Strength classification of flat glass for better quality – validation of method by well-defined surface defects and strength testing

• The current project was a collaborative project between the involved partners: RISE Glass, Lund University and Acoustic Agree. It is funded by ÅForsk (Grant No. 19-479). The project is a follow-up project from a Smart Housing Småland (Grant No. 2016-04218) pre-study where we used a nonlinear acoustic wave (NAW) to determine the damage value in float glass simultaneously with four-point bending tests. Glass is a brittle material whose strength is primarily determined by its surface characteristics i.e., the presence of flaws, defects or cracks on the surface. The strength of glass is greatly limited by stress-concentrations at the crack tips generating very high stresses when the glass is under load. The size and distribution of surface defects vary greatly, this gives a great variation of strength of glasses so that conventionally very large safety measures must be employed for glass products. If these defects and/or cracks could be detected in a non-destructive way, it would be beneficial for glass manufacturers as well as final building users. Nonlinear acoustic wave (NAW) techniques can be used to detect defects in materials. In these methods, acoustic waves are transmitted through an object and nonlinear effects, caused by the defects in the material, is analysed from the signal obtained at the receiver. The aim of the current project was to establish a calibration and a clear correlation between nonlinear acoustic wave measurements and the ultimate strength of annealed glass samples with controlled defects. Controlled defects were made as indentation imprints with a microindenter, equipped with a Vickers diamond head, in the middle of float glass samples with the dimensions 4×100×100 mm3. The applied loads were 0.5N, 1N, 2N, 5N and 10N. The indents were inspected with a microscope in order to see the cracks and the depth of the indents were also determined. The formed defects (cracks) were detected with NAW technique. Analysing the waves after propagating in the glass the nonlinear content in the wave was analysed. Due to the objects damage, the propagated wave distorts proportionally to the damage. After the NAW-inspection the strength of the glass samples were tested with ring-on-ring tests. Using the results from NAW-inspection, a clear correlation between the nonlinear response and the indenter load was found. There was also an obvious correlation between the failure load on the ring-on-ring-tests and the indenter load. The standard deviation for the ring-on-ring-tests for the 1N, 2N, 5N and 10 N was low but for the 0.5 N load was very high. A possible explanation is that the indenter imprint in most of the cases only gave rise to plastic deformation and in some samples, cracks were formed too. There were visible cracks for all the higher indenter loads and thus a lower scatter of the results. The main conclusion of the project is that it is possible to detect small cracks, which cannot be seen with the naked eye, with NAW technique and it can be directly correlated to the strength of the glass.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Teknisk mekanik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Applied Mechanics (hsv//eng)

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