Glass delayed fracture

Glass that has been under stress for a period of time may fracture suddenly. Such delayed fracture is not common in metals (except in cases of hydrogen embrittlement of steels) but sometimes does occur in polymers. It is often called static fatigue. The phenomenon is sensitive to temperature and prior abrasion of the surface. Most important, it is very sensitive to environment. Cracking is much more rapid with exposure to water than if the glass is kept dry (Figure 15.11) because water breaks the Si-O-Si bonds by the reaction — Si-O-Si—H H2O -> Si-OH + HO-Si. 

This is a case of delayed fracturing which is the result of a large subsurface bubble that may form during the glassing cycle and does not show up in any of the... 

The above theory implies that if the surface energy term y is reduced, then 9pr will also be reduced. This led Orwan [80] to suggest that delayed fracture of glass occurs by adsorption of environmental species which lowers the surface energy, and hence the stress required to cause fracture. The same principles were later adopted to account for the SCC of metals. If environmental... 

Brittle fracture mechanics applied to glass and ceramics has proved to be a v aluable research tool. In particular, the v(K ) relationship enables the quantitative description of the mechanism of a delayed fracture.The development of linear elastic fracture mechanics have lead to the possibility of predicting time to failure of brittle materials subjected to stresses lower than their impact strengths. 

Elastic modulus Up to the fracture stress, glass behaves, for most practical purposes, as an elastic solid at ordinary temperatures. Most silicate-based commercial glasses display an elastic modulus of about 70GNm", i.e. about 1/3 the value for steel. If stress is applied at temperatures near the annealing range, then delayed elastic effects will be observed and viscous flow may lead to permanent deformation. 

Polybutadiene-PMMA (80 20) core-shell particles of 0.18 pm diameter have been shown to reduce the rate of embrittlement of polycarbonate degraded by physical ageing just below the glass transition temperature [132]. The unmodified polycarbonate became brittle (as assessed by 80% of specimens showing brittle fracture) after 5 h at 135 °C in air, whereas a blend containing 10 wt% of the core-shell particles withstood these conditions for 800 h before embrittlement. By using more thermally stable particles with a poly(n-butyl acrylate) core, embrittlement was further delayed to times greater than 4500 h,... 

The measurement is carried out as follows. The mass of the fibre (only the part that contributes to deformation under gravity is considered) and its section are determined, then its elongation with time is measured using an optical microscope (or using a dilatometer sensor) allowing for dL/dt to be calculated and hence the viscosity to be determined. For a viscosity regime over 10 Pa s, the method becomes difficult since there is a risk of fracture of the glass specimen under elevated applied loads and deformations are smalL Moreover, delayed elasticity (Section 6.4.4) and structural relaxation (Section 6.5.4) interfere with the viscous deformation. Then only an apparent viscosity is extracted. 

All bridgework will contain one or more pontics. If the porcelain is completely covering the pon-tic then the heat within the alloy cannot escape. Within hours of being fabricated the porcelain will continue to develop stress around its circumference until the interior of the alloy reaches room temperature. Occasionally the stress may be sufficient to produce a crack in the porcelain. This time-delay type of fracture can be alleviated by leaving a collar of metal on one side of the pontic. This band will prevent the formation of hoop stresses in the porcelain that cause the cracks. Similar to the glass-to-metal seals, adherence of the porcelain to the alloy is via a chemical bond between the porcelain and alloy. The critical sequence is as follows ... 

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