Composites are made of two or more dissimilar materials which have different physical and chemical properties. These materials are combined to obtain the most desirable properties from the individual elements. The individual components within the finished composite remain separate and distinct, thus differentiating composites from solid alloys and mixtures. There are several types of composite materials such as fiber reinforced composites (FRC), laminated composites and particulate composites. In fiber reinforced composite materials, the polymer matrix is reinforced with high strength fibers. Due to the versatility of FRCs, they have a wide range of applications in aerospace, automotive, medical, electronic and sports equipment industries.
Phases Of FRC Materials
Fibers
Fibers have extremely high tensile strength in their longitudinal direction as the microscopic crystals are aligned along the fiber axis. They serve as the primary load bearing members of the composite. Carbon, boron, fiberglass, aramid, and wood are commonly used fiber materials.
Matrix
A single fiber in itself cannot be used as a structural member. To form a reliable structural member, a group of fibers must be used. Matrix is predominantly used to bind the fibers together and maintain their structural alignment. Matrix in itself cannot carry load, but it distributes the forces to all fibers evenly, thus preventing stress concentration in a single fiber. Low density materials like epoxy, polyester and thermoplastic are commonly used for matrix.
Interphase Region
This is a thin region around individual fibers which determines the bond strength between fiber and matrix in the composite material. This region enables load transfer and energy dissipation between fibers and the surrounding matrix material. Fiber surfaces play a crucial role in determining the strength of the interphase region. Fibers with smooth surfaces typically result in weaker interphases, whereas roughened, treated, or coated fibers significantly improve bonding with the matrix thus creating a stronger interphase region.
Lamination Process
Fibers are the load carrying components in the FRCs. A laminate is a layer of fibers oriented at a specific angle and embedded within the matrix material. The orientation angle of fibers is determined by the specific load-carrying requirements of the application, as fibers serve as the primary load-bearing elements in FRCs. The figure below illustrates various unidirectional fiber layouts within a single laminate.
The mechanical properties can be highly customized in the FRCs by using the lamination process. The single laminate will exhibit strength only along the fiber orientation. Lamination process is used to obtain multidirectional strength, multiple single laminates can be stacked and bonded together. To obtain the desired mechanical properties, fiber layers are arranged in a defined sequence, with varying orientations (e.g., 0°, ±45°, 90°).
After arranging the multiple layers of laminates, resin is applied through methods such as hand lay up (see schematic below) or infusion and the composite structure is compressed to remove air pockets using roller or vacuum bagging.
The structure is then cured using heat and pressure to solidify the resin and bond the laminate layers. The figure below illustrates numerous examples of composite laminate layups.
Advantages Of Using FRC
- Light in Weight – Both fiber and matrix materials are low density components which make the overall composite a lightweight structure.
- Directional Strength – By adjusting the fibers to orient in the desired loading direction, the composite layup can be customized to have specific directional strengths.
- Low Thermal Conductivity – Commonly used fiber materials like glass and graphite have low conductivity. The heat transfer is further hindered by the interphase region between fiber and matrix materials. Polymers used as matrix materials are poor heat conductors thus making the entire composite a low conductivity material.
- High Strength to Weight Ratio – Compared to traditional metals like steel, carbon and glass fibers have exceptionally good strength to weight ratio. Because of this, FRCs can be used in structures with high strength requirement without adding significant weight.
- Non-Magnetic – Both fiber and matrix materials are nonmagnetic making overall composite a nonmagnetic structure unlike metallic structures.
Material Property Calculations
Average material properties of the composite material can be calculated from the properties of the individual fiber and matrix components using the rule of mixtures. This is the simplest model for estimating the material behavior and it assumes that the fibers are perfectly bonded with the matrix.
In longitudinal direction:
In transverse direction:
Where, Ec is elastic modulus of the composite material. Em and Ef are the elastic moduli of matrix and fiber materials respectively. Vm and Vf are the volume fractions of matrix and fiber materials respectively.
Failure Modes
Unlike metals, composites have complex failure modes due to their unique structure. They have multiple failure modes depending on several factors like fiber, matrix, environmental conditions, and type of load applied. Below are a few modes of failure observed in FRCs.
- Ultimate fracture of the fibers.
- Microcracks developing in the matrix material.
- Interface failure between fiber and matrix (debonding).
- Separation of laminate layers
- Environmental degradation due to moisture exposure or temperature changes.
Final Thoughts
Hopefully, this article has provided insights into the general behavior and layup of composite materials. The lamination process is essential for manufacturing high-performance composite materials to meet specific load carrying requirements.
If you have questions regarding composite material behavior in your model or FEA in general, get in touch with us today!
