This article presents the basic principle, characteristics, advantages, and disadvantages of resonant ultrasound spectroscopy (RUS) methods in additive manufacturing. It focuses on the two types of RUS methods: the swept-sine method and the impulse excitation method. Representative significant results for additively manufactured complex parts obtained with the different RUS systems are also shown. The article also presents the basic principle and examples of nonlinear RUS methods.

The potential for introducing defects during processing becomes greater as the relative density of pressed and sintered powder metallurgy (PM) parts increases and more multilevel parts with complex geometric shapes are produced. This article discusses the potential defects in pressed and sintered PM parts: density variations, compaction and ejection cracks, microlaminations, poor degree of sintering, and voids from prior lubricant agglomerates. It describes the various methods applicable to green compacts: direct-current resistivity testing, radiographic techniques, computed tomography, and gamma-ray density determination. The article also discusses the methods for automated nondestructive testing of pressed and sintered PM parts: acoustic methods-resonance testing, eddy current testing, magnetic bridge comparator testing, ultrasonic techniques, radiographic techniques, gamma-ray density determination, and visual inspection.

The commonly used nondestructive testing of cast products include liquid penetrant inspection, radiographic inspection, fluoroscopic inspection and automated defect recognition, ultrasonic inspection, eddy current inspection, process-controlled resonant testing (PCRT), leak test, and electrical conductivity measurements. This article summarizes the application of these nondestructive tests to castings. It also tabulates a partial list of automotive part types and materials amenable to PCRT and lists the potential limitations to the use of PCRT.

Nondestructive inspection (NDI) methods for cast iron are used to ensure that the parts supplied perform as required by the purchaser. This article focuses on the principal nondestructive methods used to inspect for anomalies in cast irons and to determine if the volume, shape, size, or number of these anomalies exceeds the maximum allowed by the purchaser. The nondestructive methods include visual inspection, dimensional inspection, liquid penetrant inspection, magnetic-particle inspection, eddy-current inspection, radiographic inspection, ultrasonic inspection, resonant testing, and leak testing. The technique, strengths, and weaknesses of each of the nondestructive methods are also discussed.

Electron spin resonance (ESR), or electron paramagnetic resonance (EPR), is an analytical technique that can extract a great deal of information from any material containing unpaired electrons. This article explains how ESR works and where it applies in materials characterization. It describes a typical ESR spectrometer and explains how to tune it to optimize critical electromagnetic interactions in the test sample. It also identifies compounds and elements most suited for ESR analysis and explains how to extract supplementary information from test samples based on the time it takes electrons to return to equilibrium from their resonant state. Two of the most common methods for measuring this relaxation time are presented as are several application examples.

This article discusses the underlying concepts and basic techniques for performing ultrasonic fatigue tests and describes test equipment design, specimen design, and effective control over test variables. It reviews the results obtained with ultrasonic fatigue test methods with respect to strain-rate-dependent material behavior. The article also provides information on the applications of the ultrasonic fatigue test.

Acoustic-emission inspection detects and analyzes minute acoustic-emission signals generated by discontinuities in materials under applied stress. This article discusses the types of acoustic emissions (continuous-type emissions and burst-type emissions) and applications, including laboratory testing, production testing, and structural testing. The article includes a section in which the characteristics of acoustic emission inspection are compared with other nondestructive testing methods. Further, it briefly reviews the key elements of the acoustic-emission instrumentation, which includes the acoustic-emission resonant sensor.

This article describes the fracture mechanics in fatigue. It discusses the fatigue crack growth rate (FCGR) testing that consists of several steps, beginning with selecting the specimen size, geometry, and crack length measurement technique. The two major aspects of FCGR test analysis are to ensure suitability of the test data and to calculate growth rates from the data. The article presents an analysis of the crack growth data. Optical, compliance, and electric potential difference are the most common laboratory techniques, and the article reviews their merits and limitations. Forced-displacement, forced-vibration, rotational-bending, resonance, and servomechanical systems for various loading conditions are also discussed.

Spark source mass spectrometry (SSMS) is an analytical technique used for determining the concentration of elements in a wide range of solid samples, including metals, semiconductors, ceramics, geological and biological materials, and air and water pollution samples. This article discusses the basic principles of spark source technique; SSMS instrumentation such as ion source, electric sector, and magnetic sector; sample preparation; and test procedures of SSMS. Some of the related techniques to SSMS are laser ionization mass spectrometry and laser-induced resonance ionization mass spectrometry. The ions produced in SSMS are detected by either the photometric method or electrical detection method and quantitatively measured by techniques such as internal standardization techniques, isotope dilution, multi element isotope dilution, and dry spike isotope dilution. The detected spark source spectrum contains all the elemental data of the tested sample. Finally, the article exemplifies the applications of SSMS.

Vibrothermography, also known as sonic thermography, sonic infrared (IR), thermosonics, and vibroacoustic thermography, is a nondestructive evaluation (NDE) technique for finding cracks and delaminations through vibration-induced heating. This article describes the four parts of the vibrothermography process: vibration of the specimen by a transducer; conversion of vibrational energy into heat by a crack, delamination, and other contacting surfaces; conduction of the heat to an external surface; and infrared detection of the heat with a thermal camera.

This article covers defect formation and classification, followed by a brief description of the most common nondestructive testing (NDT) methods used for postbuild inspection. Descriptions of the established and emerging NDT techniques for in-process monitoring (IPM) and in-process inspection (IPI) in additive manufacturing (AM) also are provided, highlighting the advantages and limitations. The article concludes with a list of the main NDT methods and techniques used. As qualification and certification of AM parts is an urgent matter for the AM industry, a description of the current work carried out for developing standards is also included.