Glass has been used as a building material for a long time. The first glass uses were limited to filling window frames, mainly to illuminate rooms with natural light. In recent years, the popularity of glass in construction has increased significantly [1] because of the growing trend to bring as much natural sunlight into buildings as possible [2]. In recent years, glass has been used to construct elements that can safely carry snow and live loads, in addition to their own weight and wind loads. Glass facades have almost become a tradition in office buildings, as well as glass canopies over entrances, e.g. shopping malls, elevator casings, balustrades and stairs, which have also gained popularity.
The increasingly popular treatment of glass as a construction material requires the use of laminated glass, in which two or more glass panes are permanently joined together by a film (interlayer) [3]. Laminated glass is also called safety glass (the so-called VSG, ger. Verbundsicherheitsglas) [4]. The use of VSG glass is dictated by its behaviour after glass failure, which is radically different from monolithic glass. This unique behaviour occurs because the film between the glass sheets holds the glass fragments in place when fractured [5] avoiding the risk of injury to people in the vicinity. A safe “failure response” is achieved when the structure in a failure state can maintain sufficient structural integrity and load-bearing capacity to evacuate people and safely replace damaged components [6].
The structural behaviour of glass in an elastic state can be determined with reasonable accuracy using FEM software [7,8,9,10,11]. However, the post-breakage behaviour of laminated glass is still not well-recognized. This is because analytical and numerical models in the post-breakage phase should be developed on the basis of statistical processing of test results which are currently limited. Currently, the only reliable way to determine the post-breakage capacity of elements made of glass is by destructive laboratory testing. Therefore, the issue of the post-breakage capacity of glass is challenging and requires in-depth experimental and numerical analysis.
The issue of post-breakage load-bearing capacity was the subject of several laboratory studies [12,13,14]. According to the tests carried out in [15], the fractured elements were able to carry a limited load for some time; which in theory, would allow for the safe evacuation of people. Research into the post-breakage load capacity of laminated glass is a complex issue, due to the time and high cost of conducting such research. In addition, no standardised testing methodology currently exists for determining the post-breakage load capacity of an element made of laminated glass.
Numerical analysis of the behaviour of laminated glass after failure is a demanding task and requires an entirely different approach to the problem. Attempts to numerically simulate the behaviour of laminated glass at the moment of fracture have been undertaken in several publications. In [16], using a finite element mesh, a digital image processing tool was used to create a corresponding glass breakage pattern at the time of sample failure. In articles [17, 18], an attempt was made to reproduce the response of the glass to a hard body impact using the combined FEM and DEM (discrete element method). Using this approach, a realistic crack network was obtained and subsequently compared with camera recordings during the experimental study. Using numerical simulations, major cracks were obtained at similar locations and times with respect to the experimental tests.
Attempts to increase the load-bearing capacity after the failure of laminated glass elements can be found in publications [19, 20]. Laminated glass beams were reinforced in the tension zone with steel tendons and then subjected to four-point bending tests. Reinforcement with steel tendons was activated after the first cracks in the glass. After cracking, the steel element was able to transfer part of the tensile stresses, positively affecting the post-breakage load capacity. Other examples of reinforced beams made of laminated glass can be found in [21, 22]. Before the lamination process, bars made of GFRP, CFRP (Glass Fiber Reinforced Polymer and Carbon Fiber Reinforced Polymer) and steel were placed in the interlayer. In the case of this type of reinforcement, it was found that the rod inserts effectively increase the post-breakage load capacity of the beams, ensuring their ductile failure.
Another example is reinforcing glass by laminating inserts made of GFRP in the vicinity of openings [23, 24] and subjecting the samples to in-plane loading. The studies showed that the breaking load of the GFRP-reinforced samples more than doubled compared to the reference samples. The reinforcement did not increase the cracking force of the tested samples.
As part of the ongoing project financed by the National Center for Research and Development (NCBR) within the LIDER XI Program, the idea of laminating a steel woven mesh to glass laminates is being investigated [25]. The steel mesh is designed to increase the load-bearing capacity of the sample in the post-breakage phase thus increasing the safety of people in the buildings. The purpose of the study is to experimentally determine the influence of embedded steel woven mesh on the post-breakage load capacity of laminated glass subjected to in-plane load.
This study used a commercial soda–lime–silicate (SLS) float glass, which is the most popular glass type used in building construction. Architectural elements made of glass mounted with point fixings usually require the application of heat-treated glass due to the high stress concentrations around the holes [26]. For this reason, fully tempered glass was used to produce samples.
An ethylene-vinyl acetate (EVA) interlayer was used in the lamination process of the glass panels. EVA film is increasingly used as an alternative to the polyvinyl butyral (PVB) interlayer. This is due to its low lamination temperature and the fact that no autoclave is required in the manufacturing process, making it a more cost-effective alternative to the standard PVB interlayer [27]. In addition, EVA shows better resistance to moisture (it is not hygroscopic) and is less likely to delaminate than PVB, so it is ideal for wet conditions, for example, external balustrades and glass canopies. In addition, it should be mentioned that EVA film shows a lower tensile strength than PVB. Its strength is between 9.5–10.0 MPa, while, in the case of PVB foil, it is 20.8 MPa [28].
The reinforcement between the two layers of EVA Clear foil was made of EN 1.4301 woven steel mesh [29]. It is an austenitic, chlorine-nickel, low-carbon steel.
All of the steel elements of the test stand were made of steel grade S355 [30]. The 20 mm diameter bolts were made of grade 8.8 steel, resulting in a shear capacity of 117 kN [31]; this is much higher than the tensile strength of the samples assessed in preliminary studies, which were made before main tests.
In order to reduce the stress concentrations in glass due to the direct contact with steel elements, 2.5 mm thick aluminium spacers were used [32].
Laminated glass elements were used in the experimental campaign (Figure 1). The dimensions of the samples were 190 × 300 mm2 and consisted of two panes made of fully tempered glass (8, 10, and 12 mm in thickness) and two layers of EVA Clear film (1.52 mm in thickness). A steel woven mesh was placed between two layers of EVA Clear film before the lamination process. In each of the samples, three holes with a diameter of 26 mm were made using a waterjet technique. Two of them were at the bottom of the sample, while the third was at the top. Such a design of the hole locations was intentional, to force the sample failure in the upper hole. The exact location of the holes can be seen in Figure 1.
Shift of components panes was found in some samples, see Figure 2. This feature occurred in the production process. The shift affected the position of the holes relative to each other and their non-collinearity which may influence the in-plane load transfer of both panes in the experiments. The standard [33] describes the displacement tolerance of component panes in laminated glass relative to each other, amounting to a maximum of 2 mm for elements with a length not exceeding 1000 mm.
The tests were carried out on reference samples and reinforced samples. The reference samples without reinforcement (REF) only consisted of glass panes and interlayers, while in the case of reinforced samples (MESH), a steel woven mesh was placed between two interlayers over the entire surface of the sample, before the lamination process. The steel mesh consisted of wires (0.35 mm in diameter) at a spacing of 1×1 mm2. In both cases, the thickness of the interlayer (3.04 mm) was the same. A visual comparison of the reference and reinforced samples is presented in Figure 3. The study considered three thicknesses of glass pane (8, 10 and 12 mm), divided into two subtypes – reference and reinforced samples. This led to 6 test series of 6 samples each, for a total of 36 samples (Table 1).
Overview of the tested samples
Test Series | Glass Thickness (mm) | Interlayer |
---|---|---|
G8-REF | 2 × 8 mm | 2 × 1.52 mm |
G8-MESH | ||
G10-REF | 2 × 10 mm | |
G10-MESH | ||
G12-REF | 2 × 12 mm | |
G12-MESH |
The configuration of the tensile strength experiment is shown in Figure 4. The experimental setup consisted of two handles and four mounting plates. The handles were made of two sheets, with dimensions 60 × 140 mm2, and a hole of 24 mm diameter. In the lower part of the handle, there was a pin for attaching the handle to the testing machine. The lower part of the sample was fastened to the holders with two fixing plates of dimension 130 × 150 mm2. The upper fixing plates had dimensions of 60 × 150 mm2. The appropriate number of holes necessary for mounting the test sample was made in the lower and upper mounting plates. Five 20 mm diameter screws were used to assemble the sample. The upper handle and the upper mounting plates were used to introduce displacement during the experiments. The tensile strength test of the laminated glass was carried out in a displacement-controlled testing machine with a displacement speed of 10 mm/min. The tests were performed at room temperature and a relative humidity of 50%.
During the tests force and displacement of the cross-head were monitored. Load cell and cross-head displacement signals were recorded and stored in an external data acquisition device. In addition, a linear differential transformer (LVDT) with a measuring length of ±12 mm was mounted to the upper handle, which was used to measure the local displacement of the tested samples. In the sample preparation phase, the value of the relative shift was controlled. All signals were acquired using an external data acquisition device with 50 Hz data acquisition. Pictures of the test stand are shown in Figure 5.
Figure 6 shows exemplary results for a selected series (G12 series, glass thickness 12 mm). The graph compares force-displacement diagrams for a reference sample (REF) and a sample with embedded steel woven mesh (MESH). The values on the horizontal axis were derived from the cross-head displacement. The results of other test series were analogous in terms of global behaviour.
The behaviour of the samples during testing can be divided into three main phases: elastic, force drop and post-breakage. In the first phase, the relationship between the load and the displacement is almost linear and refers to the perfectly linear-elastic response of the composite materials to the load. As a result of the uniformly increasing load, the stress in the glass increases until a limit stress value is reached, and the glass fractures in a brittle manner. During testing, the stiffness of the sample in the elastic phase was the same for reference and reinforced samples. Due to the arrangement of the holes (see Section 2.2), the origin of the fracture is located at the top hole.
Following the glass failure, there is a sudden drop in the force due to the almost complete loss of tensile stiffness of fully tempered glass due to fracture (second phase). After this stage, progressive degradation of the sample occurs due to further cross-head movement (third phase). The post-breakage behaviour differs for the reference (REF) and reinforced samples (MESH). The latter shows increasing load due to the presence of the reinforcing mesh, while in the case of the reference sample, the load is almost constant and relates almost completely to the stiffness of the inter-layer. Fully tempered glass breaks into many small pieces and bears tensile loads only for a short time with the interlayer, contributing to the load transfer. Because of this, the force does not drop to zero. Then, due to the separation of the cracked glass fragments, the interlayer is the only component of the laminate that allows the sample to carry a tensile load. It was noticed that samples with embedded steel mesh retained higher values of residual force after glass breakage.
In the second phase, two modes of laminated glass failure were observed. The first mode occurred when two panes broke simultaneously (Figure 7a). The second mode involved non-simultaneous breakage of the panes. The fracture of a single pane (Figure 7b) caused a force drop and then a force increase to the value at which the second glass pane failed. The reason for the two failure modes is the initial, relative shift of the glass panes in some samples which was revealed before testing (see point 2.2). In the case of the first model, the force was successfully introduced to both panes, while the second mode is the result of the inequality of forces transmitted by the panes and the impossibility of their equalization due to the low stiffness of the interlayer and the aluminium spacers.
In the third, post-breakage stage, the samples exhibited behaviour where the force increased to a certain level and then slowly decreased until the interlayer was degraded entirely and the upper screw ultimately passed through the sample (Figures 7c and 7d). For all specimens, the load increased after the force drop was observed. The specimens reinforced with embedded steel woven mesh showed higher post-breakage capacity. The post-breakage capacity is assumed to be the ratio of the maximum force after glass fracture
Figure 8 shows schematic representations of the global behaviour of the samples during the experiments. Figure 8a shows the first failure mode, where two glass panes break simultaneously, while Figure 8b shows the second failure mode (progressive glass breakage). The notation in the index “
The average values of forces and displacements at characteristic points in the global force-displacement behaviour are listed in Table 2 along with the corresponding standard deviations. During the tests, 20 out of 36 samples failed according to the first failure mode, which is 56% of the total number of samples. The remaining 44% comprise 16 samples which fractured according to the second failure mode.
Results of the experimental study
Test Series | |||||||||
---|---|---|---|---|---|---|---|---|---|
G8-REF | 8.76 ± 0.13 | 1.00 ± 0.07 | 5.71 ± 2.18 | 0.82 ± 0.30 | 11.45 ± 2.87 | 1.92 ± 0.41 | 0.06 ± 0.01 | 1.24 ± 0.10 | 96.34 ± 2.02 |
G8-MESH | 13.30 ± 3.18 | 0.92 ± 0.43 | 7.76 ± 2.2 | 0.92 ± 0,35 | 13.00 ± 2.67 | 2.05 ± 0.55 | 2.52 ± 0.34 | 4.80 ± 0.22 | 87.20 ± 10.04 |
G10-REF | 11.10 ± 1.83 | 1.00 ± 0.39 | 12.27 ± 3.37 | 1.17 ± 0,27 | 14.98 ± 4.64 | 2.04 ± 0.68 | 0.08 ± 0.02 | 1.17 ± 0.13 | 98.29 ± 10.04 |
G10-MESH | 20.30 ± 3.71 | 1.69 ± 0.10 | 15.20 ± 4.17 | 0.9 ± 0.27 | 21.88 ± 9.57 | 1.59 ± 0.28 | 2.32 ± 0.46 | 4.92 ± 0.22 | 85.43 ± 12.04 |
G12-REF | 18.79 ± 5.17 | 1.47 ± 0.16 | 11.75 ± 0 | 1.7 ± 0 | 21.05 ± 0 | 2.02 ± 0 | 0.06 ± 0.01 | 1.23 ± 0.13 | 99.68 ± 11.73 |
G12-MESH | 20.72 ± 2.85 | 0.87 ± 0.29 | 29.18 ± 5.46 | 1.97 ± 0,02 | 32.97 ± 3.07 | 2.60 ± 0.04 | 2.15 ± 0.45 | 4.67 ± 0.61 | 89.55 ± 8.28 |
The first value obtained from the tests was the force at which the glass fractured (
It was discovered that when there was a progressive breakage of glass panes (the second failure mode), the
In the fracture phase, the average force
For the maximum force in the post-breakage phase
The maximum displacement at sample failure
The effect of strengthening the laminated glass with a steel mesh is revealed as soon as the glass is fractured. Figure 9 shows the percentage decrease in force after glass breakage from
Figure 10a shows the average values of
Based on the values presented in Table 2, the effect of reinforcement on the samples’ post-breakage load capacity was determined. Figure 10b shows the ratio of globally averaged
The lack of a linear relationship in the index may be due to the high standard deviations of the
Figure 11 shows the average displacement
The article presents the results of experimental studies on the tensile capacity of laminated glass samples with embedded steel woven mesh subjected to in-plane loading. The research involved experimental tests in a custom-made experimental set-up. The following conclusions can be drawn from the conducted research:
The woven steel mesh embedded in the interlayer positively affects the response of the laminate to a sudden loss of force transfer caused by glass breakage. The load increase in the post-breakage state was observed for all samples. However, the reinforced samples showed significantly higher post-breakage load-bearing capacity. It was found that a residual force index for the reinforced samples was higher by almost 300% in comparison to the reference (unreinforced) samples.
It should be emphasised that the results and conclusions obtained from this research only apply to the tests carried out by the authors of this article. The results may vary depending on the test set-up and geometrical (and material) properties of the samples. Testing the post-breakage load capacity of glass laminates is a very complex issue, and further research is required on other configurations and under different environmental conditions.