Edge machining effects on the failure of polymer matrix composite coupons

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Abstract

This papers deals with the influence of the machining procedure of carbon/epoxy and glass/epoxy composite coupons on their mechanical performances. First, a literature review is presented to summarize the rather small number of studies dedicated to this subject. Then, machining procedures are detailed and evaluated by surface roughness measurements and microscopic observations. Finally, results of Iosipescu shear tests and ring tension tests are reported and discussed. The main conclusion of this study is that there is a significant effect of machining procedure on composite coupons strength for certain sets of parameters but that surface information (roughness, microscopic observation) is not enough to explain them. Further work is necessary on this subject to derive relevant machining quality criteria with a view to mechanical performance.

Introduction

The design of reliable and cost-effective polymer-matrix composite structures requires the accurate determination of design allowables such as elastic moduli, strengths, etc. To determine these properties, small coupons are often used that require preparation including edge cutting/machining. However, it is of primary importance to understand how the coupons surfaces are affected by the cutting operations. Indeed, industrial composite components undergo very limited cutting or machining operations. Therefore, measuring design allowables on coupons having an important ratio between cut surfaces and tested volume may result in underestimated design allowables, leading to overdesign of the structures. Besides, understanding the effects of cutting on resulting composite properties will also lead to a better knowledge of mechanical property reduction due to local machining of industrial components (drilling, contouring etc.)

Section snippets

State of the art

Surprisingly, not very many contributions have addressed the above problem; most of the studies tend to concentrate on the relationship between cutting conditions and surface quality, similarly to what is generally used to assess the machinability of metallic materials. Nevertheless, the heterogeneous and anisotropic nature of fibre composites strongly affects the mechanisms of chip formation, as is clearly detailed by Wang et al. [1]. As a consequence, severe local damage can be caused by

Materials

The composite materials used in this study were:

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    Composite n°1: unidirectional E-glass/epoxy panels made from Vicotex® BEM10 prepreg with a 52% volume fraction;

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    Composite n°2: unidirectional T300J carbon/epoxy panels autoclaved from Structil© CTE013.2DA508T300J prepreg with a volume fraction of 60±2%;

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    Composite n°3: E-glass/epoxy tube obtained by filament winding. The winding angle is 55° with respect to the tube axis, the internal and external diameters are, respectively, 55 and 67 mm. The fibre

Machined specimens analysis

In order to assess the damage produced by the cutting action on the specimens surfaces, different observations have been performed. First, surface roughness measurements have been performed to check whether the statements from the literature survey that surface roughness is not a relevant damage indicator for composites are true. Then, optical microscopy has been used to observe macroscopic and mesoscopic damage. Finally, scanning electron microscopy (SEM) osbservations have been performed to

Mechanical testing: procedures and results

Two types of tests have been performed: Iosipescu shear test on the V-notched specimens and diametral tension on the ring specimens.

Conclusion

The present study is the first, to the knowledge of the authors, to investigate the effect of machining conditions on the strength of Iosipescu and thick ring composite specimens. The main conclusions are:

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    as also mentioned by previous authors, surface roughness is not a relevant indicator to assess machining quality of fibre composites, particularly when mechanical performance is the issue;

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    it is extremely difficult to relate microscopic surface observations to loss of mechanical behaviour. This

Acknowledgements

The present authors would like to thank Dr Marc Legrand (LTVP, ENSAM Lille) for providing the carbon/epoxy panels, Mr Marc Wary (ENSAM Metz) for the SEM observations and Dr Peter Davies (Ifremer) for his support of the work on ring testing.

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