Mechanical Properties of Tempered Glass

Theoretical and actual strength of tempered glass
The mechanical strength of tempered glass is generally expressed by compressive strength, flexural strength, tensile strength and impact strength. From the perspective of mechanical properties, glass is widely used because of its high compressive strength and hardness. However, due to its low tensile strength and flexural strength, as well as its great brittleness, the application of glass is limited.
The theoretical strength of tempered glass is equal to 11.76GPa according to Orowan assumption. The average strength of glass fiber without serious defects on the surface can reach 686MPa. The tensile strength of glass is generally 34.3~83.3MPa, while the compressive strength is generally 4.9~1.96GPa. However, in fact, the bending strength of window glass and bottle glass is only 6.86 MPa, which is 2 to 3 orders of magnitude different from the theoretical strength.
The low actual strength of tempered glass is due to the brittleness of the glass and the existence of microcracks and uneven zones in the glass. As the glass will not flow under the stress, the microcracks on the surface will expand rapidly, and the stress concentration will lead to fracture. In order to improve the mechanical strength of glass, methods such as annealing, tempering, surface treatment and coating, crystallizing, and making composite materials with other materials can be adopted. These methods can greatly improve the mechanical strength of glass. Some can double the bending strength of glass, and some can even increase it dozens of times.
The main factors affecting the mechanical strength of tempered glass are:
① Relationship between glass strength and chemical composition
The bond strength between glass structures with different chemical compositions is also different, which affects the mechanical strength of glass. The order of effect of various oxides on the tensile strength of glass is:
  CaO>B203> BaO>Al2O3>PbO> K2O>Na20> (MgO,Fe2O3)
The order of the effect of various oxides on the compressive strength of glass is:
  Al2O;> (MgO,SiO2 ,ZnO)>B2O:>Fe2O:>(CaO,PbO)
② Defects in glass
Macroscopic defects such as solid inclusions, gaseous inclusions, chemical inhomogeneity, etc., cause internal stress due to their chemical composition inconsistent with the main glass.
The phenomenon increases, so that more strain energy is accumulated at the defect. The relationship between glass strength and temperature