Journal of Nondestructive Evaluation

This work describes a numerical study on non-destructive evaluation of interlayer disbond defects in aerospace grade Fibre Metal Laminate sheets (FMLs). A recently proposed infrared non-destructive testing setup is considered, where a continuous laser is moved over the material surface, while the thermal footprint of the moving heat source is acquired, e.g. by an infrared thermal camera. Interlayer disbonds are then detected by analysing the features of the acquired thermograms. The experimental feasibility of this approach has been recently proved. The present work proposes a numerical simulation of the NDT approach, where the material thermal response is analysed and correlated to defects signatures. The numerical study has in particular investigated the influence of a number of different features on the defect detectability, and on the accuracy of defect edges and position identification. Such features comprise different FML materials (GLARE, CARAL, Ti-Gr), laser heat deposition and regions of data analyses.

High precision ultrasonic time-of-flight measurement is a well known part of non-destructive evaluation used in many scientific and industrial applications, for example stress evaluation or defect detection. Although ultrasonic time-of-flight measurements are widely used there are some limitations where high noise and distorted ultrasonic signals are conflicting with the demand for high precision measurements. Cross-correlation based time-of-flight measurement is one strategy to increase reliability but also exhibits some ambiguous correlation states yielding to wrong time-of-flight results. To improve the reliability of these measurements a new machine learning based approach is presented based on experimental data collected on tightened bolts. Due to the complex structure of the bolts the ultrasonic signal is influenced by boundary conditions of the geometry which lead to high number of the ambiguous cross-correlation results in practice. In this particular application, bolts are in practice evaluated discontinuously and without knowledge of the time-of-flight in the unloaded condition which prevents the use of all other available comparative preprocessing techniques to detect time-of-flight shifts. Three different preprocessing strategies were investigated based on variations in the bolting configurations to ensure a machine learning based model capable of predicting the state of the cross-correlation function for different bolting parameters. With this approach, we achieve up to 100% classification accuracy for both longitudinal and transversal ultrasonic signals under laboratory conditions. In the future the method should be extended to become more robust and be applicable in real-time for industrial applications.

Magnetic flux leakage (MFL) testing has been widely used as an efficient non-destructive testing method to detect damage in ferromagnetic materials. It’s of great importance to improve the testing capability of MFL sensors. In this paper, a micro magnetic bridge method in MFL of high sensitivity is proposed to detect micro-cracks. This method consists of a micro magnetic bridge core and an induction coil. Furthermore, a novel micro magnetic bridge probe (MMBP) of higher spatial resolution is designed and developed with $$10~\upmu \hbox {m}$$ width between the two sides of this MMBP in the testing magnetic bridge. The lift-off effect of this new MMBP is studied via finite element method and experimental verification. The results show this MMBP can achieve high sensitivity only when working with a micro-lift-off value. To examine the detecting capability of this MMBP, micro-cracks in magnetic particle inspection sensitivity testing pieces are all inspected, and the lowest depth value is only $$7~\upmu \hbox {m}$$ . The MMBP in this paper improves the testing capability of MFL to the micrometre scale and can be widely used to detect grinding micro-cracks in bearing rings.

The efficiency of transient wave generation in a thermoelastic silicon layer excited by a pulsed laser is considered. First a principle-based transfer matrix formulation with relaxation effect, also referred to as the generalized dynamic theory of linear thermoelasticity, is used in obtaining transfer functions between the input heat field and the elements of the thermoelastic state vector. The second sound effect, through this relaxation time term, is included to eliminate the thermal wave travelling with infinite velocity as predicted by the diffusion heat transfer model. By employing the fast Fourier transform (FFT) algorithm, the transient response of a silicon thermoelastic layer under a thermal excitation (by a pulsed laser) is investigated to quantify the conversion efficiency from thermal to mechanical energy. The transient acceleration, stress, heat, temperature, and mechanical power flux responses are presented. The pulse duration of the laser excitation is submicrosecond level and, consequently, a large number of modes of motion are excited. Rigid body singularities are eliminated by considering the higher order time derivatives of the state variables. A layer made of bulk silicon under this laser excitation is considered and it is found that the amplitude ratio of the applied heat field to the propagating heat flux at the data points is in the order of 10°. The ratio of the applied power (heat flux) to the generated mechanical power flux is in the order of 10°. The resulting rigid body motion of the layer due to the laser excitation is excluded in calculating the mechanical power.

Based on a theory of elastic wave propagation in arbitrarily oriented transversely isotropic media, which has been presented recently, the radiation characteristics of ultrasonic transducers in these media are determined. Using the directivity patterns for normal and transverse point sources on the free surface of such (semi-infinite) materials—the derivation is based on the reciprocity theorem—the radiated wave fields are obtained by the method of point-source-synthesis, i.e., by superposing the wave fields of numerous point sources located within the transducer aperture. Since ultrasonic inspection of anisotropic materials, especially weld material in nuclear power plants, suffers from the well-known effects of beam splitting, beam distortion, and beam skewing, valuable information in view of an optimized inspection is provided. Focusing on transversely isotropic weld material specimens, numerical evaluation is performed for several grain orientations with respect to the transducer-normal. The approach presented is particularly useful in view of an appropriate extension to inhomogeneous welds and the consideration of time-dependent RF-impulse functions.

The current text presents a parametric study of two active thermography routines namely, Pulse and Lock-in as applied to carbon fiber reinforced plastics (CFRP) composites; using a Taguchi design of experiment approach. A set of controllable factors are highlighted and selected for each technique at different levels. Three factors have been identified for the pulse thermography (specifically; defect aspect ratio, pulse period, and experimental duration), and two factors for the Lock-in mode (that is lock in frequency and period); each factor can be manipulated at three different levels. The analysis reveals the effectiveness of the Taguchi design of experiment in consolidating the number of factorial experiments, and in quantifying the results and the associated sensitivity for each factor (its dominance), using a signal to noise ratio criterion. The analysis of variance and analysis of means show that the aspect ratio is not a controlling parameter for the pulse thermography, with the pulse time being the most dominant. Moreover, it decides on the optimal settings for each testing mode. These settings are further validated using additional CFRP artificial sample with eight and six layers of laminates.

A method is described to quantitatively characterize the corrosion status of steel samples under insulation. The method uses backscatter X-rays to obtain a density vs. depth profile of the sample, from which the mass absorption coefficient, density, and thickness of the rust layer are evaluated. From these data, the iron content of the rust layer is computed, and the steel losses are expressed in either wall thickness or in the mass per unit area. Rust with a thickness of less than 1 mm can be detected but not quantified. The upper limit for quantitative expression of steel losses is approximately 6 mm when an X-ray tube operated at 160 KV is used.

Many of historical structures have degenerated in time by environmental effects, earthquakes, and winds because of the inadequate preservation. The preservation of historical heritage is considered a fundamental issue in the cultural life of modern societies. The protective measures can be supplied if the actual behaviour of the structures is known. The paper presents the results of ambient vibration test and operational modal analysis carried out on the historical masonry bell-tower of the Hagia Sophia church in Trabzon, Turkey. The bell-tower is about 23 m high and dates back to the XIII century. The study includes also the initial analytical model of the tower constituted by the geometrical survey. The experimental measurements are performed using two measurement setups in different times. In the first setup twelve uniaxial accelerometers are used, while in the second setup four triaxial accelerometers with one uniaxial reference are used with the aim of determining the bending and torsional mode shapes as well as natural frequencies and modal damping ratios of the tower. The analytical model of the tower is developed by using solid brick elements, and a relatively large number of finite elements have been used in the model to obtain a regular distribution of mass. The first five natural frequencies and corresponding mode shapes are determined from both theoretical and experimental modal analyses and compared with each other. A good harmony is attained between mode shapes, but there are some differences between natural frequencies. The sources of the differences are introduced in terms of variations in the elasticity modulus of walls, cracks on upper walls, and boundary conditions on base level.

An ultrasonic technique to determine the acoustoelastic coefficients of Rayleigh waves in steel alloys is described. The technique is based on the measurement of the time of flight of Rayleigh waves over a fixed surface distance as a function of applied stress. Measurements on AISI 1080 carbon steel, AISI 4130 alloy steel, and 316L stainless steel specimens are reported. Time of flight resolution and repeatability as well as temperature effects are discussed insofar as they influence the applicability of ultrasonic methods to the measurement of applied and residual biaxial surface stresses in steel.