Elsevier

NDT & E International

Volume 89, July 2017, Pages 44-55
NDT & E International

Feature guided wave inspection of bond line defects between a stiffener and a composite plate

https://doi.org/10.1016/j.ndteint.2017.03.008Get rights and content

Abstract

Adhesive bonding is widely used in aerospace composite structures. A continuous well-cured bond can offer good joint strength and improved fatigue and impact resistance, and is therefore crucial to the performance of the entire structure. This paper explores the feasibility of using feature guided waves (FGW) for rapid screening of the bond line between a stiffener and a carbon fiber reinforced polymer (CFRP) composite panel. Such FGWs are capable of focusing the wave energy along the stiffener and the bond layer, with limited radiation to the adjacent plate. The Semi-Analytical Finite Element (SAFE) approach is employed to understand the modal properties of FGWs that exist in the structure, and criteria are suggested to choose proper mode-frequency combination that is sensitive to adhesive defects. A shear horizontal type FGW mode is identified to be well suited, as it is easy to excite, and propagates with little dispersion and relatively low attenuation, while it retains sufficient energy around the bond layer. Both 3D Finite Element (FE) simulations and experiments are performed to study the interaction of the selected FGW mode with defects in the adhesive bond, and the results show excellent agreement. The reflection behavior and the wave-defect resonance phenomenon are investigated, which demonstrate the capability of the FGW for the bond line inspection.

Introduction

Fiber reinforced polymer composites have been increasingly incorporated into passenger aircrafts for decades, due to their high strength to weight ratio and good stiffness properties. In these composite structures, adhesive bonding is the most common means to fix skins to reinforcing elements, such as spars and stiffeners, since it offers uniform stresses (i.e. low stress concentration) across the bond line, increased joint strength, and good sealing performance. Also, compared to mechanical fastenings and welding, the bonding allows complex shapes and dissimilar materials to be joined with ease, and smoother external surfaces are attained. However the adhesive bonds are susceptible to failure where the composite structures are under fatigue loading and exposed to variation in temperature and/or humidity during the in-service use. This can lead to crack formation, reduced interfacial strength, and even more severe deterioration of the entire structure [1]. Therefore, there is considerable interest in assessing the bond quality in the aerospace industry [2], [3], [4], [5], [6], [7], [8], [9].

In the present work, we investigate a particular case - the bonding between a stiffener and a composite panel, being typical components to compose airplane wings and the fuselage. The normal practice to test such bond is to use ultrasonic C-scan to implement a point-by-point inspection on the other side of the stiffened panels, which is tedious and sometimes infeasible when the panels are difficult to access or bonded to honeycomb cores. Ultrasonic guided wave testing can be an effective alternative for bond diagnosis due to its capability of long-range inspection with a fixed transducer position and the flexibility in selecting sensitive mode-frequency combinations. It has been reported that proper guided waves propagating in a parent material can be used to monitor the condition of its bonded components of different materials, such as patches or spars on skins [8], [10], and the sensitive guided modes would normally provide high energy levels in adhesive bonds at operational frequencies.

One particular branch of guided waves confined to local structural variations - feature guided waves (FGW) have attracted much interest in the recent years and been studied in the literature extensively [9], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Guided waves actually not only exist in the regular waveguides, like plates or cylinders, but also exist in the complex-shaped structures, such as metal welds, bends, and stiffeners. Due to the influence of geometric variation, the topographical feature can be a special ‘local’ waveguide, and the energy of proper guided waves can be concentrated in the feature. The physics of such energy trapping effect is that the guided modes in the separate feature have similar mode shapes to the adjacent plate but possess slower phase velocities, causing the wave energy to be constrained in the ‘slower medium’, arising from the total internal reflection [11], [21]. The FGW phenomenon can be very promising for quick inspection of defects in or around long-range features on plate-like structures, as it allows guided waves retaining most of the energy at the features to propagate long distances and also to particularly interrogate such features rather than the entire structure. In this study, we explore the potential of exploiting FGWs for rapid screening of the adhesive bonds between stiffeners and composite skins for possible occurrence of bond line defects, where both geometric variation and anisotropic material properties would influence the propagation of guided waves, thus adding complexity to the FGW based non-destructive testing (NDT).

The paper starts with modal studies of a stiffener attached to a composite panel by using the Semi-Analytical Finite Element (SAFE) method, which allows for solving problems for waveguides with arbitrary cross section [11], [22], [23], [24], [25]. Multiple potential feature guided modes have been identified. The criteria are then suggested to select proper modes that are sensitive to adhesive defects and also have little dispersion and low attenuation. Following that, the defect scattering study using the selected mode is carried out by three-dimensional Finite Element (FE) simulations, and validated by experimental measurements. Finally the paper concludes with the direction for future work.

Section snippets

SAFE modeling

To identify and understand guided modes existing in a stiffened composite panel, the well-known Semi-Analytical Finite Element (SAFE) approach was applied. This approach solves an eigenvalue problem of the solid with constant cross section to obtain wavenumbers along the propagation direction (i.e. x3 axis in Fig. 1) at the chosen frequency. In the SAFE modeling, only the discretization of the cross section is required, and the harmonic behavior is imposed along the waveguide. The formalism for

Selection of FGWs sensitive to adhesive defects

This section evaluates the performance of different FGWs in the composite structure and selects proper mode(s) for the bond line inspection. To achieve this, following criteria are suggested: (1) sufficient power flow in the bond layer; (2) little dispersion; (3) low attenuation; (4) easy to generate. They will be discussed in details in this section.

Defect scattering study using selected mode

In order to further evaluate the suitability of the selected SH(3) mode to the damage detection, and to confirm the optimal frequency range, its scattering by a local disbond has been studied in this section by the 3D FE simulation, to be compared with experimental measurements shown later.

Fig. 8 presents typical time snapshots of the scattered wave field of the SH(3) mode, with the color contour indicating the relative magnitude of the resultant displacement. A notch-like defect (marked in

Experimental setup and procedure

The experimental setup is shown in Fig. 12. The composite plate (dimensions: 500 mm×900 mm×1.6 mm) was manufactured by laminating 8 plies unidirectional carbon epoxy prepregs (Hexcel Inc.), containing the T800S carbon fibers embedded in the M21 epoxy resin, with a quasi-isotropic [0/45/90/45]s stacking sequence. The bonded T-shaped aluminum stiffener has the same cross-sectional geometry as that had been studied in the modeling, and its length is 900 mm. A two-component epoxy adhesive consisting

Conclusions

Intended for the quick inspection of the bond line between a stiffener and a composite panel, FGWs confined to the stiffener region have been investigated by using the SAFE method. Through the modal analysis, the performance of different FGW modes existing in such structure has been evaluated, and the criteria to select proper mode sensitive to adhesive defects are suggested. A SH type FGW (the SH(3) mode) has been identified to be well suited. The mode has sufficient power flow in the bond

Acknowledgment

This work was supported by the Start Up Grant (SUG) from the Nanyang Technological University. The authors are grateful to Dr. Madis Ratassepp for helpful discussions.

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