Date of Award

Spring 2023

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Lingyu Yu

Abstract

The ability to inspect and detect damage within a structure without permanent damage has broad appeal in a range of engineering applications from aerospace structures to nuclear facilities. Non-destructive evaluation (NDE) using guided ultrasonic Lamb waves has growing potential and popularity in these fields. Lamb waves are known for being sensitive to material properties and various defects across the thickness such as cracks in metallic structures and delamination or debonding in composite structures. Research has also been done on the development of fully non-contact system that generates and detects Lamb waves, particularly focused on the use of pulsed laser for generation and laser doppler vibrometer for sensing. In the past, the NDE system has been set up for situations where access to both sides of the sample is available. This creates the limitation where situations where access to only one side is permitted cannot be accurately re-created. Another limitation lies in the majority of sensing with laser doppler vibrometer being achieved through velocity measurement. In addition, research of NDE is often focused on the detection and evaluation of the damage leaving the mitigation of the damage less discussed, if identified. Therefore, this thesis is aimed at addressing these subjects.

In part I this thesis explores the upgrading and development of a laboratory testing configuration that accommodates a variety of fully non-contacting Lamb wave NDE system consisting of a pulsed laser and a scanning laser doppler vibrometer. The original system was re-designed and re-configured to accommodate both same-side (with both pulsed laser and scanning laser doppler vibrometer access to the same surface of the test plate) and opposite-side inspection through optical re-directing of laser from pulsed laser. Special considerations are given to high energy invisible laser from the pulsed laser being used in this study. The system has been tested with a range of samples constructed of simple or complex geometries, including thin foils and small diameter tubing. The results show that Lamb waves can be actuated and detected at frequencies up to 4 MHz.

In part II this thesis explores a potential solution to stress corrosion cracking by use of an engineered carbon fiber reinforced polymer patch. Patch design, material selection, and a detailed installation procedure are explored and developed in this study. To evaluate the patch integrity as well as the crack mitigation with the patch, a set of tests have been designed. We first performed effectiveness testing to verify the load bearing ability of the patch. After that, an environmental chamber that can accommodate temperature (room to 50°C), humidity, and marine environment (salt water corrosivity) was designed and selected samples were placed for an extended period for exposure. We have evaluated and shown that the proposed patch can increase the load bearing ability of the stainless-steel substrate. Secondly, we have successfully developed a controlled environment for evaluating the durability of the patch when exposed to a variety of environmental conditions including temperatures reaching 50°C and humidity reaching 85%.

In summary, this thesis work explores a fully non-contact Lamb wave NDE System that can provide same-side or opposite-side inspection for a variety of structures with varying geometries. A composite patch repairing methodology is also explored to mitigate the structural integrity if cracking occurs. We showed that the patch has satisfactory integrity with clear load bearing increase for patched samples of up to 500 lbf and can prevent the crack growth. In the future, further exploration can be done by investigation into expanding the frequency limits of the SLDV sensing system, performing studies into the detection of and integrity verification of coatings applied to thin foils and tubes, and review possibilities on expanding the durability of the bonded CFRP patches.

Rights

© 2023, Andrew Philip Campbell

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