As the commercial and military aircraft fleets age, additional resources are required to ensure their airworthiness. As the aircraft become older, the more likely they are to develop structural damage that may lead to unscheduled repairs or, in the worst case, accidents. Fatigue and corrosion are the two main causes of structural damage in aging aircraft and this research examines the use of a Superconducting QUantum Interference Device (SQUID) as a tool for Nondestructive Evaluation (NDE) to detect and characterize these aging aircraft problems. The primary advantage of using SQUIDS in NDE over other techniques is the ability to detect second layer cracks and corrosion commonly found in aircraft structures.

In general, verification of a NDE method means demonstrating, through experiment and/or calculations, the ability to distinguish signal from noise for the flaw types and sizes and instrument/flaw configuration’s expected in the actual inspection. A common approach to quantify and validate the capabilities of an inspection technique is to conduct a probability of detection (POD) analysis. There are basically two ways to conduct this type of analysis. The first is experimentally, which requires a large number of samples with a range of flaw characteristics being examined by several inspectors. The second is analytically, which requires construction of a model to simulate the inspection process which is run for a range of samples and testing conditions. Due to the large number of parameters defining a method, experimental results alone are usually inadequate but are still required to describe system parameters that cannot be modeled analytically. With mathematical modeling, the response of the measurement system to the anomalies of interest (e.g., cracks, corrosion, and voids) can be simulated if enough is known about the field-flaw interaction that generates the response function. Due to the complexity of the mathematics of these interactions, idealized models are normally used but these still provide sensitivity analysis information useful in evaluating the inspection technique.

The following is a development of a measurement model simulating the scanning of a SQUID over a sample containing a crack. We model a closed crack through a special development of boundary integral equations, which is used to calculate corresponding magnetic fields, simulating what the SQUID instrument measures. The measurement model will be used later in a POD analysis.