Nonconstrained length effects on the compressive behavior of thick laminated composites

Abstract This study reports the nonconstrained length effects on the properties of thick woven fabric composites under static compression loading. The materials examined were carbon/epoxy composites laminated with 10, 20, and 30 plies of the woven prepreg. A pressure rolling setup was designed and employed in the present materials to compact the laminate and to remove the air possibly trapped in between the plies. To apply the compressive load, an end‐supported, end‐loaded fixture was used. The end‐supported clamps were designed with different heights at both ends for the tested specimens, such that there are various free lengths (14, 26, 38, and 50 mm) in the central portion of the specimen defined as nonconstrained length. The growth of cracks was monitored and recorded in real‐time by picture and video cameras. The continuous monitoring of the failure progression provides detailed information about the onset and growth of cracks in various specimens under different nonconstrained lengths. Compared with the critical test length, various nonconstrained lengths can result in different loading curves with or without postfailure portion. The influence of the nonconstrained length on modulus, strength, the loss of stiffness, and energy absorption density has been examined. Two major modes of failure are fiber kink‐band and buckling, and the appearance of the modes depends on the nonconstrained length. Finally, the relationships among the nonconstrained length, the resulting properties, and the induced failure modes are discussed. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers