Elsevier

Composite Structures

Volume 139, 1 April 2016, Pages 30-35
Composite Structures

Experimental evaluation of the low-velocity impact damage resistance of CFRP tubes with integrated rubber layer

https://doi.org/10.1016/j.compstruct.2015.11.069Get rights and content

Abstract

The impact performance of structural components made of fibre-reinforced plastic is often one of the limiting properties during the design process. To improve the damage resistance regarding transverse low-velocity impact loading, a rubber layer (KRAIBON®) is integrated into the composite laminate of tubular carbon/epoxy specimens. Numerous impact tests, using two different rubber compounds and three different layups, are carried out. The specimens are impacted using a modified Charpy pendulum. Force–time histories have been used to determine the damage threshold load. To visualise damage such as delaminations and inter-fibre failures, the impacted samples have been examined using microsectioning.

It is shown that a significant improvement in impact damage resistance can be achieved by integrating a rubber layer into a carbon/epoxy laminate.

Introduction

In many industries, the low impact damage resistance of Fibre-Reinforced Plastics (FRPs) is one of the major obstacles to the application of FRPs. Even a “low-energy impact” can lead to a severe weakening of the load bearing capacity as a result of Inter-Fibre Failures (IFF) and delaminations caused by the concentrated out-of-plane loads. Particularly a low-velocity impact loading causes damages inside the laminate, which are difficult to detect by visual inspections, the so-called barely visible impact damages (BVID). Such a low-velocity impact can be caused for example by tool drops during maintenance or by foreign object impacts like hail or stone chip. The fact, that a hardly detectable damage inside a FRP-laminate can have a major impact on the load bearing capacity of the structure, makes it necessary to have precise knowledge of the fracture behaviour of the composite laminate.

In addition to the efforts to improve the calculation methods and failure criteria for a better prediction of the damage initiation and propagation as a result of impact loads, the improvement of the impact damage resistance of FRPs has been the subject of the investigations of many researchers for a long time. Nash et al. [1] give an extensive overview of different methods to improve the impact and post-impact performance of carbon fibre-reinforced composites (CFRP). Good results could be achieved by increasing the toughness of the matrix. This can be accomplished for example by the addition of liquid rubber [2], [3] or thermoplastic particles [4], [5]. Furthermore, the use of thermoplastic interlayers leads to good results, for example for layers consisting of thermoplastic fibres [6], [7] or thermoplastic films [8], [9]. The aim of these methods is to modify either the entire matrix material or the interfaces between the individual layers to improve the impact resistance determining material properties such as the interlaminar shear strength (ILSS), the interlaminar fracture toughness (ILFT) and the matrix fracture toughness without a significant increase of mass. However, these approaches lead in some cases to the degradation of other material properties such as the Younǵs modulus and often the procedures for implementing these approaches are very laborious.

In many applications of FRP only a small area of the structure is faced to a high risk of impact loading. These are for example the leading edges of wings and propeller blades or the downtube of a bicycle frame. For this reason the local use of a surface layer or an interlayer in impact-prone areas can be advantageous. This method is not in competition with the above mentioned methods, but can rather be understood as a supplement. A very common method is to use thin metal layers to protect the underlying laminate. This type of composite is called fibre-metal laminate (FML). An overview of numerous experimental and numerical studies on this method is given by Chai [21]. A disadvantage of this approach is the high density of the metal layers and the accompanying large increase in areal weight. A positive aspect is the additional protection against erosion. Experimental studies of Sarlin et al. [10] show a decrease in damaged area and studies of Düring et al. [11] show a significant increase in damage threshold load by the additional use of a rubber layer in combination with a steel layer. This application is mainly used in the field of aviation.

The increasing use of CFRP for bicycle frames in particular in the field of mountain bikes makes the topic “impact performance” also interesting for the sports equipment industry. In the worst case a low-velocity impact can lead to a non-visible damage within the laminate resulting in a reduction in lifetime of the bicycle frame. However, especially in the sports equipment industry, externally mounted protective layers are not desired for optical reasons. Investigations regarding this topic have been carried out by Kaiser [12]. He used a transparent protective film (ScotchguardTM PU 8591E) to improve the damage resistance. The investigations show, that already at the lowest investigated impact energy of 0.5 J no improvement in impact resistance can be achieved.

In the present work a rubber layer is integrated inside a CFRP laminate to improve the impact resistance by a better load distribution in the laminate. Moreover, by placing the rubber layer within the laminate the demand for the non-visibility is satisfied.

In order to assess the effectiveness of this approach and to validate numerical models, extensive experimental studies are needed. Most of the experimental impact-tests were carried out on plates. However, the majority of lightweight structures has curvatures. Based on analytical and numerical investigations of Ramkumar et al. [13] and Kim et al. [14] it can be concluded, that curved structures are significantly more susceptible to damage caused by impact loads than flat plates. Experimental investigations of Ehrlich [15] confirm this knowledge. This finding leads to the conclusion that experimental studies should be carried out with a specimen’s geometry close to the applicatiońs one. Since the aforementioned impact-prone areas are highly curved and so far only a few impact tests with tubular samples were carried out [18], the experimental investigations were performed with tubular specimens and not with plates. To improve the damage resistance of these tubular specimens, two different rubber compounds were studied, wherein the rubber layer was placed at three different positions within the laminate.

In the past few years many impact tests on FRP were performed. Some surveys of impact tests have been published by Agrawal et al. [16] and Cantwell et al. [17]. Detailed descriptions of the impact test methods, damage detection methods and impact mechanics in general are provided by Abrate [19], [20]. To perform low-velocity impact tests two methods have been established. Whereas the more frequently used method is the drop-weight impact-tower, the second established approach uses a pendulum impact tester (Charpy pendulum). In the present work a modified Charpy pendulum is used. The damage inside the laminate after the impact is examined by microsectioning.

The aim of the present work is to determine the increase of the impact damage resistance by integrating a rubber layer into a CFRP laminate and to provide a basis for a subsequent verification and improvement of numerical models and failure criteria. The focus is primarily on identifying the impact energy, which the respective laminate stands without damage and not on the determination of differences with regard to the extent of damage or the absorbed energy.

Section snippets

Specimen preparation and materials

The cylindrical specimens (length: 200 mm, outer diameter: 60 mm, thickness: 1.5–2 mm) were made of 10 unidirectional plies of carbon fibre/epoxy prepreg (Krempel KUBD 1507). According to the manufacturer, the epoxy resin system “BD” is a toughened modified one with high impact strength.

The protective layer (KRAIBON®) with a nominal thickness of 0.5 mm is made of an Ethylene Propylene Diene Monomer (EPDM). It was integrated during the prepreg lay-up process without any additional treatment. The

Test procedure

Fig. 1 shows schematically the test device consisting of a modified Charpy impact pendulum and a Polytec data management system. The Charpy pendulum is equipped with a piezoelectric load cell (Bruel & Kjaer Type 8200) to detect force–time histories and a spherical impactor with a diameter of 14 mm. The force signal can be recorded at sample rates up to 204.8 kHz. The tubular specimen is supported by a specimen support, comprising half of the circumference of the specimen. A gap (40 mm) in the

Results and discussion

In this section, experimental force–time histories and section cut views of the different specimen types are compared to assess the capability with respect to an improvement in impact damage resistance.

Conclusions

In the present study, a rubber layer (KRAIBON®) has been integrated into a carbon/epoxy prepreg laminate to improve the impact damage resistance. Two rubber compounds with different properties were placed at three different positions within the laminate. The impact tests have been performed with a modified Charpy pendulum.

By evaluating the force–time histories it has been shown, that the tolerable impact energy mainly depends on the rubber compound, while the softer rubber compound tends to

Acknowledgements

This work has been partially funded by the BMWi under the project KF 2054304-GZ9. Furthermore, the authors would like to thank the Chair of Physical Metallurgy and Materials Technology of the Brandenburg University of Technology Cottbus-Senftenberg (BTU-CS) for the support and the Kraiburg GmbH for the rubber and the material data.

References (22)

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