Ultrasonic and Advanced Methods for Nondestructive Testing and Material Characterization

The following sections are included:

• PREFACE

• CONTENTS

It will be shown that it is possible to completely characterize an ultrasonic nondestructive flaw measurement system and simulate the measured flaw signals. This characterization includes the pulser/receiver, cabling, and transducers present as well as the propagating and scattered acoustic/elastic wave fields. All the elements of this comprehensive ultrasonic measurement model will be discussed as well as practical methods for obtaining those elements through a combination of models and measurements.

Thick (> 40 μm) ceramic films as piezoelectric and ultrasonic transducers (UTs) have been successfully deposited on metallic and non-metallic substrates by a spray technique. In the film fabrication a composite consisting of piezoelectric powders well mixed with solution of high dielectric constant is directly sprayed onto the substrate. It is then dried, fired or annealed by heat. Multiple coating is used to achieve preferred thicknesses. A corona poling is utilized to achieve the piezoelectricity of the film. The top electrode is accomplished by a painting method. All fabrication processes may be carried out on-site and by handheld devices. Integrated ultrasonic longitudinal, shear, surface and plate wave transducers have been made. The same technology has been also used to fabricate flexible transducers consisting of a thin substrate, a piezoelectric ceramic film and electrodes. The flexibility is realized owing to the porosity of piezoelectric film and the thinness of the piezoelectric film, substrate and electrodes. All transducers have been tested at least up to 150°C. Their applications for non-destructive testing of different materials are demonstrated.

Real-time and in-line ultrasonic diagnostics of polymer extrusion and injection molding processes together with the high temperature ultrasonic probes used are presented. For polymer extrusion, melt quality, filler concentration, and dispersion monitorings are the examples. Clad ultrasonic rod probes consisting of a core and a cladding fabricated by a thermal spray technique are used. For injection molding, diagnostics of melt flow front, average flow speed, filling completion, and solidification are illustrated. Sprayed high temperature ultrasonic thick film transducers integrated onto the mold inserts are employed.

A broad overview of the field of laser-ultrasonics is presented. This overview draws from developments at the Industrial Materials Institute of the National Research Council of Canada as well as elsewhere. The principles of generation and detection are presented, stressing a few key characteristics of laser-ultrasonics: the material is actually the emitting transducer and transduction is made by light, thus eliminating any contact. These features carry both advantages and limitations that are explained. Another feature, which has been an impediment, is actually the complexity of the "laser-ultrasonic transducer", but in spite of this complexity, it can be made very reliable for use in severe industrial environments. It also can be very cost effective for a number of applications. Four applications that are now used in industry are presented: the inspection of polymer matrix composites used in aerospace, the thickness gauging of hot steel tubing in production and the measurement and characterization of thin layers in microelectronics by two different approaches. Technological aspects, such as interferometer design, detection lasers and others are also discussed. As an overall conclusion, laser-ultrasonics that was for a long time a laboratory curiosity has definitely now made its transition to industry. Nevertheless, developments should continue to improve performance, to make it well adapted to specific inspection or characterization tasks and more affordable.

A new methodology of guided wave based nondestructive testing (NDT) is developed to detect crack/corrosion damage in metallic structures without using prior baseline data. In conventional guided wave based techniques, damage is often identified by comparing the "current" data obtained from a potentially damaged condition of a structure with the "past" baseline data collected from the pristine condition of the structure. However, it has been reported that this type of pattern comparison with the baseline data can lead to increased false alarms due to its susceptibility to varying operational and environmental conditions of the structure. To develop a more robust damage diagnosis technique, a new concept of NDT is conceived so that defects such as crack and/or corrosion can be detected without direct comparison with previously obtained baseline data. The proposed NDT technique utilizes the polarization characteristics of the piezoelectric wafer transducers attached on the both sides of the thin metallic structure. Crack/corrosion formation creates Lamb wave mode conversion due to a sudden change in the thickness of the structure. Then, the proposed technique instantly detects the appearance of the defects by extracting this mode conversion from the measured Lamb waves even at the presence of changing operational and environmental conditions. Numerical and experimental results are presented to demonstrate the applicability of the proposed technique to crack/corrosion detection.

Acoustic nonlinear imaging has brought about significant improvements in image quality by taking advantage of the nonlinear components. This article reviews works of acoustic nonlinear imaging for biological tissues in Nanjing University, including acoustic nonlinearity parameter B/A imaging, tissue harmonic imaging by using the multi-phase-coded-pulse technique and super harmonic imaging. Theoretical analysis and experimental imaging of biological tissues by using these methods are presented.

An angular spectrum decomposition method (ASM) for calculating second-harmonic propagation in fluid-like media is discussed. This method is based on a perturbation solution of Westervelt's equation, which can be augmented to take into account inhomogeneity of the medium. ASM can be applied to the analysis of tissue harmonic imaging (THI), a diagnostic method that utilizes second harmonics for imaging. One of the advantages of THI is the second harmonic's ability to improve image quality by reducing aberration caused by inhomogeneity in the body wall. The body wall aberration can be modeled as a localized phase screen in the wave propagation path. The results show that the interactions of angular spectrum components of the fundamental field generate a second-harmonic field that is relatively immune to the aberration. An experiment was performed to demonstrate this phenomenon by using a phase screen created from low density Polyethylene material. The measurements were in close agreement with calculations using ASM.

High-frequency ultrasound (HFUS) measurement techniques have been developed for application to diagnostic and regenerative medicine. The ultrasonic propagation and scattering properties of several biological materials have been measured over frequency ranges between 15 and 65 MHz. Ex vivo measurements of murine pulmonary arterial walls show the potential role of HFUS in monitoring pulmonary arterial hypertension. Similarly, ex vivo measurements of bovine cartilage and poly(ethylene glycol) based hydrogels indicate that HFUS techniques may permit real-time, online monitoring of functional tissue-engineered cartilage. Lastly, ex vivo measurements of human coronary arteries with atherosclerotic plaque have been performed to develop an in vivo tool (VH™ IVUS) for management of coronary artery disease. Additional discussion concerning standards and technologies for HFUS as well as opportunities to improve understanding of the interaction of HFUS and tissue is provided.

The application of receiver operating characteristic (ROC) methods for computer-aided detection systems is reviewed. A statistical framework for ROC analysis is presented and different methods of evaluating the performance of computer-aided detection systems are derived.

Most detection systems that are used today are dependent on a separation threshold, which, in many cases, can be chosen by the operator without restrictions. The separation threshold separates the range of output values of the detection system into different target groups, thus, conducting the actual detection process.

In the first part of this chapter, threshold specific performance measures, e.g. sensitivity and specificity, are presented. In the second part, a threshold-independent performance measure, the area under the ROC curve, is reviewed. Only the use of separation threshold-independent performance measures provides detection results which are overall representative for computer-aided detection systems.

This chapter provides the interested scientist with all information needed to conduct ROC analysis and to integrate algorithms performing ROC analysis into detection systems while understanding the basic principles of detection.

Waveguide sensors and buffers have been applied to measuring three of the four principal measurands of interest in industrial process control, namely, liquid level, flow, and temperature. Buffers are used when the measurand is at high temperature, which sometimes occurs in NDE/NDT situations. Waveguide sensors can also sense liquid density and viscosity. Problems include isolating the sought measurand from interfering variables, and eliminating acoustic or ultrasonic noise conveyed by a pipe or other structural element that acts as an unwanted waveguide, channeling interference to a transducer and reducing the signal to noise ratio at the transducer.

In this chapter, various electromagnetic NDE techniques such as eddy current, magnetic, micro-magnetic, potential drop and microwave techniques for materials characterization are covered. For each technique, principle, instrumentation along with sensors, capabilities, applications, and recent advances towards quantitative characterization of defects, microstructures and residual stresses in engineering materials are given. Special emphasis is also given to numerical modelling, sensors and signal processing for automated NDE.

Pulsed terahertz nondestructive evaluation is an emerging technology with significant potential for inspecting nonconductive aerospace materials for hidden flaws and damage. This new technology is potentially useful for inspections that have previously been identified as either extremely difficult or impossible with current inspection technologies. Characteristic of a difficult-to-inspect material is the Sprayed-On Foam Insulation of the Space Shuttle External Tank. Recent measurements demonstrate that a terahertz nondestructive evaluation system is able to detect voids and unbonds in this material. Other viable applications of pulsed terahertz technology include examination of metallic surface roughness, measurement of paint thickness, and corrosion detection under Space Shuttle tiles. This chapter discusses the development of a measurement system for performing these inspections and the results of measurements on a variety of materials and structures.

The method, technology, and examples of high speed time domain terahertz (a.k.a. T-Ray) imaging non-destructive examination (NDE) for security, aerospace, and building structure are discussed. T-Ray imaging can be utilized for non-contact transmission and/or monostatic reflection inspection of non-conductive materials such as plastics, foam, composites, ceramics, paper, wood and glass. The method can be used to generate 2 and 3 dimensional sub-surface structural images. Spectral content analysis enables material characterization. Example subsurface homeland security images of concealed items in baggage and on personnel are shown. Voids and disbonds present in space shuttle external tank sprayed on foam insulation are shown. We tabulate attenuation and penetration characteristics through a selection of building materials, and demonstrate the ability of T-ray instrumentation to sub-surface image building structures such as wall framing and interior wiring and conduits.

Active infrared thermography refers to the group of methods employed to inspect the integrity of materials or systems through the use of an external energy source and an infrared detector. The external stimulus can be of many forms such as warm or cold air, heat pulses, periodic thermal waves, or mechanical oscillations, e.g. ultrasounds. The way data is captured and processed, as well as the typical applications differ according to the excitation source. This chapter presents a review of three of the most common active techniques in the field of thermography: lock-in thermography, pulsed thermography and vibrothermography.

Sandia National Laboratories Airworthiness Assurance Nondestructive Inspection Validation Center (AANC) provides independent and quantitative evaluations of new and enhanced inspection technologies, to developers, users, and regulators of aircraft. In this chapter, we present the results from two recent thermography probability of detection (POD) experiments. The first POD study involves sonic infrared (IR) imaging for detecting fatigue cracks in Inconel^® and titanium. The second POD study involves pulsed thermography using a low cost uncooled infrared camera for detecting flaws in composite honeycomb structure.

Sonic Infrared Imaging is a novel NDE technique that combines ultrasonic excitation and infrared imaging to detect defects, such as cracks, delaminations and disbonds, in materials and structures. The ultrasonic excitation is typically a fraction-of-a-second-long pulse, which causes friction heating in the defects. The temperature changes in the target due to the heating are simultaneously being imaged by an infrared camera. Thus, defects in a wide variety of materials can be identified. This technique provides a sensitive, fast, and wide-area NDE method.

Multiple nondestructive inspection (NDI) techniques were applied to detect and quantify the hidden corrosion in aircraft lap joints. The inspection data is presented in raster-scanned images. In this chapter, the use of Dempster-Shafer (DS) theory to fuse multi-frequency and pulsed eddy current inspection data is presented. The NDI images are first discriminated by iteratively trained classifiers. The basic probability assignment (BPA) is defined based on the conditional probability of information classes and data classes, which are obtained from a teardown inspection followed by a X-ray thickness mapping process and the NDI measurements respectively. The DS rule of combination is applied to fuse multiple NDI inputs. The remaining thickness is estimated by applying locally weighted regression of the DS-fused results.

Multi-site fatigue damage, hidden cracks in hard-to-reach locations, disbonded joints, erosion, impact, and corrosion are among the major flaws encountered in today's extensive array of aerospace vehicles and civil structures. These damage scenarios, coupled with new and unexpected phenomena, have placed greater demands on the application of advanced nondestructive inspection (NDI) and health monitoring techniques. Reliable, structural health monitoring systems can automatically process real-time data, assess structural condition, and signal the need for human intervention. Prevention of unexpected flaw growth and structural failure could be improved if on-board health monitoring systems are used to continuously assess structural integrity. Such systems would be able to detect incipient damage before catastrophic failures occurs. Condition-based maintenance practices could be substituted for the current time-based maintenance approach. Other advantages of on-board distributed sensor systems are that they can eliminate costly, and potentially damaging, disassembly, improve sensitivity by producing optimum placement of sensors with minimized human factors concerns in deployment, and decrease maintenance costs by eliminating more time-consuming manual inspections. This chapter focuses on developments in mountable sensors and how they can be integrated into such a Structural Health Monitoring (SHM) system to guide condition-based maintenance activities.

Nondestructive Testing (NDT) techniques assess the conditions of rolling bearings without affecting their structural functionality, thus have played an important role in bearing health diagnosis. Among the various NDT techniques investigated for bearing applications, vibration and acoustic-based techniques are the most popular, since vibration and acoustic signals measured from bearings contain physical information that directly reflect the defect severity. In this chapter, a review of such NDT techniques is presented, and related sensors and sensing principles are discussed.

Recent advances in sensors, microelectronics, adaptive signal processing and predictive technologies have significantly shaped the fundamental approach to dealing with traditional problems within the aerospace community. Aircraft diagnostics, prognostics and health management (DPHM) is increasingly becoming a main stream approach to aircraft maintenance within an advanced operational autonomic logistics structure. Driven by the requirement for increased safety, reliability, enhanced performance and aircraft availability at reduced cost, key sensor technologies such as electrorheological fluids, shape memory alloys, piezoelectric materials, magnetostrictive and electrostrictive materials, triboluminescent materials, optical fibres, carbon nanotubes, comparative vacuum monitoring and MEMS/NEMS are expected to play a major role in the development of such DPHM systems. Such advanced sensors often referred to as smart sensors are further expected to provide functionality that is not matched by current technologies and nondestructive evaluation techniques within in an on-line in-situ environment. Regardless of the extensive number of sensors and sensory systems that could potentially be employed or integrated within a DPHM system, only a selected few illustrating near commercial exploitation are introduced in this chapter. Furthermore, the DPHM and on-line structural health monitoring concepts and its impact on aircraft operations and maintenance is briefly introduced.

This chapter aims to show how some relevant industrial applications of Non Destructive Testing and Evaluation (NDT/NDE) can benefit of some recently emerged Computational Intelligence (CI) techniques. The problems under study are generally formulated as inverse problems whose typical ill-posedness and ill-conditioning (in their numerical counterpart) can be dealt with such tools like innovative signal processing techniques (i.e., Independent Component Analysis, ICA, wavelet representation, Support Vector Machines, Fuzzy Systems, Neural Networks). Some practical examples for the solution of relevant inspection problems will be discussed. The advantages of using the CI approach will be clarified and assessed on applications of NDT/NDE in both civil engineering and industrial problems.

The integrity of materials used in applications such as nuclear power plants and aircraft engines is tested periodically for defects that could potentially lead to part failures during critical operations. Ultrasonic flaw detection plays an important part in the nondestructive evaluation (NDE) of these materials. This chapter presents signal processing approaches for enhancing defect detection in materials consisting of grain-like microstructures. Grain scatterers create noise processes with properties similar to those created by flaw scatterers, which makes for a challenging detection problem. This chapter highlights a particular flaw detection approach, referred to as split-spectrum processing (SSP), and explains its performance relative to classical approaches, such as Wiener and matched filtering. Analytical and simulation results are presented to demonstrate the ways in which SSP uses statistical properties of scatterer phase spectra to enhance flaw detection. In addition, examples are included to demonstrate SSP implementations for automatic flaw detection.

For thin layered materials, such as fiber reinforced titanium, or some composite structure, the determination of thickness of individual layers can be difficult with ultrasonic testing method because the reflected signals from thin layers are highly overlapped. The popular methods using cross-correlation and homomorphic deconvolution are reviewed. It is suggested that the model based method can be more suitable for this problem. The EM (expectation-maximization) maximization) algorithm is proposed for estimating the model parameters A detailed implementation of the algorithm is presented and comparison with the homomorphic method clearly shows that the EM algorithm is more suitable for an accurate estimate of thickness of multi-layered materials.

The chapter deals with restoration of medical ultrasonic images by means of deconvolution. The so far published relevant approaches are reviewed and shortly described. The main focus is given to homomorphic deconvolution. Basic concepts and practical realization of homomorphic filtering are given for both one-dimensional and two-dimensional signal processing. Finally, application of homomorphic filtering to medical ultrasound signals is explained and examples of deconvolved phantom and clinical images are given.

A technique for the analysis of full wavefield data in the wavenumber/frequency domain is presented as an effective tool for damage detection, visualization and characterization. Full wavefield data contain a wealth of information regarding the space and time variation of propagating waves in damaged structural components. Such information can be used to evaluate the response spectrum in the frequency/wavenumber domain, which effectively separates incident waves from reflections caused by discontinuities encountered along the wave paths. This allows removing the injected wave from the overall response through simple filtering strategies, thus highlighting the presence of reflections associated to damage. The concept is first illustrated on analytical and numerically simulated data, and then tested on experimental results. In the experiments, full wavefield measurements are conveniently obtained using a Scanning Laser Doppler Vibrometer, which allows the detection of displacements and/or velocities over a user-defined grid, and it is able to provide the required spatial and time information in a timely manner. Tests performed on an aluminum plate with artificially seeded slits simulating longitudinal cracks show the effectiveness of the technique and its potential for application to the inspection of a variety of structural components.

The critical NASA problem of foam release from the foam-based thermal protection system of the Space Shuttle external tank during ascent has spurred the advanced development and tailoring of NDE methods to inspect the foam for flaws that might facilitate release. NDE methods under consideration include terahertz, microwave, shearography and xray backscatter imaging. In parallel with the development of waveform-based methods such as terahertz, a sophisticated broadband signal and image processing software package has been developed. One of the unique aspects of the software is the inclusion of a merit assessment algorithm to "grade" various signal processing methods by providing a quantitative measure well correlated to the ability to subjectively distinguish flaw data from noise or background data. This chapter describes the software and provides a case history for its focus and use to process and analyze terahertz imaging results on a foam sample standard containing seeded voids.

The detection of welding flaws by means of nondestructive inspection methods remains open to the development of new algorithms and methods of inspection. One of the most widely used techniques is radiographic analysis, which requires interpretation by trained inspectors. Unfortunately, manual inspection is subject to various factors that can alter performance in the detection of the faults. An automated welding fault segmentation algorithm is presented using a set of digitized radiographic images. The result of the study has allowed the development of the following scheme: first, use the median filter to reduce noise; second, apply the bottom-hat filter to separate the hypothetical faults from the background; third, determine the segmented regions by binary thresholding; fourth, use the filters provided by morphological mathematics to eliminate over segmentation; and fifth, use the watershed transform to separate the internal regions. The results of the study have generated a general ROC curve on a set of 10 images with an area A_z = 93.6%.

The following sections are included:

• SUBJECT INDEX