Quasi Isotropic

‘QUASI-ISOTROPIC’ DEFINITION “isotropic” means that the material stiffness and doesn’t depend on dırectıon(same all dırectıon). 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 constant. Quasi-isotropic laminates behave like isotropic materials at the laminate level though their individual plies are orthotropic or anisotropic with respect to the reference coordinates. In case of quasi-isotropic laminates, only the elastic in-plane properties are isotropic. The strength properties, however, will vary with direction. QUASI- ISOTRPIC RULES 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 QUASI-ISOTROPIC MATERIAL ADVANTAGES  Use unidirectional ply orientations to make material strong and stiff in all direction  “quasi-isotropic” laminates provide essentially “isotropic” design parameters which often make life much easier for analysts.  A quasi-isotropic layup [0/45/90/-45] is able to support axial, transverse, and shear loading DESIGN OF QUASI- ISOTROPIC MATERIAL •It must have three layers or more. •Individual layers must have identical stiffness matrices and thickness The layers must be oriented at equal angles. For example, if total number of layers is ‘n’, the angle between two adjacent layers should be 360º/n. If a laminate is constructed from identical sets of three or more layers each, the condition on orientation must be satisfied by the layers in each set, for example, (0º±60º) or(0º/±45º/90º) 

0 deg. layers - provide axial strength and stiffness, ideal for beams and colums that have the design purpose of resisting axial loads.

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+/- 45 deg. layers - provide shear/torsional strength and stiffness, ideal for torsion shafts and shear webs (I-beam webs).

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90 deg. layers - provide transverse strength and stiffness, used primarily as a consolidating layer (keep everything together) and to provide most of the structural resistance in pressure vessels.