrial problem considered in R5 the design and life assessment procedures used by British Energy in the UK.     

改善镍基合金铸造性能的研究

研究了稀土元素用于改善镍基合金铸造性能的可能性和效果。试验结果表明，在镍基合金中加入适量的稀土，可提高合金的流动性，降低其热裂倾向，对合金的线收缩率影响不大。     

长期使用荷载下钢筋混凝土结构的钢筋锈蚀——总结

薄壁楔形工字型构件的性能主要由板的宽厚比、构件长细比和楔率决定。对这三者的交互作用效应进行了理论性研究。分析中构建了一个考虑几何非线性的非线性有限元模型,选择具有不同翼缘和腹板宽厚比的大量构件(不同的构件长度和楔率),得出了完整的极限强度-长细比曲线,并且分析了不同的破坏模式。基于设计目的,考虑不同的力和弯矩,描绘出一系列交互曲线,基于这些曲线提出了一个通用公式。这些曲线简化了现有的有效宽度的复杂计算,提供了更为弹性的设计方法。     

Fatigue crack growth analysis using 2-D weight function

A method for crack growth analysis of planar cracks under arbitrary Mode I loading is presented in the paper. The method is based on the point-load (2-D) weight function used for the calculation of stress intensity factors. An algorithm for the analysis of fatigue crack growth of planar cracks, and validation results supporting the entire methodology is also discussed. Application examples of the proposed method for crack growth analysis under arbitrary Mode I stress fields are presented as well.     

Analysis of crack interactions at adjacent holes and onset of multi-site fatigue damage in aging airframes

Focusing on the geometry of one hot spot in airframes, this paper discusses the onset of the interaction of two collinear cracks at adjacent holes and defines the onset as a criterion for multi-site fatigue damage failure. The finite element method is used to calculate the stress intensity factors at the tips of two collinear cracks at adjacent holes growing towards each other. The stress intensity factor is found to increase rapidly at the onset of interaction. Since a rapid increase in stress intensity factor results in a rapid and unstable growth of the crack, the onset of the interaction is proposed as the point where the multi-site fatigue damage starts. A criterion to avoid multi-site fatigue damage locally is then established based on the separation distance of two crack tips at the onset of the interaction. To speed up the simulation of crack growth under multi-site fatigue damage with the finite element method, a semi-empirical criterion is derived to determine the time at which the stress intensity factors at the tips of the cracks correlate. The numerical examples show that the proposed criterion saves simulation time while incurring negligible relative error in the computation of the final crack length.