Microstructure and tensile properties of carbon–epoxy laminates produced by automated fibre placement: Influence of a caul plate on the effects of gap and overlap embedded defects

Abstract

a b s t r a c tAutomated fibre placement (AFP) enables the trajectory of unidirectional composite tape to be optimized, but laying down complex shapes with this technology can result in the introduction of defects. The aim of this experimental study is to investigate the influence of gaps and overlaps on the microstructure and tensile properties of carbon-epoxy laminates. First, a comparison between a hand-layup and AFP layup, draped and cured under the same conditions, shows equivalent microstructures and tensile properties. This provides the reference values for the study. Then, gap and overlap embedded defects (more or less severe) are introduced during manufacturing, on two cross-ply layups [(0°/(90°)5/0°] and [(90°/0°)2/90°]. Autoclave cure without a caul plate results in local thickness variation and microstructural changes which depend on the defect type. This has a strong influence on mechanical performance. Use of a caul plate avoids these variations and in this case embedded defects hardly affect tensile properties.

Manufacturing-Induced Imperfections in Composite Parts Manufactured via Automated Fiber Placement

“…Typical primary imperfections within a layup causing fiber angle deviations are gaps and overlaps [48][49][50][51], wrinkles and folds [21,30,37,39,52], pores and inclusions [53] as well as ramps (ply-drop off) [30,54]. The tooling [51,[55][56][57] and processing [21,30,[56][57][58]] significantly influence the development of out-of-plane fiber waviness.…”

“…Potter et al [30] further identify the corner regions of prepreg parts as locations where typically out-of-plane fiber waviness emerges during the autoclave process. Lan et al [56,57] show in their work that the use of a rigid plate instead of a flexible vacuum bag during the autoclave process reduces the formation of fiber waviness. Li et al [51,55] find that out-of-plane fiber waviness caused by isolated gaps and overlaps is mainly influenced by the ply thickness.…”

“…Within the transitions zones of gaps and overlaps (transition from pristine to defective laminate) fiber angle deviations and fiber waviness may occur [48,51,69,[71][72][73][74][75][76]. The imperfection size is the determining factor for the formation of fiber waviness [48,56,57,71,77]. The resulting stress localizations promote damage initiation in the laminate [27,39,49,50,61].…”

Experimental investigation of the effect of defects in Automated Fibre Placement produced composite laminates

“…They modelled the out-of-plane waviness and ply thickness variations caused by gaps and overlaps and predicted the strength knockdowns for various defect combinations. Lan et al studied experimentally the effect of isolated gaps and overlaps in various defect sizes on the tensile [7], in-plane shear and compressive [8] properties. They confirmed results reported by Li et al [6] that defects in composites have a much smaller influence on the mechanical properties when a caul plate was used during laminate curing.…”

“…However, when the tooling geometry becomes more complex, gaps between the tows may be created (Figure 1a) or the tows overlap slightly (Figure 1b). These two types of defects, which have been also studied previously by other authors [5,7,8], were selected for isolated (only gaps or overlaps) and combined (gaps and overlaps) studies to investigate their effect on the mechanical performance. It should be noted that due to the AFP manufacturing method of laying down unidirectional material, the local defect orientation is inherently parallel to the fibre direction.…”