Hardness Vs. Toughness – What Matters Most in Bullet Resistance?

Bulletproofing is the latest iteration in a long line of systems designed for damage prevention. It reflects the complex relationship between innovations in weaponry and defence, where advancements in one typically lead to steps forward in the other. But the underlying protection concept of bullet resistance is unique in that it went against centuries of conventional wisdom in armouring and led to something of a paradigm shift in warfare.

A Brief History of Armour: Building Up to Bullet Resistance

The history of humans exploiting materials to protect ourselves from harm extends back beyond record. But we know that developments in personal protection arose as a result of specific threats. Leather armour offered little protection from forged metal weapons. Likewise, chainmail might stop a cutting edge but offered little support against a blunt force.

The consensus with each of these developments was that hard armour performed best. Steel plates, for instance, were the gold standard in plating for centuries as they were heavily resistant to localized deformation and could readily resist deformation from cutting, indentation, or scratching. These excellent mechanical properties came at the expense of weight, which would prove fatal on a 20^th Century battlefield where mobility was critical.

Bullet Resistance: What’s the Difference Between Hardness & Toughness?

Hardness is not an intrinsic property. It is a quality we use to define materials as a function of how well they resist localised deformation. To find out how hard a material is, you expose a small portion of it to a load using a hard implement (i.e. an indenter) and measure the resulting impression left in the material. This gives you a good indication of how well it will perform in given conditions; in this case, its degree of bullet resistance. The goal with hard bulletproofing materials is to resist penetration by shattering incoming projectiles. Success here depends on a complex range of interconnected factors, including bullet calibre, velocity, and plate thickness. Abrasion-resistant (AR) steel grades composed of a medium-carbon alloy perform well at minimum thicknesses of ¼”, but—again—this all depends on the situation.

Additionally, deliberately shattering the projectile means there is a risk of peripheral shrapnel generation which can harm bystanders. In some scenarios, plates may completely fragment a bullet but not prevent penetration, meaning shrapnel is projected through to the perceived “safe side” of the plate.

So, hardness has its merits but it isn’t the be-all-and-end-all. Toughness, by contrast, describes the property of being able to absorb energy and deform plastically without succumbing to fracture. This is a better measure of bullet resistance is it can be defined as the amount of energy per unit volume that a material can absorb before it ruptures. As bullet resistance is energy-dependent, this provides greater insight into performance under dynamic conditions; such as those presented by an active shooter.

Hardness and toughness are discrete but connected qualities that, together, provide a good level of bullet resistance. But they are often induced at the expense of one another, with high toughness AR grades of steel typically losing a degree of hardness to promote greater toughness.

The solution to the problem of bullet resistance is to leverage a lightweight composite designed specifically for high hardness and toughness in the event of impacts by rapid projectiles.

Interested in details about our bullet resistant products?

At Armortex, we design cost-effective security solutions designed to combat threats specific to your field. If you would like to learn more about how we tackle the difficult challenges of selecting materials with the right bullet resistance for the job, contact us today.