Eddy Current (EC) inspection is a non-destructive evaluation (NDE) method commonly used for flight quality assurance of commercial, military, and rocket engine hardware. EC inspection is used to detect surface or near-surface anomalies, such as cracks, in metallic hardware by sensing changes in coil impedance. Understanding the relationship between crack size and system response is the basis for determining the detection capability of an EC system and establishing find/no-find decision rules. For a given EC system, the relationship between the EC response and crack size can be evaluated by scanning a set of standard specimens with a known number and distribution of crack sizes. The results are analyzed using the a versus a analysis method to quantify the relationship between the EC response (i.e., a) and crack size (i.e., a) and develop a Probability of Detection (POD) curve, which defines the crack size that can be detected with a specified level of reliability. Available POD or statistical software packages have the ability to perform an a versus a analysis. However, the validity of the analysis, resulting POD curve, and any parameters of interest (e.g., a_(90), a_(90/95)) can be significantly impacted by the choice of analysis inputs. This paper provides a practical overview of the a versus a methodology and illustrates how some of the analysis inputs impact an a versus a analysis.
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