Electromagnetic signals at microwave and millimeter-wave frequencies are well suited for inspecting dielectric materials and composite structures in many critical applications. This article presents a partial list of reported nondestructive testing (NDT) application areas for microwaves. It discusses the advantages and limitations of inspection with microwaves. The article discusses the physical principles, including reflection and refraction, absorption and dispersion, scattering, and standing waves. It provides a discussion on terahertz (THz) imaging for nondestructive evaluation (NDE). The article concludes with information on ground-penetration radar (GPR) that uses electromagnetic radiation and detects the reflected signals from subsurface structures.

This article addresses the specialized aspects required to accurately quantify the behavior of soft materials, including polymers and polymeric composites, using the split-Hopkinson pressure bar (SHPB). It details some of the specialized SHPB techniques that facilitate testing soft materials. These techniques include the data-reduction techniques and assumptions required to use polymer pressure bars, the importance of sample-size considerations to polymer testing, and temperature-control methodologies to measure the high-strain-rate uniaxial stress response of polymers and other soft materials.

This article describes the techniques involved in measuring the high-strain-rate stress-strain response of materials using a split-Hopkinson pressure bar (SHPB). It focuses on the generalized techniques applicable to all SHPBs, whether compressive, tensile, or torsion. The article discusses the methods of collecting and analyzing compressive high-rate mechanical property data. A review of the critical experimental variables that must be controlled to yield valid and reproducible high-strain-rate stress-strain data is also included. Comparisons and contrasts to the differences invoked when using a tensile Hopkinson bar in terms of loading technique, sample design, and stress-state stability, are discussed.

This article provides an overview of the characteristics of Rayleigh waves plus methods for generation and detection of waves, including using piezoelectric transducers or noncontact techniques such as lasers, electromagnetic acoustic transducers, or air-coupled ultrasonics. It reviews the methods for using Rayleigh waves for defect detection and materials characterization, alongside nonlinear ultrasonic inspection and surface acoustic wave (SAW) microscopy. The article concludes with information on the standards that use Rayleigh waves for nondestructive evaluation (NDE) of different structures.

Guided wave testing (GWT) is a method of nondestructive evaluation for the inspection of pipelines. This article focuses mainly on explaining GWT as it is applied in routine established use, covered by standards, in the oil and gas industry and also introduces some of the other contexts in which its use is growing in other evolving applications. It discusses the various guided wave modes and their selection criteria. The article provides information on considerations for mode control and the sensitivity of the GWT to the defect. It also shows some examples of advanced GWT.

This article provides a discussion on the generation of an incident wave with the help of the stored-torque torsional Kolsky bar and explosively loaded torsional Kolsky bar. It examines the procedures followed for measuring the waves in these bars. The article compares the compression Kolsky bar with the torsional Kolsky bar. It includes information on the various application areas of torsional Kolsky bar: limitations on strain rate, low- and high-temperature testing, quasi-static and incremental strain-rate testing, and localization and shear-banding experiments.

This article describes the basic features of electromagnetic acoustic transducers (EMATs) and discusses their existing and some potential uses within the field of ultrasonic nondestructive evaluation (UNDE). It provides sufficient basic and practical information to make an informed choice when considering the transducer to be used for any particular UNDE application. The article describes how different types of EMATs operate and presents their fundamental and some practical limitations. It summarizes the representative literature for electromagnetic acoustic transducer UNDE applications. Some successful uses of EMATs are mentioned to illustrate the depth, range, and potential of commercial EMAT applications. The article concludes with information on the commercial sources for EMAT systems and components.

Ferromagnetic resonance (FMR) is used in the identification of the magnetic state of materials, the quantitative determination of static magnetic parameters, and the determination of microwave losses. This article describes the theory of ferromagnetic resonance and provides information on reflection spectrometers, microwave spectrometers, and ferromagnetic anti-resonance spectrometers used for measuring FMR. It also discusses the applications of FMR and provides several detailed examples.

This article introduces the principal methodologies and some technologies that are being applied for nondestructive evaluation of composite materials. These include ultrasonic testing (UT), air-coupled UT, laser UT, ultrasonic spectroscopy, leaky lamb wave method, acousto-ultrasonics, radiography, X-ray computed tomography, thermography, low-frequency vibration methods, acoustic emission, eddy current testing, optical holography, and shearography. The article presents some examples are for fiber-reinforced polymer-matrix composites. Many of the techniques have general applicability to other types of composites such as metal-matrix composites and ceramic-matrix composites.

This article introduces the principal methodologies and some advanced technologies that are being applied for nondestructive evaluation (NDE) of fiber-reinforced polymer-matrix composites. These include acoustic emission, ultrasonic, eddy-current, computed tomography, electromagnetic acoustic transducer, radiography, thermography, and low-frequency vibration methods. The article also provides information on NDE methods commonly used for metal-matrix composites.

Split-Hopkinson pressure bar (SHPB) testing is traditionally used for determining the plastic properties of metals (which are softer than the pressure bar material) at high strain rates. However, the use of this method for testing ceramic has various limitations. This article provides a discussion on the operational principle of the traditional SHPB technique and the relevant assumptions in the derivation of the stress-strain relationship. It describes the inherent limitations on the validity of these assumptions in testing ceramics and discusses the necessary modifications in SHPB design and test procedure for evaluating high-strength brittle ceramics. The article includes information on the maximum strain rate that can be obtained in ceramics using an SHPB and the necessity of incident pulse shaping. It also reviews the specimen design considerations, interpretation of experimental results obtained from SHPB testing of ceramics, and effectiveness of the proposed modifications.

X-ray topography is the general term for a family of x-ray diffraction imaging techniques capable of providing information on the nature and distribution of imperfections. This article provides a detailed account of x-ray topography techniques, providing information on the historical background and development trends in x-ray diffraction topography. The discussion covers the general principles, components of systems, and applications of x-ray topography techniques, namely conventional X-ray topographic techniques and synchrotron x-ray topographic techniques.

Impact tests are used to study dynamic deformation and failure modes of materials. This article discusses low-velocity impact experiments in single-stage gas guns. It describes surface velocity measurements with laser interferometric techniques. The article details plate impact soft-recovery experiments, pressure-shear friction experiments, and low-velocity penetration experiments. It reviews two types of plate impact soft-recovery experiments: normal plate impact and pressure-shear plate impact experiments. The article provides information on low-velocity penetration experiments, which include the setup for direct penetration experiment (rod-on-plate) and the reverse penetration experiment (plate-on-rod). It also considers high-temperature plate impact testing and impact techniques with in-material stress and velocity measurements.

Laser-ultrasonics is a particular implementation of ultrasonic nondestructive inspection in which ultrasound is generated and detected by lasers. This article discusses the various mechanisms that ensure ultrasound generation and explains the possibility to get the equivalent of phase-array by numerical processing of an array of previously acquired laser-ultrasonic signals. The article describes the ultrasound generation by thermoelastic mechanism and ablation or vaporization. It illustrates the principle of optical detection of ultrasound with confocal Fabry-Perot interferometer and photorefractive two-wave mixing interferometer. The article concludes with information on the industrial applications of laser-ultrasonics, including thickness measurement, flaw detection, and material characterization.

This article describes methods for analyzing impact-damaged composites in the aircraft industry. These include C-scan and x-radiography methods and optical microscopy. The article reviews brittle-matrix composite and tough-matrix composite failures. It explains the different types of composite failure mechanisms such as thermoplastic-matrix composite failure mechanisms, untoughened thermoset-matrix composite failure mechanisms, toughened thermoset-matrix composite failure mechanisms, dispersed-phase and rubber-toughened thermoset-matrix composite failure mechanisms, and particle interlayer-toughened composite failure mechanisms.

Measuring the thickness of thin films can be accomplished in many ways. This article focuses on the optical method of single-wavelength ellipsometry, two multiple-wavelength methods of reflectometry and spectroscopic ellipsometry for measuring the thickness of thin films. The general capabilities, principles and applications of ellipsometry and reflectometry are discussed in terms of nondestructive methods.

This article reviews the dynamic factors, experimental methods and setup, and result analysis of different types of high strain rate shear tests. These include high strain rate torsion testing, double-notch shear testing and punch loading, drop-weight compression shear testing, thick-walled cylinder testing, and pressure-shear plate impact testing.

Microwaves (or radar waves) are a form of electromagnetic radiation with wavelengths between 1000 cm and 1 mm in free space. One of the first important uses of microwaves in nondestructive evaluation was for components such as waveguides, attenuators, cavities, antennas, and antenna covers (radomes). This article focuses on the microwave inspection methods that were subsequently developed for evaluation of moisture content in dielectric materials; thickness measurements of thin metallic coatings on dielectric substrates; and detection of voids, delaminations, macroporosity, inclusions, and other flaws in plastic or ceramic materials. It also discusses the advantages and disadvantages and the general approaches that have been used in the development of microwave nondestructive inspection.