NDT / NDT Exclusives

New Resonant Inspection Method for MIM Parts

November 24, 2010
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The nondestructive testing Drop Test Fixture allows manufacturers to avoid the possible consequences of a quality concern: expense, lost time and a damaged reputation.

Typical acoustic signature, power spectrum to 50 kHz, for a metal part, shown as processed by NDT-RAM with criteria ranges (indicated with green “boxes”) at seven natural resonant frequencies. Source: The Modal Shop Inc.

After defective parts have been sorted with RI, complimentary traditional NDT techniques may provide a means for subjective diagnosis on the smaller subset of “rejected” parts. This is useful for determining a defect’s root cause and ultimately improving the production processes.

Resonant Inspection (RI) is commonly used for quality assurance testing of powder metal components, providing a volumetric whole body inspection sensitive to both external and internal structural flaws or anomalies. This technique measures a metal component’s mechanical resonances and compares an individual part’s signature to a template generated from a statistically significant sample space of “good” parts. Traditionally it has been limited to small and medium components with sizes ranging from about a “dime to a dinner plate” given the requirements to fixture and excite the part effectively. However, using a new methodology that employs a drop testing fixture, very small components, such as those commonly manufactured by powder metal MIM processes, can be 100% inspected reliably, quickly and cost-effectively.

Resonant Acoustic Method (RAM NDT), a form of RI, is a nondestructive testing technique that evaluates structural integrity by striking a part with an impact and analyzing its mechanical resonances from the acoustic ringing produced. This technique can be easily automated to eliminate human errors with fast throughput, providing cost effective, 100% inspection with minimal disruption to production. With a large number of successes on the production lines of powder metal and cast parts, RAM NDT is the simple and effective solution to manufacturers’ zero PPM challenge.

Traditional NDT techniques, for example magnetic particle or dye penetrant testing, focus on detecting and diagnosing defects. They use visual or imaging techniques that scan for indications of specific defects at specific locations. For production line quality inspection, identifying the type and/or location of defect itself is secondary to identifying the defective parts themselves. While diagnosing specific defects is applicable when evaluating and inspecting some systems, such as using ultrasonics to inspect gas pipelines, it is not appropriate for high volume 100% inspection of manufactured metal parts. For these production lines it is of primary importance to detect if a part is non-conforming rather than why. Therefore, an end-of-line go/no go objective inspection, such as by RAM NDT, which provides a reliable 100% sort, is preferred to a subjective diagnosis, perhaps useful for defect root cause analysis.

Resonant inspection (RI) measures the structural response of a part and evaluates it against the statistical variation from a control set of good parts to screen defects. Its volumetric approach tests the whole part, sensitive to both external and internal structural flaws or deviations, providing objective and quantitative results. This structural response is a unique and measurable signature, defined by a component’s mechanical resonances. These resonances are a function of part geometry and material properties and are the basis for RI techniques. By measuring the resonances of a part, one determines the structural characteristics of that part in a single test. Many of the traditional NDT techniques can detect these flaws as well, but often only RI can detect all in a single test, throughout the entire part (including deep sub-surface defects), in an automated and objective fashion.

After defective parts have been sorted with RI, complimentary traditional NDT techniques may provide a means for subjective diagnosis on the smaller subset of “rejected” parts. This is useful for determining a defect’s root cause and ultimately improving the production processes.

Science of Resonant Inspection

Modal analysis is defined as the study of the dynamic characteristics of a mechanical structure or system. All structures, even structures such as metal gears or similar parts that are apparently rigid to the human eye, undergo elastic deformation as a result of applied forces. The structure itself deforms in a distinct, specific pattern. This structural dynamic behavior is defined by the mass, stiffness and damping of a given part’s material properties and geometry. Specifically, all structures have mechanical resonances, where the structure itself amplifies any energy imparted to it at certain frequencies. For example, tuning forks or bells will vibrate at very specific frequencies, their natural frequencies, for relatively long periods of time with just a small tap. The sound that is generated is directly due to these natural frequencies. In fact, any noise made by a structure is done so by its vibration. RAM NDT utilizes this structural dynamic behavior to evaluate the integrity and consistency of parts.

The natural frequencies are global properties of a given structure and the presence of structural defects causes shifts in some or all of these resonances depending upon how the flaw interacts with the specific deformation pattern. For example, a crack will change the stiffness in the region near the crack and a variation in density or the presence of porosity will change the mass. The resulting frequency shifts are measurable if the defect is structurally significant with respect to the either the size or location of the flaw within a specific resonance mode shape. With some defects, a shift in resonant frequency can also be noticed audibly, such as a cracked bell that obviously does not ring true.

Practical Application of RAM

The RAM technique performs resonant inspection by impacting a part and “listening” to its acoustic spectral signature with a microphone. The controlled impact provides broadband input energy to excite the part and the microphone allows for a non-contact measurement of the part’s structural response. The part’s mechanical resonances amplify the broadband input energy at its specific natural frequencies, indicated as peaks in the frequency spectrum (shown below the “black box” signal processor), measured by the microphone above the background noise in the test environment.

Good parts (structurally sound) have consistent spectral signatures, for example, the mechanical resonances are the same among part samples, while bad parts (structurally different) are different. Deviations in peak frequencies or amplitudes constitute a structurally significant difference that provides a quantitative and objective part rejection. NDT-RAM processes the individual spectra, evaluating these changes compared to a baseline template for the given part. The results are displayed on the industrial PC workstation, with the pass/fail decision communicated to the system PLC.

Figure 1. Small MIM parts used in medical instruments. Source: The Modal Shop Inc.

RAM Applied to Small MIM Parts

RAM NDT has typically utilized a mechanical impactor to provide an impulsive force into the part specimen. However, this impactor can be ineffective for smaller parts, such as small MIM parts commonly used within medical devices as shown in Figure 1. Given the obvious need for 100% inspection of components used within medical instruments to ensure quality, a new drop test fixture design allows for the application of RAM NDT with a reliable excitation technique and sorting mechanism for very small powder metal parts.

The drop test fixture design, shown in Figure 2, uses gravity instead of an electromechanical impactor to generate the impulse force required for RAM NDT. Small MIM parts can be collected manually or with a bowl feeder and dropped through a tube directing the part into a chamber where it impacts the surface of a piezoelectric force transducer. This triggers the measurement of the part’s acoustic resonant frequency signature by the microphone before the specimen comes to rest in a collector at the bottom of the chamber. Depending on the pass/fail result the collector sorts the good parts from the bad by rotating one direction or the other.

Figure 2. NDT-DTF Drop Test Fixture and laptop controller. Source: The Modal Shop Inc.


The Resonant Acoustic Method with the drop test fixture for small part inspection offers MIM manufacturers a fast, objective, whole body test that can be configured quickly to work on any number of small parts. 100% testing with RAM NDT removes the need for time consuming, subjective human visual inspection.
The benefit of removing doubt on whether shipped parts are defect free allows Plant Managers, Quality Managers, and Process Engineers more time to concentrate on high value projects without worrying about individual part quality contaminating production shipments. RAM NDT also saves substantial costs as compared with 100% visual inspection labor.
Additionally, fully inspected parts reduce liability and quality related expenses like external sorting, product recalls, and customer initiated penalties that can easily run in the tens of thousands of dollars per occurrence.

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Charles J. Hellier has been active in the technology of nondestructive testing and related quality and inspection fields since 1957. Here he talks with Quality's managing editor, Michelle Bangert, about the importance of training.
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