Experimental characterization and finite element modelling of strain-rate dependent hyperelastic properties of PVB interlayers

Laminated glass provides safety in an impact or explosion event by way of a polymer interlayer to which glass fragments adhere upon fracture. The mechanical deformation of the interlayer defines how the impact energy can be absorbed to prevent calamities by flying glass debris, penetration of a blast wave, lacerations, etc. The PVB interlayer used in safety glass shows highly nonlinear viscoelastic material behaviour, with a great sensitivity to temperature and deformation rate. Although various material models for PVB can be found in literature, few publications discuss the full range of its mechanical behaviour and none are found to describe a material model that is valid in a wide range of deformation rates and up to high elongations. Such material model is necessary for the numerical study of the post-fracture response in a dynamic event. The article describes the mechanical behaviour of PVB interlayer and the constitutive models by which the polymer can be represented under different load cases. Tensile experiments of Saflex® PVB are presented for a wide range of deformation rates and up to tearing of the specimens. Subsequently, a method to calibrate a hyper-viscoelastic material model for the interlayer by numerically simulating the tensile tests is developed. The resulting material models are valid up to the tearing strain of the interlayer and are accurate within a specified range of deformation rates and temperatures

Numerical simulation of the EN 12600 Pendulum Test for Structural Glass

In modern-day architecture, transparent glass units are omnipresent as large façades, windows, floors and balustrades. To ensure safety in an accident, glass panels must successfully pass the 'human impact' test, described by the international standard EN 12600. This test setup consists of a steel frame in which the test plate is clamped with prescribed force; and the pendulum impactor, hanging from a steel cable. The impactor weighs a total 50 kg and is built up from a rigid steel core to which two small tyres are mounted. The window panels are assigned a qualification number as they remain intact, fracture without losing integrity or fragment completely in impacts from different drop heights. As experimental testing is expensive and time-consuming, there is an interest in numerical modelling to predict a qualifying glass panel, which is already allowed by the German standard DIN 18008-4. Several modelling approaches allow the impact simulation for intact glass panels. This paper presents a detailed numerical model for the pendulum impact which enables realistic simulation of impactor, frame and test plate, to be valid also for the post-breakage safety assessment of laminated glass. The model shows good correspondence for static compression of the tyres and for impact against a pressure plate. Further comparison is made for the impact on a laminated glass panel that remains intact. Although less suited for structural design qualification, the detailed model can be used for future simulation of the post-breakage response of laminated glass panels

Experimental Characterization and Finite Element Modelling of Strain-rate Dependent Hyperelastic Properties of PVB Interlayers

Laminated glass provides safety in an impact or explosion event by way of a polymer interlayer to which glass fragments adhere upon fracture. The mechanical deformation of the interlayer defines how the impact energy can be absorbed to prevent calamities by flying glass debris, penetration of a blast wave, lacerations, etc. The PVB interlayer used in safety glass shows highly nonlinear viscoelastic material behaviour, with a great sensitivity to temperature and deformation rate. Although various material models for PVB can be found in literature, few publications discuss the full range of its mechanical behaviour and none are found to describe a material model that is valid in a wide range of deformation rates and up to high elongations. Such material model is necessary for the numerical study of the post-fracture response in a dynamic event. The article describes the mechanical behaviour of PVB interlayer and the constitutive models by which the polymer can be represented under different load cases. Tensile experiments of Saflex® PVB are presented for a wide range of deformation rates and up to tearing of the specimens. Subsequently, a method to calibrate a hyper-viscoelastic material model for the interlayer by numerically simulating the tensile tests is developed. The resulting material models are valid up to the tearing strain of the interlayer and are accurate within a specified range of deformation rates and temperatures

Added value of micro-tomography measurements in mechanical characterization of materials: some case studies in engineering applications

The research group Mechanics of Materials and Structures at Ghent University in Belgium is doing research on the experimental and computational mechanics of (composite) materials. Micro-tomography is used as a measurement tool to improve insight in mechanical testing and to provide input for finite element simulations. In this contribution, several case studies in engineering applications will be discussed: • Laminated glass is widely used in all building applications. The EN12600 standard is a testing standard to assess the resistance of laminated glass against "human impact", as it frequently happens that people accidently walk into a glass door or tumble into a large glass window or glass fence. As the "human impact" should be replaced by a standardized impactor, an inflated rubber tyre is used to that purpose. In modelling the EN12600 standard, the interaction of the tyre with the laminated glass is important. Micro-CT imaging has been used to reconstruct the tyre model (Figure 1), • All large OEM companies in automotive industry are actively doing research on weight reduction of cars. One of the considered options is (partially) replacing metallic components by composite components. However, cycle times in automotive industry are extremely short (about 1 minute per part) and 50,000 to 100,000 parts per year are target numbers that should be reached. Therefore, novel composite material combinations have to be designed that can be processed very fast. Figure 2 shows a dry 3D woven fabric with commingled glass/polyester yarns that can be consolidated very fast in a heated press. The Micro-CT images are used to characterize the fabrics and reconstruct the "as-woven" geometry, • Figure 3 shows a CT-scan of a 3D printed PA12 (polyamide-12) material. In this research, relations are sought between the internal microstructure of the 3D printed material, the sequence of layer-by-layer deposition and the resulting mechanical properties of the 3D printed material

Study on the mechanical response of glass facades under air blast loading

Many buildings housing strategic companies or federal services employ a large glass curtain wall so as to have (literally and figuratively) a ‘transparent’ image towards the public. However, in case of a blast event, resulting glass shards impose major threats. With our research, we hope to mitigate these threats by increasing knowledge about the post-failure behaviour of laminated glass. Therefore, blast experiments and advanced numerical modelling will be conducted

Simulating dynamic glass fracture in an air blast event

In modern-day architecture, large glass facades often define the skin of prestigious buildings. When these buildings are considered a target for terroristic attack, great care is required in the structural design as to saveguard human life. Above all, the window components need to be considered, since glass shard debris is the main cause of injury in any urban impact event. Typically, laminated glass panes are used in blast resistant glazing systems. These consist of 2 glass plies, bonded together by one or more polymer interlayers to retain glass fragments upon fracture. The efficiency of a laminated glass against impact loading depends on a number of factors: ply thicknesses, viscoelastic properties of the interlayer material and bonding strength between layers. The frame connection also plays an important role. Ideally, the window should remain in its frame without glass fragments being propulsed and without tearing of the interlayer. Then the questions are: - How and when does the glass fracture? - How are the fragments kept together by the interlayer? - How much of the impact energy can be dissipated by the window without causing danger to humans? Or practically, up to which load is a glazing panel able to withstand the attack? It may be clear that destructive testing is a complex and expensive manner to assess the applicability of a structural wall configuration. Computer simulation offers a tool that can save time and costs, and the possibility to gain a deeper insight in the quantification of the main failure mechanisms: brittle cracking, delamination, interlayer tearing and frame connection failure.Our aim is to develop a numerical modelling technique for laminated glass under impact and blast loads, validated by laboratory experiments. The finite element model takes into account non-linear material behaviour and allows for localised fracture, using upcoming techniques such as the cohesive zone method

Numerical and experimental study of the peel test for assessment of the glass-PVB interface properties in laminated glass

In an impact event and for post-fracture behaviour, the adhesion between the glass plies and the interlayer is key to achieving the required safety performance and residual load-bearing capacity. Surprisingly, no specific values for the adhesive properties are prescribed by international standards or available in literature. However, direct measurement or calculation is not possible for the existing test methods. In this article, a finite element framework based on the cohesive zone method is developed to characterise the adhesive properties and study the influential factors in a 90° peel test. The resulting values are of similar magnitude as in other publications in the field, but no certainty can be established over them, mainly because large-strain behaviour of PVB interlayer is still not well documented for the observed range of strain rates. Also, it is seen that no direct relation exists between the peel force and Mode I characterisation of the glass/polymer interface, due to plastic deformation of the aluminium backing foil