Spontaneous fracture of tempered single-pane safety glass
Examination of damaged hardened glasses by SEM / EDX
Large format glass fronts are a design element of modern architecture. Often, tempered safety glass is used. In this material, cracks can be formed years after installation and spontaneous fracture can occur. The optical picture of a spontaneous break already provides the glass expert with valuable information on the cause of the damage. However, a clear determination of the cause usually requires more precise analyses of the defective hard glass with techniques such as SEM / EDX.
Detection of particle composition
Spontaneous fractures are often already recognized by their appearance, the so-called butterfly fracture. Figure 1 shows the optical image of such a butterfly fracture. In the center of the picture, a particle inclination is visible as the starting point of the spontaneous fracture.
A closer examination of the particle with the aid of scanning electron microscopy (SEM, figure 2) provides information about its diameter (about 200 μm) and morphology. In addition, the differences in brightness that are clearly visible in the backscattered electron image (figure 2) show a material contrast between the particles and the toughened glass.The areas marked in the backscattered electron image were therefore used to determine the elemental composition of particles and glass more precisely with energy dispersive X-ray spectrometry (EDX). Figure 3a and b shows the results of this analysis. The significant Ni and S signals show that nickel sulfide is the main constituent of the particle. Nickel sulphide particles can be introduced into glass panes by impurities in the molten glass. By phase transformations and thermal expansion, these particles can lead to stress and, at worst, even after years, to the fracture of the glass panes. To avoid such glass fracture in the installed state, modern tempered glass panes are subjected to a so-called hot storage test before their delivery. Panes that pass this test show a significantly reduced probability of a spontaneous fracture later on. Nevertheless, the combined SEM-EDX analysis in the example above clearly identified the damage mechanism of spontaneous glass fracture. This greatly facilitated the subsequent handling of the damage.