Damage detection in CFRP by coupling acoustic emission and infrared thermography

doi:10.1016/j.compositesb.2015.09.011

Composites Part B: Engineering

Volume 85, February 2016, Pages 68-75

https://doi.org/10.1016/j.compositesb.2015.09.011 Get rights and content

Abstract

Acoustic emission (AE) and infrared thermography (IT) are simultaneously combined to identify damage evolution in carbon fibre reinforced composites. Samples are subjected to tensile static loads while acoustic emission sensors and an infrared camera record the acoustic signals and the temperature variations respectively. Unsupervised pattern recognition procedure is applied to identify damage mechanisms from acoustic signals. Thermodynamic arguments are introduced to estimate global heat source fields from thermal measurements and anisotropic heat conduction behavior is taken into account by means of homogenization technique. A spatial and time analysis of acoustic events and heat sources is developed and some correlation range in the AE and IT events amplitude are identified.

Introduction

Nondestructive experimental characterizations allow nowadays promising perspectives for studying the damage process in high-performance engineering materials such as composites. In the particular case of the aeronautic industry, Acoustic Emission (AE) and Infrared Thermography (IT) appear as widely used and advanced techniques for the damage monitoring in materials and aerostructures. With acoustic emission technique, the energy released during a damage process is detected by piezoelectric sensors in form of transient elastic waves. The study of acoustic emission descriptors allows then to identify specific damage mechanisms of composites [1], [2]. This identification is done by the use of Unsupervised Pattern Recognition technique that has been used successfully by different authors in the case of glass fibre composites [3], [4]. Infrared thermography is an optical measurement technique that provides 2D surface thermal fields. Acquired thermal fields can thus reveal and localize overheating related to degradation mechanisms (see works on carbon-epoxy of [5], [6], [7] and on glass-epoxy of [8]). Since temperature may be affected by external exchanges and/or by heat diffusion inside the material itself, the dissipation sources represent even a more accurate indicator of damage [9], [10].

Some authors have naturally suggested to combine these both techniques. Ref. [11] has shown that it is possible to correlate the damage zone of a glass fibre-thermoplastic composite by both techniques in quasi-static tensile test. Some others correlations between AE and thermal events were thus put in evidenced, mainly in the context of fatigue behavior. For instance, it was noted by Refs. [12], [13] a progressive appearance of hot areas on the composite surface as the number of acoustic hits increases. Mean surface temperature and AE cumulative counts or energy were also corroborated. The time curves of these data exhibit simultaneously inflection points corresponding to transitions between damage modes of the materials [14]. In Ref. [15], crack growth monitored by thermography and the ratio of rise time to amplitude of the waveform exhibit similar trends.

Even if these preliminary studies have confirmed the interest in coupling AE and IT, they only propose a global analysis of AE events, without distinguishing specific mechanisms involved. Regarding thermal aspects, they are mostly limited to temperature-based observations and thus potentially influenced by the surrounding. Few attempts to provide further interpretation are generally restricted by strong assumptions: either a dissipation estimated only from the rate of heat absorption [14] or the single consideration of the thermo-elastic coupling [15], heat conduction inside the material being neglected in all cases.

This work focuses on the damage mechanisms in unidirectional carbon fibres laminates subjected to axis and off-axis static tensile loads. The aim is to investigate correlations between AE acquisitions interpreted with pattern recognition technique and dissipation fields accounting for anisotropic conduction heat behavior. From this, a relevant spatial and time analysis of acoustic events and heat sources can thus be developed.

Section snippets

Materials and experimental work

A carbon fibre reinforced laminate is considered in this study. The composite is made of 14 unidirectional plies [0]₁₄ of prepreg Hexply^® with resin M10R and carbon high strength (CHS) fibres Toray^® T700S and cured in an autoclave at 125 °C during 90 min at a pressure of 2 bars. ( $2 m m$ thickness, fibre volume $f_{V}$ of 60%). The cured plate is cut according to the standard ISO 527-5 [16] so as to obtain two composite directions (0° and 90°). Samples dimensions are $250 m m$ length, $20 m m$ (for 0° tests)

Acoustic emission pattern recognition

AE data are in general complex in nature. Regarding damage in composite materials, conventional graphical analysis is not sufficient to provide arguments to discriminate different mechanisms involved. In such a case, automated statistical techniques can help to identify correlation between data. In this way, this study uses Unsupervised Pattern Recognition (UPR) techniques to cluster in classes AE signals with similar acoustic signatures. Such post-processing is done with Noesis software [17].

Heat source identification

The heat source determination is established within the framework of quasi-static processes and small perturbations and is based on classical principles of the thermodynamics of irreversible processes. Following assumptions are assumed for the infrared image processing to obtain the heat source estimation: the temperature variation has no influence on the microstructure state, internal coupling sources are neglected, external heat does not depend on the time, coefficients ρ, C and conductivity

Results

For the correlation analysis between AE and IT, two analysis steps are considered. In a first step, we intend to correlate heat sources and acoustic events produced by matrix cracking. As explained in Section 3, this damage mechanism generates acoustic hits of low amplitude and energy in regards to the interface failure and fibre breaking. The second part of the analysis focuses obviously on interface failure and fibre breakage mechanisms. This work is done in both senses: find heat source

Conclusions

In this study, combined analysis of carbon-epoxy composite subjected to axis and off-axis tensile tests is done by means of acoustic emission and infrared thermography measurements. On the one hand, acoustic emission technique is used for damage detection, both to identify and locate the damage events during the load. The use of Unsupervised Pattern Recognition algorithm allows to discriminate physical mechanisms associated to different signals: three damage mechanisms are found in specimens at

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Damage detection in CFRP by coupling acoustic emission and infrared thermography

Abstract

Introduction

Section snippets

Materials and experimental work

Acoustic emission pattern recognition

Heat source identification

Results

Conclusions

Compos Part B

Compos Sci Technol

Compos Part B

Compos Part B

Compos Part B

Compos Part B

Polym Test

Compos Part A

Mech Res Commun

Int J Fatigue

Compos Part B

Compos Part B