The Mechanical Properties of Architectural Glass, The Self-explosion Mechanism of Tempered Glass And The Technology Sharing for Reducing The Self-explosion Rate

As practitioners in the field of architectural glass, we often encounter a series of technical issues related to glass. Among them, the problem of self-explosion of  Tempered glass is particularly emphasized in our production and manufacturing process. The following text was written by our professional technical consultant, summarizing the mechanical properties of architectural glass, the forms of failure, the mechanism of self-explosion of tempered glass, and the methods to reduce the self-explosion of tempered glass

Introduction

Architectural glass is a typical brittle material. A thorough understanding of its mechanical properties is of great significance for correct design and construction.This article reviews the mechanical properties of architectural glass, with the expectation of playing a beneficial role in the application of architectural glass.

Mechanical Properties

1 .Elastic Architectural glass is a completely elastic body. So far, no visible plastic deformation of it has been detected worldwide.Therefore, when designing and constructing architectural glass, its periphery should be in contact with soft materials such as rubber strips or sealants, and must not come into direct contact with metal materials such as aluminum profiles or steel profiles.

2. Brittle architectural glass has a large number of microcracks on its surface, which makes it extremely brittle and has extremely poor fracture toughness, manifested as sudden fracture when it fails.Therefore, under normal circumstances, architectural glass cannot be used as an engineering structural material.

3. Strength Dispersion Due to the presence of a large number of microcracks on the surface of architectural glass, its strength is closely related to the crack size, and the crack size and quantity exist randomly, which makes the strength dispersion of architectural glass relatively large.When designing and using architectural glass, a larger safety factor should be taken into account. Generally, for glass, a safety factor with a failure probability no greater than 0.1% should be adopted.

4. Strength Value: The failure of architectural glass is closely related to the propagation of surface cracks. The initial crack location and crack direction at the time of failure should be distinguished during design.The direction of force application determines the strength of architectural glass.

The strength of architectural glass is classified into large area strength, edge strength and end face strength.

Failure forms

1 .Bending failure Under the action of external forces such as wind load, architectural glass shows the bending failure of thin plates. Generally speaking, the design of tempered glass requires a strength of 84MPa, which is the bending strength.

Architectural glass does not have compressive strength, shear strength or tensile strength. Therefore, when designing, it is meaningless to calculate the compressive stress, shear stress and tensile stress of architectural glass

2. Impact Damage Under the impact of the human body or objects, architectural glass is prone to damage, that is, the impact resistance of the glass is relatively low. Therefore, improving its impact resistance is a key issue that should be considered in the production of architectural glass.

3. Thermal Cracking Under the action of temperature difference stress, building glass is highly prone to thermal cracking.Since the initial crack of glass thermal explosion starts from the edge of the glass plate, the fine processing of its edge has a significant effect on improving the glass's resistance to thermal explosion. Meanwhile, heat treatment of the glass will also significantly enhance its resistance to thermal explosion.

Tempered glass self-explosion

The bending strength and impact resistance of tempered glass self-explosion architectural glass are relatively low, and it is highly prone to thermal cracking, which limits its wide application.

Heat treatment of glass, namely tempering treatment, can increase its bending strength by 2 to 3 times and its impact strength by 3 to 4 times. There is no problem of individual thermal cracking.

The excellent performance of tempered glass has greatly expanded the application of architectural glass.However, tempered glass also has a distinct drawback, namely self-explosion of tempered glass.Only by thoroughly understanding the mechanism of tempered glass self-explosion can tempered glass be correctly designed and used.There are many reasons for the spontaneous explosion of tempered glass, and the most important one is the expansion of nickel sulfide particles.The glass contains nickel sulfide inclusions, which generally exist as crystals (NiS). At room temperature, there is a thermodynamic tendency for phase A to transform into the opposite phase, accompanied by a volume expansion of 2% to 3%.Nickel sulfide particles exist in flat glass, which is why they also exist in semi-tempered glass and tempered glass.

However, flat glass and semi-tempered glass do not have the phenomenon of self-explosion. Only tempered glass does have the phenomenon of self-explosion. The reason is that the thermodynamic tendency of the phase transformation of nickel sulfide particles from a to A is insufficient. Certain kinetic conditions must be met to achieve this phase transformation, which in turn causes the self-explosion of the glass.Flat glass is annealed glass and there is no stress inside.Semi-tempered glass and tempered glass, after quenching, have internal stress and belong to prestressed materials.

The internal stress states of semi-tempered glass and tempered glass are shown in Figure 1.

As can be seen from Figure 1, the internal stress distribution trends of semi-tempered glass and tempered glass are consistent, with the outer surface under compressive stress and the inner surface under tensile stress. The difference between the two is that the surface compressive stress and internal tensile stress of tempered glass are both greater than those of semi-tempered glass. Nickel sulfide particles in glass have the kinetic conditions for phase transformation only when they are located in a sufficiently large tensile stress region. Because the phase transformation of nickel sulfide particles is accompanied by volume expansion, a sufficiently large tensile stress causes the volume expansion of nickel sulfide particles.

The dynamic conditions are provided. This is the reason why flat glass and semi-tempered glass do not experience self-explosion while tempered glass does. Nickel sulfide particles in glass are randomly distributed. If they are located at the area with the maximum tensile stress of tempered glass, these particles may become the ignition point for the self-explosion of tempered glass. The self-explosion of tempered glass caused by nickel sulfide particles often has a crack shape at the burst point similar to that of a butterfly, which is called a butterfly-shaped crack. Some self-exploding tempered glasses have a colored particle in the middle of the explosion point, which is believed to be a nickel sulfide particle. These two characteristics are often used as the criteria for determining whether tempered glass self-explodes. The volume of nickel sulfide particles is different before and after the self-explosion of tempered glass. Before the explosion, the volume is small and not easy to be seen. After self-explosion, its volume increases, the location is determined, and it is very easy to be seen. This is also one of the reasons why the self-explosion of tempered glass is not easy to predict. The self-explosion crack of tempered glass is shown in Figure 2.

The self-explosion of tempered glass caused by nickel sulfide particles has the characteristics of initiative, spontaneity and no external cause, and is truly true Self-explosion. Two conditions are required for nickel sulfide particles to cause the self-explosion of tempered glass. One is the magnitude of the tensile stress at the location where the nickel sulfide particles are situated. The second is the size of nickel sulfide particles. The larger the size of nickel sulfide particles, the smaller the tensile stress it requires. That is, for different tensile stresses, nickel sulfide particles have critical sizes. In tempered glass, the greater the tensile stress, the smaller the critical size of nickel sulfide particles, the more self-exploding nickel sulfide particles are produced, and the greater the probability of self-explosion of tempered glass.

In addition to nickel sulfide particles, flat glass also contains stones, bubbles and impurities. Glass is a typical brittle material, and its mechanical behavior follows fracture mechanics. Stones, bubbles and impurities in the glass will form cracks in the glass, which is the weak point of tempered glass, especially the tip of the crack is the stress concentration area. If stones, bubbles or impurities are in the tensile stress zone of tempered glass, or if it is subjected to tensile stress under load, it may cause tempered glass to shatter.

According to Chinese standards, the surface stress of tempered glass should not be less than 90MPa, while American standards stipulate that the surface compressive stress of tempered glass should be greater than 69MPa. It is highly worthy of study whether the surface compressive stress of tempered glass in our country can be reduced to be consistent with or close to the standards of the United States. 

If feasible, it will greatly reduce the self-explosion rate of tempered glass. Reducing the limit value of surface compressive stress may cause the fragments of tempered glass to be larger. However, even if the surface compressive stress of tempered glass is very high and the fragments are very small, it cannot be guaranteed that all the fragments exist in a split state. In many cases, the fragments are cracked but not broken, forming a "tempered glass cover". The result is not much different from that of larger fragments, and in some cases, the harm is even greater. Therefore, It can be considered to reduce the limit value of the surface compressive stress of tempered glass. 

Moreover, the standard for semi-tempered glass in our country stipulates that the limit value of surface compressive stress should not exceed 60MPa, while the standard for tempered glass stipulates that the limit value of surface compressive stress should not be less than 90MPa. If the surface compressive stress of the glass is between 60MPa and 90MPa, it neither belongs to semi-tempered glass nor tempered glass and is considered a substandard product. 

From this perspective, the limit value of the surface compressive stress of tempered glass should also be reduced. If it is difficult to connect the limit value of the surface compressive stress of semi-tempered glass with that of tempered glass, at least the limit value of the surface compressive stress of tempered glass can be reduced to narrow the gap between the two. During the processing, transportation, storage and construction of the surface and edges of the glass, defects such as scratches, cracked edges and broken edges may occur, which can easily cause stress concentration and lead to the self-explosion of tempered glass. There are already a large number of microcracks on the surface of glass, which is also the fundamental reason why the mechanical behavior of glass conforms to fracture mechanics.

 These microcracks will expand under certain conditions, such as the action of water vapor loads, etc., all of which may accelerate the expansion of microcracks. Under normal circumstances, the propagation rate of microcracks is extremely slow, which is manifested as the strength of the glass being a constant value. However, there is a critical value for microcracks on the glass surface. When the size of the microcracks approaches or reaches the critical value, the cracks expand rapidly, causing the glass to break. If there are microcracks close to the critical size on the surface and edges of the glass, such as scratches, cracks, and chipped edges caused during processing, transportation, storage, and construction, which are relatively large in size, the microcracks on the surface or edges of the glass may rapidly expand under extremely small loads, eventually leading to the breakage of the glass.

To this end, the quality of edge processing of tempered glass should be improved, and the requirements for edge processing should be clearly defined, such as complete edge grinding on both sides or incomplete edge grinding on three sides, to avoid scratches and bumps on the edges and surface of the glass.Theoretical analysis and experiments show that the tempering degree of the edge of tempered glass is relatively low. Therefore, the edge of tempered glass should be given priority protection.For point-supported curtain wall glass, if holes are drilled in the glass, the edges of the holes must be finely ground, preferably to a polished finish, as the edges of the glass holes are areas where stress is concentrated.During the production process of tempered glass, heating and cooling are required. Uneven processing along the glass plate surface and asymmetry along the thickness direction will lead to uneven stress along the plate surface and asymmetrical stress distribution along the thickness direction of tempered glass. All these may cause self-explosion of tempered glass.Uneven stress along the surface of tempered glass can cause local tensile stress in the glass. If this tensile stress is too large and exceeds the breaking strength of the glass, the glass will burst. The stress distribution along the thickness direction of the glass plate should be symmetrical, that is, the upper and lower surfaces are under compressive stress, and the middle surface is under tensile stress.

The magnitude of the compressive stress on the upper and lower surfaces, the thickness and variation of the stress layer are completely symmetrical. The ability of the glass plate to withstand positive and negative wind pressure is the same.If the stress distribution along the thickness direction of the glass plate is asymmetrical, the ability of the glass plate to withstand positive and negative wind pressure will be different. One side will have a stronger load-bearing capacity, while the other side will have a weaker one. That is, the glass may break under a smaller load. In severe cases, the glass plate will deform under no load, causing distortion of the image of the curtain wall glass. To this end, the uniformity of surface stress and the symmetry along the thickness direction of tempered glass should be improved.Especially for the tempering of Low-E glass, more attention should be paid to the symmetry of stress along the thickness direction. Because the difference in the absorption of thermal radiation by the upper and lower surfaces of Low-E glass will cause the temperature difference along the thickness direction of the glass plate during heating, and this difference will eventually lead to the asymmetry of the stress of tempered glass along the thickness direction.

At present, in the process of glass tempering, the method of forced convection is adopted to eliminate this unfavorable factor.The internal stress of tempered glass is uneven and there is a large stress gradient, which can cause self-explosion, manifested as the size of the fragments varies greatly.There are five measurement points for surface compressive stress, and the average value is taken.The limit values of the difference between the maximum and minimum values of the five measurement points should be added to characterize the uniformity of the compressive stress on the surface of tempered glass.Reducing the surface size of tempered glass plates can lower the self-explosion rate of tempered glass.At present, in China, the application of architectural glass shows a trend of larger and larger plate surfaces. The larger the size of tempered glass and the thicker the glass plate, the greater the probability of self-explosion.

In a Tempered glass plate, as long as there is one self-explosion point and it eventually leads to the self-explosion of the tempered glass, regardless of the size of the tempered glass plate, the entire tempered glass plate will shatter. The larger the glass plate is, the more unfavorable factors such as impurities, nickel sulfide particles, edge processing defects, surface scratches, and uneven stress that can cause self-explosion of tempered glass will be. Under the same load, the probability of self-explosion will increase. Therefore, the size of tempered glass plates should be restricted based on the thickness and quality grade of the flat glass.

Conclusion

Both "Tempered Glass for Building Doors, Windows and Curtain Walls" (JG/T455-2014) and "Technical Code for Application of Building Glass" (JGJ113-2015) have made clear regulations on the production of tempered glass and the application of building glass. This article conducts a comprehensive review of the mechanical properties of building glass and provides some supplementary explanations for understanding the above standards.