A team of researchers from Pusan National University (PNU) (Busan, South Korea) recently investigated the mechanics behind a phenomenon known as “contact flaws,” which occurs when glass windows that are exposed to harsh environmental stresses for extended periods of time develop tiny cracks on their surfaces that grow when they encounter water droplets. The PNU team’s study was published in the Journal of the European Ceramic Society.
Glass windows often endure external environmental factors such as wind, rain, and humidity, which lead to the formation of microcracks in their surface. These microcracks then grow in size when aggravated by water droplets and humidity.
When water comes in contact with these surface flaws, it penetrates the microcracks and slowly dissolves the silicon-oxygen bonds. As a result, the mechanical strength and optical properties of glass structures gradually degrade, a process known as stress corrosion or subcritical crack growth (SCG). The depreciation of mechanical performance due to SCG raises safety concerns for skyscrapers and high-rise buildings.
While scientists have recently taken an interest in the effects of SCG as it pertains to two widely-used types of soda-lime glass, annealed and tempered glass, most studies have either focused on establishing crack growth rates—with comparatively little known about the effect of water on the propagation of crack flaws—or on the microcrack propagation dynamics of annealed, but not tempered, glass.
To address this research gap, the PNU research team led by Prof. Sanghu Park explored the mechanisms associated with water-promoted cracking behavior around surface micro flaws in both annealed and tempered glass.
“Thermally tempered glass is of great commercial importance; it is used in displays, protection covers, vehicular windows, and others,” explains Park. “The crack propagation of tempered glass is different from that of annealed glass by water droplets. So, it's really important to understand how water droplet promotes cracking behavior in different types of glasses.”
To mimic surface microcracks, the research team created artificial flaws on a glass surface using a Vickers indenter, an instrument used to test the hardness of a material. The samples were then exposed to atmospheric humidity and water to analyze crack growth. The team discovered that the maximum length of crack growth was much lower in tempered glass than annealed glass. A subsequent 24-hour water immersion text revealed the formation of radial cracks in annealed glass and lateral cracks in tempered glass.
The comparative study provides new insights into the origin of glass cracks which can be useful for glass designers, engineers, and architects.
“With an increase in typhoon and storm frequencies due to global warming, we need to find out new ways to protect the exterior of glass windows,” says Park. “We tried to understand the interactions between water and surface cracks to help solve safety problems associated with glass windows and structures.”
Source: Pusan National University, www.prnewswire.com.