Eddy Current Probe for Residual Stress Measurement

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Solution Overview

Problem

Existing methods for non-destructive testing of residual stress around cold-worked holes in metal airframe structures are inadequate due to human errors, tooling issues, and material variations, which can lead to unpredictable results and reduced fatigue life of fastener holes.

Innovation Solution

An eddy current probe and test method that measures compressive residual stress by differentiating between the effects of residual stress and plastic deformation using a four-coil differential probe, which induces eddy currents and analyzes the in-phase and out-of-phase output voltages to quantify residual stress without being influenced by plastic deformation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Duration of action of moving object

If cold working process is used to induce residual compressive stresses, then fatigue life and damage tolerance are improved, but measurement and validation of the process becomes difficult

Engineering Contradiction:

Improvefatigue lifeVSAvoidresidual stress measurement

Core Design Contradiction:

Duration of action of moving objectVSDifficulty of detecting and measuring

Solution Approach 1:

The patent replaces traditional mechanical measurement methods with electromagnetic (eddy current) sensing to detect residual stress. The eddy current probe uses electromagnetic induction to sense changes in electrical conductivity caused by residual stress, avoiding direct mechanical contact and providing non-destructive measurement capability.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent exploits the relationship between residual stress and electrical conductivity parameters. By measuring changes in electrical conductivity through eddy current testing, the system indirectly measures residual stress levels, transforming a difficult mechanical parameter measurement into an electrical property measurement.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If traditional testing methods are used, then process validation is required, but human errors and tooling issues lead to unpredictable results

Engineering Contradiction:

Improveprocess validation reliabilityVSAvoidtesting result consistency

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The eddy current testing system is self-calibrating and automatically compensates for variations. The differential probe configuration inherently compensates for tooling wear and material variations by comparing measurements, reducing dependence on operator skill and tooling condition.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The system provides immediate feedback through the differential measurement process, where deviations from expected residual stress patterns are detected and can trigger re-testing or process adjustment, ensuring consistent validation results.

Inventive Principle:

Principle #23Feedback

3Measurement precision

If four-coil differential probe is used to differentiate residual stress from plastic deformation, then measurement accuracy is improved, but device complexity increases

Engineering Contradiction:

Improveresidual stress measurement accuracyVSAvoidprobe structure

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The probe is segmented into four separate coils arranged in a differential configuration. This segmentation allows each coil to sense specific magnetic field components, and the differential arrangement cancels out signals from plastic deformation while preserving residual stress signals.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The four coils are positioned asymmetrically around the fastener hole to create a measurement configuration that is sensitive to circumferential residual stress but insensitive to radial plastic deformation. The asymmetric arrangement optimizes the differential measurement capability.

Inventive Principle:

Principle #4Asymmetry

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The method effectively measures small changes in conductivity to determine the degree of cold work expansion, allowing for the validation of proper cold working processes and extending service intervals by accurately assessing residual stress levels, with a signal-to-noise ratio greater than 60 and the ability to distinguish between low and high cold-worked holes.

Implementation Method 1

An eddy current probe and test method measures compressive residual stress around a cold-worked fastener hole in a metal piece

Methodology Applied

Scientific EffectEddy currents: Eddy Currents

Implementation Method 2

The test method measures the electrical conductivity within the region of the residual stress around the hole, which is proportional to the residual stress, due to the piezoresistive effect

Methodology Applied

Scientific EffectPiezoresistive effect: Piezoresistive Effect

Data Source

PatentUS11125723B2Eddy current testing for measuring residual stress around cold-worked holes

Publication Date: 2021.09.21 SOUTHWEST RES INST

AI summary

A method of testing the residual stress around a cold-worked hole in a test material. The test uses two identical eddy current coil pairs, each pair having a driving coil and a pickup coil. One coil pair is placed into the cold-worked hole. The other coil pair is placed into a non-cold worked hole in a reference material. When the coils are activated, and with a properly chosen phase difference between the excitation signal and the output signal, the in-phase component of the output voltage signal can be correlated to residual stress around the cold-worked hole.