When working with carbon fibers, known for their excellent strength and stiffness-to-weight ratios, it is important to understand how fiber orientation affects the strength and stiffness of carbon fiber composite (CFRP) laminates. There are different approaches to the laminate design of composite panels, where the laminates are oriented at an angle to produce different structural properties. These properties are: isotropic, quasi-isotropic and anisotropic.
Isotropic refers to a material whose strength and stiffness are the same when measured in any direction through the material. Examples of isotropic substances are glass and metals.
Quasi-isotropic refers to a material that is isotropic but only in a plane. In other words, strength and stiffness are equal in all directions within the plane of the part. Many CFRP laminates fall into this category.
Anisotropy refers to materials that have different strengths and stiffnesses in different directions through the material. For example, wood is harder along the grain than across it. Carbon fiber laminates made with fibers that are all oriented in one direction are extremely anisotropic.
Preparation of quasi-isotropic carbon fiber laminates
When the orientation of the fibers in the layup is balanced, CFRP laminates have quasi-isotropic properties such that their strength and stiffness remain constant regardless of the direction in which the material is loaded. Whether individual carbon fiber plies or plies are made from woven fabrics or unidirectional carbon fibers, they can be assembled into isotropic materials when combined into well-designed laminates.
When the orientations of the plies are balanced, a quasi-isotropic laminate is made such that the tensile stiffness of the laminate is the same in each in-plane direction. Typically, quasi-isotropic sheets are made using a carbon fiber braid with plies oriented at 0º, 90º, +45º and -45º with a minimum of 12.5% in each of these four directions. Unidirectional layers with orientations of 0º, 60º and 120º can also achieve quasi-isotropic properties.
Laminates can be quasi-isotropic and don’t seem to play by the rules. For example, as mentioned above, 0º/-45º/ +45º/90º laminates are quasi-isotropic. However, if each layer is oriented at the same angle to its original orientation (eg +60º), the laminate will remain isotropic. +60º reorientation of laminate to form 60º/15º/-75º/-30º laminate. In addition to the orientation angle, high-quality carbon fiber laminates have other design requirements: the stack must be symmetrical. Additionally, each sheet will typically contain the same fiber-to-resin ratio; have the same layer thickness; the same fiber type and geometry
Quasi-isotropic carbon fiber board design
In a typical quasi-isotropic carbon fiber sheet, the sheet must contain at least 4 layers, and each layer must contain the same stiffness and thickness. Additionally, fiber orientation must be balanced and symmetrical. In addition to collectively providing quasi-isotropic properties, fibers in different orientations have specific strengths
0º Layer: Provides axial strength and stiffness, ideal for beams and columns that must withstand axial loads.
+/-45º ply: Provides shear and torsional strength and stiffness, ideal for torsion shafts and shear webs such as I-beams.
90º Layer: Provides lateral strength and stiffness, ideal for creating consolidation layers that hold everything together and provide the strength of a pressure vessel.
Quasi-isotropic advantage
Carbon fiber sheets can be produced inexpensively by not using quasi-isotropic laminations. For example, if a laminate is built using only 0 and 90 degree carbon fiber layers, the laminate will be weaker along the 45 degree diagonal, with 50% more deflection in off-axis bending and tension than it would be. Quasi-isotropic carbon fiber sheet. In most applications, 0º/90º sheets work well. However, if stiffness along the 45-degree axis is required, quasi-isotropic carbon fiber sheets will provide superior performance. Quasi-isotropic carbon fiber sheets can be made from unidirectional or woven plies with the appropriate lamination direction to produce a stiff material with strength in all directions.
