“Spall” is chipping or fragmentation from a solid body due to anything from erosion to a forceful impact. In ballistic glass, spall is fragmentation of the glazing as a result of a bullet impact.
As you can imagine, flying shards of glass on the safe side of the pane can absolutely be a danger to anyone within the fragmentation radius. This is why it is critically important to know what kind of bullet spalling you can expect from bulletproof glass and your low- and no-spall product options.
A Demonstration of the Dangers of Spalling
In the image below, you can see spalling on the secure side of the bulletproof glass. The glazing has stopped the bullet, but the spall is hitting a thin piece of metal 18 inches behind the glass.
If you expect to have personnel or sensitive items near the glass, we recommend considering a low- or no-spall glazing option to reduce or eliminate spalling.
What Causes Bullet Spalling in Ballistic Glass?
When a bullet enters the pane of glazing, a tremendous amount of energy must be dispersed in order to stop the bullet. When the bullet meets the first layer on the “threat” side, it may explode with fragmentation—this is spalling.
Subsequent layers experience less and less energy. The final layer, on the “secure” side of the glass will spall depending on the type of material used in the construction of the glazing panel. Some materials produce more spall than others. Some, like acrylic and actual glass, can produce a significant spall. Other materials, such as laminated polycarbonate, typically produce no spall at all.
UL 752 Acceptable Bullet Spalling Parameters
UL 752 is the gold standard for bullet resistance. For most people, the important numbers are the levels of bullet resistance. What many people don’t know is that the UL 752 also outlines allowances for spalling in bullet-resistant glass systems.
To meet the UL 752 standard, any spall must not cause injury to someone approximately 18 inches behind the secure side. This is determined by placing a 1/8 inch corrugated cardboard panel, called a witness plate,18 inches behind the secure side. If any material fragments embed into or damage the cardboard, the material fails to meet the UL 752 standard.
There are additional parameters for allowable spalling in specific situations that are not covered in this article. To learn more about spalling, the level of spalling various products might produce, and what that means for your security project, talk to a security professional at TSS.
Low- and No-Spall Bulletproof Glass Options
If you’re in the market for ballistic glass systems, spalling should be a consideration.
When comparing your bulletproof glass options it is important to understand that both acrylic and glass-clad polycarbonate (GCP) windows can spall, while laminated polycarbonate windows typically do not spall.
For applications where you expect to have at least 18 inches of distance between personnel and the secure side of your bulletproof windows, a low-spall glazing option minimizes the spalling that may occur.
The advantage of a low-spall glazing is the presence of actual glass on both the threat and secure side of the window. This provides the best durability, scratch resistance, and allows for a wider range of cleaning products to be used. The addition of a glass pane on the secure side however does increase the cost of low-spall glazing.
When you expect personnel to be in close proximity to a bulletproof window, a no-spall glazing option is the right choice. A polycarbonate back-sheet on the secure side of a no-spall GCP bulletproof window absorbs the energy transferred from the bullet’s impact. The back-sheet is a flexible protective barrier against any potential spalling. See the below illustration for an example of no-spall GCP in a live-fire demonstration.
In a laminated polycarbonate window, there is a polycarbonate sheet on both the threat and secure side that is a protective barrier against spalling.
While this no-spall option provides superior protection, the softer polycarbonate panel on both the threat and secure side is less resistant to everyday wear than actual glass and can be damaged by ammonia-based cleaning products.
For a comprehensive list of our various products, please review our specifications page, which has the glass-clad polycarbonate (GCP) products labeled low-spall and no-spall as applicable.
The Experts At TSS Can Help You Choose the Right Spall Level
When purchasing GCP for your ballistic applications, you want to make sure you’re choosing a product that will keep your people safe. This includes taking spalling into consideration and weighing the pros and cons of the various styles of glazing. Make sure you download the free Ballistic Glazing Solutions Guide for more details about GCP and other bullet-resistant glass.
The best way to make sure you’re making the right choice is to get an expert to weigh in on your application and threat level. The ballistic experts at Total Security Solutions can help you understand your options and find the solution that is right for your situation. Reach out to us if you have any questions or if you need information about your specific security project.
