Lamb Wave Crack Detection in Fastener Countersinks

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Solution Overview

Problem

Current non-destructive evaluation methods for detecting cracks near fasteners in thin-walled structures, such as aircraft skin, are limited by their inability to detect smaller cracks without removing fasteners, require skilled operators, and are time-consuming and expensive, especially when using ultrasonic or eddy current techniques.

Innovation Solution

The use of Lamb waves generated and detected at symmetrical positions relative to a fastener to calculate a symmetry parameter, which indicates the presence of cracks without the need for fastener removal or highly skilled technicians, utilizing a system with a Lamb wave generation device, detection device, and computing system to process signals and determine crack presence.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If ultrasonic methods with phased array are used to detect smaller cracks in the countersink, then measurement precision is improved, but device complexity and ease of operation worsen due to required surface preparation, contact media, and highly trained operators

Engineering Contradiction:

Improvecrack detection capabilityVSAvoidsurface preparation requirements

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent replaces traditional mechanical contact-based ultrasonic methods with electromagnetic acoustic transducers (EMATs) that generate and detect Lamb waves through electromagnetic interaction with the conductive material. This substitution eliminates the need for mechanical contact media and surface preparation, resolving the contradiction between measurement precision and device complexity

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the physical state and parameters of wave propagation by using Lamb waves (guided waves in thin plates) instead of traditional bulk ultrasonic waves. This parameter change allows detection through the fastener structure without requiring surface access or contact media, improving ease of operation while maintaining crack detection capability

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If eddy current methods are used to detect cracks, then measurement precision is improved for cracks extending 100 mil past the rivet head, but ease of operation worsens because fasteners must be removed first

Engineering Contradiction:

Improvecrack detection capabilityVSAvoidfastener removal requirement

Core Design Contradiction:

Measurement precisionVSEase of operation

Solution Approach 1:

The patent transitions from traditional eddy current methods that detect surface-breaking cracks to Lamb wave methods that propagate through the bulk structure in multiple dimensions. This dimensional change allows detection of cracks within the countersink region without requiring fastener removal, resolving the contradiction between measurement precision and ease of operation

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

3Measurement precision

If laser generated interferometry techniques are used, then measurement precision is improved for detecting low intensity waves, but ease of operation worsens because the low intensity waves are difficult to measure in an industrial environment

Engineering Contradiction:

Improvewave detection sensitivityVSAvoidindustrial environment measurement difficulty

Core Design Contradiction:

Measurement precisionVSDifficulty of detecting and measuring

Solution Approach 1:

The patent changes the intensity and detectability parameters by using EMATs that generate higher intensity Lamb waves compared to laser-generated waves. These enhanced waves are easier to detect with standard sensors in industrial environments, resolving the contradiction between measurement precision and detection difficulty

Inventive Principle:

Principle #35Parameter changes

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

This method allows for the detection of cracks near fasteners without removing them, reducing operational complexity and costs, and does not require highly skilled operators, providing a simple and effective indication of crack presence through the symmetry parameter calculation.

Implementation Method 1

A first Lamb wave can be generated at a first location, wherein the first Lamb wave propagates towards a fastener in the structure

Methodology Applied

Scientific EffectLamb wave propagation: Waveguide

Implementation Method 2

At a first detector position, a first signal corresponding to the first Lamb wave can be detected subsequent to its interaction with the fastener

Methodology Applied

Scientific EffectWave detection:

Data Source

PatentUS10234432B2Device and method to detect cracks in the countersink of a fastener

Publication Date: 2019.03.19 THE BOEING CO

AI summary

Disclosed are methods and systems to nondestructively detect cracks at or near a countersink of a fastener, such as a rivet, in a thin walled structure using Lamb waves. Generation and detection of Lamb waves at symmetric locations relative to the fastener provides signals used to calculate a symmetry parameter. The symmetry parameter represents a comparison of the relative amplitudes of the detected Lamb wave signals and provides a simple indication of whether a crack exists.