Hygrothermal Damage Monitoring of Composite Adhesive Joint Using the Full Spectral Response of Fiber Bragg Grating Sensors.

Chow-Shing Shin,Tzu-Chieh Lin

doi:10.3390/polym14030368

Abstract

Adhesive joints in composite structures are subject to degradation by elevated temperature and moisture. Moisture absorption leads to swelling, plasticization, weakening of the interface, interfacial defects/cracking and reduction in strength. Moisture and material degradation before the formation of defects are not readily revealed by conventional non-destructive examination techniques. Embedded fiber Bragg grating (FBG) sensors can reflect the swelling strain in adhesive joints and offer an economical alternative for on-line monitoring of moisture absorption under hygrothermal aging. Most of the available works relied on the peak shifting phenomenon for sensing. Degradation of adhesive and interfacial defects will lead to non-uniform strain that may chirp the FBG spectrum, causing complications in the peak shifting measurement. It is reasoned that the full spectral responses may be more revealing regarding the joint’s integrity. Studies on this aspect are still lacking. In this work, single-lap joint composite specimens with embedded FBGs are soaked in 60 °C water for 30 days. Spectrum evolution during this period and subsequent tensile and fatigue failure has been studied to shed some light on the possible use of the full spectral response to monitor the development of hygrothermal degradation.

Highlights

The advantageous specific stiffness and specific strength and the excellent corrosion resistance of fiber reinforced composites lead to their increasing replacement of metallic materials in adverse environments
In a previous paper [82], we have shown that tensile loading on virgin specimens progressively shifted the fiber Bragg grating (FBG) spectra towards longer wavelengths without changing the spectral shape initially
The development of the full spectral responses of uniform period FBG sensors embedded in epoxy adhesive single-lap joints during hygrothermal aging at 60 °C for 30 days of the joint specimens and their subsequent tensile and fatigue failures has been studied

Summary

Introduction

The advantageous specific stiffness and specific strength and the excellent corrosion resistance of fiber reinforced composites lead to their increasing replacement of metallic materials in adverse environments. Examples of these include applications in the petroleum and gas extraction sector [1,2], tidal turbine blades [3], wind turbine blades [4], air [5,6] and land transport vehicles [7] as well as cooling towers [8]. The coupled effect of temperature and moisture affects the adhesive joint more than the composite [15] Service loading conditions such as impact, occasional overload and fluctuating loading will further aggravate the damages. If such degradation went undetected, serious structural failures and catastrophic outcome might follow

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