·Table of Contents
·Reliability and Validation 2
Reliability Assurance for Automated Eddy Current Inspection SystemsCarlos Pairazaman,
Customer site support manager for Veridian Engineering, Oklahoma City, Oklahoma, USA
Branch Chief, Processing Engineering, United States Air Force, Tinker Air Force Base, Oklahoma, USA
Ward D. Rummel
is an independent consultant for D&W Enterprises, LTD., Littleton, Colorado, USA.
The protocol for establishing and maintaining system operational integrity involves
The methods and procedures for establishing and validating system design CAPABILITIES; for assessing REPRODUCIBILITY of additional system modules; and for revalidation of operating modules (REPEATABILITY) are described and discussed.
Repetitive response from multiple flaws of equal size results in broadening of the response distribution. This broadening is the result of flaw to flaw variations as well as measurement variations and are accounted for by using multiple flaws in the generation of a typical POD curve. The spread between the upper limit of the noise and the lower limit (signal and noise) of the flaw response enables repetitive detection and discrimination / identification of flaws of that size without false calls (Type II errors). The practical threshold detection and discrimination limit is at that small flaw size at which the signal and noise responses converge without overlap and detection / discrimination can be attained. It is wise to maintain a signal / noise margin (safety factor) in practical applications to allow for unanticipated variations in the NDE procedure.
|Fig 1: Repetitive response from a single flaw||Fig 2: Repetitive response from flaws of equal size|
Small sample sizes assume that the flaws selected are representative of the population of flaws to be detected and that flaw to flaw variance in application is bounded by the flaws selected for assessment. Various thrusts have been directed to modeling the flaw to flaw variance and have been successful for simple flaw configurations. In many applications, this variance is accounted for by including a margin (safety factor) in detection requirements and by follow-up data collection and analysis of signal responses from service hardware. For complex configurations, larger margins may be used to address difficulties in validating margin assumptions.
When a small sample size is accepted as being representative of the population, repetitive measurements can be made on the selected flaws to establish the signal / measurement variance for each flaw size sampled. FIGURE 2 illustrates the broadening of response due to multiple measurements from flaws of equal size. This method produces a data set that can be used to establish a discrimination threshold and for plotting a POD curve. Although the number of measurements can provide a high measurement confidence level based on the small sample set, the measurements are not fully independent and thus less rigorous than that obtained by independent measurements on independent cracks. The POD curve generated is a measurement curve and is representative of most important elements of the characterization task, but may not fully describe a capability if the response from service flaws of equal size varies significantly from the selected small sample set.
The second part of NDE procedure optimization is in setting the acceptance level for the signals provided. For large flaws, the signal and noise are well separated and the threshold decision level can be easily set to provide clear discrimination as shown schematically in FIGURE 3.
|Fig 3: Signal and noise separation for large flaws which provide clear signal discrimination||Fig 4: Theresholdlevel set too high with resultant misses.|
If the threshold decision level is set too high, flaws will be missed. This condition may be imposed when signal and noise separation would otherwise allow clear discrimination as illustrated in FIGURE 4. This is a condition often experienced when the threshold signal level is set on a slot in a "calibration" specimen and consideration is not given for the reduced response of a crack of a size that is equal to that of the slot.
The limit of capability of an NDE procedure is reached when the signal and noise separation approaches zero. When signal and noise overlap, both misses and false calls will result as illustrated in FIGURE 5. In this case, if the threshold level is set to assure detection, the number of false calls will increase. A level of false calls can be tolerated if a secondary procedure (usually NDE) is applied to resolve false calls and provide the required discrimination. CAUTION: Applying the same NDE procedure cannot resolve false calls since the same signal and noise conditions are application. Likewise, an NDE procedure with lower discrimination capabilities does not provide resolution. This error has been frequently observed in the use of visual inspection to resolve eddy current findings.
|Fig 5: Overlap of signal and noise results in both misses and false calls.|
When an optimized signal, noise and discrimination (acceptance) level are established, the capability of the inspection process can be characterized in the form of a Probability of Detection (POD) measurement as shown in FIGURE 6. For an automated system, performance RELIABILITY, at the capabilities level established, is dependent on REPRODUCING the conditions which reproduce the same signal level response to the same size flaw (REPRODUCIBILITY) and on REPEATING the same distribution of signal and noise responses to the same flaw sizes that were used in establishing CAPABILITY (POD). The RELIABILITY of an inspection procedure may thus be viewed in terms of REPRODUCIBILITY and REPEATABILITY.
|Fig 6: Typical probability of detection(POD) curve|
REPRODUCIBILITY in an inspection process is primarily dependent on the "calibration" of the inspection system. Most classical measurements are made with the aid of a reference "calibration" artifact or "standard". Calibration "standard" artifacts are "measured" by reference to a master standard that is traditionally retained as a as a national resource. Control is approached by international agreements to provide a common basis for exchange in commerce. It was therefore logical that a reference slot has evolved as a calibration artifact for most NDE measurements and physical measurement of slot size may be traceable to a national "master standard". Slots are economical to produce with available technology and are commonly specified in establishing and applying NDE procedures. Traceability of reference calibration artifacts (slots) are assumed when they are used in validated NDE procedures. Users are cautioned that apparent consistency in slot size and configuration (mechanical / geometric consistency) may not provide a consistent NDE response and NDE response verification is required before artifact acceptance. Unfortunately, a single slot is often used for reference and set-up and linearity of response of the NDE procedure is assumed. This practice partially satisfies the REPRODUCIBILITY requirement, but is incomplete in providing confidence in end to end system response. A system (instrument, cables, probes and accessory commodities) "calibration" which includes not only a baseline set point, but a response linearity verification (characteristic response hereafter referred to as "linear") is necessary to provide high confidence in the REPRODUCIBILITY system response / output.
Modern electronic instruments are produced with attention to linear response, but periodic validation of the response linearity is required to maintain confidence in instrument / system output. It is often assumed that classical electronic "calibration" performed by an independent "calibration laboratory" assures linearity of response. Since the electronic instrument constitutes only a part of the NDE system, an end to end system linearity response verification is an essential part of a validated NDE procedure. A change in eddy current probes or cables can significantly change the response linearity. When an NDE procedure is intended for application at different locations or with multiple instruments, a "master gage" "calibration" specimen and sensor probe (transducer) are required to establish and document the output variability (or conformance) due to "calibration" practices. It is necessary to establish output variability bounds and to assure performance remains within those bounds for each inspection procedure. This is an essential requirement for both manual and automated eddy current systems.
REPEATABILITY is assumed to be constant for a fully automated inspection system. That assumption can be supported for scanning and measurement processes which have no variance. Automation of a variable process will continue to produce a variable output. For instrumented methods, the inherent REPEATABIITY of measurements is assumed to be constant due to the ability of electronic devices to produce the same decision for the same signal levels (alarms, recording devices etc.). A variation in part surface finish, a variation in scanning speed, a variation in a scan raster or a variation in part geometry will be reflected in variations in REPEATABILITY. For automated scanners or robots, parts wear may result in an increased variance in the scan path and hence a decrease in REPEATABILITY. It is important to note that REPEATABILITY is a multi-dimensional characteristic and depends on the precision of probe location, orientation and on consistence of probe, cable and instrument response.
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