Engineering Fundamentals | PA-EF-004

Understanding Spall

One of the primary objectives of bullet-resistant glazing is not
only to resist penetration by the specified ballistic threat, but
also to address what happens on the protected side of the glazing
during impact.

Overview

When a projectile strikes a glass-containing ballistic laminate,
the impact can fracture the glass and potentially release fragments
from the protected side. This phenomenon is commonly referred to as
spall.

Understanding spall is important because two glazing systems capable
of resisting the same ballistic threat may behave very differently
on the protected side after impact.

The appropriate level of spall protection depends on the performance
specification, laminate construction, application, and overall
security requirements of the project.

What Is Spall?

During a ballistic impact, extremely high localized stresses are
introduced into the glazing.

Glass layers may fracture as the laminate absorbs and distributes
the projectile’s energy. Depending on the construction, fragments
of glass or other material can be released from the rear, or
protected, side of the glazing.

This rear-face fragmentation is generally referred to as
spall.

Spall should not be confused with complete projectile penetration.

A laminate may successfully resist penetration by the specified
ballistic threat while still producing rear-face fragments.
Conversely, a construction can be specifically engineered to
control or prevent hazardous fragment release while also providing
the required ballistic resistance.

These are related, but distinct, aspects of ballistic performance.

Why Does Spall Occur?

Glass is a hard and relatively brittle material.

During ballistic impact, complex stress waves travel through the
laminate as energy is transferred among the individual layers.
Glass may fracture extensively even when the overall laminated
assembly successfully prevents penetration.

Interlayers help retain fractured material and maintain continuity
within the laminate, but rear-face behavior depends on the complete
construction.

Factors that can influence spall behavior include:

• Glass type and thickness
• Number and sequence of glass plies
• Interlayer type and thickness
• Presence and location of polycarbonate or other polymers
• Overall laminate architecture
• Projectile characteristics and impact energy
• Number and location of impacts
• Applicable test requirements

For this reason, spall performance cannot be determined simply by
looking at the overall thickness of a laminate.

Engineering Principle

Stopping the projectile and controlling what happens on the
protected side are separate engineering objectives. A complete
ballistic glazing design considers both.

Low-Spall Construction

Low-spall laminates are engineered to
control and reduce rear-face glass release
during a ballistic event.

These constructions use carefully selected glass, interlayers, and
laminate geometry to retain fractured material and limit the amount
of spall projected toward the protected side.

Unlike a true no-spall construction, some controlled rear-face
glass release may still occur.

Low-spall systems may be selected when engineers are balancing
ballistic performance with considerations such as weight, optical
quality, exposed-surface durability, maintenance, thickness, and
overall system design.

The important point is that

low spall describes rear-face behavior; it does not mean lower
ballistic protection
.

A properly designed and tested low-spall laminate can provide the
specified ballistic resistance while allowing a defined or
acceptable degree of rear-face fragmentation.

Adding a Spall Shield

There is another approach that sits between a conventional
low-spall construction and what we commonly think of as a
traditional no-spall laminate.

A spall shield may be added to the protected-side
glass surface of an otherwise low-spall construction when
additional rear-face fragment protection is required.

The underlying laminate can retain the basic characteristics of
its low-spall construction while the added spall shield helps
contain fragments that might otherwise be released from the
protected-side glass surface.

This gives engineers another tool for addressing rear-face
performance without necessarily changing the laminate to a
traditional no-spall glass-clad polycarbonate construction.

It is important, however, to evaluate the complete tested
construction. Adding a spall shield should not be assumed to create
a particular performance classification unless that configuration
has been evaluated against the applicable requirements.

No-Spall Construction

No-spall laminates are engineered to

prevent hazardous rear-face fragment release during the specified
ballistic event
.

Traditional no-spall constructions commonly incorporate
polycarbonate toward the protected side of the laminate, with a
mar-resistant polycarbonate surface serving as the exposed interior
face.

Polycarbonate contributes toughness, energy absorption, and
penetration resistance while providing a non-glass protected-side
surface that helps prevent fractured glass from being released into
the occupied area.

No-spall performance may be required because occupants or equipment
are positioned close to the glazing, but

proximity is not the only reason no-spall protection may be
specified
.

Government, defense, detention, institutional, OEM, or other
project-specific requirements may call for no-spall performance
regardless of occupant proximity.

The required construction should therefore be determined by the
applicable performance requirements rather than solely by where
occupants are expected to stand.

Low Spall, Spall Shield, and No Spall Are Different Engineering
Approaches

It can be tempting to think of these constructions as a simple
progression:

Low Spall → Spall Shield → No Spall

But that can imply that one is inherently better than another.

A more useful way to think about them is as

different engineering approaches to rear-face protection
.

A low-spall construction may be appropriate when controlled
rear-face fragmentation is permitted.

A spall-shield construction may provide additional fragment
containment while preserving other characteristics of the
underlying laminate.

A traditional no-spall construction may be appropriate when
hazardous rear-face fragment release must be prevented and the
overall construction meets the project’s other requirements.

The correct solution depends on the specification and complete
application.

Spall Performance Is Part of a Larger Design Decision

Rear-face protection is important, but it is rarely the only
consideration in laminate selection.

Engineers may also need to evaluate:

• Ballistic threat
• Applicable test standard
• Weight
• Overall thickness
• Optical quality
• Exposed surface material
• Scratch and abrasion resistance
• Environmental exposure
• Cleaning and maintenance
• Thermal performance
• Framing compatibility
• Edge engagement
• Panel size
• Long-term service life

Changing the protected-side construction to improve spall
performance can affect some of these other characteristics.

This is why laminate selection should be based on the complete set
of project requirements rather than on a single performance
characteristic.

How Is Spall Evaluated?

Spall requirements depend on the applicable ballistic standard,
specification, and test method.

Depending on the test protocol, evaluation may involve determining
whether material is released from the protected side and whether
that material produces evidence of potentially hazardous rear-face
fragmentation.

The exact definition of acceptable performance must therefore come
from the applicable standard or project specification rather than
from the terms low spall or no spall alone.

That distinction is important.

Those terms are useful for describing laminate behavior and
construction families, but the

test result and applicable performance criteria ultimately
establish whether a glazing system satisfies the requirement
.

Applications

Low-spall systems may be appropriate where controlled rear-face
glass release is permitted and the design places greater emphasis
on other requirements such as exposed glass durability, maintenance,
optical performance, weight, thickness, or system configuration.

Systems incorporating additional spall protection may be used when
the project requires greater rear-face fragment control while
retaining other characteristics of the underlying laminate.

No-spall systems may be required where the applicable specification
calls for protection from hazardous rear-face fragments, regardless
of occupant proximity.

The building or market type alone should not determine which
construction is appropriate.

The performance requirement should drive the construction.

Key Takeaways

• 
  Spall is rear-face fragmentation produced during ballistic
  impact.
  
• Projectile penetration and rear-face spall are related but
  separate performance considerations.
• Low-spall laminates are engineered to control and reduce
  rear-face glass release; they are not inherently lower in
  ballistic protection.
• A spall shield can provide an additional method of controlling
  protected-side fragmentation in certain laminate constructions.
• Traditional no-spall systems commonly use polycarbonate toward
  the protected side to prevent hazardous glass release.
• No-spall protection may be specified regardless of occupant
  proximity.
• Low-spall, spall-shield, and no-spall constructions should be
  viewed as different engineering approaches rather than a simple
  hierarchy of good, better, and best.
• The applicable test standard or project specification ultimately
  determines acceptable rear-face performance.
• Spall performance should be considered together with weight,
  optics, durability, maintenance, thickness, framing, and the
  other requirements of the complete system.

Continue Learning

• PA-EF-001 — What Is Bullet-Resistant Glass?
  Review the fundamental principles behind ballistic glazing and
  the different families of bullet-resistant constructions.
• 
  PA-PT-002 — Low-Spall Bullet-Resistant Laminates
  
  Explore how low-spall laminate systems are engineered and where
  they may be appropriate.
• 
  PA-PT-003 — No-Spall Glass-Clad Polycarbonate
  
  Learn how glass-clad polycarbonate systems use protected-side
  polycarbonate to provide ballistic protection while controlling
  rear-face fragmentation.
• 
  PA-MC-001 — Understanding Polycarbonate in Security Glazing
  
  Explore why polycarbonate’s toughness and energy-absorbing
  characteristics make it an important component in many ballistic
  glazing systems.
• PA-TS-001 — Understanding UL 752
  Learn how ballistic-resistant equipment is evaluated against
  defined projectile threats and test conditions.