Significance and Use
4.1 IEM Applications and Capabilities—IEM has been successfully applied to a wide range of NDT applications in the manufacture, maintenance, and repair of metallic and non-metallic parts. Examples of anomalies detected are discussed in 1.1 and 6.2. IEM has been proven to provide fast, cost-effective, and accurate NDT solutions in nearly all manufacturing, maintenance, or repair modalities. Examples of the successful application focuses include, but are not limited to: sintered powder metals, castings, forgings, stampings, ceramics, glass, wood, weldments, heat treatment, composites, additive manufacturing, machined products, and brazed products.
4.2 General Approach and Equipment Requirements for IEM:
4.2.1 IEM systems are comprised of hardware and software capable of inducing vibrations, recording the component response to the induced vibrations, and executing analysis of the data collected.
4.2.2 Hardware Requirements—Examples of a tabletop impact excitation system and a production-grade drop excitation system are shown in Fig. 1 and Fig. 2, respectively. IEM systems include: an excitation device (for example, modal hammer / impact device / dropping system) providing an impulse excitation to the object, a vibration detector (for example., microphone), a signal amplifier, an Analog-to-Digital Converter (ADC), an embedded logic, and a data User Interface (UI). Tested parts can typically be on any surface type, but they can also be supported (for example, foam support, held with an elastic) in consideration of possible damping influences. The following schematics show the basic parts for an impact excitation approach (Fig. 3) and a drop excitation approach (Fig. 4).
FIG. 1 IEM Tabletop Testing System Using a Non-Instrumented Impactor
FIG. 2 Production-Grade Drop Excitation System
FIG. 3 Schematic of Impact Excitation Approach
FIG. 4 Schematic of Drop Excitation Approach
4.3 Constraints and Limitations:
4.3.1 IEM needs a change in structural integrity to properly sort different parts. This means that parts with only cosmetic issues, such as a visual surface anomaly would still need be inspected with a focused visual inspection.
4.3.2 The location of a flaw or specific flaw type characterization is challenging. As IEM measures the whole-body response of a part, location and categorization of defects usually requires additional data (such as additional nondestructive and destructive evaluation) and analysis.
4.3.3 Large raw material or process variation, or both, may limit the sensitivity of IEM without some method for compensating for those variations.
4.3.4 Groups of parts with a wide range of physical temperatures are not good subjects for IEM without some method for compensating for those variations. Temperature affects the natural frequencies, so stabilization of temperature is desired for parts testing. Data can be taken over a large range of temperatures, as long as the parts are stable during the testing.
4.3.5 IEM is a volumetric inspection method. Sensitivity to defects will be driven by the size of the defect relative to the size and mass of the part. For example, a small hairline crack of a certain length that may be detectable in a 0.5 lb part may not be detectable in a 100 lb part.
4.3.6 The expected useful frequency range of the part to be tested must be considered when selecting and configuring an IEM examination. Many IEM systems are limited to detecting frequencies up to 50 kHz, but more modern systems have demonstrated detection of frequencies up to 150 kHz on some parts. Parts with small dimensions or parts made from certain materials, or both, may have resonance spectra that fall partially or entirely outside of the frequency range of some IEM systems. The physics of energy distribution from the impulse and attenuation from interfering harmonic modes can also cause a reduction in signal-to-noise ratio at the higher end of IEM frequency ranges.
4.3.7 Materials that resonate poorly or dampen vibrations are typically not good candidates for IEM examination.
Scope
1.1 This practice covers a general procedure for using the Impulse Excitation Method (IEM) to facilitate natural frequency measurement and detection of defects and material variations in metallic and non-metallic parts. This test method is also known as Impulse Excitation Technique (IET), Acoustic Resonance Testing (ART), ping testing, tap testing, and other names. IEM is listed as a Resonance Ultrasound Spectroscopy (RUS) method. The method applies an impulse load to excite and then record resonance frequencies of a part. These recorded resonance frequencies are compared to a reference population or within subgroups/families of examples of the same part, or modeled frequencies, or both.
1.2 Absolute frequency shifting, resonance damping, and resonance pattern differences can be used to distinguish acceptable parts from parts with material differences and defects. These defects and material differences include, cracks, voids, porosity, material elastic property differences, and residual stress. IEM can be applied to parts made with manufacturing processes including, but not limited to, powdered metal sintering, casting, forging, machining, composite layup, and additive manufacturing (AM).
1.3 This practice is intended for use with instruments capable of exciting, measuring, recording, and analyzing multiple whole body, mechanical vibration resonance frequencies in acoustic or ultrasonic frequency ranges, or both. This practice does not provide inspection acceptance criteria for parts. However, it does discuss the processes for establishing acceptance criteria specific to impulse testing. These criteria include frequency acceptability windows for absolute frequency shifting, scoring criteria for statistical analysis methods (Z-score), Gage Repeatability & Reproducibility (R&R) for diagnostic resonance modes, and inspection criteria adjustment (compensation) for manufacturing process and environmental variations.
1.4 This practice uses inch pound units as primary units. SI units are included in parentheses for reference only and are mathematical conversions of the primary units.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
