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Is Kevlar® Stab Proof? How To Test Kevlar Strength

Xomety X
By Team Xometry
February 13, 2024
 12 min read
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Kevlar® is often associated with bulletproof vests. Its molecular structure also makes it viable for stab-proof vests. Kevlar fibers’ exceptional tensile strength allows them to flex and distribute the force of a stabbing motion, reducing the risk of penetration. 

This article will discuss Kevlar’s strength, and the methodologies employed to test and measure its protective value against stabbing forces.

How Strong Is Kevlar?

Kevlar is an incredibly strong and durable synthetic fiber known for its exceptional tensile strength, making it a popular choice for various applications, including stab-proof and bullet-resistant vests. Pound for pound, the tensile strength of Kevlar is significantly higher than that of steel, making it one of the strongest synthetic fibers available. The material's high tensile strength allows it to absorb and disperse the energy of a bullet impact, preventing penetration. 

When used in stab-proof vests, Kevlar demonstrates remarkable resistance to knife attacks. The fibers in Kevlar are designed to flex enough to distribute the force of a stabbing motion but not enough to allow the blade to penetrate the fabric. The layered arrangement of Kevlar fibers in these vests is designed to maximize resistance to punctures, slashes, and stabs.

Kevlar's strength-to-weight ratio is exceptionally high, making it perfect for many unconventional situations. This lightweight characteristic makes Kevlar an ideal choice for wearable protective gear, ensuring comfort without compromising safety.

To learn more, see our full guide on What is Tensile Strength?

Is Kevlar Stab Proof?

No, Kevlar is not entirely stab-proof. Kevlar vests that are advertised as “stab-proof” are realistically designed to resist penetration and minimize the risk of injury from sharp objects. While Kevlar is highly effective against slashing motions and punctures, it can’t provide 100% protection against all stabbing forces. The term "stab-proof" is more of a sales tactic than a guarantee. It's crucial to understand that the effectiveness of Kevlar in stab-proof vests may vary based on the design, thickness, and specific threats the vest is intended to face. 

How To Test Kevlar Strength

There are specific and unique procedures to test Kevlar’s tensile strength and overall durability. Here is a step-by-step instruction list for testing Kevlar’s strength:

  1. Cut uniform samples of Kevlar material, ensuring consistency in size and shape. Samples should be free of defects or irregularities that could impact the test results.
  2. Utilize a universal testing machine designed for tensile strength tests. Ensure the machine is calibrated according to relevant standards and specifications.
  3. Secure one end of the Kevlar sample to the fixed grip of the testing machine. Attach the other end to the moving grip and make sure it’s properly aligned. 
  4. Determine the testing parameters, including the test speed and load rate, based on relevant standards or testing requirements.
  5. Initiate the test by gradually increasing the tensile force on the Kevlar sample. Monitor and record the load and elongation data throughout the test.
  6. Capture key data points such as breaking force, elongation at break, and any visible changes in the material during the test.
  7. Conduct multiple tests on different Kevlar samples to ensure the reliability and consistency of the results.
  8. Calculate the average tensile strength and other relevant parameters based on the data from multiple tests.
  9. Evaluate the Kevlar's performance characteristics, considering factors like: tensile strength, modulus of elasticity, and elongation properties.
  10. Compare the results with industry or product standards to assess whether your Kevlar meets specified requirements.
  11. Document all testing procedures, parameters, and results in a comprehensive report for future reference or quality assurance purposes.

How Does Kevlar Work?

Kevlar employs its exceptional tensile strength and unique molecular structure to provide effective protection in applications like stab-proof vests. 

Kevlar is composed of long-chain polymer molecules with intermolecular hydrogen bonds, creating a strong and flexible structure. The repeating aromatic units in its molecular chain contribute to the material's resilience and enable it to resist stretching and pulling forces. Kevlar's ability to absorb and disperse energy is crucial to its effectiveness as a protective material. When subjected to force, the hydrogen bonds between the polymer chains help distribute the energy throughout the material.

In stab-proof vests, multiple layers of Kevlar are strategically arranged to maximize resistance to punctures, slashes, and other cutting motions. The arrangement of the fibers spreads and disperses the force of a stabbing motion over a wide area of material, minimizing the risk of penetration. This prevents the blade from cutting through the fabric and reaching the body.

What Machine Is Used To Test Kevlar?

The best machine to test Kevlar is a universal testing machine (UTM). It is specifically designed for conducting tensile strength tests on materials like Kevlar. 

A UTM applies controlled tension or compression to a material sample and then measures its response. During the test, a Kevlar sample is mounted in the UTM, and a gradually increasing tensile force is applied until the material reaches its breaking point. The UTM records key parameters such as breaking force, elongation at break, and other mechanical properties. These data points are crucial for evaluating the material's strength and suitability for specific applications.

Is Kevlar the Only Material Used for Body Armor?

No, Kevlar is not the only material used for body armor. While it is common and well-known in this role, several other materials and technologies are employed in body armor to achieve specific performance characteristics. Things like aramid fibers, polyethylene fibers, ceramics, composites, and metal alloys are utilized based on the product’s specific requirements, balancing factors such as: weight, flexibility, and protection level.

What Is Kevlar Material?

Kevlar is a brand name for para-aramid synthetic fiber, well-known for its mix of strength, flexibility, and resistance to wear and tear. Its versatility has led to widespread use in everything from protective gear to industrial conveyor belts, tires, and cables and recreational products like: racing sails, bicycle tires, and canoe paddles.

Kevlar is composed of repeating chains of para-phenylenediamine and terephthaloyl chloride. Its molecular structure includes strong covalent bonds and hydrogen bonds which contribute to its strength and durability. Its crystalline structure and hydrogen bonding give Kevlar a unique combination of tensile strength and resistance to impacts.

Despite its strength, Kevlar is flexible and can be woven into fabrics, making it a go-to option for tough fabrics. Kevlar's strength-to-weight ratio is high, providing robust protection without adding significant weight to the end product. It exhibits resistance to many chemicals, contributing to its durability and suitability for various environments. Kevlar maintains its strength and structure even at high temperatures. 

When Was Kevlar Invented?

In 1965, Stephanie Kwolek was researching polymers at DuPont when she made the discovery that would lead to Kevlar. Her work was focused on developing lightweight and durable fibers for use in tires. Kwolek synthesized a polymer solution that displayed unusual properties. Despite the skepticism of her colleagues, she continued exploring the potential of this unique material. The resulting polymer, later known as Kevlar, became a game-changer in various industries due to its exceptional tensile strength, flexibility, and wear resistance.

DuPont recognized the commercial potential of Kevlar and began commercial production in 1971. Initially used as a replacement for steel in racing tires, Kevlar found more applications over the years. Its introduction had a profound impact on sectors such as: aerospace, automotive, and especially protective gear. 

How Is Kevlar Made?

Kevlar is made through a multi-step chemical process that involves polymerization and spinning to create strong and flexible fibers. The process begins with the polymerization of para-phenylenediamine and terephthaloyl chloride monomers through a reaction. The result is a liquid crystal solution, known as a polymer dope, with long-chain polymeric molecules that form the basis of Kevlar. The polymer dope is then subjected to a spinning process. In this step, the liquid solution gets extruded through tiny holes to create fibers. The spinning process aligns the polymer molecules, contributing to the exceptional tensile strength of Kevlar.

After spinning, the fibers pass through a coagulation bath, typically consisting of a liquid solution. This bath solidifies the fibers and removes the solvent, transforming the liquid crystal solution into a solid form. The solid fibers go through a drawing process where they get stretched to align molecular chains even further. Drawing enhances the strength and stiffness of the fibers again. The drawn fibers are subjected to heat treatment to stabilize their molecular structure and enhance their thermal and mechanical properties. This heat treatment process is crucial for reaching Kevlar’s ultimate strength and flexibility.

Once the Kevlar fibers are produced, they can be cut to the proper lengths and woven into a fabric. This fabric forms the basis for knife-proof vests or other protective gear. In the case of knife-proof vests, multiple layers of Kevlar fabric are often strategically layered to maximize resistance to punctures, slashes, and stabs. The layering and weaving techniques are designed to disperse the force of a stabbing motion, minimizing the risk of penetration.

What Are Real World Applications of Kevlar?

Kevlar finds use across diverse industries. Below is a comprehensive list of its biggest real-world applications:

  1. Body armor, including bulletproof vests and stab-proof vests 
  2. Aircraft and spacecraft components
  3. Tires, brake pads, and as reinforcement in composite materials for automotive parts
  4. Bicycle tires, racing sails, protective helmets, and other sports gear
  5. Conveyor belts, cables, and other strong and chemically resistant industrial equipment
  6. Mobile phone cases, camping gear, and outdoor equipment
  7. Wind turbine components
  8. Firefighting gear 
  9. Boat sails, kayak hulls, and other marine components
  10. Concrete structures, cables, and building components

What Is the Kevlar Standard for Bulletproof Vests?

In the United States, the standard for bulletproof vests that utilize Kevlar is specified by the National Institute of Justice (NIJ). For stab-resistant vests, standards such as the Home Office Scientific Development Branch (HOSDB) in the UK or NIJ Spike Level standards are relevant.

The NIJ Standard-0101.06 outlines the performance requirements and testing methods for ballistic-resistant vests. NIJ standards classify bulletproof vests into different levels of protection (I, II, IIIA, III, and IV) based on the caliber and velocity of projectiles they can effectively stop. Kevlar vests are tested and rated based on whether they meet or exceed these standards. 

Military organizations may have their standards for body armor, often exceeding civilian standards. The United States military, for example, has specific requirements and testing protocols for body armor used by its personnel. In many cases, Kevlar alone does not meet military demands.

To learn more, see our full guide on ASTM International Standards.

What Is the Tear Strength of Kevlar?

The tear strength of Kevlar is high due to its strong molecular and intermolecular bonds. The intermolecular hydrogen bonds and crystalline structure enhance the material's resilience. Kevlar is renowned for its exceptional tensile strength, making it resistant to stretching, pulling, and tearing forces. Despite its high strength, Kevlar is flexible, allowing it to absorb and distribute energy. The combination of flexibility and toughness contributes to its ability to resist tearing and punctures.

The tear-resistant nature of Kevlar makes it a suitable choice for protective gear, including body armor. Kevlar's tear strength ensures that the material can withstand harsh conditions and repeated use without compromising its protective capabilities. Kevlar is widely used in industrial settings where tear resistance is crucial for longevity and performance. Applications such as conveyor belts and cables benefit from the material’s tear strength, ensuring durability in demanding environments. In textile applications, Kevlar is often used as reinforcement in composites and fabrics. Its tear strength enhances the overall durability and longevity of products, so it finds its way into many industries. 

To learn more, see our full guide on Tear Strength Testing.

Is Kevlar Available for All Industries?

Yes, Kevlar’s remarkable properties make it good for a plethora of industries. It is strong, flexible, and resists wear and tear, making it a go-to material for everything from protective gear to aerospace, automotive, sports equipment, and various industrial applications. Its unique combination of properties makes it a valued material for diverse uses.

Are Yachts Made of Kevlar?

Yes, some yachts incorporate Kevlar in their construction. Yachts are often constructed using composite materials and sometimes include Kevlar fibers as a reinforcement component. It can add strength to the hull without adding excessive weight. Additionally, Kevlar does not suffer from the same oxidation problems as other strong materials like steel. Such resistance further enhances the boat’s longevity. 

Yachts, especially those used for racing or other high-performance purposes, gain a competitive edge from Kevlar's impact resistance. The material helps absorb and disperse energy during collisions or encounters with debris in the water. Its superior strength and impact resistance allow vessels to become faster and more agile. Kevlar's lightweight nature is advantageous in yachts that must minimize weight to optimize performance and fuel efficiency. The material's strength allows for the creation of thin but strong hull sections.

Kevlar's flexibility enables innovative yacht design. Builders can incorporate Kevlar fibers into composite laminates, adapting to specific design requirements and achieving a balance between strength and flexibility. 

Summary

This article presented Kevlar's strength, explained it, and discussed whether or not Kevlar is stab proof. To learn more about Kevlar, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

  1. Kevlar® is a registered trademark of E. I. DuPont de Nemours and Company

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.