The results of the experimental study are presented individually for each connection type. Firstly, the crack progression and failure mechanisms are described by means of schematic illustrations of the typically observed behaviour of a representative test specimen for each connection type at + 23 °C. The focus is set on the relevant observed cracks in the thin glass, the adhesive failure modes and the ultimate fracture of the specimen. Secondly, the temperature-dependent load-bearing behaviour for each connection type is illustrated with comprehensive force–displacement charts. The results for each connection type are plotted in separate diagrams. For a better comparison, all diagrams use the same scales on both x- and y-axis. Subsequently, the strength performance of the individual connection types is described with initial fracture load and maximum load capacity. Thirdly, the influence of the composite build-up on the structural load-bearing performance is evaluated by employing a thicker interlayer core and the use of chemically strengthened glass cover layers.
Crack progression and failure mechanisms
Mechanical fastener connection
The load-bearing behaviour of the mechanical connection is nearly linear (Fig. 9). The initial slope of the force–displacement curve is attributed to the movement of the alignment joints paired with the movements required to fully engage fastener threads with the PMMA material. Initial cracks developed with small rounded crack formation in the thin glass at the threads of the fastener close to the exterior stainless steel block having no influence on the load-bearing behaviour (stage 1). Following stage 1 fracture, crack formation spreads along the length of the screw inside the thin glass as glass offers reduced fracture toughness compared to the PMMA interlayer core that remains intact (stage 2). Only minor deviations from linear load-bearing behaviour were observed until stage 3 fracture with rounded crack development, which propagate with increasing force at the end of one screw. This generates an interim drop in force. The ultimate failure occurs by sudden rupture of the glass–plastic-composite starting at the end of the thread, splitting the specimen.
The initial fracture in stage 1 occurs due to the increased stress concentration within the first few engaged threads of the fastener (Fakhouri et al. 2014; Kloos and Thomala 2007). Subsequent crack formation in stage 2 can be explained by the similar force transfer phenomenon in deformed rebars for materials such as concrete, where the compression bearing of angled ribs along the bar are used to transfer tensile forces. This radial compression struts induce a balancing tension ring of stress in the encapsulating material (Tepfers 1976) that causes the stage 2 fracture. The ultimate failure can be attributed to concentrated stresses at the end of the threads leading to tensile crack development (stage 3), which increases towards ultimate failure with crack propagation, until the fracture of the PMMA. However, the thread remained intact and showed no signs of damage until the ultimate failure.
Adhesive epoxy connection
The adhesive epoxy connection exhibited a stiff behaviour until brittle failure (Fig. 10). Due to its high Young’s modulus (Eepoxy = 2312 N/mm2), the PMMA and epoxy are similarly stiff (EPMMA = 2337 N/mm2). This results in glass cracking perpendicular to the tensile load, as the insert behaves as fully embedded in a uniform material. Preliminary cracks in stage 1 were typically located in the thin glass layers, at the ends of the inserts. Cracks gathered in a similar location as the insert tab was pulled out, after which new cracks in the thin glass appeared near the bottom edge of the insert (stage 2). The subsequent cracks identified in stage 3 originated with the bridging and propagation of earlier cracks. This was closely followed by a complete fracture of the glass–plastic-composite along the arched fracture line created by previous cracking.
The connection’s insert tab disengaged from the glass–plastic-composite section along the fracture line, but it was not removable due to crack bridging in glass, or incomplete cracks through the PMMA. The portion of the glass–plastic-composite and adhesive joint above the failure crack line remained mostly undamaged and still firmly adhered to the stainless steel insert.
Location of adhesive failure is visualised by the red shaded region in Fig. 10 and was observed along the crack line. The loss of adhesion at the bottom occurred at the epoxy-stainless steel interface at rupture, whereas adhesive failure modes at the faces were observed in the epoxy-PMMA interface. This allowed for the release of the insert tab from the glass–plastic-composite during ultimate failure. Furthermore, partial cohesive failure was observed along the bottom crack line by partial pull out of the stainless steel insert.
Adhesive acrylate connection
The adhesive connection with the acrylate joint exhibited linear behaviour until the point of ultimate failure (Fig. 11). Initial cracks (stage 1) of the acrylate specimens were observed in the thin glass along the bottom line perpendicular to the load. In stage 2, the cracks were located at the edges of the inserts and did not extend past the perimeter of the adhesive joint. Striations developed in stage 3 at the adhesive faces of the insert indicating adhesion loss on both PMMA and stainless steel interfaces. Soon after stage 3, the adhesive suddenly failed in a cohesive failure mode. The test specimen remains largely intact.
The failure in the adhesive appeared in all specimens on at least one of the insert faces. At rupture, fracture at the glass surface was increased substantially. This may be attributed to the quick displacement of the insert tab as it is suddenly pulled out at ultimate failure. The insert still sticked to the glass–plastic-composite after testing. The observed failure mechanisms indicate ultimate failure of the joint caused by the lack of strength of the acrylate.
Adhesive PU connection
The tensile behaviour of the adhesive PU connection denotes increased flexibility (Fig. 12). The failure sequence began with adhesive failure at the adhesive-PMMA interfaces indicated by the loss of clarity (stage 1). Subsequent cracking of the thin glass along the bottom side of the insert (stage 2) perpendicular to the force was observed. Stage 3 followed shortly with a cohesive failure of the adhesive at the boundary line created by the area of adhesive still in contact with the PMMA walls. The cohesive failure did not instantaneously lead to ultimate failure due to low force levels. Instead, the failure gradually progressed from stage 3 to ultimate failure by almost complete tearing. During this transition, the intense tear correlated with nearly full loss of connection resistance. The residual load capacity within the force–displacement curve captured the gradual sliding of the insert out of the PMMA cavity and was not of significant importance as the connection was considered to have reached ultimate failure at the apex of the force–displacement curve. Overall, the low stiffness of the adhesive and early adhesive failure resulted in minimal fracture of the glass–plastic-composite specimen during testing.
Temperature dependent load-bearing behaviour
The unified force–displacement charts (Fig. 13a–d) of the test series characterise and compare the load-bearing behaviour of the investigated connection types at different temperatures. The strengths of the connections and corresponding statistics are discussed in Sect. 4.3.
Mechanical fastener connection
The test series of the mechanical fastener connection at + 23, + 40 and + 60 °C represent a linear load-bearing behaviour (Fig. 13a). Individual specimens only showed crack developments at a force level between 2 and 4 kN that resulted in a significant drop in force, followed by a further increase in load leading to ultimate failure. The ultimate force level characterizes the final failure and maximum capacity load of the PMMA interlayer core.
Adhesive epoxy connection
The adhesive epoxy connection exhibited a linear behaviour until sudden failure (Fig. 13b). At + 40 and + 60 °C, an increasing scatter between the individual tests was observed. The specimens developed significant cracks at the edges leading to earlier partial loss of adhesion at epoxy-PMMA interface on the side faces of the insert (Fig. 14). In general, even at + 40 and + 60 °C, the epoxy adhesive exhibited high stiffness however, paired with adhesion loss at the interface with the PMMA interlayer core at lower load levels.
Adhesive acrylate connection
Significant temperature dependence of the adhesive acrylate connection can be observed in the force–deflection charts (Fig. 13c). The adhesive softens already at + 40 °C due to relatively low glass transition temperature (9–46 °C). This leads to increased displacements until ultimate failure. Adhesive failure modes were detected between acrylate and stainless steel at the bottom of the insert and between the acrylate and PMMA on the side faces (Fig. 15). At + 60 °C, limited force transfer is observed. This is due to yielding of the adhesive joint after which the insert is pulled out at relatively low force level with similar failure modes as described in Sect. 4.1.
Adhesive PU connection
The adhesive PU connection exhibited lowest stiffness compared to the other adhesive joints (Fig. 13d). This is in agreement with the material properties in Table 1. The glass transition temperature (9–34 °C) is around room temperature. Therefore, at elevated temperature even lower stiffness and capacities due to increased softening of the adhesive are anticipated. This directed the exclusion of the adhesive for further testing.
Comparison of strength
The evaluated results of the first test series (Fig. 16) allow for a comparison in strength with corresponding statistics. It includes the initial fracture load and maximum load capacity of the connection. The post-fracture load reserves (marked by the arrows) are evaluated as the additional capacity up to the maximum load capacity after the initial fracture load. This quantification can be employed in fail-safe design concept for practical applications. The complete statistics of the individual test series are summarised in Table 3.
The strength of the fastener remains in the same order of magnitude for all tested temperatures (maximum change of -15%). This confirms consistent load-bearing behaviour that can be attributed to the relatively low change in Young’s modulus of the PMMA interlayer core at temperatures up to + 60 °C. Amongst adhesive connections, adhesive stiffness, strength and adhesion were the main parameters dictating the initial fracture and strength. In general, the lower the adhesive stiffness, the lower the overall strength of the connection regarding both initial fracture load and maximum load capacity. This influence of adhesive stiffness is clearly visible for the acrylate connection by the reductions of -66% for initial fracture and -71% for maximum load capacity caused by temperature effects. At + 23 °C, the rigid epoxy showed lower initial fracture strength (1.33 kN) than the more flexible acrylate (2.16 kN). It can be concluded that the adhesive joint requires a certain level of flexibility to reduce stress concentrations in the thin glass of the composite that is decisive for the initial glass fracture. At + 40 °C, the epoxy connection attained the highest overall initial strength of 2.99 kN representing an advantageous balance of stiffness and strength. At + 60 °C, the strength decreases to 2.05 kN due to earlier initial fracture formation. The low stiffness and tearing of PU resulted in very poor overall performance (initial fracture: 0.87 kN and maximum capacity: 0.88 kN).
All connections, except adhesive PU connection, show an increase in load capacity following the initial fracture. Hence, post-fracture load reserves ranging from 1.52 to 2.64 kN for fastener, from 0.99 to 2.54 for epoxy and from 0.44 to 1.89 kN for acrylate connection were quantified (Table 3). The PU adhesive connection did not offer noteworthy load post-fracture load reserves due to tearing of the adhesive.
Influence of composite build-up
The influence of composite build-up is represented in similar strength comparison (Fig. 17 and Table 4).
The mechanical fastener connection shows an increase in initial fracture load of + 76% and maximum capacity of + 75%. The failure characteristics match the aforementioned ones for the thinner build-up however, at increased load levels.
The adhesive epoxy connection shows a significant strength increase (initial fracture load + 114% and maximum load capacity + 37%), whereas the adhesive acrylate connection shows equivalent performance to the thinner build-up (initial fracture load -5% and maximum load capacity + 9%). The initial failures for both adhesives matched the aforementioned characteristics of the thinner build-up. The increased initial strength of the adhesive epoxy connection and low amount of cracking in the test specimen demonstrated a reduction in glass stresses. However, the ultimate failure occurred mainly due to adhesion loss (Fig. 18a). The acrylate adhesive connection failed by mixed adhesive and cohesive failure (Fig. 18b). This justifies the limited increase in maximum load capacity for both adhesive connections, as the load-bearing capacity of the joint becomes decisive.
Overall, the mechanical fastener connection demonstrated superior performance compared to the adhesive connection types, regarding both initial fracture load and maximum load capacity. Hence, the choice of the preferred variant belonged to the mechanical connection and additional testing with chemically strengthened thin glass (1.1CSG–10PMMA–1.1CSG) was carried out.
By the use of chemically strengthened glass, the initial fracture load was enhanced by additional + 53% compared to annealed glass (+ 169% compared to 1ANG–6PMMA–1ANG) whereas the maximum capacity that is defined by the PMMA strength remained at an equal level (Table 4). An initial fracture load of 5.07 kN and ultimate load capacity of 6.65 kN were reached with the mechanical fastener connection leading to a high strength relatively to the small connection and offering a noteworthy post-fracture load reserve. This evaluation quantifies the influence of the glass build-up and the use of chemically strengthened glass cover layers on the strength of glass–plastic-composite panel connections.