Microwave Detection of Stress-Induced Fatigue Cracks in Steel and Potential for Crack Opening Determination

Abstract

Fatigue crack detection in metals is an important practical issue in many industries. In this paper the results of detecting fatigue cracks, using the dominant mode approach, employing flange-mounted, open-ended, rectangular waveguides at several microwave frequencies are presented. The goal of this investigation has been to demonstrate the capability of this approach for detecting stress-induced cracks under various static loads. In addition, a correlation between the features of the measured crack characteristic signals and crack opening has been sought. The results show that at all of the investigated frequencies, cracks from being nearly closed to having openings of up to 0.0508 mm are detected effectively. Furthermore, it is found that the interaction of the flange edge with a crack results in features that can be used to enhance crack detection robustness significantly (i.e., increased probability of detection). Several features associated with these measured crack characteristic signals are shown to correlate linearly with crack opening. Such simple correlations may then be used to estimate a crack opening closely after it has been detected using this approach. A complete discussion of the results is also provided in this paper.     

Detection and localization of multiple cracks in a stepped shaft

The presence of a crack in a shaft causes a slope discontinuity in the elastic line of the shaft. There are crack detection techniques, available in the literature, exploiting the slope discontinuity arising because of the crack in the shaft. Steps present in a shaft are expected to interfere with the results obtained through these identification techniques based upon slope discontinuity. It would be even more difficult to identify a crack if it is near a step as both the step and the crack will cause slope discontinuities. A multi‐crack identification technique has been developed (Singh S. K. and Tiwari R. (2010). Mech. Machine Theory 45, 1813–1827; Singh S. K., Tiwari R. and Talukdar S. (2009). IUTAM Proc. in Recent Trends in Rotor Dynamics, March 23–26, IIT Delhi, India) which uses shaft‐forced responses at several frequencies to identify the number of cracks and their locations over the shaft. The algorithm uses normalization of quadratic coefficients obtained from measured responses of a cracked shaft and from simulated responses of the intact shaft for detecting the slope discontinuity. In the present work, the effect of steps in the shaft on crack identification has been analysed. Cracks are assumed to be both near the step and far from the step. The identification algorithm works well for both the simulation cases.     

An engineering approach to fatigue analysis based on elastic‐plastic fracture mechanics

Nowadays cast iron components are widely used in highly stressed structures. Component lifetime is strongly influenced by inhomogeneities caused by the material's microstructure and the manufacturing process (graphite particles, (micro‐)shrinkage pores, inclusions). Inhomogeneities often act as a fatigue crack starter. Lifetime until failure may be divided into stages for crack initiation, short and long crack growth. Initiation of a crack of technical size (a ≈ 1mm) is often dominated by the growth of short cracks. The paper presents an approach to analyse the mechanically short fatigue crack growth based on elastic‐plastic fracture mechanics considering the closure behaviour of short cracks. The effective J‐integral range is used as a crack driving force. Finite element analysis results as well as analytical solutions to approximate the crack driving force are presented. The application of the approach is successfully demonstrated for cast iron material EN‐GJS‐400‐18‐LT using data from fatigue tests, microstructure and fracture surface analyses to assess the fatigue life.     

某筏板基础大体积混凝土裂缝控制技术

针对某综合楼出现的大体积混凝土裂缝问题进行了探讨,并从设计、原材料、浇筑、养护等方面综合介绍了裂缝控制的措施,为大体积混凝土的设计和施工提供参考。     

地铁隧道结构病害原因分析及修复方法

对城市地铁隧道结构中常见的病害类型及其产生的原因进行了系统分析,进而探讨了不同病害对隧道结构安全稳定性的影响机制,在此基础上,结合工程实例提出了各类病害相应修复方法,并提出了修复治理隧道混凝土结构病害的建议,以供参考。     

混凝土裂缝成因及治理分析

针对混凝土低抗拉强度的弊端,分析了混凝土裂缝产生的原因,从设计,材料,混凝土浇筑及成型、混凝土养护等角度出发,提出了混凝土裂缝的预防措施,并着重对已经产生裂缝的混凝土构件的治理措施进行了研究分析,以满足混凝土结构的安全及使用要求。     

谈大体积混凝土施工中裂缝的控制

结合工程实例,详细归纳了大体积混凝土主要施工方法和控制要点,并进行了混凝土热工计算,通过对大体积混凝土温度进行全面控制,减少了裂缝的产生,提高了工程质量。     

E-beam lithography of computer generated holograms using a fully vectorial 3D beam propagation method

A fully vectorial 3D beam propagation method (BPM) has been applied to obtain a required pattern of computer generated hologram (CGH) with a variable profile of four phase levels. The computer reconstruction of the CGH image having one and two focal spots was performed by application of the fully vectorial 3D BPM method. After transferring the CGH by EBL technique an adequate phase profile was obtained. Inter-level parameter method was developed to obtain the estimated an electron beam dose required for the even topographical patterning. Using this method, an EBL exposure dose determined to achieve the required relief amplitude of 1.29 μm was 43 μC/cm². The manufactured holograms showed that the overall proposed production process, from the 3D BPM computer simulation to e-beam lithography, can be used to obtain good quality product with reasonable time and computational resources.     

Flame propagation enhancement by plasma excitation of oxygen. Part II: Effects of O2(aΔg)

The isolated effect of O₂(a¹Δ_g) on the propagation of C₂H₄ lifted flames was studied at reduced pressures (3.61 kPa and 6.73 kPa). The O₂(a¹Δ_g) was produced in a microwave discharge plasma and was isolated from O and O₃ by NO addition to the plasma afterglow in a flow residence time on the order of 1 s. The concentrations of O₂(a¹Δ_g) and O₃ were measured quantitatively through absorption by sensitive off-axis integrated-cavity-output spectroscopy and one-pass line-of-sight absorption, respectively. Under these conditions, it was found that O₂(a¹Δ_g) enhanced the propagation speed of C₂H₄ lifted flames. Comparison with the results of enhancement by O₃ found in part I of this investigation provided an estimation of 2-3% of flame speed enhancement for 5500 ppm of O₂(a¹Δ_g) addition from the plasma. Numerical simulation results using the current kinetic model of O₂(a¹Δ_g) over-predicts the flame propagation enhancement found in the experiments. However, the inclusion of collisional quenching rate estimations of O₂(a¹Δ_g) by C₂H₄ mitigated the over-prediction. The present isolated experimental results of the enhancement of a hydrocarbon fueled flame by O₂(a¹Δ_g), along with kinetic modeling results suggest that further studies of C_nH_m + O₂(a¹Δ_g) collisional and reactive quenching are required in order to correctly predict combustion enhancement by O₂(a¹Δ_g). The present experimental results will have a direct impact on the development of elementary reaction rates with O₂(a¹Δ_g) at flame conditions to establish detailed plasma-flame kinetic mechanisms.