• Meyland, Martin Jensen

Abstract

Glass has been known to humanity for thousands of years and is considered one of the most important materials that have enabled the development of modern society. Widely used in construction, it often constitutes a significant part of buildings’ facade, where it allows for daylight and views while protecting the occupants against the weather. In extreme events, such as blast loading, the exterior glazing can also offer the first line of defence if the design is accordingly, e.g. using a laminated glass design. Although bombings in urban areas are rare, it has been shown that the consequences can be disastrous if the glazing design is not resistant to blast loading. However, it is challenging for engineers to quantify this resistance at such high loading rates. An essential parameter to assess in this regard is glass strength.<br/>Glass is a linear elastic, brittle material that fails suddenly and rapidly once its resistance is exceeded. The glass strength depends on the load duration and rate of loading due to sub-critical crack growth. The present thesis investigates the tensile strength and stiffness (Young’s modulus) of glass at loading rates relevant to blast loading. Furthermore, and application of these mechanical material properties in the simulation of glass fracture under blast loading using the explicit Finite Element Method (FEM) is presented.<br/>State-of-the-art knowledge about the dynamic fatigue behaviour of soda-lime-silica glass has been obtained through a comprehensive literature review. The review revealed that the tensile strength significantly increases with increasing strain rate, while a very limited amount of data characterise the glass at the extremes of strain rates. Furthermore, a good agreement between the experimental data and most load duration factors defined by various national and international Standards for glass was observed. <br/>To investigate glass at high strain rates, a novel experimental setup was designed and built, which is a modification of the well-known Split-Hopkinson Pressure Bar (SHPB). The modifications enabled high-speed cameras for fracture assessment and non-contact optical full-field deflection measurements using the Stereo Digital Image Correlation (Stereo-DIC) technique in a ring-on-ring bend testconfiguration. The high strain rate material characterisation at 48 s−1 (∝ 4.3 · 106 MPa s−1) included annealed float glass and also thermally tempered glass to study the effect of residual stresses. A significant strength enhancement was ascertained compared to a quasi-static strain rate, while the residual stresses did not affect the glass’ strain rate sensitivity. Also, the glass strength showed an expected increase with increasing residual compressive surface stress. The performed Stereo-DIC deflection measurements did not find a strain rate sensitivity in the glass’ Young’s modulus. <br/>The gained mechanical properties of the glass at high strain rates formed the basis to develop a rate-dependent progressive material damage model for explicit FEM. The focus was on a simple implementation, practicable for general engineering practice, using the element deletion technique to simulate the dynamic fracture of large-sized, thin-walled monolithic glass panes under blast loading. Global quantities such as a glass pane’s loadbearing capacity and post-fracture response were predicted and compared to results from blast tests found in the literature.

Topics

• impedance spectroscopy
• surface
• simulation
• glass
• glass
• crack
• strength
• fatigue
• tensile strength
• lime