Mechanical performance of carbon-fibre- and glass-fibre-reinforced epoxy I-beams: III. Fatigue performance

This paper is the third in a series which describe in detail the static and fatigue mechanical behaviour and fractographic observations of the failure of composite I-beams. Experimental investigations into the fatigue respons e of both unnotched and web- and flange-notched continuously reinforced car bon-fibre/epoxy and glass-fibre/epoxy I-beams are discussed in the present paper. A four-point flexural configuration was used to test the I-beams, ea ch of which had the same multidirectional stacking sequence consisting of a balanced layup of 0, +45 and -45 degrees plies. In no cases were the beams fatigued above the loads at which buckling of the compression flange would occur under static testing. The unnotched carbon/epoxy and glass/epoxy bea ms did not exhibit any detectable damage within 1.2x10(6) and 8x10(6) cycle s, respectively. The notched beams, on the other hand, failed as a result o f the tensile and compressive stresses which were present around the web no tches. Damage mechanisms, including delamination, matrix microcracking and fibre fracture, were all present at final failure around the web notches, a lthough the extent of the different mechanisms varied during the fatigue li fe of the I-beams. (C) 1999 Elsevier Science Ltd. All rights reserved.