RAM Flaws Out of Products

May 1, 2004
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The conversion of machined parts to powdered metal and cast parts comes at the same time that manufacturers face almost no-tolerance for defects and demand for high throughput

Most powdered-metal component suppliers are already doing spot testing via magnetic particle technique on batches of parts from a given production run. The problem starts when a customer, say an automotive manufacturer, experiences field failures. The result is that these component suppliers are put on "parts-hold" and they have to pay for both containment and 100% field inspection on the customer site. At the risk of permanently damaging the company's reputation and losing both existing and new business, significantly larger part batches are subjected to magnetic particle inspection, with 100% of the production lots inspected via a 300% visual part sort-where each part is visually inspected by three separate technicians.

Often, everyone who can be pulled from another job is pulled in to help out during this time of crisis. To ensure necessary quality, 100% end-of-line part inspection must be implemented; traditional nondestructive testing (NDT) techniques that are typically used include magnetic particle, liquid penetrant, eddy current and X-ray, or purely visual inspection. These can be painstaking, subjective manual processes. As a result, rarely does the 100% inspection continue, and the cycle of "flawed-parts roulette" continues.

Providing relief and security for the high volume manufacturer, resonant acoustic method (RAM) NDT offers reliable inspection, with quantitative, objective results. This technique can be automated to eliminate human error with fast throughput for 100% inspection and minimal disruption to production. RAM NDT is a volumetric, resonant inspection (RI) technique that measures the structural integrity of each part to detect defects on a component level.



History

Initially, the basic visual inspection by the operators themselves served as the primary means of monitoring part acceptability. More sophisticated NDT techniques evolved, and magnetic particle inspection eventually became a de facto standard for testing ferrous metallic components such as castings, forgings and, more recently, powdered metals. This subjective and visual technology has remained essentially unchanged for the past 50 years, yet continues to be the most common inspection tool for such parts.

Traditional NDT techniques focus on detecting and diagnosing defects. They use visual techniques or imaging to scan for any indication of defects. For those companies who spot test, identifying the type of defect itself is secondary to identifying the defective parts. While diagnosing specific defects is applicable when evaluating and inspecting some systems, such as gas pipelines, it is not appropriate for high volume 100% manufactured part inspection. For these components, 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, is preferred as compared to a subjective diagnosis.

Image-based or visual methods include magnetic particle testing (MT), ultrasonic testing (UT), eddy current/electromagnetic testing (ET), dye penetrant testing (PT), X-ray/radiographic testing (RT) and visual testing (VT). The fundamental difference between these traditional NDT techniques and RI is this scanning methodology. Scanning methods are manual and require subjective interpretation by an operator. As a result, the operator requires a certain level of technical training and/or certification to properly diagnose such indications of defect and infer the effects on the functionality of a part. Additionally, whenever such a technique requires the judgment of an operator, overall reliability suffers. In Juran's Quality Handbook, Dr. Joseph M. Juran states that operators average only 80% reliability. This statistic is a reflection of the human interpretation factor, not the accuracy of the techniques themselves. None of these scanning techniques allow for efficient, cost effective or reliable quality control testing of 100% of manufactured parts of any appreciable volume. It should be noted that in some cases eddy current techniques can be implemented as a "whole part" test by using an encircling coil, easily automated with high throughput. However, in these cases the effectiveness of ET's flaw detection is reduced, limited to detecting on certain types or configurations of surface flaws.

Resonant inspection, conversely, 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, both for 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 Rl techniques. By measuring the resonances of a part, one determines the structural characteristics of that part in a single test. Many traditional NDT techniques can detect these flaws, but often only resonant inspection can detect all these flaws in a single test, throughout the entire part and including deep sub-surface defects, in an automated and objective fashion.

After defective parts have been sorted with RI, complementary traditional NDT techniques may provide a means for subjective diagnosis on the smaller subset of parts. This is useful for determining a defect's root cause and ultimately improving the production processes. The ASME has published standards that detail each of the traditional NDT methodologies.



Theory

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 deformation. These deformations can be described using modal analysis. 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, which are their natural frequencies, for long periods of time with just a small tap. The sound that is made is directly due to these natural frequencies. In fact, any noise generated by a structure is done so by vibration, which is simply a pattern of summed sinusoidal deformations. RAM NDT uses this structural dynamic behavior to evaluate the integrity and consistency of parts.

For illustrative purposes, consider the single degree-of-freedom (SDOF) mass, spring, damper system in as seen in the chart, Mass, Spring, Damper System. It has one DOF because its state can be determined by one quantity, the displacement of the mass. The elements of this simplified model are the mass, stiffness and damping. The energy imparted into the system by the excitation force is stored in the system as kinetic energy of the mass and potential energy of the spring and is dissipated by the damping.



Resonant Acoustic Method

Resonant inspection is basically experimental modal analysis simplified for application to high-volume production manufacturing and quality control testing. The generic, step-by-step procedure is as follows:

1. Excite the part with a known and repeatable force input. This force is typically generated by a controlled impact or actuator providing broadband or sinusoidal energy over the appropriate frequency range of analysis.

2. Measure the structural response of the part to the applied input force using a dynamic sensor such as a microphone or accelerometer (vibration pickup) and a high-speed analog to digital converter (ADC) with appropriate anti-aliasing filters.

3. Process the acquired time data with a Fast Fourier Transform (FFT) for analysis in the frequency domain.

4. Analyze the consistency of the frequency spectrum from part to part by comparing to a spectral template created from known good parts. Mechanical resonances are indicated as peaks in the frequency spectrum of the response. "Good" parts (structurally sound) have consistent spectral signatures (the mechanical resonances are the same among parts) while "bad" parts are different. Generally these templates are set up to evaluate the consistency of the frequency and amplitude of 10 or fewer peaks. Any deviation in a range of peak frequency or amplitude constitutes a structurally significant difference that provides a quantitative and objective part rejection.

The Resonant Acoustic Method 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 structural response. The part's mechanical resonances amplify the broadband input energy at its specific natural frequencies, measured by the microphone above the background noise in the test environment.

Gross defects can often be distinguished directly by the human ear, but human hearing is subjective and limited to a maximum of approximately 20 kiloHertz (kHz). By analyzing data beyond 20 kHz, to upwards of 50 kHz, much smaller defects can be detected, even across production lots given reasonable process control. Typically, these defects cause resonant frequency shifts. These shifts are a function of how the specific defect affects the mechanical resonance, which depends upon the specific defect location with respect to the deformation pattern of the resonance. Fortunately, mechanical resonances are global properties of a structure, and generally a defect will alter at least one resonant frequency. For this reason, it is good practice to set up multiple criteria ranges for analysis.

An additional signal processing tool for improving analysis and sorting of good parts vs. defective parts is implemented with a time-delay function. Often, a defect may not cause a substantial shift in resonant frequency, but instead reduces the structure's capability to "hold its tone" over time. By delaying the structural response measurement (many times just milliseconds), the resonant peak is not measurable from defective parts because the energy decays too rapidly. The peak in the frequency spectrum disappears. A practical example of this is a cracked bell that, when struck, does not ring for an extended period of time as a "good" bell would.

RAM NDT's basic measurement procedure allows for automation and high part testing throughput. No part preparation is required such as magnetizing, cleaning or immersion. Expendable costs associated with such preparation, such as chemicals and waste removal, are eliminated. The single impact and non-contact response measurement (via microphone) can be made as a part is moving down a conveyor, often as fast as one part per second. The parts do not need to be physically stopped; nor are they required to be precisely located with expensive robotics on contact actuators and vibration pickups. Simple guides are typically adequate to position the part for impact and allow flexibility to test many different types of parts or geometries. Given this capacity for automation and throughput, and its quantitative analysis with objective results, RAM NDT is ideal for plant floor, high-volume quality control-test applications.

Successful implementation of RAM NDT depends upon proper set up of the accept/reject criteria ranges in the part template. Each type and/or geometry of part requires a separate template. Parts need to be tested in the same manufacturing state. Typically, templates can be set up quickly with just a few dozen parts in less than 30 minutes. This sample set should include both good parts (ideally with at least several from different production batches) and parts with the expected variety of flaws. It is recommended to validate the template and resulting part sort with a larger statistical data set of a few 100 pieces. Often other NDT techniques, for example magnetic particle inspection, are complementary in this regard, or destructive evaluations are commonly used for correlation as well. After the specific part's template is verified for accuracy, large volumes of parts can be 100% tested quickly and reliably.

System validation can be performed using a controlled set of known parts. Parts of a given type, both good and defective, are kept as "standards" and run through the automated system for validation on a regular basis. Across batches over time, signatures often show trends where mechanical resonances shift due to acceptable variations in material properties (density, etc.) or process variations (heat treatment, etc.) By investing time upfront with this type of system validation procedure, process engineers and technicians have a better understanding of their parts and manufacturing processes and ensure the reliability of their inspection system.

The RAM NDT technique serves quality minded manufacturers who are dissatisfied with visual detection techniques such as magnetic particle, liquid penetrant or X-ray, which are time consuming, costly and subjective. RAM NDT allows for simple integration of a turnkey system that is a reliable, fully automated method for quality control and process improvement. This rapidly growing technique creates an economical, on-line inspection system that provides for zero defect product supply. Unlike previous implementation of resonant inspection that are excessively complicated and costly to automate, RAM NDT is fast, simple and reliable, and easily re-configurable. As a result, powdered metal and casting manufacturers around the world have proven the benefits of RAM NDT resonant inspection over their traditional inspection techniques. NDT

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