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ASNT 2025 6-9
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ABSTR ACT
There is a need for innovative methods to provide
accurate sizing of discontinuities in structures,
to guide maintenance actions and better inform
engineering of the structure state. This paper
highlights the development of the capability to size
the length and depth of cracks in multilayer fastener
sites using bolt-hole eddy current (BHEC) techniques.
Technical efforts include improvements to model
calibration, liftoff compensation, and the inversion
process. In addition, an expanded set of surrogate
models was developed that address crack sizing
for titanium, aluminum, and steel structures across
multiple frequencies and for varying hole diameters.
A comprehensive crack sizing evaluation study was
performed under a wide range of test conditions,
demonstrating improved sizing capability over
using peak amplitude. Crack length estimates were
found to have less error than crack depth estimates,
although crack depth is the more critical parameter for
informing maintenance actions.
KEYWORDS: bolt-hole eddy current inspection, BHEC, model-
based inversion, sizing, uncertainty
1. Introduction
There is a need for innovative methods to provide accurate
sizing information on discontinuities, like fatigue cracks in
metallic structures (Lindgren et al. 2007 Lindgren and Knopp
2016 Aldrin and Lindgren 2018). The value of a nondestructive
evaluation (NDE) sizing capability lies in its ability to acceler-
ate subsequent maintenance actions, improve the quality of
material review board evaluations, provide critical data to life
prediction modelers, and support long-term digital twin and
NDE 4.0 initiatives. Practical examples of NDE sizing capa-
bilities include sizing cracks in welds using time-of-flight dif-
fraction (TOFD) methods, using eddy current testing to size
cracks in aircraft propulsion components, and evaluating the
material state of turbine blades using resonance ultrasound
spectroscopy (RUS) (Aldrin and Forsyth 2017). This paper
presents progress on the characterization of discontinuities
such as fatigue cracks in metallic multilayer fastener sites using
bolt-hole eddy current (BHEC) techniques. Specific benefits of
accurately sizing bolt-hole discontinuities include streamlining
repairs (e.g., managing shallow scratches and gouges sepa-
rately from larger fatigue cracks) and delivering quantitative
data to engineering on the structure state.
1.1. Background on Model-Based Inversion for BHEC
NDE of Cracks in Metallic Structures
Given the complexity of many NDE sizing tasks, the devel-
opment of specialized algorithms and procedures has been
pursued to support inspectors as part of the data review
process. While there is a growing desire to leverage emerging
artificial intelligence (AI) and machine learning (ML) capabil-
ities, successful applications have been limited to cases where
the sizing task is relatively straightforward or where very large,
high-quality empirical datasets have been acquired (Aldrin
and Lindgren 2018). Frequently, such large datasets with
well-understood discontinuities are rarely available or are too
costly to practically produce.
As an alternative to AI/ML methods, model-based
inversion—incorporating an iterative, first-principles physics
model—has been proposed to solve complex NDE character-
ization problems. A number of efforts have been embarked
upon over the decades to develop model-based inversion
solutions for eddy current NDE applications (Sabbagh and
UNCERTAINTY EVALUATION OF CRACK
SIZING CAPABILITY INCORPORATING MODEL-
BASED INVERSION APPLIED TO BOLT-HOLE
EDDY CURRENT INSPECTIONS
JOHN C. ALDRIN†*, DOYLE MOTES‡, MALLORI HUGHES‡, DAVID FORSYTH‡, ELIAS SABBAGH§, HAROLD A.
SABBAGH§, R. KIM MURPHY§, GEORGE NUXOLL††, CHRISTINE KNOTT‡‡, AND ERIC A. LINDGREN‡‡
ME
|
TECHPAPER
† Computational Tools, Gurnee, IL 60031
‡ TRI Austin, Austin, TX 78746
§ Victor Technologies LLC, Bloomington, IN 47401
†† UniWest, Kennewick, WA 99336
‡‡ Air Force Research Laboratory (AFRL/RXNW), Wright-Patterson AFB, OH
45433
*Corresponding author: aldrin@computationaltools.com
Materials Evaluation 83 (8): 42–56
https://doi.org/10.32548/2025.me-04536
©2025 American Society for Nondestructive Testing
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