Air-coupled ultrasonic C-scan technique in impact response testing of carbon fibre and hybrid: glass, carbon and Kevlar/epoxy composites

https://doi.org/10.1016/j.jmatprotec.2004.07.143Get rights and content

Abstract

The ultrasonic air-coupled C-scan technique and X-ray radiography were used to detect impact damage in thin carbon fibre/epoxy composite plates. The sizes of damage zones were found in good agreement for both techniques employed. The presence of a polymer film on the sample surface has been shown to be useful in detecting the impact damage. Air-coupled C-scan technique was selected for defect size estimation in further studies of impact and post-impact response of hybrid laminates for naval structures. Reinforcements of carbon and glass, woven Kevlar/carbon and Kevlar/glass fabrics have been studied. Minor differences in post-impact compression performance of laminates, in which carbon fibres have been partly replaced by glass fibres, may indicate that regarding the impact behaviour, carbon fibre laminates can be safely replaced by cheaper carbon/glass or Kevlar glass laminates and woven Kevlar/carbon laminates by Kevlar/glass ones.

Introduction

Advanced polymer composites are finding increased use in a wide range of both low- and high-technology engineering applications. Composites offer a number of distinct advantages over more conventional engineering materials, such as aluminium and steel. These include higher specific strengths and stiffness, superior corrosion resistance as well as improved fatigue properties. Coupled with these improvements in overall performance, it is the fact that the cost of manufacturing components from fibre-reinforced plastics is often less than that of more conventional metals. This is also true in the aerospace sector where complex load-bearing shapes can be produced in a limited number of steps, saving the time in both joining and assembly [1]. Laminated polymer composites have also been widely used in shipbuilding industry for four decades now.

Composite materials do, however, suffer from some serious limitations. Perhaps the most significant among these is their response to localised impact loading such as that imparted by a dropped tool or runway debris. In recent years, many researches have been undertaken in an attempt to better understand the impact response of these materials and to predict the post-impact load bearing capability [2]. However, the damage zone in a composite structure is generally complex in nature and consequently, very difficult to characterise. Typical impact damage appears in the form of matrix cracking, fibre matrix debonding, fibre shear-out and fibre fracture. In particular, the internal delaminations, often generated due to the relatively low interlaminar shear strength is not visible on the impact surface and will grow under subsequent compressive loading, which can lead to a significant reduction in post-damage performance. Due to this reason, in part, the current design criteria limit allowable strain is at a very low level (e.g. 0.3%, which is an eighth to a third of the fibre failure strain) so that much of the weight-saving potential is lost. Therefore, it is of vital importance to have better understanding of the impact response of composite laminates and of their structural performance in the presence of impact damage in order to realize their potential [3].

Impact response of laminated composites is characterised by the size of impact damage produced in the material. It is, therefore, necessary to use the effective technique of impact damage development. The main technique used to evaluate the impact-induced damage area is the ultrasonic C-scanning technique [4]. However, classical ultrasonic C-scans are processed in water tank to ensure good coupling between the transducers and the tested samples, so implying contact and also a risk of water penetration in the specimen. In the current work, a recent technique has been used in which water is no longer necessary as a coupling medium owing to the replacement of traditional transducers by new air-coupled transducers. This is an important achievement in NDT testing, which facilitates and in some cases, enables performing the tests on real structures. The air-coupled ultrasonic C-scans performed in the present study were conducted on the experimental set-up, which uses original, air-coupled, ultrasonic transducers made at the LMP (Laboratoire de Mecanique Physique) and manufactured by a French company (Fogale-Nanotech) [5]. No contact with the tested composite plates is required using this system.

In the present study, the experiments have been performed first on thin (1 mm) carbon fibre laminates made of unidirectional fibres in order to estimate and compare the results of impact damage testing by X-ray and air-coupled ultrasonic C-scan techniques, and to justify the selection of ultrasonic technique for further studies on more complex materials. Then the behaviour of hybrid laminates—candidates for use in shipbuilding—has been studied in terms of impact damage size and post-impact residual compressive strength.

Characterisation of overall performance of hybrid laminates is a complex task, and therefore has rarely been studied so far. There exists a great deal of variables: type and form of fibres (unidirectional, woven), number of plies, stacking sequence, fibre volume fraction, technological parameters, etc. It is, therefore, necessary to conduct extensive and detailed studies of different laminates in order to select the best ones in terms of required properties as well as cost effectiveness. The present study is limited to the characterisation of impact resistance in terms of damage surface area and impact damage tolerance expressed by residual compressive strength after impact of three different laminates: hybrid Kevlar, carbon and glass/epoxy and a carbon laminate for comparison. The term damage tolerance refers to a system's ability to perform post-impact. Residual strengths in tension, compression, bending and fatigue are reduced to varying degrees depending on the dominant damage mode. As delaminations are the dominant mode of low-energy impact damage in carbon fibre laminates, large strength reductions in compression occur for these materials [6]. It was, therefore, justified to study damage tolerance in terms of residual compression strength after impact in the first place.

The results presented in this paper are a part of a broader study of new laminates for small naval structures. It is particularly important to introduce new fibrous composite materials into Polish shipbuilding industry since it is still dominated by traditional cheap glass/polyester laminates or materials reinforced with expensive carbon or Kevlar fibres, whereas it is the hybrid reinforcement of the composites that offers new solutions in terms of improved performance and reduced price of the materials.

Section snippets

Experimental techniques for impact damage assessment

The damaged zone in a composite structure is generally complex in nature, consisting of fibre breakage, delaminations and matrix cracking. In most cases, particularly with low velocity impacts, a large part of the induced damage is internal and cannot be detected simply by examining the surface of the laminate. Since glass/epoxy and Kevlar/epoxy composites transmit light, damage can be observed in such laminates using strong backlighting. The size and shape of delamination and the presence of

Materials

Two types of laminates have been fabricated in this study. For initial, comparative studies of impact damage development by means of air-coupled C-scans and X-ray technique, thin (g = 1 mm) carbon fibre laminates were fabricated from continuous, unidirectional carbon fibre tape (0.3 mm thick), impregnated with epoxy resin Epidian 6 (produced by Organika Sarzyna plant, Poland) by hand lay-up technique following the stacking sequence (0°, ±45°, 90°). The surface of the laminates was covered with

Ultrasonic and X-ray tests of thin carbon fibre laminates

The results of C-scan and X-ray tests of thin, carbon laminates have been shown in Fig. 2, Fig. 3, Fig. 4. At impact energy of 0.6 J slight indentation of the surface of the specimen has been visible. Ultrasonic C-scanning revealed the presence of a small damage zone (Fig. 2) but no damage was observed by X-radiography.

At incident energies of 1.8, 3 and 5.8 J, the area of disbonding between the polyethylene film and the composite surface layer has clearly been observed. The C-scan images

Impact damage response of hybrid laminates

The reasons for different behaviour of the laminates can be explained when considering the plots of residual compression strength after impact versus impact damage area, (Fig. 11) and comparing with the analysis of macro and micrographs of the damage area. It can be seen that poor performance of Kevlar/glass laminates in terms of extent of delaminations is compensated by very good damage tolerance in terms of compression strength after impact, much better than for Kevlar/carbon composites and

Conclusions

The experiments performed on thin carbon laminates lead to the following conclusions.

  • (1)

    Since the measurements of damage area revealed by two methods are in good agreement, ultrasonic air-coupled C-scanning technique and X-radiography can be used to assess the impact damage area in carbon fibre/epoxy composites.

  • (2)

    In order to observe delaminations in X-ray radiograms, a penetrant opaque to X-rays must be injected in the damage zone.

  • (3)

    Provided there is suitable equipment, the air-coupled technique seems

References (19)

  • Yasunobu Hirai et al.

    Impact response of woven glass fabric composites – 1. Effect of fibre surface treatment

    Compos. Sci. Technol.

    (1998)
  • M.O.W. Richardson et al.

    Review of low velocity impact properties of composite materials

    Composites

    (1996)
  • G. Zhou

    Damage mechanisms in composite laminates impacted by a flat-ended impactor

    Compos. Sci. Technol.

    (1995)
  • W.J. Cantwell et al.

    The impact resistance of composite materials – a review

    Composites

    (1991)
  • B. Hosten et al.

    Etude et caractérisation de transducteurs a couplage par air pour l’évaluation et le contrôle non destructifs des matériaux. Instrumentation Mesure Métrologie

    (2001)
  • G. Dorey

    Impact damage tolerance in advanced composite materials

    Seminar on Advanced Composites

    (1986)
  • S. Abrate

    Impact on laminated composite materials

    Appl. Mech. Rev.

    (1991)
  • T.H. Gan et al.

    The use of broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging

    Ultrasonics

    (2001)
  • F.L. Mattews et al.

    Composite Materials. Engineering and Science

    (1994)
There are more references available in the full text version of this article.

Cited by (0)

View full text