Composites Materials

What are Composites?

Composite materials are engineered materials made from two or more constituent materials with significantly different physical or chemical properties that remain separate and distinct at the macroscopic level within the finished structure. Essentially, we're talking about combining different materials – like a strong fiber (reinforcement) embedded in a binding material (matrix) – to create something that has properties superior to those of the individual components acting alone. The key is that the individual components retain their identity within the composite, contributing their strengths to the final product.

History

The concept of combining different materials for enhanced properties isn't new. Nature has been doing it for years – think of wood, which is a natural composite of cellulose fibers in a lignin matrix. Historically, humans have also utilized composite principles. Mud bricks reinforced with straw date back thousands of years, providing increased strength and preventing cracking. Ancient Egyptians used linen and plaster for mummy cases, and Mongols utilized layers of wood and sinew to create incredibly strong and flexible bows. The modern era of engineered composites began in the early 20th century with the development of Bakelite (a phenolic resin) and later, fiberglass in the 1930s, which saw significant use during World War II. The aerospace industry's demand for lightweight and high-strength materials spurred further advancements in the latter half of the 20th century, leading to the sophisticated composites we see today, like carbon fiber reinforced polymers.

Matrixes and Reinforcement 

The magic of composites lies in the synergy between its two primary components: the matrix and the reinforcement. 

Matrix: This is the continuous phase that surrounds and binds the reinforcement together. Its primary roles are to transfer stress to the reinforcement, protect the reinforcement from environmental damage, and maintain the shape of the composite. Common matrix materials include polymers (like epoxy, polyester, and thermoplastics), metals (such as aluminum and titanium), and ceramics. The choice of matrix depends on the desired properties of the final composite, such as temperature resistance, toughness, and processing requirements. 

Reinforcements: These are the discontinuous phase embedded within the matrix, providing strength and stiffness to the composite. They are typically in the form of fibers (like fiberglass, carbon fiber, aramid, and natural fibers), particles, or flakes. The type, orientation, and volume fraction of the reinforcement significantly influence the mechanical properties of the composite. For example, continuous, aligned carbon fibers in an epoxy matrix can create a material with exceptionally high tensile strength and stiffness. 

Application and Benefits 

Composite materials have revolutionized numerous industries due to their unique combination of properties, leading to a wide range of applications and significant benefits: 

High Strength-to-Weight Ratio: This is a key advantage, allowing for lighter structures without sacrificing strength, leading to fuel efficiency and easier handling in various applications. 

Corrosion Resistance: Many composites, particularly those with polymer matrices, are resistant to corrosion from moisture, chemicals, and salt, making them ideal for marine environments. 

Design Flexibility: Composites can be molded into complex shapes, offering greater design freedom compared to traditional materials. This allows for optimized aerodynamic or hydrodynamic designs. 

Durability and Fatigue Resistance: Composites can withstand repeated loading and unloading without significant degradation, leading to longer service life in demanding applications. 

Composites in the Marine Industry 

The marine industry has embraced composite materials, recognizing their unparalleled ability to withstand the demanding and corrosive marine environment. Unlike traditional materials like steel and wood, which are vulnerable to rust, rot, and marine organisms, composites – particularly fiber reinforced polymers – offer exceptional durability and longevity in saltwater and freshwater applications. This inherent resistance translates to significantly reduced maintenance costs and extended service life for marine assets. 

Beyond durability, a significant advantage of utilizing composites in the marine industry is the scalability of production enabled by molding techniques. Once a mold is created – whether for a hull, deck or a small part of a boat – identical parts can be replicated with high precision and consistency. This replicability allows for efficient series production, bringing down manufacturing costs per unit and enabling the adoption of composite technologies across various segments of the marine market. 

Manufacturing Processes 

The way composite materials are made is crucial to achieving the desired properties and shapes. Numerous manufacturing processes exist, each suited for different types of composites, part geometries, and production volumes. 

In the marine industry two of these processes are widely used across a vast range of boats types and sizes, named: 

Hand Lay-up/Spray-up: These are open-mold techniques where reinforcement materials are placed in a mold, and resin is applied manually or by spraying. They are suitable for large and complex parts, often used for boat hulls and prototypes. 

This is a widely used process due to its simplicity and faster coverage in the case of spray-up, however it is a very labor-intensive process which results in a lower fiber to resin ratio affection strength and weight and with higher emissions. 

It’s highly useful to smaller parts where replicability is desired and weight and strength are not crucial. 

Infusion: A closed-molding process where dry reinforcement materials are laid up in a mold, and a vacuum bag is sealed over the top. Once a vacuum is drawn, resin is pulled into the laminate through strategically placed feed lines, thoroughly wetting the fibers. 

Although it involves a higher initial setup cost, with vacuum pumps, bagging materials and a few consumables, it provides higher fiber to resin ratio, resulting in a stronger, lighter and stiffer part along with low void content and lower emissions as well as more consistent quality, once the resin flow can be controlled. All these factors make the infusion process the go to manufacturing process for larger and more critical parts, such as hull, decks and superstructures. 

Author: Herbert Berckenhagen - Senior Structural Engineer