SAMPE Journal - May/June 2012 - (Page 48)

Tech Tidbits Dr. Scott W. Beckwith SAMPE International Technical Director and President, BTG Composites Inc. swbeckwith@aol.com Quasi-Isotropic Composite Materials/Structures Design – Part I The term “quasi-isotropic” is often used in the world of composite materials and structures design. To an engineer knowledgeable of metallic materials, the term “isotropic” infers that the material stiffness and strength in all directions are essentially the same. While it is true that some metal forming or casting processes induce some “preferred” orientation directions due to the fabrication process, in essence, the primary properties (Young’s modulus, tensile strength, compression strength) are independent of direction when it comes to loading or testing. Hence, the term “”quasi-isotropic” is a term applied to fiber-reinforced composite materials when the orientation and lamination stack-up sequence is such that the resultant material behaves like an isotropic material. That is to say, it does not matter which direction one pulls the material in that its stiffness and strength will be essentially constant. So the first question is – what material orientation and lamination sequence might lead to being “quasi-isotropic” with respect to continuous fiber-reinforced composites. It turns out that there are essentially two that are well established and used frequently by designers. Those lay-ups are: • Laminations using 0, +45, -45 and 90 degree layers, and, • Laminations using 0, +60 and -60 degree layers There are other “quasi-isotropic” rules in addition to just selecting the orientation angles. The first is that the layer combination must be balanced: • For every +q angle above the laminate mid-plane, there must be a -q angle below the laminate mid-plane The second rule pertains to symmetry of the location of each laminate orientation: • For every layer above the laminate mid-plane, there must be the same exact layer located below the mid-plane – and – furthermore, each of these layers must have: o The same fiber-resin volume fractions, o The same layer thickness, o The same fiber type and geometry, and, o Be located the same distance from the mid-plane Hence, a “quasi-isotropic” laminate might look like the following two examples: • 0/+45/90/-45/-45/90/+45/0 (8-plies) • 0/+60/-60/-60/+60/0 (6-plies) Note that the 8-ply laminate using the 0/±45/90 angles would be thicker than one using a 6-ply laminate using the 0/±60 angles – assuming similar construction materials. One might therefore think a 6-ply system would be more useful in satellite structures where a more lightweight structure is required because it is 25 percent thinner and thus lighter. Hence, “quasi-isotropic” laminates provide essentially “isotropic” design parameters which often make life much easier for analysts. The above discussion notes that there are, however, rigid stacking rules and material characteristics which must be adhered to. In the next Part II we will discuss some concerns regarding (a) combining certain angle sets together, (b) using “quasi-isotropic” design as a “black aluminum” design approach, and (c) failing to use composites to their maximum potential in order to counter highly direction induced loads within the structure. 48 SAMPE Journal, Volume 48, No. 3, May/June 2012

Table of Contents for the Digital Edition of SAMPE Journal - May/June 2012

SAMPE Journal - May/June 2012
Contents
President’s Message
Technical Director’s Corner
Improving mechanical Performance of Automotive Underbody Shield with Unidirectional Tapes in Compression-Molded Direct-Long Fiber Thermoplastics (D-LFT)
2012 SAMPE Lubin Award
SAMPE Europe’s SETEC 2012, Lucerne
2012 SAMPE Fellows
Europe News & Views
SAMPE Europe’s SEMAT 2012, Munich
The SAMPE Foundation
SAMPE Journal Editorial Calender
SAMPE Proceedings
Co-molded, Metallic components for Thermal Barrier Protection in TRM Composite Valves for Internal Combustion Engines
Corporate Partners
Welcome SAMPE’s Newest Members
Materials & Products
SAMPE LinkedIn Communities
SAMPE Tech 2012|Charleston, SC
Perspectives
Industry News
Tech Tidbits
SAMPE 2013|Long Beach Call for Papers
SAMPE 2012|Baltimore
SAMPE 2012 Exhibition
SAMPE Asia 2012 Review and Photos
SAMPE Books & CD’s Order Form
SAMPE 2012 Exhibitors
SAMPE 2012 Exhibitor Listings
Advertiser’s Index
Resource Center
Membership Application
Industry Events Calendar
Chapter Meetings Dates and Contacts

SAMPE Journal - May/June 2012