A nondestructive testing technique based on magnetic flux leakage is presented to inspect automatically the stay cables with large diameters of a cable-stayed bridge. Using the proposed inspection method, an online nondestructive testing (NDT) modular sensor is developed. The wreath-like sensor is composed of several sensor units that embrace the cable at equal angles. Each sensor unit consists of two permanent magnets and a hall sensor to detect the magnetic flux density. The modular sensor can be installed conveniently on cables with various diameters by increasing the number of sensor units and adjusting the relative distances between adjacent sensor units. Results of the experiments performed on a man-made cable with faults prove that the proposed sensor can inspect the status signals of the inner wires of the cables. To filter the interfering signals, three processing algorithms are discussed, including the moving average method, improved detrending algorithm, and signal processing based on a digital filter. Results show that the developed NDT sensor carried by a cable inspection robot can move along the cable and monitor the state of the stay cables.

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Inspection method of cable-stayed bridge using magnetic flux leakage detection: principle, sensor design, and signal processing

Electromagnetic methods are commonly employed to detect wire rope discontinuities. However, determining the residual strength of wire rope based on the quantitative recognition of discontinuities remains problematic. We have designed a prototype device based on the residual magnetic field (RMF) of ferromagnetic materials, which overcomes the disadvantages associated with in-service inspections, such as large volume, inconvenient operation, low precision, and poor portability by providing a relatively small and lightweight device with improved detection precision. A novel filtering system consisting of the Hilbert-Huang transform and compressed sensing wavelet filtering is presented. Digital image processing was applied to achieve the localization and segmentation of defect RMF images. The statistical texture and invariant moment characteristics of the defect images were extracted as the input of a radial basis function neural network. Experimental results show that the RMF device can detect defects in various types of wire rope and prolong the service life of test equipment by reducing the friction between the detection device and the wire rope by accommodating a high lift-off distance.

Non-Destructive Detection of Wire Rope Discontinuities from Residual Magnetic Field Images Using the Hilbert-Huang Transform and Compressed Sensing

As an important load-bearing component, steel wire ropes (WRs) are widely used in complex systems such as mine hoists, cranes, ropeways, elevators, oil rigs, and cable-stayed bridges. Non-destructive damage detection for WRs is an important way to assess damage states to guarantee WR’s reliability and safety. With intelligent sensors, signal processing, and pattern recognition technology developing rapidly, this field has made great progress. However, there is a lack of a systematic review on technologies or methods introduced and employed, as well as research summaries and prospects in recent years. In order to bridge this gap, and to promote the development of non-destructive detection technology for WRs, we present an overview of non-destructive damage detection research of WRs and discuss the core issues on this topic in this paper. First, the WRs’ damage type is introduced, and its causes are explained. Then, we summarize several main non-destructive detection methods for WRs, including electromagnetic detection method, optical detection method, ultrasonic guided wave detection method, and acoustic emission detection method. Finally, a prospect is put forward. Based on the review of papers, we provide insight about the future of the non-destructive damage detection methods for steel WRs to a certain extent.

A Review of Non-Destructive Damage Detection Methods for Steel Wire Ropes

Deformation characteristics and stress responses of cement-treated expansive clay under confined one-dimensional swelling

Wire rope inspection by nondestructive testing methods, sensors and signal processing techniques are mainly reviewed in this paper. Owing to the difference of physical mechanism and testing principles, magnetic flux leakage, eddy current, acoustic emission and ultrasonic guide wave testing as well as other inspection methods for steel wire rope are summarized. Then, the commonly and frequently used testing sensors of inductive coil, hall element, magnetoresistive sensors and others are compared in the perspective of their corresponding operating principles, development situation, advantages and disadvantages. Furthermore, signal processing techniques including the signal filtering techniques such as the time and frequency analysis methods, quantitative data processing methods such as the machine learning and defect classification are studied. Finally, the challenges and future developing trends of wire rope inspection in practical applications are discussed.

A Review of Wire Rope Detection Methods, Sensors and Signal Processing Techniques

The invention relates to quick preparation device and method of a saturated soil sample. Saturated clay is extruded by utilizing a load-vacuum combined consolidation principle so as to obtain a saturated clay sample with a low water content (the water content is lower than a plastic limit). The device comprises a sleeve, a base, a piston, a vacuum cavity and a fixing sleeve, wherein the sleeve is sequentially and respectively provided with a slide-fit piston, a cylindrical porous stone internally embedded with a horizontal pressure sensor and the fixing sleeve; the piston, the base and the sleeve are respectively provided with upper draining holes, lower draining holes and side draining holes; and a vacuum extraction hole is arranged on the vacuum cavity. The draining process of the saturated clay is accelerated under the extruding effect of the piston through the negative pressure (about 101.325kPa) condition of the vacuum cavity, and meanwhile, horizontal stress and vertical stress applied on the saturated clay are obtained through the horizontal pressure sensor and a vertical pressure sensor so as to further obtain a coefficient k0 of earth pressure at rest (a specific value of the horizontal pressure to the vertical pressure applied to the soil sample under the condition of no lateral deformation).

Quick preparation device and method of saturated soil sample

Wire ropes, employed extensively in coal mine hoists and transportation systems are subject to damage due to wear, corrosion and fatigue. The extent of damage and the carrying capacity of ropes are closely related to the sense of safety by staff and equipments. Magnetic flux leakage detection method (MFL), as an effective method, is these days widely used in detection of broken strands of wire ropes. In order to improve the accuracy of detection of flaws in wire ropes by magnetic flux leakage (MFL), the effect of the distance between a sensor and the surface of a wire rope (i.e., lift-off) on detection by magnetic flux leakage was investigated. An analysis of the main principles for the choice of lift-off is described by us and a new method that improves the structure of the detector is proposed from the point of view of the design of a magnetic circuit, to restrain the impact of fluctuations of sensor lift-off. The effect of this kind of method is validated by simulation and computation. The results show that the detection sensitivity is markedly increased by this method. Furthermore, the signal-to-noise ratio (SNR) can be increased by over 28%. This method will lend itself to offer reliable scientific information to optimize the structure of excitation devices and improve the accuracy of MFL detection.

Key technique of a detection sensor for coal mine wire ropes

In this study consecutive consolidated isotropically drained triaxial tests for the coefficient of earth pressure at rest (K0) were carried out to investigate its rules of evolution as well as its strength characteristics for normal, consolidated saturated silt under high pressure. The tests results indicate that: 1) for normal, consolidated saturated silt, K0 values increase as the consolidation stress increases at high pressure levels, while the nonlinear characteristics of K0 are inconspicuous compared to cohesive soils; 2) the Jaky and Roscoe equations, used to calculate K0, are only suitable for certain soils, but cannot represent these values for normal, consolidated saturated silt due to the variation in bilinear strength at high pressure; and 3) there are close relations between the nonlinear characteristics of K0 and the void ratio, measured in the tests. Both share the same functional form while under pressure. Based on our experimental results, we developed an empirical linear model to interpret the rules of nonlinear variation for the coefficient of earth pressure at rest.

Coefficient of earth pressure at rest for normal, consolidated soils

The effect of test methods and stress paths on the experimental value of the coefficient of earth pressure at rest, K 0 , was investigated under high pressures. The results indicate that the rigid pressure chamber and flexible lateral confining pressure medium method gives a stress ratio at the initial stage that is not the real K 0 . Moreover, K 0 increases during the loading process becoming greater at high pressures. In the unloading process, however, K 0 increases only at the initial stage but decreases thereafter. In addition, the incremental magnitude definition, K 0 =d σ 3 /d σ 1 , gives higher values than the total magnitude definition, K 0 = σ 3 / σ 1 , under loading. This is also true during initial stages of unloading. The experiment results also indicate that earth pressure at rest in deep and thick soils can be estimated by a power function of axial and confining pressures. It is necessary to choose the appropriate K 0 to avoid some accidents.

Coefficients of earth pressure at rest in thick and deep soils

The invention relates to a device and a method for testing the coefficient of a static earth pressure. The device comprises a base, a pressure chamber, a piston, a radially-deformed resistive strain gauge and a hydraulic sensor, wherein a sample is arranged on the base; the pressure chamber is covered on the sample base; the piston is arranged in the centre of the top part of the pressure chamber and is matched with the pressure chamber in a sliding way; the radially-deformed resistive strain gauge sensor is fixed on the middle section of the sample; and the hydraulic sensor is connected to the base. The section of the piston is as large as and of the same shape as that of the sample to be specifically tested in the pressure chamber. The internal volume of the pressure chamber is adjusted by nesting a plurality of sleeves which are all equal in height and thickness. The volume of the pressure chamber is varied to improve the rigidity thereof, reduce the amount of compression of liquid in the pressure chamber and finally obtain the pressure chamber meeting a condition on the ratio Kopt of the volume of the optimal pressure chamber to the volume of the sample, namely the condition that the maximum of the variable of the diameter of the sample corresponding to different liquid pressures does not exceed 10 microns at the premise of a certain volume of the initial liquid within the pressure chamber, thereby improving the testing accuracy of the coefficient K0 of the static earth pressure.

Xiaodong Zhao

Papers

Quick preparation device and method of saturated soil sample

Key technique of a detection sensor for coal mine wire ropes

Coefficient of earth pressure at rest for normal, consolidated soils

Coefficients of earth pressure at rest in thick and deep soils

Device and method for testing coefficient of static earth pressure