The resonant acoustic method offers thecapability of 100% part inspection in ahigh-volume production environment.

Richard Bono and Scott SorensenThe Modal Shop Inc.Cincinnati, Ohio

The resonant acoustic method is a “wholepart” inspection technique that measuresthe structural integrity of each part to de-tect flaws on a component level. Capabili-

ties include materials ranging from ductile iron topowder metal to ceramics, and part sizes rangingfrom less than an ounce up to 50 lb (23 kg). The tech-nique is easily automated to eliminate human error,with fast throughput providing cost-effective 100%inspection with minimal disruption to production.The nondestructive testing — resonant acousticsmethod (NDT-RAM) is now the standard in-line in-spection on production lines for powder-metallurgyand cast parts. It is a simple, effective solution to thechallenge of achieving zero parts per million defects.

Resonant inspection principleThe principle of resonant inspection is simple:

every part has a unique resonant vibration signature,or pattern, that reflects its structural integrity. A de-viation from the expected signature of resonancescan indicate the presence of a flaw. For example, con-sider a bell or tuning fork. When you strike eitherpart, it vibrates and emits a sound. An instrumentthat “rings true” produces a consistent sound, whichcorrelates to the structural integrity of the instrument.

This is the basis for NDT-RAM technology. Whenstruck by an impactor, parts such as gears, brake an-chors, and rotors emit many resonant frequenciesas part of their structural response. This unique,measurable signature of resonances is comparedwith and analyzed against both a good and badproduct. Just as a cracked bell will not ring true likea structurally sound bell, if a part such as a gear iscracked, lacks the correct density, or misses othercharacteristics of a structurally sound product, theflaw will be exposed when its resonant signaturedeviates from what has been identified as goodproduct. Figure 1 graphically shows the steps in theRAM inspection process.

The resonances of a structure are a function ofpart geometry and material properties, and are de-fined by mass, stiffness, and damping ability. These

resonant frequencies can be measured in most rigidmaterials, including most metals and ceramics.NDT-RAM systems detect frequency shifts that canbe caused by imperfections such as cracks, porosity,and voids, as well as variances in nodularity (ductilecast irons), dimensions, geometry, weight, density,and manufacturing processes. Typical flaws and de-fects adversely affecting structural characteristicsare given in the table for powder metallurgy, duc-tile iron, and ceramic parts.

Quantitative analysisRAM tests the entire part for both external and

internal flaws. It provides an objective, quantitativeanalysis that eliminates errors involving human in-terpretation and judgment through proven dynamicmeasurement equipment.

In operation, a dynamic sensor captures thesound waves, and a high-speed analog-to-digitalconverter translates the sound into measurable data.The structural resonance of a defective part shifts,

ADVANCED MATERIALS & PROCESSES/JULY 2008 25

RESONANTACOUSTIC METHODDELIVERS DEFECT-FREE PARTS

Fig. 1 — Steps in the Resonant Acoustic Method (RAM) testing process includefrom left to right: Impact the part, measure the response, process the data, and quantify the results. An industrial instrumented impact hammer taps each partwith a measured and repeatable force. The impact causes the part to ring; audibleand inaudible vibrations are measured by the microphone. A smart digital controller performs a Fast Fourier Transform (FFT) on the measured data, andNDT-RAM software compares results to acceptable limits and accepts or rejects thepart accordingly.

Typical structural defects detected by RAM inspectionPowder metallurgy parts Ductile iron Ceramics

Cracks Cracks CracksChips Nodularity ChipsVoids Porosity Voids

Hardness Hardness SizingInclusions Inclusions Inclusions

Heat treatment Heat treatment DelaminationsDecarb Cold shuts PitsOxides Stresses Agglomerates

Contaminants Contaminants ContaminantsMissed operations Missed operations Missed operations

and the shift is identified when compared with predefineddata. In effect, NDT-RAM listens to the structural responseof a part and evaluates it against the statistical variation froma control set of good parts to screen anomalies.

The criterion for the predefined data is established by ex-perimental analysis of a statistically significant sample set.The resonant signatures of both good and bad products arecaptured to provide objective, measurable variables for com-parison. Templates are established for each part type. Anexample of a template is shown in Fig. 2. The NDT-RAMsystem is programmed with these templates, and requiresminimal human interaction for operation or maintenance.Because no special part fixturing or preparation is needed,parts can be tested as fast as one per second, keeping pacewith automated production lines.

Comparison with other methodsTraditional NDT techniques focus on detecting and diag-

nosing defects. They are based on visual techniques or im-aging to scan for any indication of defects. Scanning methodsinclude magnetic particle testing (MT), ultrasonic testing(UT), eddy current/electromagnetic testing (ET), dye-penetrant testing (PT), and X-ray/radiographic testing (RT).

These methods often are manual and require subjectiveinterpretation by an operator. Although diagnosing and/orimaging specific defects are applicable when evaluating anindividual part or system, they are not appropriate for high-volume, 100% manufactured part inspection. In these cases,it is of primary importance to detect whether a part is non-conforming, rather than why. Identifying the type of defectitself is secondary to identifying the nonconforming parts.Therefore, an end-of-line "go/no go" objective inspectionsuch as by NDT-RAM is preferred over that of a slower sub-jective diagnosis.

After defective parts have been sorted with NDT-RAM,complementary visual or imaging nondestructive testingtechniques provide a means for subjective diagnosis on thesmaller subset of parts. This is useful to determine the rootcause of a defect and ultimately improve the product de-sign or production process.

One of the benefits of RAM is that it is easy to justify fi-nancially. It has a low overall cost of ownership because itdoes not require additional materials to operate; no solu-tions, chemicals, or films are needed. It aids in profitability byachieving zero defects and lowering overall production costsby reducing variances in the production process.

Because it is fast enough to test 100% of the parts, thesystem works as a process monitor in addition to part quality

26 ADVANCED MATERIALS & PROCESSES/JULY 2008

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PLACE TO START.” Fig. 2 — Parts template in NDT-RAM software used for evaluatingparts against known good samples.

inspection, and insures against quality-related ex-penses while functioning as a process control mon-itor. If the system starts failing multiple parts, it al-lows manufacturing personnel to adjust theupstream process before a large number of faultyparts is produced and must be scrapped.

Nodularity testingOne common application for NDT-RAM is testing

ductile iron for acceptable levels of nodularity. Castiron contains a high carbon content, and during so-lidification of the metal, the carbon content can formirregularly shaped graphite flakes that disrupt thecrystalline structure, causing cracks and brittleness.In ductile iron, the graphite forms spherical, roundednodules that inhibit the formation of cracks and pro-vide enhanced ductility and machinability. Thehigher the nodularity, the higher the ductility. Figure3 illustrates the difference in microstructure of duc-tile iron with high nodularity (93%) and poor nodu-larity (55%).

Very often, part designers specify a minimumlevel of acceptable nodularity (typically >85%) toensure proper ductility. This can be especially im-portant in safety-critical automotive parts such asbrake and suspension components. The traditionalinspection technique for testing this is ultrasonic(UT), which measures the velocity of sound wavespassing through material. Rounded graphite nod-ules do not impede the acoustic energy, while flakygraphite particles do.

Thus, higher ultrasonic velocity correlates withhigher nodularity. However, UT is a localized testthat looks only at one specific section of a part, andit requires a coupling liquid bath for testing.

With NDT-RAM, resonant frequency correlatesdirectly with nodularity, and no coupling agent isrequired. Lower nodularity levels result in reducedresonant frequency characteristics. Resonant inspec-tion detects these shifts in resonant frequency char-acteristics, resulting in cost-effective, reliable, high-volume automated inspection. Additionally, becauseresonant acoustic inspection is a volumetric tech-nique, it checks the entire part for nodularity (aswell as for other flaws such as cracks and voids).

Sinter brazingSinter brazing is a common joining process for

powder metallurgy parts, as it allows productionof complex components via conventional PM tech-nology. The technique involves assembling multipleparts prior to sintering, adding a braze compound,and sintering at temperatures above the melting

Fig. 3 —Microstructureof ductile ironwith high nodularity(93%), left, andpoor nodularity(55%), right.

ADVANCED MATERIALS & PROCESSES/JULY 2008 27

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point of the brazing alloy.When processed properly,sinter brazing is very cost-effective and produces a verystrong joint. Several exam-ples of brazed powder metalparts are shown in Fig. 4.

Certain defects are commonamong sinter-brazed PMparts, including subcompo-nent misalignment during

initial assembly and incomplete braze material infil-tration. Inadequate infiltration is typically causedby an improper braze alloy or damaged braze pel-lets (for example, a slug with 50% of its materialchipped away).

Another root cause of poor sinter brazing ismissing the braze pellet altogether. Other processvariances that can lead to inadequate braze jointsinclude improper furnace settings and dew point.

All of these defects can result in poor joints, re-duced part strength, and ultimately part failure.With RAM, resonant frequency shifts are measuredfor an accurate, reliable 100% inspection. By com-paring these measurements to visual and destruc-tive separation pull-force evaluations, it can easilyand reliably detect poor sinter-brazed joints. The ca-pability of automating NDT-RAM makes it supe-rior to subjective visual inspection or inefficient sep-aration-force testing.

Ceramic partsCeramic parts ranging from ceramic washers and

filters to personal body armor plates are also suitable

for NDT-RAM. Typical structural defects in these com-ponents include cracks, chips, and delaminations.

Ceramic parts are manufactured in a similarmanner to powder metallurgy parts: Ceramic pow-ders are pressed into a green compact and then sub-jected to a sintering process. Density gradients andnon-uniform shrinkage can cause distortion orstresses that can lead to fracture or other structuraldefects during the process. Shock or vibration togreen parts during handling prior to sintering canalso lead to significant structural defects.

As a result, 100% part inspection via resonant in-spection is often necessary, particularly with safety-critical parts or in medical equipment components.In the case of personal body armor, plates are oftendestructively tested by shooting them with a pro-jectile. If a sample set from a production lot stopsthe bullet, then the samples are discarded and theremaining production units from that lot are ac-cepted. On the other hand, if just one of the lotsample plates is damaged by the projectile, the entirelot is rejected. Given the high cost of the plates and,more important, the value of human life that theyprotect, a reliable and effective 100% inspectionmethod is critical.

With RAM, every part can be tested. Structuraldefects or anomalies induce measurable resonantfrequency shifts that are detected readily, again re-sulting in reliable and cost-effective inspection.

For more information: Scott Sorensen, The Modal ShopInc., 3149 East Kemper Road, Cincinnati, OH 45241; tel:800/860-4867, or 513/351-9919; ssorensen@modalshop.com; www.modalshop.com.

Fig. 4 —Examples of

sinter-brazedpowder

metallurgy partswith braze-plug

locations markedby red circles.

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