Multi-frequency local wavenumber analysis and ply correlation of delamination damage

Highlights

• Chirped guided wavefields are recorded via vibrometry on a delaminated composite.

• Local wavenumber technique is applied to the data over multiple frequencies.

• A multi-frequency delamination depth correlation method is described and applied.

• The results are multi-frequency determination of damage size, shape, and depth.

Abstract

Wavenumber domain analysis through use of scanning laser Doppler vibrometry has been shown to be effective for non-contact inspection of damage in composites. Qualitative and semi-quantitative local wavenumber analysis of realistic delamination damage and quantitative analysis of idealized damage scenarios (Teflon inserts) have been performed previously in the literature. This paper presents a new methodology based on multi-frequency local wavenumber analysis for quantitative assessment of multi-ply delamination damage in carbon fiber reinforced polymer (CFRP) composite specimens. The methodology is presented and applied to a real world damage scenario (impact damage in an aerospace CFRP composite). The methodology yields delamination size and also correlates local wavenumber results from multiple excitation frequencies to theoretical dispersion curves in order to robustly determine the delamination ply depth. Results from the wavenumber based technique are validated against a traditional nondestructive evaluation method.

Introduction

The expanded use of carbon fiber reinforced polymer (CFRP) composite materials is expected to continue in the aerospace industry due to the benefits of high strength, low weight, and tailorability [1]. Large-scale use of composite materials also leads to unique inspection challenges since strength-reducing damage can occur internally with little surface indication [2], [3]. Nondestructive evaluation (NDE) techniques are required to accurately detect and quantify any existing damage that may be exist due to manufacturing defects, low velocity impacts (such as falling tools), and in-service events (such as impact from ground vehicles or bird strike, for aircraft). Commonly used traditional NDE techniques for inspecting composite materials include ultrasonic C-scans and thermographic imaging [4], [5], [6].

Numerous authors have also studied guided wave based techniques for detecting damage in plate-like composite components, such as those commonly used in aerospace. Sparse piezoelectric sensor arrays have been studied for detection of composite damage in structural health monitoring (SHM) type setups [7], [8]. These prior studies have shown that sparse sensor arrays have promise for locating damage regions in composites using a small number of sensors, but may be limited in ability to size and characterize damage. Laser Doppler vibrometer (LDV) and air-coupled ultrasound techniques have been suggested as non-contact measurement approaches to further characterize composite damage in a type of hybrid NDE/SHM method where in situ piezoelectric sensors could be used to excite guided wavefields that are recorded for further analysis [9], [10]. Current commercially available single laser beam scanning LDV systems, including 3-dimensional (3D) LDV systems that have a single laser beam for each of the three laser sensor heads, can be time consuming and require point-by-point wavefield capture [11]. However, future multi-beam LDV systems with a grid of laser beams per scanning head may allow for a practical, rapid method for wavefield capture [12], [13].

While time domain wavefield data can qualitatively reveal the interaction of waves with any existing damage, a variety of post-processing wavefield analysis techniques have been explored as a means to obtain quantitative damage information from the wavefield data. Recent techniques reported in the literature for wavefield data processing include wavenumber domain filtering, frequency-wavenumber and space-wavenumber analysis, energy analysis, instantaneous wavenumber, and local wavenumber techniques [14], [15], [16], [17], [18], [19], [20], [21], [10], [22], [23]. Ruzzene used frequency-wavenumber filtering to remove incident waves from LDV wavefield data for a metallic plate to observe reflections from damage [14]. Michaels et al. later utilized both air-coupled and scanning LDV techniques to record wavefield data in metallic and composite plates. The LDV data was found to be more broadband and contained less ringing. The team then used frequency-wavenumber filtering to remove incident waves, enhancing wave scattering from crack and delamination damage [15]. Yu and colleagues used frequency-wavenumber and space-wavenumber analysis to identify and study guided wave modes created due to the presence of damage, including trapped waves above delamination damage [16], [17]. Leckey et al. used space-wavenumber analysis to study mode conversion and propagation in multi-modal scenarios with up to eight different guided wave modes interacting with damage in metal plates. Energy analysis methods have been used by both Sohn et al. and by Leckey and Seebo to observe trapped guided waves above delamination damage. Leckey and Seebo identified differences in trapped energy for guided waves interacting with multilayer (multiple ply) delamination damage versus single layer damage and less severe multilayer damage [21]. Instantaneous and local wavenumber techniques have been explored as methods for processing wavefield data to quantitatively characterize delamination damage in CFRP composites and thinning type damage in metals [10], [22], [23].

This paper expands on prior work using the local wavenumber technique to characterize delamination damage in CFRP composite laminates. Previously, the local wavenumber technique was used to quantitatively determine the size and depth of an idealized single delamination type damage (Teflon insert) for a single excitation frequency [23]. The technique was also used to characterize impact delamination damage in a CFRP composite laminate at two different excitation frequencies [10]. In the latter work, the technique allowed for approximate delamination sizing and a rough association of wavenumber to delamination depth, but a direct measure of ply depth and delamination characterization was not rigorously performed for the complex, multi-ply delamination damage. The work presented in this paper utilizes chirped guided wave excitation to enable multi-frequency local wavenumber analysis in order to more robustly characterize impact induced delamination size and depth in a CFRP aerospace composite.

The order and content of the remainder of this paper is composed as follows: Section 2 details post-processing wavefield analysis procedures including a new multi-frequency local wavenumber approach and a proposed method for identifying delamination ply depths. Section 3 describes an experimental case to which the wavefield analysis procedures are applied and discusses the effectiveness of the techniques in determining delamination sizes and depths. Section 4 summarizes the findings presented in the paper.