·Table of Contents
·Workshop - Reliability
Application Parameters - A Definition and Their Role in NDE ReliabilityMatthew J. Golis, Advanced Quality Concepts, PO Box 141388, Columbus, Ohio 43214 (614) 268-0518
Email : email@example.com
Thus, NDT technique reliability, R, as it relates to a singular type of discontinuity can be represented using the conceptual model shown below where f, g and h are still-to-be-determined functions dependent upon IC, AP and HF, respectively.
This conceptual model merely suggests that some function, f, dependent upon the technique's intrinsic capabilities, is representative of the technique's reliability under idealized conditions. When application parameters (deviations to idealized assumptions) are considered, the idealized reliability will be reduced in accordance with the nature of another function, g. When human factors associated with manual inspections are considered, the effective reliability is reduced even further, following function h.
A conceptual drawing of this relationship is shown to the right. On the left is an idealized level of reliability, established by the Intrinsic Capability of the inspection technique being used. On the right is the predicted level of reliability derived from the estimated negative impacts of specific application parameters and the degrading effects of human factors.
The purpose of this paper is to define what constitutes an application parameter. The nature of the functional relationship g(AP) is driven by the general types of formats considered applicable to expressing reliability performance. The characterization of each of these three functions (and identification of ways in which they can be linked in a quantitative manner) was discussed in depth during the second Reliability Workshop and still remains to be done. This paper presents the definition for an Application Parameter and how they might be evaluated and used to realistically predict the most probable reliability of a specific NDT technique used for a specific application.
An inspection using the ultrasonic method of NDT, for example, would have an intrinsic capability based, in part, on the electronic instrument's performance characteristics, e.g., noise level, amplitude dynamic range, time-base dynamic range and frequency response. However, the intrinsic capability for a specific technique must include additional variables such as the transducer's performance characteristics (conversion constants and frequency response) and any constraints programmed into the instrument to modify the appearance or utility of received signals (e.g., reject, filtering, and gate threshold settings).
In assessing Intrinsic Capability, we assume the test material is homogeneous and isotropic. Surfaces are smooth and flat. Acoustic wave velocity and impedance are known and assumed constant. The useful range of the ultrasonic instrument is the region above the electrical noise level and below the upper limit or saturation point of the instrument. The acoustic wave characteristics in this ideal material would be predictable based on material wave velocity, transducer size, pulser output and transducer conversion coefficients (d33 & g33). The simulated discontinuity would be oriented at an optimum angle (perpendicular), comprised of an air-backed flat, smooth surface of diameter equal to the diffraction limited extent of the acoustic wave (possibly l /10 or so). Static readings could be taken from any direction. Given these variables, it is possible to predict the ultrasonic wave's geometrical beam characteristics (directivity function) and its signal strength, relative to the initial energy input. The signal sensitivity could be estimated and its operational dynamic range would be at its maximum.
In the case of Ultrasonic NDT, attenuation and anisotropy are the major material characteristics that degrade the passage of wave fronts. Attenuation has two materials-related components[Note that beam spread due to diffraction is considered an idealized beam characteristic and thus is part of the Intrinsic Capability estimate. Although it relies on the acoustic wave velocity in the test material, it is not considered a material-dependent attenuation Application Parameter.]. One is scatter due to inherent reflectors, e.g., grain boundaries and inclusions. The other is non-elastic absorption in some materials, e.g., plastics and rubber. Each of these distributive features reduces the sound energy available for detecting discontinuities.
Scatter from material texture, grain boundaries, small inclusions and voids also degrade the wave's coherence, and in addition, introduces backscatter noise into the detected signal. In the case of materials that exhibit backscatter, the effective noise level of the ultrasonic instrument increases and therefore the available operating range of the instrument/transducer combination is reduced. Engineered materials such as composites exhibit differential propagation characteristics (anisotropy) in accordance with the materials used and orientations of reinforcing fibers or aggregates. Such material effects are often summarily included in measures of material propagation characteristics such as bulk wave group velocity and signal level attenuation.
Another wave-coherence degrading effect is caused by rough surfaces upon entry and upon internal reflection from exterior surfaces. Each interaction of an ultrasonic wave with a rough or irregular surface contributes to the deviation of the wave front from that of the coherent (in-step) waves initially created by the pulse excitation of the transducer.
Inspection Equipment Characteristics
The capability of the inspection equipment is the basic reference for overall system reliability. A piece of equipment, coupled with a transducer-type sensor, establishes whether or not an inspection can be conducted. It must have a sufficient useful dynamic range with a suitable level of sensitivity for each specific application. The ideal instrument will be capable of showing the presence of the smallest physically detectable indication from a test target. Presumably, this will be at or about the diffraction limit of a crack tip. When a reference reflector, e.g., flat-bottomed or side-drilled hole, is used to set the sensitivity of an ultrasonic instrument, this merely serves to establish the operating level of interest.
The useful operating range of the instrument in a specific application is the region between the reference level (or some specified percentage thereof) and where the instrument becomes saturated, e.g., at the top of the viewing screen or where the gain can no longer increase signal response. The lower extreme of the overall operating range in a specific application is limited by the instrument's electrical noise or the backscatter noise from the test material. The upper extreme will be limited by the effect that material and interface attenuation needs to be overcome by the gain capabilities of the system.
In other applications where orientation, morphology and stresses are unknown, the detection probabilities can decrease precipitously. In such cases, the response affects of different discontinuity characteristics may be approximated using the statistical probabilities of their respective occurrences. The figure to the right schematically shows the effects of the degrading influences of material noise, material attenuation, discontinuity orientation and shape and their attendant reductions in system operating range. The issue of exactly how to define the ultimate sensitivity of a system remains an open question and is represented by the question mark, (?).
Geometrical Constraints Characteristics
In addition to the material, equipment and discontinuity characteristics, constraints introduced through inspection protocols and physical encumbrances can also degrade the reliability of NDT. For example, in ultrasonic shear-wave inspection, only a limited set of transducer aspect angles is used. Scanning of test parts may be in accordance with a less than 100% sampling scheme. Test part curvature in pipes and nozzles degrades coupling effects and beam consistency. Even some corrective measures, such as contoured shoes, limit the ability to rotate transducers in order to enable off-normal detection of discontinuities. The more convoluted the test part surface, the more the effectiveness of the inspection is compromised. These conditions must also be considered part of the collection of application parameters which affect the reliability of an NDT activity.
|ULTRASONIC METHOD VARIABLES|
|Contributing Element||Intrinsic Capability||Application Parameters|
|Material||Homogeneous||D Noise (Scatter)|
|Isotropic||D Attenuation (Absorption & Scatter)|
|Smooth Surfaces||D Surface Roughness|
|Flat Surfaces||D Surface Curvature|
|Acoustic Impedance||D Acoustic Impedance|
|Instrument1||Electrical Noise Level|
|Unfiltered Signals||D Filter Settings2|
|Wavelength in test part|
||Best Case Scenario
Orientation, Right Angle
Size, Diffraction Limited
Flat and Smooth
Change in DZ (material & stress)|
D Flatness or Smoothness
||Limits of constraint
||Scan Intervals & Rates
|| Aspect Angle(s)
||Surface Access (inherent & external)
|EDDY CURRENT METHOD VARIABLES
Encircling Coil (Absolute)
|Contributing Element||Intrinsic Capability||Application Parameters|
|Smooth - Straight - Round||Dh : coupling, fill factor, noise|
|Conductivity Constant, s||D s : inclusions, temperature, heat treat noise|
|Permeability Constant, mr = 1||D mr, : inclusions, heat treat, work harden noise|
|Instrument||Electrical Noise Level||D Environmental (elec/mag) noise intrusion|
|Exciting Current||D Induction heating drift|
|Unfiltered Signals||D Filter Settings, thresholds, phase selection|
|Coil||Fill Factor, h = 1|
|Number of Turns|
|Discontinuity||Best Case Scenario s = 0, Perpendicular, On Surface Size ~ TBD||D Orientation|
D Surface to subsurface
|Coil Positioning||Aligned with axis|
|Static Readings||Throughput Rates|
Examples using other NDT methods and scenarios could be developed, but the general approach would be the same as that used for Ultrasonic and Eddy Current NDT. Having reviewed the potential approach, how should the term Application Parameter be defined?
An Application Parameter is any physical condition or state identified as pertinent to the physical aspects of an NDT technique, which when changed, alters the performance of the technique from that expected under idealized conditions.
Application Parameters are largely related to material and discontinuity features different from those assumed in an idealized inspection. They can be modified by operational settings related to instrument settings designed to ease or modify signal interpretation or selectivity. They can be related to operational procedures that require use of specific, but limited, sensor positions caused by physical impediments and economically attractive scanning practices.
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