The applications of composite parts are endless. Given the design freedom and variation which is attainable with different combinations of materials, almost any application and material requirement can be addressed with composites.
Material & Process Selection
The selection of the material and manufacturing process is directly related to the desired properties and use of the finished part. When considering the applications of a composite part, here are some elements to consider during material and process selection:
Stiffness & Strength
The stiffness and strength of a part will be a function of the part composition and the direction of load. Most composite materials are what is known as anisotropic – meaning they have differing mechanical properties in different directions. For example, a composite part with reinforcement fibers parallel to the direction of the applied load, will be orders of magnitude stiffer and stronger than a part where the reinforcement fibers are perpendicular to the applied load. For structural applications, that allows designer to reinforce parts in directions where higher stresses are anticipated. In addition, composite parts allow for certain areas which are not loaded to have fewer plies and areas of higher stresses (bolt holes, joints, and bends) to have additional reinforcement material. This helps drastically bring down the weight of a part without large machining costs. When designing a composite part for stiffness and strength here are some factors to consider:
- Reinforcement: Material, orientation, thickness, plies, content (% vol.)
- Matrix material: Type, content (% vol.), coverage/distribution
- Core Material: Type, thickness, adhesion
Simple analysis techniques can be used to give a rough, upper limit on the stiffness of a composite part. Using the rule of mixtures, an upper limit on the elastic tensile stiffness and density of cured fiber and resin can be established based on the reinforcement material – matrix volume ratio. This model assumes that loads are applied parallel to the fiber direction and that reinforcement material is uniformly distributed in the matrix material. This is known as the same strain, different stress model for a composite:
Where E is the elastic modulus and ρ is the density
These properties can then be used in a simple beam calculation to determine the upper limit on the bending stiffness for a composite beam with a core. This model assumes that the cured reinforcement and matrix skins are far stiffer than the core material, and that there is no slip between the core and laminate skins. It also assumes that shear properties of the core material do not dictate the behaviour of the loaded structure.
The moment of inertia for the composite beam shown below can be given as:
This model can then be used with the properties of the composite skins established from the rule of mixtures and used with standard engineering beam tables to determine an approximate stiffness of a panel. This makes assumptions (such as no transverse expansion/ contraction from Poison Effect) which are not necessary true, but offers a simple method of approximating and comparing stiffness’s and final part weights to metal counterparts. More accurate methods of analysis are available but require more in-depth calculations.
Chemical and UV Resistance
Composite materials provide good resistance to many industrial chemicals and offers exceptional protection from UV light damage.
When selecting a composite matrix material, it is important to ensure there is compatible and that corrosion or chemical attack won’t damage the part. Several online resources are available to see which composite material will meet the needs of your application. The chemical resistance will change based on trade name for resin and the concentration and temperature of the reactive compounds. A few resources and comparisons to standard engineering materials are presented below:
UV light from the sun is known to damage and degrade surface coatings like paint. In addition, in some materials like polypropylene, UV light can cause mechanical properties such as stiffness and strength to be severely reduced.
Pigmented UV resistant gel coats can maintain their color for far longer than paint surfaces in areas of direct sunlight. Specialty resin systems such as polyester resins based on neopentyl-glycol are also available to extend the life of a pigmented surface.
UV stabilizers are often used in extreme applications where long term exposure is expected and the pigment of the composite part is very critical.
The manufacturing processes selected will have a significant effect on the final structural and geometric properties of the part. In addition, the repeatability and variation amongst parts will be larger in some manufacturing techniques compared to others.
Effects on Structure, Performance and Geometry
The strength, stiffness and weight of a composite part is directly related to the control available to the manufacturer in maintaining correct ratios between fibers and resin, vacuum pressure and temperature. In hand laminations, resin is often added onto dry fibers using a roller and is controlled by the laminator. Too much resin and the part will be heavier than anticipated. Too little resin and the parts structural integrity can be compromised.
A study from the Department of Mechanical Engineering at Washington State University conducted by Dave Kim (et al), looked at the variation present in the physical and structural properties of glass reinforced composites in the manufacture of recreational yacht hulls. The study looked at comparing a hand lamination (HL) vs. a low pressure vacuum infusion (VL) (20” Hg) vs. a high pressure vacuum infusion process (VH) (28” Hg) for a typical glass fiber laminate used on recreational yachts.
Effect on physical properties of a sample laminate as a function of manufacturing methods (from Dave Kim et al)
Form the data available, it is evident that hand lamination techniques resulted in parts with a larger proportion of resin content when compared to a vacuum infusion process. In addition, the void space in hand laminates is 3.5 time larger than in a high pressure vacuum infused laminate. This void space is a result of interstitial air which is entrapped in the laminate due to the plies not being compressed together. Voids can also be caused by air bubbles being created as resin is brushed on.
Entrapped air and excess resin also resulted in poor thickness control in this experiment. There is only a 3% variation in thickness between a 20” Hg sample vs a 28” Hg sample, whereas there is a 40% difference in thickness between a hand lamination sample and a 20” Hg sample. This data clearly suggests that the application of even a small vacuum can greatly improve the thickness consistency between parts.
Effect on sample thickness as a function of manufacturing method (adapted from Dave Kim et al)
Air entrapment and voids were seen to causes a drastic reduction in both the tensile stiffness and tensile strength of the samples.
Effect on strength, stiffness and strain to failure as a function of manufacturing methods and vacuum pressure (adapted from Dave Kim et al)
In this study, there was a 60% increase in strength when comparing a high pressure infusion composite to a hand laminate.
When samples were examined upon testing, millimeter scale air voids were seen in the laminate structure of the hand laminated samples.
Air entrapment in a hand laminated sample (from Dave Kim et al)
From this study it is clear that the lamination method and anticipated properties are directly related, and vacuum infusions techniques offer superior part quality over a hand laminated part.