MECHANICAL PERFORMANCE OF CARBON-FIBER AND GLASS-FIBER-REINFORCED EPOXY I-BEAMS .2. FRACTOGRAPHIC FAILURE OBSERVATIONS

This present paper is the second in a series which together detail the static behaviour, fractographic observations, fatigue behaviour and f inite element predictions of composite I-beams subjected to mechanical loads. Fractographic observations associated with the mechanical beha viour under static load of both unnotched and web- and flange-notched continuously reinforced carbon-fibre/epoxy and E-glass-fibre/epoxy I-b eams ave discussed. Ultrasonic scanning, X-radiography and both optica l and scanning electron microscopy have been used to elucidate the pre sence of different damage mechanisms and the directions of delaminatio n growth in different regions of the beams. The principal damage mecha nisms 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 compr essive flange was observed prior to failure in all cases. Failure of t he unnotched I-beams initiated from a buckle on the compressive flange and the subsequent damage was predominantly in the form of delaminati on. The main delaminations were along the interfaces between the separ ate 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 degrees-+45 degrees interfaces, i.e. the relative fibre angle between the adjacent plies is 90 degrees. Failure of the notched I-beams init iated from a shear-loaded circular cutout within the web. The critical damage mechanism was matrix cracking in local 90 degrees/90 degrees p lies which were subject to local tensile stresses. Fibre fracture and component failure resulted from this matrix cracking. (C) 1996 Elsevie r Science Limited