SAMPE Journal - July/August 2012 - (Page 24)

Feature Article Out-of-Autoclave Sandwich Structure: Processing Study S. Nagarajan, V. G. K. Menta and K. Chandrashekhara Missouri University of Science and Technology, Rolla, MO T. R. Berkel The Boeing Company, St. Louis, MO J. Sha and P. Wu Spirit AeroSystems, Wichita, KS D. Pfitzinger GKN Aerospace, St. Louis, MO Abstract Composite sandwich structures offer several advantages such as reduced weight, high bending/torsion stiffness, superior thermal insulation and excellent acoustic damping over conventional structural materials. One failure mechanism in a composite sandwich structure is the debonding of the composite facesheets from the core structure. A well-formed adhesive fillet at the interface of the honeycomb core cell walls and the laminate is a significant factor in preventing bond failure. Improved adhesion strength with a well-formed adhesive fillet at the interface of the honeycomb core walls and laminate is the aim of sandwich composite manufacturing, and is one area of investigation in this work. Out-of-autoclave (OOA) processing of composites offers several key benefits compared to autoclave processing such as lower manufacturing cost resulting from a lower capital cost and lower energy consumption. In the present work, the effects of moisturized adhesive film, core type, material system and variable levels of vacuum on adhesive fillet formation are evaluated. Aluminum honeycomb and vented aluminum honeycomb are used as core materials in this study. Two material systems of prepregs and adhesive films, Cycom 5320 prepreg with FM 309-1M adhesive, and MTM45-1 prepreg with MTA-241/PK13 adhesive are used to manufacture the panels and evaluate the performance. A quantitative investigation of the adhesive fillet geometry is carried out for all the panels. Manufactured panels are evaluated for peel strength and flatwise tensile strength in accordance with ASTM standards. Various combinations of core material, moisture content, material system and vacuum level are evaluated for effects on adhesive strength and adhesive foaming. Introduction Composite sandwich structures are employed in aerospace and marine industries as well as in other applications that require high strength, high in-plane and flexural stiffness coupled with a reduction in weight. Sandwich composites are made of relatively stiff, strong and thin facesheets (which absorb most of the in-plane loading and bending) separated by a thicker flexible core (which absorbs most of the shear load) allowing for improved mechanical performance and reduction in weight when compared to conventional composites. Design and application of sandwich composites depends on thorough characterization and understanding of the sandwich constituent materials and also the structure as a whole under quasi-static and dynamic loading1,2. Sandwich composites used in the aerospace industry are typically manufactured using high pressure autoclave processing which has several disadvantages such as large capital investment cost, long cycle time, high operating cost and low energy efficiency3. The main constraint however is the ability to manufacture large composite structural components which require autoclaves of sufficiently large sizes. An example mentioned by Hou et al.4 is NASA’s Constellation program Ares V cargo launch vehicle which requires large composite structures with sizes up to 10 m in diameter. In the past few decades, several studies have been devoted to developing non-autoclave manufacturing techniques that can significantly reduce the manufacturing cost of composites. The “out-ofautoclave (OOA) vacuum bag only process” has emerged as one viable manufacturing technique. OOA offers several benefits such 24 as low investment cost and greater design flexibility to manufacture large, highly loaded structural members with complex geometries. The OOA technique utilizes atmospheric pressure of 14.7 psi (101 kPa) and temperatures of up to 350°F applied to a vacuum bag. The issues with OOA curing are in the elimination of porosity and the compaction of the lay-up to give a high resultant fiber volume fraction. In sandwich structures, the dominant failure mode is local instability of the facesheet, resulting in either de-bonding or buckling from the core. The failure of the structure due to de-bonding at the facesheet-to-core interface has been studied by numerous authors. Studies conducted by Grove et al.5 have shown that higher debonding energy can be obtained with larger, regular shaped adhesive fillets between the honeycomb cell walls and the skin. Larger fillets are also known to absorb a considerable amount of energy upon fracture and thereby increase the fracture toughness value. Hayes et al.6 have shown that apart from size, quality of the fillet also plays a role. Adhesive fillet failure is observed to be one of the frequent and critical failures observed in helicopter rotor blades. Rion et al.7 have shown that failure occurs in the adhesive meniscus when low weight adhesive is used. Authors observed that the mechanical performance of sandwich structures is dependent upon the quality of the core-tofacesheet adhesive bond. In the case of honeycomb cores, the cell walls provide a relatively small area for bonding and hence the adhesive bond and adhesive fillet formation is of great importance in this design. Formation of a well formed adhesive fillet at the interface of the SAMPE Journal, Volume 48, No. 4, July/August 2012

Table of Contents for the Digital Edition of SAMPE Journal - July/August 2012

SAMPE Journal - July/August 2012
Table of Contents
President’s Message
Technical Director’s Corner
Development of New Lightweight Hybrid Sandwich Cores using FDM Technology
Corporate Partners
Europe News & Views
SAMPE Europe’s SETEC 2012, Lucerne
SAMPE Journal Editorial Calender
Welcome SAMPE’s Newest Members
Materials & Products
Perspectives–The Art of a Well-Crafted BHAG
Out-of-Autoclave Sandwich Structure: Processing Study
SAMPE LinkedIn Communities
SAMPE Proceedings
SAMPE 2013|Long Beach Call for Papers
SAMPE Tech 2012|Charleston, SC
Industry News
Composite Payload Fairing Structural Architecture Assessment and Selection
SAMPE 2012|Baltimore-A Review
SAMPE 2012|Baltimore Photo Gallery
The SAMPE Foundation
Advertiser’s Index
Resource Center
SAMPE Membership Application
SAMPE Books & CD’s Order Form
Industry Events Calendar
Chapter Meetings Dates and Contacts

SAMPE Journal - July/August 2012