Vitro Certified^TM Newsletter

Breaking It Down
Why Did the Glass Break?

By Timothy Bellovary, Technical Services Engineer, Vitro Architectural Glass

Its beauty can turn heads.

From the functional to the fantastic, glass delivers on numerous levels in contemporary building designs—aesthetically, performance-wise, quality and energy efficiency, to name a few.

It’s no wonder that architects make it the centerpiece of building designs. As the focal point of high-visibility structures, expansive glazing areas and large insulating glass units (IGUs) are taking up more space on curtain walls, storefronts and interior building components than ever before.

Despite its numerous appearance and performance advantages, though, glass has its breaking point.

Primary Causes of Glass Breakage

Although a relatively rare occurrence, glass breaking after being installed in high-rise buildings raises several questions beyond just the immediate need to replace the unit or units. Was it a one-time event, or part of a pattern of similar breaks in that particular glass product? Did the glass shatter into hundreds of pieces or simply crack? Was the glass properly installed? What role did weather play?

Although these questions are typically part of a more in-depth investigation, any incident always demands an answer to this fundamental question: Why did the glass break?

Like many brittle materials, glass, which has much higher compressive strength than tensile strength, breaks or fails when stresses approach or exceed the material’s limits. Put another way, when an external force applied to a glass product exceeds its maximum strength, the product will fail.

A prevalent contributing factor of breakage is edge damage. Even minor defects on the edge of a glass panel can reduce its strength by more than 50 percent, compromising its ability to resist both thermal and mechanical loads.

Although edge damage in and of itself doesn’t break the glass—after all, the glass may remain functional throughout its service life—it creates a weak point that may make the glass susceptible to conditions that increase the likelihood of breakage.

In some instances, the cause of breakages can be traced to inclusions, which can be non-homogenized, or not fully melted, batch materials or contaminates such as nickel and aluminum. Two types of inclusions are nickel sulfide and aluminum oxide “stones.”

Many inclusions “behave” the same way glass does: They expand in heat and contract in cold. Although most inclusions do not directly cause breakages, their size, composition and location are the primary factors in the small percentage of inclusion-related incidents that actually occur.

Identifying the Break Origin

When there is a pattern of repeated glass breaks, it’s critical to explore the reasons for such incidents. Typically initiated by the building owner or glazing contractor, this investigation must start with identifying the origin of the break. This is possible only if pieces of the broken glass can be retained or re-assembled into the original orientation.

Identifying the break origin can provide hints about the following:

• Mode of glass failure—Was it mechanical or thermally induced stress?
• The stress or tension level at which the breakage occurred.
• Other contributing factors—Were there digs (deep, short scratches) resulting from glass-to-glass or glass-to-metal contact? Did a projectile hit the glass? Is there edge or surface damage?

To find the origin of a break, the first step is to assess its direction by inspecting the fracture lines, or “Wallner lines,” in the glass. These rib-shaped marks, distinguished by a wave-like pattern, begin at the break origin and radiate along break branches, and almost always project into the concave face of these lines.

It’s often helpful to make a basic diagram (see Figure 1) of the fracture lines. This can be easily done by examining the direction of the Wallner lines on individual pieces of glass. The origin of the break can be determined by:

• Drawing arrows (indicating fracture line direction) pointing into the concave face of break wave markings in the glass edge.
• Tracing point-to-tail of arrows back to the break origin.

Figure 1
Diagram of Fracture Line Direction

Failure Modes

Knowing the origin of the break sets the stage for determining the mode, or type, of failure. The lack of crystallographic planes in glass can allow certain break patterns to be observed and then associated with modes of failure. Two common modes are mechanical and thermal.

Mechanical Stress

The most common contributing factor of breakage, mechanical stress, occurs when forces resulting from bending or impact exceed the Modulus of Rupture (MOR), a critical metric in stress analysis. There are low-stress and high-stress mechanical tension breaks:

• Low-stress tension breaks are experienced most frequently by residential window and IGU manufacturers. The origin of the break is typically at damaged areas of the edge or surfaces near the edge, such as digs, scratches or chips. In many cases, breakage from damaged glass occurs after the initial edge damage is incurred, such as during IGU fabrication, sashing operations, transportation, job-site handling or storage, or the installation process.

In Figure 2, the break origin is not 90 degrees to the edge of the glass, indicating a tension break caused by bending. Low-stress, mechanical tension breaks often occur from bending at less than 1,500 psi.

Figure 2
Low-Stress Mechanical Tension Break

• High-stress tension breaks share one characteristic with low-stress tension breaks: The break origin is not 90 degrees to the edge of the glass, suggesting a tension break caused by bending. However, additional branching of the crack within two inches of the break origin (see Figure 3) indicates that the stress at breakage was likely higher than 1,500 psi.

High-stress breakage due to impact at the glass edge or surfaces near the edge is typically the easiest to identify, as multiple fractures emanate from the point of impact in a spider web fashion (see Figure 9). This breakage pattern is a tell-tale sign that either the glass edge or surface was struck by a foreign object with significant force.

Figure 3
High-Stress Mechanical Tension Break

Thermal Stress

Thermal stress results from several factors, including weather, the method of glass installation and the insulation around the glass. Breaks caused by thermal stress occur because of large temperature differences, most commonly between the edge and center of the glass. For instance, when the sun hits the glass and warms up its center, but the edges remain cold, the thermal gradient produces increased stress, potentially leading to breakage. This temperature gradient can be even more severe when glass is glazed into concrete and other heavier framing that acts as a heatsink, which keeps the edges cooler for a longer period of time. Thermal stress breaks often originate at the edge of the glass and form virtually 90-degree angles to the edge and surface of the glass.

As with mechanical stress, there are two types of thermal stress breaks: low stress and high stress.

• Low-stress thermal breaks are often indicated by a single break line starting at the break origin point at or near the glass edge and propagating two inches or more before branching into more break lines (see Figure 4). Damaged glass edges are the most frequent cause of low-stress thermal breakage.

Figure 4
Low-Stress Thermal Break

• High-stress thermal breaks appear as a single break line starting at the break origin point at or near the glass edge and generally branching into additional breaks within two inches of the origin (see Figure 5). This indicates a breakage brought on by conditions that cause high thermal stress, such as severe outdoor shading on parts of the glazing; heating registers located between the glass and indoor shading devices; closed, light-colored drapes located close to the glass; or glazing in massive concrete, stone or similar framing.

Figure 5
High-Stress Thermal Break

NOTE: Most thermally induced stress breaks in architectural glazing applications occur in the winter, when it is typically colder overnight and the low-angle sun in the morning heats the center of the glass more quickly than the edges.

Analyzing the Break Origin

A reliable method for estimating the stress level of a break at failure is a mirror radius measurement. Radius dimensions are determined by crack propagation velocity characteristics.

A crack propagates itself through glass with increasing velocity as it moves further from the point of origin. If an object has sufficient energy to propagate a crack through the thickness of the glass, then a “spider web” pattern will form. The lack of crystallographic planes in glass will cause this type of crack to proceed perpendicular to the applied tension stress.

Near the point of origin, a smooth, mirror-like appearance on the fracture face indicates a low crack velocity. However, as velocity increases (due to higher tension stress), the fracture face takes on a frosted look; then, at the highest velocity, it assumes a ragged or hackled appearance.

Mirror radii appear in various forms, depending on the stress level of the fracture (see following illustrations).

Figure 6 shows break origins resulting from high tensile stresses, such as bending or thermal stress breaks.

Figure 6
High-Stress Mirror Radii
(R = Mirror radii)

Figure 7 represents the break origins of glass fracturing at low bending stresses. In this example, a smooth fracture face forms across the thickness of the substrate. When the breaking stress is low, the mirror radius is often radial and may extend deeply into the substrate.

Figure 7
Low-Stress Mirror Radii
(R = Mirror radii)

The relationship of the radius of the mirror surface of the break origin to the stress that caused the breakage, known as “PPG/Orr’s Equation,” is expressed by the following:

The graph in Figure 8 also shows the radius-to-stress relationship at various levels of breaking stress.

Figure 8
Radius-to-Stress Relationship

A Deeper Dive

In some situations, it’s helpful to expand the investigation beyond determining the type of breakage and the stress level of the break. Pinpointing the actual source of the damage can help prevent or reduce future occurrences of breakage.

For example, a lite of glass might appear to have been broken by high, thermally induced stresses, when, in fact, a more-detailed analysis might reveal that impact damage that reduced the thermal-loading threshold of the glass may have been the actual cause.

To identify what damaged the glass in the first place, four factors are examined during this analysis:

• Impact
• Inclusions
• Thermal variance
• Pressure differentials

Impact

Identifying the nature of the breakage pattern can determine whether a foreign object hit the glass and whether the impact was perpendicular or parallel.

Depending on the severity of the impact, the immediate area surrounding the break origin might be cracked, crushed or missing (see Figure 9).

Figure 9
High-Stress Mechanical Breakage

Other sources of impact can be detected during the fabrication process or the transit and installation of the glass; these often appear at the edges. As mentioned previously, edge damage can reduce the strength of the glass by more than 50 percent, which increases the likelihood of breakage due to mechanical and/or thermal stress.

Inclusions

Any undesirable material embedded in glass is considered an inclusion. Coming in numerous shapes and sizes, inclusions such as nickel sulfide and aluminum oxide stones can have unlimited distribution densities. These densities factor in to the glass quality ratings by ASTM C1036. Quality for clear glasses ranges from Q1 (used in the production of high-quality mirrors) to Q6 (applications in which blemishes are not a concern).

Contaminates in batch materials and furnace refractory degradation are often contributing factors of inclusions. Sometimes, however, an inclusion might consist of only unmelted batch material. Regardless of the source, responsible glass manufacturers go to great lengths to ensure that inclusions are kept to a minimum.

Effects of a Nickel Sulfide Inclusion

Similar to water, nickel sulfide increases in volume as it cools. The presence of a nickel sulfide inclusion in the center residual tension stress region of thermally tempered glass can lead an investigator to believe that the breakage occurred spontaneously. In reality, the breakage was likely caused by a phase transformation that increased (from 2 percent to 4 percent) the volume of the inclusion. This growth caused additional stresses that likely led to the breakage.

Here’s how this could happen. When float glass is manufactured, the glass is intentionally cooled at a slow, controlled rate to minimize or eliminate residual surface, edge compression and center tension. During this process, called annealing, inclusions can undergo a phase transformation (α to β) and become stable, without causing breakage.

When the glass is reheated during heat strengthening or full tempering, any existing nickel sulfide stones will shrink to their smaller, high-temperature stable α form. The slower cooling cycle used to make heat-strengthened glass allows the stones to complete this phase transformation. Vitro Glass is not aware of a single confirmed case of spontaneous breakage of heat-strengthened glass due to nickel sulfide stone inclusions.

In contrast, the rapid cooling cycle used to produce fully tempered glass stops the phase transformation, thereby trapping the stones before they complete their volume growth. This can become problematic later when a glass is in-service and its temperature exposure restarts the phase transformation, triggering the volume growth of nickel sulfide stones and re-instigating conditions that can lead to breakage. Because of this potential phenomenon, Vitro Glass has a long-standing preference for the specification and use of heat-strengthened glass, except where tempered glass is mandated for safety or other purposes by code.

Thermal Variance

Another potential underlying factor that contributes to breakage is thermal variance, which could be caused by interior and exterior shading. If the temperature difference across a lite of glass is great enough, the accompanying stresses can reach levels that cause breakage. This is particularly true if the nucleation point of the crack was caused by a damaged edge or an inclusion. In this case, the break origin is often located near the edge of the glass.

The characteristics of tinted and spandrel glasses (higher heat absorption) and glass that is glazed into large concrete or steel structures (edges stay cooler longer) may contribute to the intensity of thermal-variance stresses in these applications.

In addition, a collapsed IGU may produce conditions that potentially could have an impact on thermal-variance stresses. For instance, there’s often contact between the muntin bars inside the window or between the two pieces of glass. This can create an alternative path for heat conduction, thereby establishing localized temperature gradients. The combination of contact, surface damage and localized gradients can greatly increase the likelihood of breakage.

Pressure Differentials

When the assembly and installation of an IGU occur in two different places, a change in altitude—even as little as 5,000 feet—that changes atmospheric pressure, can jeopardize the unit’s integrity, causing the seal to rapidly fail and potentially leading to breakage.

Studies show that altitude changes of only 1,000 feet accompanied by large temperature differences between the installation and assembly sites can cause IGU failures after extended periods of time. This is exacerbated at higher temperatures—when the temperature differential exceeds 70 degrees F, as each 20-degree differential increase is equivalent to an altitude increase of 1,000 feet.

Glazing contractors can offset this risk by installing capillary tubes that regulate pressure changes in IGUs. The tubes must be properly terminated at the final location by crimping or sealing to prevent moisture ingression. Because capillary tubes allow for gases to enter and exit the IGU, to maintain a pressure equilibrium, gas filling must be done at the final installation site. Care should be taken if the IGU includes an MSVD low-e coating because any moisture ingress during transportation and storage when the tube is open can be detrimental to the low-e coating.

It’s important to research framing systems and IGU spacers when altitude and temperature changes are factors.

In its pristine state, glass, as a prominent element in architectural design, can define contemporary buildings. Sometimes, however, glass breaks for no obvious reason. Whether it’s a one-off or part of a continuing pattern of incidents, glass breakage is inconvenient, potentially dangerous and costly—for building owners, construction professionals and other stakeholders.

Although architectural glass and fenestration manufacturers continue to develop advanced technologies to make their products as “break-proof” as possible, absolute perfection is simply not possible due to flaws inherent in the glass manufacturing process and the wide range of in-service environments in which glass products must ultimately perform.

Conducting “post-mortems” on glass breaks helps investigators identify the general reasons for each incident, including the type of failure that caused the break, and the potential original source of the damage. By using the techniques outlined in this article, building owners, contractors, engineers, glass manufacturers and other stakeholders may be able to accurately identify the likely origin of such failures and also potentially use that information to prevent future occurrences.

Timothy Bellovary is a technical services engineer at Vitro Architectural Glass (formerly PPG Flat Glass). In this position since 2014, he conducts annual audits of industrial production facilities and provides process improvement suggestions to elite fenestration fabricators within the Vitro Certified™ Fabricator Network. Bellovary holds a Bachelor of Engineering degree in mechanical engineering from Youngstown State University.