Delamination and Buckling in 3D Composites

Abstract

Elementary results are applied to the buckling of stitched laminates and woven composites with three-dimensional (3D) reinforcement that contain delamination cracks. The through-thickness fibers are assumed to provide continuous, linear restoring tractions opposing the deflection of the delaminated layer adjacent to the crack. With the boundary condition that the ends of the delaminated layer are clamped and with deflections permitted in one direction only, there exists a characteristic length a0 for buckling: if the length, 2a, of the delamination crack exceeds 2a0, then, when buckling occurs, it will consist of waves of period 2a, and will usually not span the whole delamination. A simple expression is derived for the minimum density of through thickness reinforcement required to suppress buckling of the delamination layer prior to failure by other mechanisms. It is shown that for typical densities of through-thickness reinforcement in current stitched laminates and 3D woven composites, the length 2a0 will rarely exceed the thickness of the delaminated layer by as much as an order of magnitude. Furthermore, the current generation of these materials are probably considerably overdesigned in the quantity of through-thickness reinforcement they contain for suppressing buckling.

Novel carbon nanotube array-reinforced laminated composite materials with higher interlaminar elastic properties

“…The microstructure-property relationship at the interphase region is critical to a successful composite and for that, it is essential to understand adhesion between fiber and matrix. It is accepted that the interlaminar fracture toughness of a laminated composite is determined principally by the properties of the matrix material including its ultimate strain, ability to develop plastic deformation and fracture toughness [3,22,25]. The fiber-matrix interphase properties are not a decisive factor in determining the interlaminar fracture toughness in composites [23,25].…”

“…These composites, still being laminated materials, can experience premature failure. Interlaminar stresses caused by cracks or defects can arise in these composites and their propagation along the intra-layer planes or plies cannot be arrested due to the lack of fibers in the transverse direction to the laminate [1][2][3][20][21][22]. This cracking process between plies is called delamination and represents a fundamental weakness of laminated composite materials [1][2][3]23].…”

“…This constitutes an attempt to mitigate delamination at the macroscopic or continuum level by reducing the buckling load or bridging the cracks. The most efficient reinforcement cases include: i. Three-dimensional (3D) woven fabrics that include a third set of tows with components in the z-direction besides those in the warp and fill directions [22,24,25]. ii.…”

The effects of bridging in a 3D composite on buckling, postbuckling and growth of delamination

“…It is, therefore, required to take into account the effects of bridging. Cox [1] researched initial buckling of one-dimensional delamination with linear bridging and found many useful results, but the results simulated by the classical beam theory appear not to be suf®cient for engineering applications, [8,9]. In this paper we extend Cox's work to a circular thin-®lm delamination constrained by linear restoring forces.…”

“…In recent years, the ®rst aspect has been widely researched, and many useful results obtained have greatly contributed to engineering design, [2±7]. The second aspect however, has not caused much attention so far, [1,8,9]. It is well known that the through-thickness tows in 3D composites can bridge the delamination cracks and prevent buckling of the adjacent delaminated layers; therefore, while the ®bers do not eliminate delaminations, they do minimize the loss of strength under subsequent compression.…”