Measurement models for ultrasonic nondestructive evaluation

The article reviews ways to use the electromechanical reciprocity relation to construct theoretical models of measurement processes in ultrasonic nondestructive evaluation. This relationship is important because it connects the change in the voltages measured at the electrical terminal of the transducer to the perturbation in the mechanical wavefield caused by the change in the propagation environment. It does so by mixing an unperturbed reference wavefield with one containing the perturbed wavefield. Two problems of progressive difficulty are explored. We begin by investigating the imaging of a one-dimensional sinusoidal fluid-solid interface using a cylindrically focused beam and continue by developing a model of the imaging of the mechanical properties of a two-dimensional, thin solid film using a confocal arrangement of point-focused transducers. This last problem uses an approximation to the thin solid film, which reduces its mechanical response to one similar to that of a membrane. Integral relations are given that can be used to form integral equations or to generate asymptotic approximations to the particle displacements and stresses in the film.     

Guided wave nuances for ultrasonic nondestructive evaluation

Recent developments in guided wave generation, reception, and mode control show that increased penetration power and sensitivity are possible. A tone burst function generator and appropriate signal processing are generally used. Variable angle beam and comb-type transducers are the key to this effort. Problems in tubing, piping, hidden corrosion detection in aging aircraft, adhesive and diffusion bonding, and ice detection are discussed. Additionally, sample configurations, inspection objectives, and logic are being developed for such sample problems as defect detection and analysis in lap splice joints, tear straps, cracks in a second layer, hidden corrosion in multiple layers, cracks from rivet holes, transverse cracking in a beam, and cracks in landing gear assembly. Theoretical and experimental aspects of guided wave analysis include phase velocity, group velocity, and attenuation dispersion curves; boundary element model analysis for reflection and transmission factor analysis; use of wave structure for defect detection sensitivity; source influence on the phase velocity spectrum, and the use of angle beam and comb transducer technology. Probe design and modeling considerations are being explored. Utilization of in-plane and out-of-plane displacement patterns on the surface and longitudinal power distribution across the structural cross-section are considered for improved sensitivity, penetration power, and resolution in nondestructive evaluation. Methods of controlling the phase velocity spectrum for mode and frequency selection are available. Such features as group velocity change, mode cut-off measurements, mode conversion, amplitude ratios of transmission, and reflection factors of specific mode and frequency as input will be introduced for their ability to be used in flaw and material characterization analysis.     

Quantitative nondestructive evaluation with ultrasonic method using neural networks and computational mechanics

This paper describes an inverse analysis method using hierarchical neural networks and computational mechanics, and its application to the quantitative nondestructive evaluation with the ultrasonic method. The present method consists of three subprocesses. First, by parametrically changing the location and size of a defect hidden in solid, elastic wave propagation in the solid is calculated with the dynamic finite element method. Second, the back-propagation neural network is trained using the calculated relationships between the defect parameters and the dynamic responses of solid surface. Finally, the trained network is utilized to determine appropriate defect parameters from some measured dynamic responses of solid surface. The accuracy and efficiency of the present method are discussed in detail through the identification of size and location of a defect hidden in solid.This work is financially supported by the Grant-in-Aid for the scientific research of the Ministry of Education, Japan.     

Flaw signature estimation in ultrasonic nondestructive evaluation using the Wiener filter with limited prior information

Flaw signals measured in ultrasonic testing include the effects of the measurements system and are corrupted by noise. The measurement system response is both bandlimited and frequency dependent within the bandwidth, resulting in measured signals which are blurred and distorted estimates of actual flaw signatures. The Wiener filter can be used to estimate the flaw's scattering amplitude by removing the effect of the measurement system in the presence of noise. A method is presented for implementing an optimal form of the Wiener filter that requires only estimates of the noise distribution parameters. The theoretical error for scattering amplitude estimation, assuming various levels of available prior information, is analyzed. Three estimation techniques, one a maximum-likelihood based method and the other two residual-sum-of-squares methods, are formulated and tested. The results demonstrate that any of the three approaches could be used to optimally implement the alternative form of the Wiener filter with limited prior information.     

Blind multiridge detection for automatic nondestructive testing using ultrasonic signals

Ultrasonic imaging has been a significant means for nondestructive testing (NDT). Recently the NDT techniques via the ultrasonic instrumentation have shown the striking capability of the quality control for the material fabrication industry. To the best of our knowledge, all existing signal processing methods require either the a priori information of the ultrasonic signature signals or the manual segmentation operation to achieve the reliable parameters that characterize the corresponding mechanical properties. In this paper, we first provide a general mathematical model for the ultrasonic signals collected by the pulse-echo sensors, then design a totally blind novel signal processing NDT technique relying on neither a priori signal information nor any manual effort. Based on the automatic selection of optimal frame sizes using a proposed new criterion in our scheme, the signature signal can be blindly extracted for further robust multiridge detection. The detected ridge information can be used to estimate the transmission and attenuation coefficients associated with any arbitrary material sample for the fabrication quality control.     

Time reversal processing in ultrasonic nondestructive testing

In this paper, we present a novel and completely different approach to focusing on defects beneath plane or curved surfaces: the time reversal mirror method. The time reversal technique is based on the concept of time reversal of ultrasonic fields and takes into account both the phase and modulus information coming from the defect. This technique is self-adaptative and requires only the presence of a target in the solid sample. In highly scattering media, it is shown that the time reversal process allows a new approach to speckle noise reduction. Experimental results obtained with a 121-channel time reversal mirror on titanium and duralumin samples are presented. They demonstrate the ability of time reversal to compensate for the distortions induced by liquid-solid interfaces of different geometries and to detect small defects in a noisy background     

Use of ultrasonic velocity for nondestructive evaluation of ferrite content in duplex stainless steels

The possibility of using ultrasonic velocity measurements for the nondestructive estimation of the ferrite content in duplex stainless steels has been explored. The variation of the longitudinal ultrasonic velocity in specially prepared duplex stainless steel coupons having a ferrite content in the range 40–55 vol% has been studied using an ultrasonic thickness/velocity meter. The longitudinal wave velocity has been correlated to the ferrite content of the samples estimated by X-ray diffraction and quantitative image analysis.     

A generic hybrid model for bulk elastodynamics, with application to ultrasonic nondestructive evaluation

Practical ultrasonic inspection requires modeling tools that enable rapid and accurate visualization; because of the increasing sophistication of practical inspection, it is becoming increasingly difficult to use a single modeling method to represent an entire inspection process. Hybrid models that utilize different or interacting numerical schemes in different regions, to use their relative advantages to maximal effect, are attractive in this context, but are usually custom-made for specific applications or sets of modeling methods. The limitation of hybrid schemes to particular modeling techniques is shown here to be related to their fundamental formulation. As a result, it becomes clear that a formalism to generalize hybrid schemes can be developed: an example of the construction of a generic hybrid modeling interface is given for the abstraction of bulk ultrasonic wave phenomena, common in practical inspection problems. This interface is then adapted to work within a prototype hybrid model consisting of two smaller finite element model-domains, and explicitly demonstrated for bulk ultrasonic wave propagation and scattering examples. Sources of error and ways to improve the accuracy of the interface are also discussed.     

Determination of density distribution in powder compacts using ultrasonic tomography

Laser-induced ultrasonic bulk wave tomography is used for density variation determination of powder metal compacts. A laser beam is used to excite ultrasonic energy, and the signals passing through the specimen are received by an air-coupled transducer. The density variations of powder metal compacts can be determined directly by the cross-sectional tomographic images of slowness obtained by using a filtered, backprojection algorithm based on measured time of flights. Interpolations with respect to sample and projection angles are used to generate the input data required for displaying a well-balanced, reconstructed image to reduce the aliasing distortions caused by insufficient input data. Results of presintered cylindrical ferrous powdered samples show that this novel approach makes the reconstruction process more cost effective than the very tedious, time-consuming, and inaccurate metallographic methods, thus making it a potentially powerful tool for studying manufacturing processes through significant parameters to obtain a more uniform density distribution.     

Fabrication of PIMNT/epoxy 1-3 composites and ultrasonic transducer for nondestructive evaluation

In this paper, PIMNT/epoxy 1-3 composites with different volume fractions were prepared by the dice-and-fill method for application in ultrasonic transducers. The theoretical and experimental properties at different volume fractions and the temperature stability of the electromechanical property were investigated. The highest electromechanical coupling factor k(t) was obtained as 0.833 with the volume fraction of 0.58 and k(t) changed little below the Curie temperature of the single crystal. Afterward, an angle-beam transverse wave ultrasonic transducer using the fabricated composite was designed and manufactured based on the simulation of the KLM model and commercial software. The assembled prototype transducer showed large improvement in two-way insertion loss, relative bandwidth at -6 dB, surplus sensitivity, and axial resolution, which were -24.3 dB, 107%, 85 dB, and 28 dB, respectively, compared with a commercial PZT-based composite transducer.     

Structural evaluation of phosphate bonded ceramic composite materials from nondestructive ultrasonic velocity and attenuation measurements

The fabrication of a ceramic consisting of a matrix of newberyite and aluminium orthophospate filled with alumina and lesser amounts of carbon and glass fibre, is described. This material, whilst combining excellent insulation properties with forgiving fracture, is easy to produce by moulding without sintering. The ceramic has been characterized by a number of complementary ultrasonic techniques in the frequency range 24 kHz to 5 MHz,Dynamic elastic moduli measurements have been found to agree well with elastic constants calculated theoretically by treating the matrix and filler as end members of a two-phase material whose properties obey the lower Hashin and Shtrikman bound. In addition the bulk modulus of the matrix computed theoretically from crystallographic data and making an allowance for porosity agreed closely with the experimental modulus. Thus ultrasound velocity (moduli) measurements combined with theory can be used for non-destructive monitoring and quality control of the ceramic composition which is subject to variation with the parameters governing the chemical reaction during preparation. The theoretical bulk modulus of the ideal (pore-free) matrix is 10.4 GPa. This matrix modulus is far less than that of the moduli of the constituent oxides in the starting mixture. The reason for this is the large expansion in the sizes of closed rings of cation-oxygen network bonds that takes place in the reaction, rather than structural weakening (breaking of rings of network bonds) by hydration.The frequency dependence of ultrasonic attenuation has been used to identify scattering regimes and thus determine the dimensions of the major scattering particles. Grain sizes determined ultrasonically for the three compositions showed excellent agreement with values determined by optical microscopy.The high frequency dependent absorption and scattering in this material, mean that good ultrasound propagation is obtained only at low frequencies. The lowest frequency at which the ultrasonic propagation and properties are dependent on the material structure alone, i.e. independent of sample size, has been established to be 2 MHz with conveniently sized test pieces of dimensions 1.5×1.5×6 cm³. Previous address: Department of Physics, Brunel University, Kingston Lane, Uxbridge, Middlesex, UK. Previous address: Department of Physics, Brunel University. Previous address: Thorn EMI Central Research Laboratories, Dawley Road, Hayes, Middlesex, the Mechanics Group, Department of Engineering, University of Reading. Now at Fison's Scientific Instruments, Manor Industrial Estate, Gatwick Road, Crawley, Surrey. UK.     

Characterization of micromachined ultrasonic transducers using light diffraction tomography

This paper demonstrates that light diffraction tomography can be used to measure the acoustic field of micromachined ultrasonic transducers (MUT) in cases in which standard methods like hydrophone and microphone measurements fail. Two types of MUTs have been characterized with the method, one air-coupled capacitive MUT (cMUT) and one waterloaded continuous wave (CW) miniature multilayer lead zirconate titanate (PZT) transducer. Light diffraction tomography is an ultrasound measurement method with some special characteristics. Based on the interaction of light and ultrasound, it combines light intensity measurements with tomography algorithms to produce a measurement system. The method offers nonperturbing pressure measurements with high spatial resolution. It has been shown that, under certain circumstances, light diffraction tomography can be used as an absolute pressure measurement method with accuracy in the order of 10% in water and 13% in air. The results show that air-coupled cMUTs in the frequency range of about 1 MHz as well as the extreme near field of a miniaturized CW 10 MHz water-loaded transducer were successfully characterized with light diffraction tomography.     

Air-coupled through-transmission fan-beam tomography using divergent capacitive ultrasonic transducers

Abstracttrasonic transducers (CUTs) with curved backplates was used to acquire signals through regions of air containing solid objects, air flow, and temperature fields. Fan-beam datasets were collected and used in a tomographic reconstruction algorithm to produce cross-sectional images of the area under interrogation. In the case of the solid objects, occluded rays from the projections were accounted for using a compensation algorithm and a priori knowledge of the object. A rebinning routine was used to pick out parallel ray sets from the fan-beam data. The effects of further reducing the number of datasets also were investigated, and, in the case of imaging solid objects, characteristic Gibbs phenomena were seen in the reconstructions as expected. However, when imaging temperature and flow fields, the aliasing artefacts were not seen, but the reconstructed values decreased with the size of dataset used. The effect of changing the kernel filter function also was investigated, with the different filters giving the best compromise between image noise, reconstruction accuracy, and amount of data required in each scenario.     

Application of ultrasonic stress measurements to nondestructive evaluation of the J integral in elastic-plastic deformation

A technique was proposed in [1] for nondestructively evaluating the J integral in cracked specimens through use of ultrasonic measurements of stress, and tested on several specimen configurations with the deformation confined to linear elastic fracture mechanics [1,2]. In this paper the application of this procedure to the evaluation of J in the presence of elastic-plastic deformation is described. Experimental results are presented for the case of an edge-cracked panel in tension, which compare favorably with numerical predictions.     

Remote nondestructive evaluation technique using infrared thermography for fatigue cracks in steel bridges

Long‐standing infrastructure is subject to structural deterioration. In this respect, steel bridges suffer fatigue cracks, which necessitate immediate inspection, structural integrity evaluation or repair. However, the inaccessibility of such structures makes inspection time consuming and labour intensive. Therefore, there is an urgent need for developing high‐performance nondestructive evaluation (NDE) methods to assist in effective maintenance of such structures. Recently, use of infrared cameras in nondestructive testing has been attracting increasing interest, as they provide highly efficient remote and wide area measurements. This paper first reviews the current situation of nondestructive inspection techniques used for fatigue crack detection in steel bridges, and then presents remote NDE techniques using infrared thermography developed by the author for fatigue crack detection and structural integrity assessments. Furthermore, results of applying fatigue crack evaluation to a steel bridge using the newly developed NDE techniques are presented.     

Progressive failure monitoring and analysis in aluminium by in situ nondestructive evaluation

Damage initiation and progression in precipitate hardened alloys are typically linked to the failure of second phase particles that result from the precipitation process. These particles have been shown to be stress concentrators and crack starters as a result of both particle debonding and fracture. In this investigation, a precipitate hardened aluminium alloy (Al 2024‐T3) is loaded monotonically to investigate the role the particles have in the progressive failure process. The damage process was monitored continuously by combining the acoustic emission method either with in situ scanning electron microscopy or X‐ray microcomputed tomography to obtain both surface and volume microstructural information. Particles were observed to fracture only in the elastic regime of the material response, while void growth at locations predominantly near particles were found to be associated with progressive failure in the plastic region of the macroscopic response. Experimental findings were validated by fracture simulations at the scale of particle‐matrix interface.     

Determination of density distribution in ferrous powder compacts using ultrasonic tomography.

Laser-induced ultrasonic bulk wave tomography is used for density variation determination of powder metal compacts. A laser beam is used to excite ultrasonic energy, and the signals passing through the specimen are received by an air-coupled transducer. The density variations of powder metal compacts can be determined directly by the cross-sectional tomographic images of slowness obtained by using a filtered, backprojection algorithm based on measured time of flights. Interpolations with respect to sample and projection angles are used to generate the input data required for displaying a well-balanced, reconstructed image to reduce the aliasing distortions caused by insufficient input data. Results of presintered cylindrical ferrous powdered samples show that this novel approach makes the reconstruction process more cost effective than the very tedious, time-consuming, and inaccurate metallographic methods, thus making it a potentially powerful tool for studying manufacturing processes through significant parameters to obtain a more uniform density distribution.