Modeling of a scan type magnetic camera image using the improved dipole model

The scan type magnetic camera is proposed to improve the limited spatial resolution due to the size of the packaged magnetic sensor. An image of the scan type magnetic camera, ∂B/∂x image, is useful for extracting the crack information of a specimen under a large inclined magnetic field distribution due to the poles of magnetizer. The ∂B/ ∂x images of the cracks of different shapes and sizes are calculated by using the improved dipole model proposed in this paper. The improved dipole model uses small divided dipole models, the rotation and relocation of each dipole model and the principle of superposition. Also for a low carbon steel specimen, the experimental results of nondestructive testing obtained by using multiple cracks are compared with the modeling results to verify the effectiveness of ∂B/∂x modeling. The improved dipole model can be used to simulate the LMF and ∂B/∂x image of a specimen with complex cracks, and to evaluate the cracks quantitatively using magnetic flux leakage testing.     

Damage detection in glass/epoxy composite structure using 8–12 GHz X-band

In this study, an electromagnetic wave 8–12 GHz X band microwave was utilized to detect various types of damage in a woven fabric composite structure. Damages, such as cracks, delamination, bubbles, and voids, were synthesized artificially in accordance to ASTM D2734. Variable thicknesses of composite structure were also analyzed, and the influence of defect on the transmitted signal was investigated. Network analyzer (ENA5701C) in X-band was utilized for this investigation. Detection was based on the changes in the electromagnetic properties, such as permittivity and permeability, and the reflection and transmission to microwaves were based on standard samples. These were implemented for each case of defect selected for this study. Experimental tests revealed that damage in the transparent glass/epoxy composite can be recognized clearly. Particularly, sample thickness was detected when the microwave was applied. This testing method can be considered an inline operation and non-destructive testing for such composites, especially during fabrication.     

Detection and measurement of surface cracks by the ultrasonic method for evaluating fatigue failure of metals

The problem of measuring the dimensions of safe cracks by the acoustic method was considered on the basis of the concept of safe cracks. The results of investigations are presented and the obtained data on the detection and measurement of the depth of surface cracks using Rayleigh waves at artificial reflectors of the surface-notch type and at real cracks and specimens, which are subjected to cyclic loads during the “growth” of fatigue cracks, are analyzed.     

Static and dynamic fracture analysis for the interface crack of isotropic-orthotropic bimaterial

In the present study, interfacial cracks between an isotropic and orthotropic material, subjected to static far field tensile loading are analyzed using the technique of photoelasticity. The fracture parameters are extracted from the full-field isochromatic data and the same are compared with that obtained using boundary collocation method. Dynamic photoelasticity combined with high-speed digital photography is employed for capturing the isochromatics in the case of propagating interfacial cracks. The normalized stress intensity factors for static cracks are greater when α=90° (fibers perpendicular to the interface) than when α=0° (fibers parallel to the interface), and those when α=90° are similar to ones of isotropic material. The dynamic stress intensity factors for interfacial propagating cracks are greater when α=0° than α=90°. For the velocity ranges (0.1<c/c _s1 <0.7) observed in this study, the complex dynamic stress intensity factor |K _D |, I increases with crack speedc, however, the rate of increase of |K _D | with crack speed is not as drastic as that reported for homogeneous materials.     

Sliding wear behaviour of Ca α-sialon ceramics at 600 °C in air

As an extension of a previous investigation on the wear behaviour of Ca α-sialon ceramics of differing microstructures at room temperature, wear testing was conducted at 600 °C in air to explore the effects of microstructure, contact pressure and sliding speed on the wear behaviour. Under all loading conditions from 1 MPa to 1 GPa, a constant high friction coefficient was observed and a severe wear process was dominant, in which the sliding contact induced cracks were observed in different microstructures. Wear particles were generated along the wear track, but no tribofilm was detected. Increasing the sliding speed from 10 to 23 cm/s was found to significantly increase wear rate. However, variations in microstructure had little impact. That is, large elongated-grained α-sialon exhibited only a slightly lower wear rate than fine equiaxed-grained α-sialon.     

Comparison of thermal fatigue behaviour of ASTM A 213 grade T-92 base and weld tubes

Super-heater tubes are subjected to alternate heating and cooling in power plants causing them to crack and eventually fail. This phenomenon is referred to as “thermal fatigue.” In this paper, a laboratory simulation for reproducing the thermal fatigue phenomenon is developed to determine the number of cycles necessary before failure occurs in super-heater tubes. The temperature and strain distributions along the specimen were computed theoretically using ANSYS software for the applied temperature condition. The thermal fatigue test was conducted for both base and shielded metal arc (SMA) welded tubes separately and both passed in the non-destructive tests. These tubes were subjected to thermal cycles from 800°C (accelerated temperature) to room temperature. Oxy-acetylene heating setup was utilized as a heating source, and a water bath was utilized for quenching purposes. The tests were carried out until open cracks were identified. Surface cracks were identified in the base and weld tubes after 90 and 60 cycles respectively. This study reveals that heating and cooling cause thermal fatigue, initiate cracks in the tubes.     

Experience in the ultrasonic testing of fastening studs in the covers of hydraulic units at hydroelectric power plants

The criteria for evaluation of the admissibility of discontinuity flaws under the threads of studs during their inspection from the end face without unscrewing them from their seat in a hydraulic unit frame are experimentally substantiated. A transducer with the optimal parameters that allows one to reveal cracks of minimal depths is selected. Ultrasonic inspection of studs with a hexahedron at the end face is used as an example and the results of the ultrasonic testing of studs with detected cracks are presented.     

Acoustic-emission testing of solebars with boxlike cross sections

The changes in the design of the 18–578 solebar with a boxlike cross section required the development of a new scheme of arrangement of acoustic-emission (AE) transducers and improvement of both technique and software. All sources that were detected during AE testing were divided according to their degree of hazard into passive and critically active ones. Location grids that specified the rejection levels were imposed on the tested zones under study. In the refined technique, linear location was used, which made it possible to preserve the reliability of the investigation results and reduce the labor intensiveness of the AE testing.     

Application of metal magnetic memory test in failure analysis and safety evaluation of vessels

Metal magnetic memory test (MMMT), which is a new subject in the field of nondestructive examination, can determine regions of stress concentration by testing the distribution of the magnetic field of metal structures so as to effectively diagnose premature defects. MMMT and other test methods are applied in the study to put a propylene purifier of a temperature-jump accident and a leaked ammonia vessel through safety evaluation. Results are as follows: The margin of safety declines after the purifier is overburnt; several stress concentrations are observed within the overburnt area and the level of stress concentration rises after one-month operation; and overpressure operation of the purifier must be strictly avoided and carefully monitored during later operation. Cracks are observed on the ammonia vessel after one year’s service. Extremely high residual stress is the primary cause of cracks. After four years in service, the residual stresses existing in the area of the base metal and weld zone are still greater than 0.5 σ _S, which results in numerous cracks due to stress corrosion. From the MMMT result of the ammonia vessel’s defects, it can be seen that the derivative of magnetic density (dH _p /dx) is an important reference variable. Within the 31 leakage points, the one whose values of dH _p /dx are above 10 occupy 67.7%, and the ones above 8 share 96.8%.     

Investigation of Behavior of Stress-Corrosion Cracks in a Pipes under Internal Pressure

Corrosion induced by ambient conditions causes generation of longitudinal cracks on outside surfaces of gas pipelines, and this can lead to breaks in main pipelines. The paper presents experimental data on behavior of cracks in sections of pipelines with aws under constant and cyclically varied (45 cycles) hydrostatic pressure, the pressure magnitude corresponding to operational parameters. Crack parameters (increasing depths and openings of cracks) were monitored by sensors of magnetic induction placed on pipes immediately over cracks. The data indicated that cracks with depths of up to 30–35% of the wall thickness did not show residual changes (in their depths and openings) after tests. Residual changes were detected in deeper cracks, which manifested as critical ows of metal near the crack edge and growth of cracks to their critical dimensions. Photographs of fractured surfaces showed that crack depths started to increase long before the ultimate fracture of a pipeline. Thus, tests conducted by applying high pressure to operating gas pipelines can lead to increases in crack dimensions without detecting them. This effect is actually detrimental to the tested pipeline's safety. The absence of residual changes in cracks with depths of up to 30–35% of the wall thickness indicates that, possibly, new limits on aw dimensions should be established for decommissioning unsafe sections of pipelines, and these limits should take account of the hazard due to stress-corrosion cracks, depending on their dimensions and inside gas pressure.     

薄板奥氏体不锈钢对接焊缝超声波探伤方法研究

通过对超声波应用于薄板奥氏体不锈钢对接焊接接头的研究和试验,开发了薄板奥氏体不锈钢对接焊接接头超声检测和质量分级方法。试块探伤试验和现场实物检测试验表明,对板厚为3～10 mm的在用压力容器奥氏体不锈钢对接焊缝及母材,辅以超声波探伤,可以及时发现奥氏体不锈钢容器在对接焊缝、封头扳边处、支座角焊缝母材等处存在的各种形式的裂纹或缺陷,有效避免出现容器的失效事故。     

Initiation and propagation laws of the glass cracks in specimens subjected to normal loading under the action of symmetric wedges

With more and more applications of glass in advanced fields of science, the demand for glass machining precision has increased greatly. More and more attention is being paid to glass cutting because precise glass parts with various shapes can be obtained at high efficiency and low cost. To improve the machining precision of part surfaces and to facilitate tool design and cutting parameter selection, the initiation and propagation laws of glass cracks in specimens subjected to normal loading by symmetric wedges were investigated. Research results show that initiation and propagation laws are the same with interior symmetric wedge angles of 30°–120°, while the laws are different with interior symmetric wedge angles equal to or more than ≥150°. The relationship between medial crack length and normal loading was also investigated when specimens were indented by symmetrical wedges with interior angles of 30°–120°. __________ Translated from Journal of South China University of Technology (Natural Science Edition), 2004, 32(7) (in Chinese)     

Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

Machine of testing the ceramic&#8217;s bending strength properties at high temperature and ultra-low speed

According to the established rules for testing ceramic’s bending strength, the falling velocity of the pressure head of the machine should be more than 0.5 mm/min. For the machine of testing the ceramic’s bending strength properties, most designs adopt the lowest falling velocity as 0.5 mm/min. In the fields such as aerospace and deep-ocean exploration, the utilization of ceramic materials that work at normal temperature, low temperature, or even high temperature and bend at an ultra-low velocity is increasing; thus, the intense requirements for the machine of testing the ceramic’s bending strength properties at high temperature and ultra-low speed (MTCBSP) and its experimental basis are put forward. This research developed an MTCBSP suitable for normal temperature and high temperature with the lowest falling velocity of 0.05 mm/min, and manufactured a drive reducer for ultra-low speed and high-temperature working condition. In the test, equipment includes a high-temperature furnace, bending die for four-point bending, and protection system of inert gas, which was placed in the high-temperature furnace to prevent the ceramic sample from being oxidized to diminish its effects. The results show that the lowest falling velocity of the pressure head of this new machine is 0.05 mm/min, and the mechanical properties of silica glasses are noticeably different at the same high temperature and the different falling velocities of 0.5 mm/min and 0.05 mm/min.     

Modeling and analysis of magnetic dipoles in weak magnetic field

The magnetic leakage field distribution resulting from linear defects of a tube sample in the geomagnetic field is modeled according to the magnetic dipole theory. The formula to compute the normal component of the weak magnetic field is deduced based on the spatial distribution of the magnetic dipole. The shape and characteristics of the zero line (an important criterion for magnetic memory testing) of the normal field is analyzed under different longitudinal magnetizations. Results show that the characteristics of the zero line should be considered when the metal magnetic memory testing method is used to find and locate the defect. __________ Translated from Transactions of Beijing Institute of Technology, 2007, 27(147): 395–398 [译自: 北京理工大学学报]     

Characteristics of the detecting equipment for the nanosecond tagged-neutron technology

Detecting equipment for the nanosecond tagged-neutron technology has been developed, and its characteristics have been studied. The principles of arrangement and operation of the readout electronics based on the selection of useful events according to specified criteria and data accumulation by a buffer-memory unit with subsequent transfer of data arrays to a remote computer for processing and visualization. The main selection criterion is the presence of signals from α- and γ detectors within the time gate and amplitude ranges in the absence of overlapped events. A prototype of a setup for testing the developed equipment was assembled and experimental studies of its characteristics were performed. The time resolution attained in the recording of α-γ coincidences is 1.0 ± 0.1 ns at an amplitude resolution of the γ detector of 3.6–3.8%. Original Russian Text ? K.A. Balygin, M.D. Karetnikov, A.I. Klimov, K.N. Kozlov, E.A. Meleshko, I.E. Ostashev, G.V. Yakovlev, 2009, published in Pribory i Tekhnika Eksperimenta, 2009, No. 2, pp. 122–132.     

Progression of fretting fatigue damage in Ti–6Al–4V

An investigation was conducted to explore the nature of fretting fatigue damage in the stages prior to crack formation. In the unique experimental apparatus employed in this study, where total slip never occurs, several locations on each test specimen exist where cracks can develop due to local contact conditions. Under the test conditions used, not all of the sites had cracks upon test completion. This study evaluated the condition of non-cracked sites on several fretted specimens in an effort to identify differences between these and sites where small cracks were observed.A single test condition of 620 MPa average applied static clamping stress and 250 MPa applied axial fatigue stress for R=0.5 was selected, which corresponds to a fretting fatigue life of 10⁷ cycles based on prior work. For specimens tested to 10⁶ cycles, or 10% of life, several destructive and non-destructive characterization methods were chosen: scanning electron microscopy (SEM), residual stress measurement and transmission electron microscopy (TEM). Each site at which crack nucleation could be expected was inspected in the SEM and was then characterized using surface X-ray diffraction to quantify the residual stresses field near that location. Then TEM foils were cut from one area on a specimen with tiny cracks and dislocation densities were observed. A novel technique was used which permitted TEM samples to be obtained from regions in close proximity on the original specimen.Comparisons were made between as-received (AR) and stress-relief annealed (SRA) specimens, on which the stress-relief was applied prior to fretting fatigue testing. SEM inspection was useful for qualitative analysis of wear debris and identification of cracks as small as 20 μm, but was unable to provide quantitative data on the level of fretting fatigue damage beyond crack size. Although differences were noted in the residual stresses for the SRA versus the AR specimens, no residual stress peaks were noted in the edge of contact regions where cracks would eventually develop. TEM observations in the vicinity of the crack nucleation region showed that the dislocation structure decayed rapidly into the specimen thickness. The cause of the dislocations was attributed to plastic deformation caused by the clamping stresses.     

Eddy-current testing of the wear resistance of laser-hardened carburized chromonickel steel and the quality of laser hardening of drill bits

Features of eddy-current testing of carburized chromonickel steel 30XH3A subjected to processing with continuous laser radiation, subsequent treatment with cold at s-196°C (reducing the content of retained austenite in the carburized layer from 25–40 to 5–10 vol %), and tempering in the temperature range 100–400°C are studied. It is shown that the eddy-current method can be used to detect the presence of a laser-hardened wear-resistant layer on the steel surface and evaluate an abrupt decrease in the abrasive wear resistance of steel hardened with laser radiation, additionally treated with cold, and then exposed to low-temperature tempering. The possibility of eddy-current testing of the quality of laser hardening of roller paths of the bearing unit of rock-drilling bits manufactured from carburized steel 20XH3A using an attachable transducer with a protruding rod core is established.     

复杂裂纹涡流检测与评估方法研究

钢轨表面及上表面产生的滚动接触疲劳裂纹通常以斜裂纹或多角度复杂裂纹的形态存在,对其检测和评估是个难题。 基于此,采用基于无线能量传输的涡流检测方法(WPT-ECT),设计新的探头结构并结合神经网络算法对裂纹进行检测和评估。 首先,有别于现有 WPT-ECT 方法,采用增大激励频率,而非串并联电容的方式,构造谐振电路;其次,根据复杂裂纹的特点,设计 由两个八字形激励线圈和两个矩形接收线圈组成的方向性探头结构;最后,充分提取检测信号的特征,并运用径向基神经网络 算法对裂纹进行识别。 仿真和实验结果表明,所提出的探头结构对任何角度的缺陷均敏感。 同时,RBF 算法对斜裂纹、T 裂纹、 Y 裂纹和 1. 2 mm 提离下的 T 裂纹的识别准确率分别为 92. 00% 、 95. 27% 、96. 64% 和 89. 50% 。     

Effect of Crystalline Orientation in the Ductile-Regime Machining of Silicon

This paper investigates the effect of crystallographic orientation in ductile-regime (DR) machining of (100) silicon wafers. Single crystalline diamond tools with 10-40 nm edge sharpness were used to machine the wafers at either constant depths of cut, or a taper mode to vary the depths of cut up to 1<<<:956>>>m. The feedrates were normalised as percentages of tool nose radii, and the machining process was performed using an ultraprecision machining system. The surface and subsurface integrity were then characterised with an atomic force microscope, a phase shift interferomter, and an ion beam system. The measured surface roughness of silicon was compared with those of copper alloys, and the calculated values. A ductile-regime was achieved when machining along the <110> directions when the maximum chip thickness of less than 05 χm. Machining conditions that formed thicker chips led to pitting, microcracks and slip lines. Such defects, which could be more than 1 χm deep, were found along the <110> directions and occasionally along the <100> directions. Surface roughness below 10nm was measured in a DR areas, but was as high as 170nm in pitted areas. When the depth of cut was of the magnitude of the tool edge sharpness, the surface finish was degraded by radial cracks in the lateral plane owing to rubbing between the tool and the workpiece. The surface finish of the silicon, therefore, was rougher than that of copper alloys that were machined using similar parameters.