Abstract
- Acoustic resonance analysis for non-destructive testing of materials in mass-production, as offered by RTE Akustik + Prüftechnik GmbH is a proven technology, having been successfully implemented in various industries. Acoustic testing technology for non-destructive testing of materials is very old: for instance, merchants excite glass or porcelain goods by tapping, thus causing them to vibrate; the object's quality is assessed by the sound produced. Cracked glass "rattles" instead of ringing.What is new is the industrial application of these methods to everyday manufacturing of various products in many industries. Modern, high-power computer systems can "audit" human hearing. Reliable integration into a production cycle of a few seconds, under mass-production conditions, is possible without any difficulties.
1. Introduction
- Nowadays, rationalisation and reliable, automated mass-production testing technology are increasingly important in order to deal economically with German manufacturing issues and with the increasing cost pressures.
The requirements of DIN ISO 9000 for demonstrated and documented product quality cannot easily be satisfied by subjective testing or by destructive testing of sampled specimens. The result of cumbersome and costly testing technologies is clients who respond too late to changes in process parameters, the "rebooting" of the production process remains a critical problem, in-line measurement and product evaluation is not implemented. With RTE's testing technology, production and process reliability can be increased significantly. By using acoustic methods, the detection of cracks and structural defects can be carried out during the manufacturing cycle itself.
Conventional procedures applied nowadays to non-destructive materials testing are in part too expensive and complex, and in part require a considerable amount of previous knowledge to be available. Non-destructive materials testing using resonance analysis, has already proven itself in everyday industrial practice as the new way forward.
2. Acoustics
- Acoustic deals with sound fields, i.e. with the description and explanation of the phenomena of sound generation, sound emission, sound propagation and sound absorption. Sound (aerial sound = tone, noise) in the narrow sense is understood as elastodynamic vibrations and waves in the medium of air, in the frequency range audible to humans i.e. approximately 20 Hz to 16 kHz.
Mechanical vibrations in a structure (solid-borne sound) and radiated vibrations in the surrounding air (audible sound) carry information. The resonances depend uniquely on the object's material, geometry and condition.
Acoustic measurement technology is very sensitive; even minor changes in the oscillatory behaviour of mechanical structures can be detected. Resonance analysis is a qualitative process that compares the actual oscillatory situation with the target one derived from a learning base. This learning base is established by using defined standard parts.
3. The acoustic resonance analysis process
- Acoustic resonance analysis is a new, non-destructive testing process that allows rapid and cost-effective 100% testing of a wide spectrum of test objects. It exploits the well-known physical effect in which a body, having been suitably excited (e.g. by tapping), vibrates in certain characteristic forms and frequencies (its natural resonances). The vibrations are, so to speak, the test object's 'language', its 'fingerprint' that can be captured with a sensor and analysed digitally.Material-specific acoustic parameters can be calculated from the resonant frequencies, and assigned to quality characteristics such as:
- Pass,
- Cracked,
- Material and structure
- hardened / tempered yes/no.
- AR evaluates the entire test object, regardless of its size. It does not sample or examine any local regions for faults.
- AR is simple to automate. Computer-aided testing is rapid, cost-effective and accurately reproducible (no subjective evaluation).
- AR is a qualitative procedure. Statements about the dimensions of defects are possible, and are based on comparable reference objects.
- AR is a dry procedure without chemistry or environmental problems.
- AR is proven and reliable ('ringing analysis' of glass and porcelain is an ancient method).
Key properties of acoustic resonance analysis (AR)
4. Practical applications
- RTE's acoustic resonance analysis is tried and tested, having been successfully applied to mass-production in various industries. Here are a few examples:
- Testing of spherical-cast brake callipers for structural changes, cracks and cavities
- Testing of cast pipes (6 m length) for cracks and cavities
- Testing of camshafts for white irradiation
- Testing of graphite electrodes for cracks
- Testing of sintered metal rings and cog belt wheels for cracks
- Testing of forged synchronous rings for cracks
- Testing of roof tiles for cracks
- Testing of glass for cracks and tensile conditions
- Testing of Al and Mg die castings
Example 1: 100 % inspection of mass-produced cast components
- Product: Spherical-cast safety parts for car brakes
The task: Structural and casting faults, cracks and cavities should be reliably detected as part of the components' total quality assurance tests. Fault localisation is only given to a limited degree. There are many different types; costly setting-up times are not tolerated. During the process inspection task on "starting up" the process, the important point is to define objective measured parameters right from the start, from which process status and thus product quality can be evaluated. The cast parts should be approved without tedious destructive testing. The ambient conditions are typical of a foundry (among others, high background noise level, graphite dust)
The solution concept: the design is 100% production line testing on a revolving transfer table, installed specially for the final tests. The measured parameter is sound, with the specimen excited to oscillate in a defined way. Recording and analysis occur in a few hundred milliseconds on the SR20 testing system. As a result of implementing a heuristic process ("reference run", "inspection run"), changing between types takes place by push-button. Monitoring by the test system of all the parameters relevant to the acoustic test leads to high system availability and test result reliability. Parameters can be altered by authorised personnel during operation via the facility's SPS, by using the "online parameter setting" function. Inputs are checked by the system for physical plausibility, so as not to endanger the testing process. Time-domain and frequency-domain parameters are calculated from the acoustic signal in real time, and compared against preset limits.
Result: cost-savings through rapid process optimisationReduction in customer complaints through assured quality standards (reliability aspect!). High process reliability (detection of structural defects and accurate evaluation of cracks: so far not possible with other methods).
 Spherical-cast safety parts for car brakes |
Example 2: Detection of the structural defect 'White irradiation' in camshafts
 Diagram 2: Temporal signal: the acoustic signal over time, 100 milliseconds are sufficient |
 Diagram 3: Frequency spectrum of the acoustic signals; each peak corresponds to one resonant frequency and can be utilised as a test feature |
White irradiation defect: Camshafts suffering from white irradiation lead to high machine tool wear during subsequent processing, and are brittle.
Task: During total quality testing of the parts, the white irradiation defect must be detected reliably. There are many different types; costly setting-up times are not tolerated. The ambient conditions are typical of a foundry (among others, high background noise level, graphite dust)
TThe solution concept: solution and implementation of the test task are through the compact, industrial SR20 test system. The measured parameter is sound, with the specimen excited to oscillate in a defined way. Recording and analysis occur in a few hundred milliseconds. Time-domain and frequency-domain parameters are calculated from the acoustic signal in real time, and checked against preset limits (comparison process). As a result of implementing a heuristic process ("reference run", "inspection run"), changing between types takes place by push-button. Monitoring by the test system of all the parameters relevant to the acoustic test leads to high system availability and test result reliability.
Result: Reduction in customer complaints through assured quality standards, high process reliability (detection of white irradiation and the achieved evaluation accuracy not possible so far with other methods!). Presentation of production-line results: 80 production-line camshaft specimens of one type and defined condition were analysed by acoustic testing. After acoustic measurement, specimen quality was verified (in part through destructive processes). The temporal signal is shown below in Diagram 2, the frequency spectrum of a defined measurement window in Diagram 3, an overview of the distribution of resonant frequencies (1 frequency line given as example) across the specimens in diagram 4. Table 1 summarises the statistics.
Conclusions: the detection of specimens 60 and 61 with resonant frequencies 8263 Hz (specimen 60) and 8258 Hz (specimen 61) from among the 80 specimens of verified quality, is statistically significant.
| Minimum | Maximum | Mean | STD | MW + 3 x STD |
| 8053 Hz | 8263 Hz | 8117 Hz | 28,9 Hz | 8204 Hz |
| Table 1: Statistics of the examined 80 production-line specimens of verified quality | ||||
 Diagram 4: Resonant frequency (as an example, frequency characteristics at ca. 8 kHz) shown for 80 production-line specimens (x-axis: specimen number, y-axis: resonant frequency in Hz) |
Example 3: Aluminium die casting - comparison against dye-penetration test
- Product: Steering housing for cars
- All the specimens that proved unusual during crack testing with the dye penetration method, also show significant differences against the mean non-defective pattern during acoustic testing. Several specimens, originally classified as non-defective as per dye penetration test, proved acoustically unusual and were re-tested for cracks by the penetration method. The result of the acoustic resonance analysis was confirmed.
- Of 20 specimens judged defective by the dye penetration test, 18 were also identified as defective through their acoustic properties. The two non-identified specimens (i.e., passed by the resonance analysis), were found to be non-defective through destructive sectioned pattern analysis.
- Of 50 'sale quality' specimens (classified as Passed by x-ray and dye penetration tests), 2 specimens were found to have unusual acoustic characteristics (Failed by resonance testing). These were thoroughly re-tested with x-rays. A clear defect (long cavity) was identified. Hence the parts were reclassified as defective and not sent out.
- Reproducibility - process comparison Resonance analysis: 100 % agreement of acoustic parameters Dye penetration test: repeat testing by different testers established a level of agreement of only a few percent between the Pass/Fail test results!
Defects: Cracks, cavities
Task: Production line testing, directly after casting
 Diagram 5: Examples of the tested property 'Decay behaviour of a resonant frequency' for 2 passed specimens (upper curves) and 2 failed ones (lower curves). The failed specimens decay significantly faster. |
Example 4: Magnesium die casting
- Product: Steering wheel framework
- There is good correlation between acoustic parameters and specimen quality.
- Analysis of parameter effect
- Specimen temperature and relaxation time (internal tensile conditions!): high
- Nest dependence: minor
- All cracked specimens were detected; this was reproducible.
- All non-defective specimens were identified as such; this was reproducible.
- 1 specimen originally classified as non-defective was acoustically unusual. Destructive verification discovered a structural fault (material separation).
Defect: Cracks
Task: Production line testing
Results of production-line experiments:
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| Diagram 6: Demonstration of the resonant effect's frequency distribution | |
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