Determining the Depth of Occurrence of Defects in Multilayer PCM Structures by Acoustic Methods Based on the Mechanical Impedance Value

Dedicated low-frequency acoustic-testing methods are widely used to detect flaws in three-, five-, and seven-layer structures with a honeycomb core made of polymer composite materials (PCM), with the impedance technique being the main one. However, this technique allows one to establish only the mere fact of the presence of a flaw but not the depth of its occurrence. To obtain information on the depth of occurrence, it is necessary to numerically measure the mechanical impedance on the surface. The complete impedance-transducer frame system has been simulated by the electromechanical analogy method in order to establish the relationship (and derive respective dependences) between the external load applied to the sensor, expressed as the total mechanical impedance on the article surface, and the change in the values of measured electrical parameters on sensor’s piezoelectric elements. Relevant dependences have been derived for the transmission factor, which is the modulus of the ratio of voltages across receiving and emitting piezoelectric elements, and for the phase shift between these voltages. By combining these dependences, hodographs have been produced that represent the graphs of the dependences in the amplitude-phase plane, similar to how information is displayed in state-of-the-art impedance flaw detectors. The dynamic contact compliance (significantly affecting the efficiency of impedance testing technique) of a dry point contact between materials of the outer PCM layers and widely distributed and commercially available impedance sensors (for example, PADI-8) has been determined. Model hodographs constructed with allowance for contact compliance were used to run an experiment on revealing delamination and starved-joint flaws at various depths in a seven-layer honeycomb structure of an aircraft engine nacelle. It has been confirmed that the depths of defects are effectively discriminated both by the magnitude of mechanical impedances and by the characteristic indication from cell walls on the C-scan over the entire article surface.     

Effect of a gap in the electromagnet-object magnetic circuit on the magnetic field of surface flaws in different steels

The dependences of the magnetic-field strength of surface flaws in various steels on the electromagnet current have been studied for different gaps in the magnetic circuit. It is shown that an anomalous ratio of the magnetic-field strengths of flaws in hard and soft magnetic materials holds in the entire range of gap changes. The dependence of the ratio of the magnetic-field strengths of a flaw, which are obtained for different gaps in the magnetic circuit, on the electromagnet current has an extremum in the region of weak currents and, at high currents, tends toward to a constant value, which is determined by the value of the set gaps. The results are considered, taking their interrelations with the magnetic properties of materials and processes of their magnetization in the presence of a demagnetization factor into account.     

Theoretical foundations of ultrasonic nondestructive testing of multilayer articles manufactured from composites with rubbery coatings: II. Scattering of elastic waves at flaws involving poorly glued joints between plastic layers and between a plastic layer and a rubbery coating

The second part of this study is devoted to the solution of the following problems: (i) excitation of plate modes with a lumped force applied to the plate’s surface, (ii) scattering of a bending wave at a small crack parallel to the plate’s surface, and (iii) scattering at a flaw involving a poorly glued joint between a rubbery coating and a plastic layer. The final formulas and numerical dependences for calculating the parameters of a crack or a cleavage are presented. A mathematical model that describes the scattering of waves by flaws in a multilayer article made from a plastic composite material has been developed.     

Studying a Signal due to the Tangential Component of Stray Fluxes from a Surface Flaw Detected with a Vibrating Induction Transducer

Analytical dependences of the amplitude spectrum of the output signal of a vibrating induction transducer on its position with respect to a flaw and on vibration-displacement parameters have been derived. The locality of inspection and its selective sensitivity toward flaws have been evaluated. The results of theoretical research have been confirmed experimentally.     

Electrochemical treatment of welded seams before penetrant testing

The effect of the abrasive treatment of welded seams performed before penetrant testing on opening and polishing of the surface flaws is studied experimentally. It has been shown that, owing to the electrochemical treatment, the mouths of flaws are cleaned of a metal layer and the microroughness of the surface and its background luminescence after development decrease considerably, thus providing high-quality luminescent penetrant testing.     

Susceptibility of Penetrant-Dye Testing as a Function of Development Temperature

The paper considers the effect of heating temperature of tested components on the stage of development in penetrant-dye tests on their susceptibility. Analytical expressions are given for calculating the optimal penetration depth due to diffusion in cylindrical flaws and threshold parameters of detected flaws with due account of the heating. By heating a tested object both before the penetrantdye deposition on the surface and on the stage of development, one can obtain optimal results in terms of the sensitivity and duration of the stage on which the penetrant is used.     

Non-destructive testing of plates based on the visualisation of Lamb waves by double-pulsed TV holography

We describe a new technique, currently under development, intended to detect the presence of flaws such as cracks and holes in thin-walled mechanical components. This technique combines ultrasonics with optics, both at a low power density, that allows to perform the tests in the non-destructive range. Lamb waves, a kind of surface acoustic waves that propagate in thin plates, are generated to explore metallic samples while double-pulsed TV holography, a whole-field interferometric technique, is used to detect them. This scheme provides maps of the instantaneous surface displacements produced by the waves, where the effects of the flaws can be visualised by contrast against the smooth propagation of the wavefront in defect free plates. Images with reasonable resolution can be achieved almost in real-time. Several examples of detection of typical flaws in plates using the proposed method complete this work.     

Use of the Harmonic Analysis of an Induction Probe's Signal for Increasing the Information Content of Test Results

The harmonic content of a probe's electromotive force when magnetic testing is conducted with the use of alternating fields is significantly affected by the possible presence of a flaw in the specimen. Magnetic biasing of the specimen with a static field enables identification of internal flaws in addition to surface flaws. The electromotive force's harmonic content depends on the flaw's dimensions and depth. This circumstance can be used to determine these parameters in a separate way. The effect of a gap between the surface of a tested specimen and the probe is significantly weaker on the topography and amplitude of higher harmonics than on the corresponding parameters of the first harmonic. This observation, which holds for both internal and surface flaws, enables testing of larger gaps.     

Comparative classification of flaws using ultrasonic-tomography methods and evaluation of the instantaneous frequency of echo signals

The results of the visualization of artificial reflectors of different shapes and a natural flaw of the faulty-fusion type in the bottom run are presented. The results of the evaluation of the instantaneous frequency, which is proposed as an additional informative index for shape classification of flaws, are presented for the same reflectors. A comparative analysis of the data that were obtained by these two methods was performed. The measurement results are represented in the form of B-scans for phased arrays, which provide a clear idea of the spatial location and configuration of reflectors, and in the form of diagrams of the dependences of the normalized frequency deviations for single dual transducers.     

Estimation of the strength and lifetime of technical objects using the acoustic-emission method

Estimation of the lifetime of a construction consists of forecasting the time remaining before failure and is based on extrapolation of the time dependences of the state parameters of a technical object to their critical values. The use for this purpose of simplified damageability models (e.g., a hypothesis of linear summation of flaws) and the absolute values of acoustic-emission (AE) parameters (e.g., activity, counting rate, duration of pauses) leads to a dependence of the estimate results on a large number of destabilizing factors and, as a result, to low accuracy or laboriousness of these techniques. An earlier suggested micromechanical model of AE parameters makes it possible to find a trade-off between simplicity and accuracy.     

Localization of Reflectors in Plates by Ultrasonic Testing with Lamb Waves

An algorithm is suggested for localizing flaws in thin-walled objects based on the analysis of dispersion characteristics of Lamb waves. It is shown that applying the technique of determining the instantaneous signal frequency by means of wavelet transform makes it possible to reconstruct extensive sections of frequency dependences of the group delay time for different modes of Lamb waves. An extra merit of the proposed method is its high noise immunity. It has been established that an optimum choice of the frequency range is required to minimize the experimental error, namely, in order to minimize the inaccuracy in determining distance one should choose the dispersion-characteristic section where the group delay time is long as compared with its nominal value while its frequency variation is maximum.     

Dependence of the magnetic field of a flaw on the distance to the surface of a tested article

The dependence of the magnetic field of a flaw on the distance to the surface of a tested article has been studied for rectangular slot-type flaws. For flaws with a small opening width, it can be considered that their magnetic field decreases in inverse proportion to this distance, but with an increase in the flaw width, the field decreases more slowly.     

Models of magnetic charges and fluxes in the problem of flaw detection with local magnetization

The results of experimental studies of the magnetic-field strength of surface flaws in magnetically soft and hard steels magnetized with a dipolar magnet are discussed. Peculiarities of the interrelationship between the formation of the magnetic fields of surface flaws and fluxes in a magnetic circuit have been determined. The obtained anomalous relationship of the magnetic-field strengths of flaws in magnetically soft and hard steels was considered in terms of a flux model. The distinctive feature of the model consists in the presence of an internal magnetization magnetic flux whose lines of force are closed within a magnetized body. A comparison of the charge and flux models is performed based on the results we obtained.     

A technique for evaluating the permissible depth of an extended surface flaw in a pressurized vessel

A technique for evaluating the permissible depth of extended surface flaws in pressurized vessels is based on the criterion equation of the diagram of resistance to cracking of a body with a thin cut and introduction of a safety factor into the equation. An equation for calculating the permissible stress concentration coefficient in the presence of a U-shaped surface flaw is presented. The finite element method is used for analyzing theoretical values of the stress concentration coefficient for different depths of flaws and the rounding radius of the flaw tip in the drum of a pressurized vessel. An approximate formula for calculating the coefficient is proposed. The permissible depth of a surface flaw in the drum of a pressurized vessel made of 09G2S steel is calculated.     

基于频谱分析的脉冲涡流缺陷检测研究

脉冲涡流检测技术是涡流检测技术的一个新兴分支.研究利用激励脉冲频率成分丰富的特点,创新性地提出了一种新的脉冲涡流差分信号特征值——频谱幅值.通过研究,得到频谱幅值与表面缺陷深度均成近似线性关系,与亚表面及厚度减薄缺陷深度成指数关系.从而提供了一种有效的基频分量频谱幅值对飞机内部隐藏缺陷识别的原理及方法,这对于飞机多层导电结构内部隐藏缺陷检测极为有效.     

多冲碰撞载荷下激光涂层的裂纹损伤及机理分析

对激光涂层进行了多次冲击碰撞试验，并通过SEM等手段分析研究了激光涂层在多冲碰撞载荷下的裂纹形成与扩展机理。研究发现：涂层裂纹多从表层起源，在涂层内应力和反复碰撞压应力的联合作用下逐步扩展；裂纹形状与材料、熔覆工艺及多冲工况相关，裂纹在表面呈横断和网状两大类，在截面呈垂直和网状扩展型两种。用应力集中理论和空位聚集理论解释了裂纹的成核与扩展机制。     

Flaw detection of alloys using the eddy-current method

A subminiature transformer eddy-current transducer (ECT) that is intended for the study of different nonuniform materials, alloys, miniature parts, printed-circuit boards, and microscopic defects has been designed. A block diagram of the transducer is given and its basic technical characteristics ensuring localization of the magnetic field on areas of approximately 50 × 50 μm are stated. A scheme that uses a computer as a generator and receiver of signals from windings is proposed. It is capable of automatically changing the filtering cutoff frequency and operating frequency of the device. The designed measuring system eliminates the main drawback of eddy-current transducers (the small area of the electromagnetic field), simultaneously significantly reducing the noise level due to the use of high-quality amplifiers and filters, and searches for defects in printed circuit boards, metal–dielectric–metal-junctions, and alloys of different metals. A measurement procedure that allows one to perform high-accuracy monitoring of flaws in different alloys is described. The eddy-current transducer was successfully tested on several objects, e.g., a 5.5-μm thick Al–Mg alloy and welded seams of 5-mm thick titanium plates, as well as other objects. The dependences of the ECT signal on the flaws in these structures are given.     

Effect of demagnetizing action of an interface on the field of surface flaws upon magnetization with an attached electromagnet

Dependences of the magnetic-field strength of flaws on a electromagnet’s current and the magnetic induction in a magnetic-circuit gap are obtained. The magnetic-field strength of flaws in magnetically hard steels is found to be higher than that of flaws in magnetically soft steels. The results obtained are explained in terms of the demagnetizing action of a surface magnetic dipole as well as the dependence of the magnetic-flux distribution over specimens on the magnetic permeability of a material and on the induction under the electromagnet’s poles.     

Processing of active thermal nondestructive testing results by the method of wavelet analysis

Wavelet transforms can be used for active thermal nondestructive testing for the identification of flaws that are located at different depths and produce Gaussian-like signals characterized by a time shift on a heated surface. The results of the application of scalar and complex wavelet analysis for increasing the signal/noise ratio in the active thermal inspection of flaws in composite materials and thermal-barrier coatings, as well as also for revealing the corrosion in metals, are described. Parent Morlét, Paul, and Mexican-hat wavelets were used.     

Structural features of crack formation during corrosion cracking

Stress corrosion cracking (SCC) causes damage to main gas pipelines due to formation and development of cracks on the outer surface of a tube. Prevention of SCC crack development and fracture of a gas pipeline is an important problem of diagnostics that can be solved in various ways. The most effective approach involves systematic use of in-tube nondestructive testing and local testing in shafts, owing to which the position of a flaw and its linear size are determined. The hazard of the flaw (residual life of a tube with cracks) is found and a decision is made about the necessity and methods of flaw repair, such as cutting out the tube, grinding the flaw, and reinsulating the tube surface, and the date of repeated nondestructive testing is set. These measures ensure timely discovery of potentially hazardous flaws and make it possible to take the necessary steps for preventing development of SCC.