AIAA JOURNAL Vol. 44, No. 4, April 2006 Core Crush Problem in Manufacturing of Composite Sandwich Structures: Mechanisms and Solutions H. M. Hsiao, ∗ S. M. Lee, † and R. A. Buyny ‡ Hexcel Corporation, Dublin, California 94568 Manufacturing of composite honeycomb sandwich structures is signiﬁcantly impacted by poor production yields caused by the core crush problem that occurs during the autoclave curing process. It is a major manufacturing defect that leads to costly part rejects because the defects are nonrepairable. This failure mechanism also constrains aircraft engineers, limiting the design range of core density and core thickness in attempts to mitigate core crush. Recent studies that have led to basic understanding of core crush mechanism are discussed. It was found that the prepreg frictional resistance is the key factor in controlling core crush. Research in the scientiﬁc community has mainly focused on resin effects in core crush. However, studies conducted show that core crush can also be signiﬁcantly reduced by controlling construction of the ﬁber tow shape and the fabric architecture. Rounder ﬁber tow or more open fabric produces rougher prepreg surface, which results in a higher prepreg frictional resistance, reducing the effects of core crush. Experimental results indicate that a developed core crush resistant prepreg increases the prepreg frictional resistance and effectively reduces core crush, without changing the resin system. Nomenclature F btm = frictional force between bottom skin prepreg ply and tool plate F P–B = frictional force between prepreg and bagging material F P– P = frictional force between prepreg and prepreg F P–T = frictional force between prepreg and tool plate F stiffness = combined stiffness of core and uncured prepreg plies F top = frictional force between top skin prepreg ply and bagging material P net horizontal = horizontal mechanical driving force for core crush Introduction C OMPOSITE honeycomb sandwich structures are widely used in the aerospace industry as panel parts in various aerospace structural applications such as ribs, ﬂaps, spars, and rudders. They are typically formed from a layup of prepreg skin plies sandwich- ing over a honeycomb core. The panel edge closeouts are often designed with honeycomb edges chamfered to a constant tapering angle (Fig. 1). Despite the aerospace industry’s long production ex- perience in manufacturing such structures, the manufacturing pro- cess still suffers from a signiﬁcant reject ratio due to a particular type of manufacturing defect, core crush, as known in the indus- try. Core crush is caused by the collapse of the honeycomb core in its weak lateral directions during autoclave curing of the sand- wich structure, especially when the core density is low (Fig. 2). It Received 7 June 2005; revision received 9 October 2005; accepted for publication 9 October 2005. Copyright c  2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rose- wood Drive, Danvers, MA 01923; include the code 0001-1452/06 $10.00 in correspondence with the CCC. ∗ Senior Scientist and Laboratory Manager, Research and Technology; currently Senior Research and Development Engineer, M/S S240, Endovas- cular Solutions, Guidant Corporation, 3200 Lakeside Drive, Santa Clara, CA 95054. † New Technology Manager, Research and Technology; currently Senior Staff, P.O. Box 748, MZ-9382, Advanced Development Programs, Lockheed Martin Aeronautics Company, Fort Worth, TX 76101. ‡ Senior Staff Scientist, Research and Technology, 11711 Dublin Boulevard. is among the costliest manufacturing problems in composite fab- rication. The crush of the panel is generally so extensive that it is beyond repair. Figures 3a and 3b show an example of a core-crush composite honeycomb sandwich panel from the side view and top view, respectively. Core crush occurs during the autoclave curing process when the honeycomb sandwich structure is subjected to pressure and heat well before the thermosetting resin in the prepreg skins is cured. The pres- sure difference between the autoclave pressure and the vacuum in the core provides the mechanical driving force for core crush. Whereas the honeycomb core is usually strong enough to resist such a pressure difference by itself in its thickness direction, it contributes relatively little in its lateral direction to counter the pressure-induced resultant force component in that direction. The resisting forces developed to prevent the core from lateral collapse are internal core pressure, the lateral compressive stiffness of the skin/core combination, and the prepreg frictional resistance. 1,2 When core crush occurs, slippage is initiated and the prepreg plies move inward with the core. The ele- vated temperature during curing facilitates the slippage of prepreg plies by lowering the resin viscosity and providing the lubricating effect. 3 Several techniques have been developed in the past to restrain the core from collapsing inward. For example, Corbett and Smith 4 used tie-down plies in contact with the core to prevent core crush in sandwich structures. These tie-down plies were extended beyond the trim line of the ﬁnished product and secured to the layup man- drel with tape. Hopkins and Hartz 5 found that Corbett and Smith’s tie-down method did not eliminate core crush because the tie- down plies could occasionally pull away from the tape. They later developed an improved tie-down method. These approaches im- pact the manufacturing process, increasing labor, production time, and material cost and, therefore, may not be the most desirable solution. Core crush is a complex phenomenon, where every component and manufacturing operation such as the following can contribute to the failure mechanism: raw material selection (ﬁber, resin), fabric/prepreg processing (tow forming, weaving, impregnation, postimpregnation, and part manufacturing (curing cycle, vacuum level, layup, bag/tool surface). Previous studies on the root cause of core crush have mainly focused on the resin effects. 6,7 The leading conclusion from these studies indicated that resin strongly controls core crush. At the similar degree of impregnation, a highly rubber- ized low-ﬂow epoxy prepreg consistently showed greater core crush resistance than a standard high-ﬂow epoxy prepreg. However, our studies 8,9 have demonstrated that core crush can also be dramatically 901 Downloaded by NATIONAL TAIWAN UNIVERSITY on February 4, 2014 | http://arc.aiaa.org | DOI: 10.2514/1.18067