The design and structural performance of laminated glass in overhead glass applications such as ceilings and skylights are very important because it affects the safety of the occupants of the building and its maintenance personnel. For such applications, the strength after fracture of the laminated glass structure is critical. The pre-breaking strength can be predicted with reasonable accuracy by a software package based on FEM technology (such as SJ Mepla), which allows the user to input the mechanical properties of the viscoelastic interlayer and glass. This contributes to the differentiation and design optimization of the pre-fractured structure strength based on the interlayer.

However, the strength of glass after fracture is still an unknown area, because the current analysis model is still inconclusive, so destructive testing is the only reliable solution. An impact test that simulates an accidental drop by a maintenance worker was conducted to evaluate the "drop" resistance of point-fixed laminated glass structures with different interlayers (ie, PVB, ionomer, rigid PVB, and EVA). In addition, according to the new requirements established by the German standard pr DIN 18008-6, at sub-zero (-20 °C), room temperature (21 °C) and at high temperatures of 50 °C. Deflection measurements were also performed to estimate the modulus value of the damaged laminate structure.

Author: Malvern Singh Ruplay Ingo Steerze

The strength of laminated glass after breaking is significantly higher than that of monolithic glass. The glass shards adhere to the interlayer, so as to obtain a certain remaining structural capacity when the glass shards are "arched up" or locked in place. This capacity depends on the degree of fragmentation of the glass and increases as the size of the fragments increases. Therefore, when the laminated glass element is made of annealed or thermally strengthened glass that is broken into large fragments, its remaining structural capacity is particularly high. The behavior after fracture also depends on the sandwich material. The most common interlayer is PVB, and its mechanical properties depend to a large extent on temperature and load duration.

At room temperature, PVB is soft and its elongation at break exceeds 250%. For higher temperatures and load durations, the shear transmission will be greatly affected1. The low stiffness of the PVB middle layer will cause a "blanket effect" (Figure 1), because of the panel's own weight, especially when the panel size is large and the support is minimal, once the glass is broken or even used vertically. Kuraray's ionomer interlayer (originally invented by DuPont) was developed to achieve higher stiffness, heat resistance and tear resistance in point-fixed ball applications. This helps achieve excellent post-fracture strength and design redundancy.

The above table shows the comparison of the mechanical properties of ionomer, PVB, and rigid PVB interlayer.

The strength after fracture is significantly affected by the fracture mode of the glass layer, supporting conditions and temperature. Even for the same type of glass, the failure mode of the glass may be a large variable, which makes it almost impossible to analyze and model the strength after failure. Therefore, engineers must rely on destructive testing for this. The strength after fracture can be classified as follows.

1. Instant breaking strength to prevent "drop-through" 2. Breaking strength test after medium duration (more than 30 minutes. According to pr DIN 18008-6)

The above two intensities were tested separately, and the same test is also given in this article.

The impact test was done at Intertek ATI Inc. York, Pennsylvania, by dropping a soft bag weighing 100 kg from a height of 1.2 meters at a test temperature of 50 ºC (typical maintenance work along his tools and trolleys) weight). The test method simulates potential loads from distress installation and/or maintenance workers. Adjust the panel at 50°C for 1 hour before testing. The test device is enclosed with an insulating plate to ensure that the test result will not change due to temperature. Just before the impact, the heat shield was removed.

Figure 2a shows how a laminate made of ionomer interlayers provides a barrier to the impactor under static load. After 15 minutes, the impactor was removed, and no interlayer tearing was observed at the rotating body (Figure 2b). In the subsequent impact test, the laminate provided "drop" resistance (Figure 3), while the laminates made of EVA, PVB, and rigid PVB in Figures 4, 5, and 6 collapsed immediately after the impactor. And the failure acts as a barrier.

In February 2015, the German standard pr DIN 18008-6 (Glass in buildings-design and construction rules-additional requirements for walk-in glazing during maintenance procedures) established the damaged performance of accessible overhead glazing The requirements for maintenance and cleaning are stricter than ever. The new regulations require that the laminated glass structure can withstand a weight of 100 kg for at least 30 minutes after the top glass breaks. First, the panel is impacted by dropping a double-tire impactor weighing 50 kg from a height of 900 mm.

Then apply a load of 100 Kg on an area of ​​200 x 200 mm for 30 minutes (Figure 7). After the uppermost glass layer is broken, the entire glass element must stay on its support for at least 30 minutes. If the sample does not fall from the holder, the impact does not penetrate the laminated glass and there are no dangerous glass fragments falling2, the test is successful.

The University of the Armed Forces of Munich, Germany conducted a post-break strength test on a point-supported glass panel (1.5 mx 2.0 m) for a typical canopy application. Prepared 9 glass panels, each with 4 different interlayers-ionomer (1.52 and 0.89 mm), rigid PVB (1.52 mm) and PVB (1.52 mm), used in 3 different temperature scenarios test. The purpose is to understand which sandwich type laminate structure can pass the requirements specified in pr DIN 18008-6 at 3 different temperatures (-20 °C, 21 °C and 50 °C). Three panels were tested for each temperature scene. Try to understand the final capacity of the laminate.

However, the maximum load of the test equipment is 400 Kg. The laminate is conditioned for at least 3 hours at each temperature. The test chamber has a temperature control mechanism from -25 °C to 25 °C. The laminated structure with all the different types of interlayers is strong enough to withstand the impact of any damage. Therefore, in each case, a center punch must be used to manually break the upper layer of the laminate. A 100 Kg concrete block was then placed on the glass plate for 30 minutes. The test is designed to understand the ultimate post-breakage performance limit, so the load is increased to 400 Kg in 100 Kg increments, with a 15-minute interval between each load increment.

The laminate with ionomer and rigid PVB interlayer can withstand a load of 400 Kg at -20 °C and 21 °C without collapsing (Figure 8), while in Figure 9, the PVB laminate cannot withstand 21 °C 100 Kg when the load is applied at °C, because it collapses within a few seconds due to the tearing of the rotating intermediate layer when the load is placed.

At a high temperature of 50 °C, damaged laminate structures with PVB and rigid PVB intermediate layers cannot withstand their own weight because they collapse soon after the two layers are damaged (Figure 10 and Figure 11).

The laminated structure made of 0.89 mm ionomer interlayer can withstand a weight of 100 Kg for more than 30 minutes and collapse when the load increases to 200 Kg. (Figure 12). Similarly, a 1.52 mm ionomer laminate can withstand a load of 200 Kg for more than 30 minutes after being broken, and collapse when the load increases to 300 Kg (Figure 13).

Deflection measurement used to estimate the modulus of damaged laminates

During the test, deflection measurements were performed on all three temperature conditions to reversely estimate the modulus value of the laminate structure. At -20 °C, the 1.52 mm ionomer laminate showed the lowest deflection after 30 minutes (Figure 14), while at 21°C, the ionomer laminate and laminate with a rigid PVB interlayer showed the lowest deflection. Performance is better than standard PVB laminate (Figure 14). 15). At 50°C, laminates with SentryGlas® ionoplast interlayers outperform rigid PVB. (Figure 16.)

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