Damage Characterisation in Composite Laminates using Vibro-Acoustic Technique

The need to characterise in-service damage in composite structures is increasingly becoming important as composites find higher utilisation in wind turbines, aerospace, automotive, marine, among others. This paper investigates the feasibility of simplifying the conventional acousto-ultrasonic technique setup for quick and economic one-sided in-service inspection of composite structures. Acousto-ultrasonic technique refers to the approach of using ultrasonic transducer for local excitation while sensing the material response with an acoustic emission sensor. However, this involves transducers with several auxiliaries. The approach proposed herewith, referred to as vibro-acoustic testing, involves a low level of vibration impact excitation and acoustic emission sensing for damage characterisation. To test the robustness of this approach, first, a quasi-static test was carried out to impute low-velocity impact damage on three groups of test samples with different ply stacking sequences. Next, the vibro-acoustic testing was performed on all test samples with the acoustic emission response for the samples acquired. Using the acoustic emission test sample response for all groups, the stress wave factor was determined using the peak voltage stress wave factor method. The stress wave factor results showed an inverse correlation between the level of impact damage and stress wave factor across all the test sample groups. This corresponds with what has been reported in literature for acousto-ultrasonic technique; thus demonstrating the robustness of the proposed vibro-acoustic set-up. Structural health monitoring, impact damage, acousto-ultrasonic testing, non-destructive testing.


Introduction
Composite materials are increasingly been used in various industries such as energy, automotive, aerospace, marine, to mention but a few, as critical structural elements. This increasing use of composite materials necessitates the need for characterising in-service damage. Composite is formed by the combination of two materials with different physical and chemical properties. They are used for structural applications due to their light weight, high specific strength/stiffness and good resistance to a corrosive environment. Damage to composite structures can be invisible to the naked eye and nondestructive testing (NDT) techniques such as ultrasound, pulsed thermography and acoustic emission are often used to detect faults, such as delamination, matrix cracking, fibre-matrix de-bonding, fibre breakage and matrix porosity [1][2][3].
More also, for the past few decades, accurate characterisation of damage in fibre-based composites has been an area of active research [3][4][5]. Advancing this goal, this paper investigates the characterisation of impact damage in fibre reinforced polymer (FRP) composites by using a variant of acousto-ultrasonic technique (AUT).
Creating a viable procedure for damage characterisation that generally works for various fibre-based composites has proven to be challenging, because of the anisotropic nature of fibre-based composites.
Moreover, impact response to composite structures can cause damage such as delamination, matrix cracking and de-bonding in the laminate.
AUT was first proposed by Alex Vary as an NDT technique for evaluating the inter-laminar shear strength of fibre composites [6]. AUT combines acoustic emission (AE) methodology and ultrasonic simulation of stress waves. In order to quantify variations in the mechanical properties, Vary proposed a measurement parameter called the stress wave factor (SWF). SWF is a descriptive parameter that correlates with the material properties of composite materials. Contrary to traditional acoustic emission technique that requires the material to be under stress, AUT does not.
In recent times, AUT has found application in the following areas for NDT of composite structures: damage detection and severity quantification [7], material property correlation of fibre-based composites [8] and naturally occurring composites [9]. Importantly, this paper addresses a different yet important question, "can the set-up for AUT be simplified for quick and economic one-sided in-service composite structure inspection?" To answer this question, a variant set-up of AUT is proposed and explored. This approach involves a low level vibration impact excitation and acoustic emission sensing for damage characterisation in composite structures.

Sample Preparation
The test samples were made from an epoxy impregnated carbon fibre laminates (prepreg) with stacking sequence, as shown in Table 1. Hand lay-up method was used to prepare the test samples, in addition to autoclave curing to improve their mechanical properties. The curing cycle in the autoclave involved temperature ramp-up stage from ambient of 20 to 121 o C at a rate of 1 °C/min, followed by a dwell stage to maintain the temperature constant at 121 o C for two hours; and then ramp-down stage, with temperature being reduced to ambient temperature of 20 o C. The internal pressure of the autoclave was maintained at 106 kPa for the entire temperature cycling operation.  Fig. 1, with the test samples in Table 1

Vibro-Acoustic Testing
The conventional AUT set-up consists of two piezoelectric transducers, as shown in Fig. 2. The transmitting transducer is an ultrasonic emitter, while the receiving transducer is an AE sensor [11].

Results and Discussion
In an AUT domain, a commonly used approach for characterising a damage in fibre reinforced composite laminate is by calculating the stress wave factor (SWF). SWF quantifies the attenuation of the material to the induced stress wave. A low SWF corresponds to a region with higher attenuation, due to sub-surface damage and a relatively high SWF indicates a region of lower attenuation.
SWF can be determined with any of the following methods: peak voltage SWF method, ringdown SWF method, weighted ringdown SWF method and energy integral SWF method [12]. Each method for SWF resulted to different values for the parameter. For this investigation, the peak voltage method was applied. The peak voltage SWF method which assumed an inverse relationship between the peakvoltage and signal attenuation, due to damage in material is represented as Eqn. (1).
where is the peak voltage. shows a significant reduction in SWF between the unimpacted samples (A0, B0 and C0) and (A1, B1 and C1), which was subjected to 2 kN quasi-static load. This response suggests damage progression in the composite laminates. However, in Fig. 5, there was only a minor difference in SWF between (A2, B2, C2) and (A3, B3 and C3), at these points there were already significant fibre breakage and matrix cracking present in the impacted area. The result obtained agree with similar published ones [7].

Conclusions
This investigation explored the feasibility of simplifying the conventional set-up of AUT testing for quick and economic one-sided composite laminate inspection. Hence, the following inferences have been deduced from the study.
• The vibro-acoustic set-up proposed is viable and robust for damage detection and characterisation in fibre reinforced polymer composite laminate.
• Furthermore, the SWF analysis plot showed an inverse correlation relationship between the level of impact damage and SWF across all the test sample groups. This agrees with what has been reported in the literature for AUT [7].