Mechanical performance of carbon-fibre and glass-fibre-reinforced epoxy I-beams: II. Fractographic failure observations

Abstract

This present paper is the second in a series which together detail the static behaviour, fractographic observations, fatigue behaviour and finite element predictions of composite I-beams subjected to mechanical loads. Fractographic observations associated with the mechanical behaviour under static load of both unnotched and web- and flange-notched continuously reinforced carbon-fibre/epoxy and E-glass-fibre/epoxy I-beams are discussed. Ultrasonic scanning, X-radiography and both optical and scanning electron microscopy have been used to elucidate the presence of different damage mechanisms and the directions of delamination growth in different regions of the beams. The principal damage mechanisms which have been identified as causing failure are delamination, matrix cracking, splitting and fibre fracture. As discussed in detail in the previous paper, a four-point flexural configuration was used. A mode of buckling that was antisymmetric across the width of the compressive flange was observed prior to failure in all cases. Failure of the unnotched I-beams initiated from a buckle on the compressive flange and the subsequent damage was predominantly in the form of delamination. The main delaminations were along the interfaces between the separate sub-components which comprise the I-beams: namely, the flange caps and C-sections and the backs of the two C-sections. These are all −45 ° + 45 ° interfaces i.e. the relative fibre angle between the adjacent plies is 90 °. Failure of the notched I-beams initiated from a shear-loaded circular cutout within the web. The critical damage mechanism was matrix cracking in local 90 ° 90 ° plies which were subject to local tensile stresses. Fibre fracture and component failure resulted from this matrix cracking.