Progresses in Fracture and Strength of Materials and Structures

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Authors: Hai Yan Xing, Ri Xin Wang, Min Qiang Xu, Jia Zhong Zhang
Abstract: Crack growth rate, as an important parameter, is widely applied in crack growth evaluation and fatigue life prediction. The objective of this paper is to study crack growth rate with the metal magnetic memory (MMM) technology which is a non-destructive testing method newly developed. The correlation between crack growth rate da / dN and magnetic memory signal Hp(y) is investigated through three-point bending fatigue testing. Samples of steel X45, with different heat treatments, quenching and thermal refining, are tested in HYG fatigue machinery. The correlation curves of da / dN ~ Hp(y) are built and the characteristics of magnetic memory signal are given. The influence of heat treatment on Hp(y) is also analyzed. The crack growth rate model of X45 based on MMM signal is presented and verified.
Authors: Xin Min Lai, Xin Zhao, Yan Song Zhang, Guan Long Chen
Abstract: The current inspection methods of spot-weld quality are difficult to achieve an ideal result especially for the stick-weld defect which is one of the most important types of spot-weld defect in the automotive body. This paper thus developed one fast-identification method to identify the joint defects quickly and efficiently based on quantitative analyzing the echo characteristics of ultrasonic curves that could reflect the spot-welding joint defects. The echo-characteristic parameters were analyzed quantitatively through the decision-making bintree of defects and ART networks. After knowledge acquiring and information processing, a fast-identification expert system (FIES) was developed to identify the automotive body spot-weld defects, through collecting the standard ultrasonic curves and quantitatively analyzing. Finally, a series of experiments were conducted to verify the proposed methods and the results showed that FIES is credible and the identification rate can exceed 95% in total test samples.
Authors: Yong Kang Zhang, De Jun Kong, Jin Zhon Lu, Ai Xin Feng, Xu Dong Ren, Xing Quan Zhang
Abstract: Residual stresses of model S1100 of crankshaft chamfer were measured by the technology of XRD. The distributions of residual stresses under mechanical peening, mechanical rolling and isothermal quenching are measured, and the tests of fatigue life were conducted. The results showed that the distribution of residual stress by machining in the crankshaft chamfer is complicated, which is at the tensile-compressive status, and it is one of the main factors to affect fatigue life of the crankshaft; isothermal quenching improves the distribution of residual stress, and tensile stress of the crankshaft chamfer is changed into the compressive stress, which may satisfy the requests of fatigue testing for 5 × 106 cycles. Although the mechanical rolling improved the residual stress distribution in the chamfer linked with the crank, tensile stresses in the chamfer connected with the linkage also increase, which influences the service life of the crankshaft.
Authors: Xi De Li, Kai Wang
Abstract: Recently, quantitatively nondestructive testing (QNDT) is becoming an accepted concept in some industries and scientific research areas. The combination of shearing speckle interferometry (SSI) and mechanical models is employed to quantitatively estimate defect characteristic parameters (DCP), such as coordinates, size, embedding depth, etc. However, quantitative calculation of DCP relies on the actual displacement slope in the mechanical models, but the slope in SSI is represented by the difference of displacements between the two neighboring points with a distance, i.e. the shearing amount. This leads to a deviation in calculating DCP. This paper will investigate the deviation of the relative displacement and derivate displacement introduced by the shearographic approximation in cases of two deformation models, one is a thin circular plate and the other a spherical pressure shell under the pressure loading. Two kinds of defects, a cavity and a crack, are embedded in the structures and their deformations are calculated by FEM.
Authors: Qiang Yong Zhang, Wen Xiang, Xiao Jing Li
Abstract: The geological radar is an advanced non-destructive geophysical prospecting technology used in detecting the invisible and abnormal underground objects. In this paper, this technology is used in the large-scale highway landslide treatment. The location and size of the cavity, fissure and fracture zone in the landslide body is identified accurately. The supporting design and reinforcement construction for the landslide is performed according to the testing results, which ensures the slope stability and the normal operation of the highway, and significant economic benefits have been achieved.
Authors: Zhi Hong Tan, Chun An Tang, Wan Cheng Zhu
Abstract: The changing behavior for infrared thermal image omen of the rock with fracture is essential for the geotechnical engineering. In order to study the behavior, the infrared thermal images for the failure process of rock with hole are carried out. The size of the rock sample is 20cm×10cm×2cm with hole at the center of the sample and the diameter of the hole is 1cm. Considering the fact that sample will effect the results of the observation for infrared thermal image during the experiment, the laminated granite sample was used to replace the cylinder or cuboid sample. The achieved results under uniaxial compression indicate that intensity of the micro ruptures have a close relation with the thermal effects. When the main fractures happen, there is a strip of high temperature that will appear at the destructed local area. During loading process, the abnormity of infrared temperature has two kinds of behaviors as follows: (1) temperature rises and falls alternately, rises before the fracture; (2) temperature falls slowly at beginning, and then rises slowly, then rises quickly before the fracture appears. Even for the same rock sample, the behaviors of the infrared phenomenon may be different during failure.
Authors: Zhe Feng Yu, Zhi Chun Yang
Abstract: A new method for structural damage detection based on the Cross Correlation Function Amplitude Vector (CorV) of the measured vibration responses is presented. Under a stationary random excitation with a specific frequency spectrum, the CorV of the structure only depends on the frequency response function matrix of the structure, so the normalized CorV has a specific shape. Thus the damage can be detected and located with the correlativity and the relative difference between CorVs of the intact and damaged structures. With the benchmark problem sponsored by ASCE Task Group on Structural Health Monitoring, the CorV is proved an effective approach to detecting the damage in structures subject to random excitations.
Authors: Zheng Zhao Liang, Chun An Tang, Leslie George Tham, Y.B. Zhang, T. Xu
Abstract: The investigation on the behavior of a specimen under uniaxial tension and the process of microfracture attracts considerable interest with a view to understanding strength characterization of brittle materials. Little attention has been given to the detailed investigation of influence of heterogeneity of rock on the progressive failure leading to collapse in uniaxial tension. In this paper, a numerical code RFPA3D (Realistic Failure Process Analysis), newly developed based on a three-dimensional model, to simulate the fracture process and associated fractal characteristic of heterogeneous rock specimen subjected to direct uniaxial tension. Specimens with different heterogeneity are prepared to study tension failure. In a relatively homogeneous specimen, the macrocrack nucleates abruptly at a point in the specimen soon after reaching peak stress. In more heterogeneous specimens, microfractures are found to appear diffusely throughout the specimen, and the specimens show more ductile failure behavior and a higher residual strength. Development of fractal theory may provide more realistic representations of rock fracture. The fractal dimension of distributed AE is computed during the fracture process. For all specimens, the fractal dimension increases as the loading proceeds, and it reaches the peak value when macrocrack nucleates abruptly. It is also found that fractures scatter more diffusely in relatively heterogeneous specimens, and the fractal dimension has a smaller value. The homogenous rock specimens have flat and smooth rupture faces which are consistent with the fractal results.
Authors: Zi Chang Shangguan, Shou Ju Li, Mao Tian Luan
Abstract: The inverse problem of rock damage detection is formulated as an optimization problem, which is then solved by using artificial neural networks. Convergence measurements of displacements at a few of positions are used to determine the location and magnitude of the damaged rock in the excavation disturbed zones. Unlike the classical optimum methods, ANN is able to globally converge. However, the most frequently used Back-Propagation neural networks have a set of problems: dependence on initial parameters, long training time, lack of problemindependent way to choose appropriate network topology and incomprehensive nature of ANNs. To identify the location and magnitude of the damaged rock using an artificial neural network is feasible and a well trained artificial neural network by Levenberg-Marquardt algorithm reveals an extremely fast convergence and a high degree of accuracy.
Authors: Xing Dong Zhao, Yuan Hui Li, Rui Fu Yuan
Abstract: AE technique is proved a efficient tool for real-time monitoring of the crack initiation and propagation during rock failure process under uniaxial compression condition. In this paper, An AE system was employed to investigate the crack propagation and failure modes of three groups of granite specimens (80mm×100mm×170mm) with the same pre-existing crack. The AE sensors can be surface mounted. By using a Geiger location algorithm, AE event location can be determined by time-of-arrival times. The propagation velocities of p-wave or s-wave of granite samples were measured. Experiments on pre-existing crack propagation of granite samples were carried out on the press machine. From the testing result, failure mode of three kinds of granite samples was mainly shear failure, while the secondary crack propagated slowly and could not influence the failure mode of granite sample. By surveying the relation of accumulative AE events and stress-strain curve, AE activity represents different characters with stress-strain changing during the total loading process, microcracking contributing to fracture propagation with strain corrosion. AE location result reflected crack initiation and propagation, which is of great importance in studying rock instability and predicting rock failure mode.

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