Browsing by Author "Gehrke, Eckhard"

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    Correlation between multistage crack growth and time-dependent strength in commercial silicate glasses Part 1. Influence of ambient media and types of initial cracks
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Gehrke, Eckhard; Ullner, Christian; Hähnert, Manfred
    The dependence of flexural strength upon loading velocity is studied regarding commercial glasses (sodium-borosilicate, sodium-aluminosilicate and soda-lime-silica glasses) in various ambient media (water, air, nonane, ethyl alcohol) after having produced various types of surface damage (emery paper, powder jet, Vickers indenter). This dependence (fatigue curves) is calculated by means of a numerical method based on data on macroscopic crack growth and compared with experimental results. Extension of the study to extremely high and low loading velocities allowed for experimental representation of multistage fatigue curves and also for their comparison with calculated results in a wide range from which some peculiarities of the subcritical growth of surface microcracks can be derived. On the other hand, the influence of ambient media on the life time of glass under stress could also be studied. It is pointed out that two factors are of great influence: the types of the initial cracks (part 1) and the treatment after damaging the surface but before performing the fracture tests (part 2 ) .
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    Correlation between multistage crack growth and time-dependent strength in commercial silicate glasses Part 2. Influence of surface treatment
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Gehrke, Eckhard; Ullner, Christian; Hähnert, Manfred
    The influence of surface treatment on the fatigue behaviour of commercial glasses is investigated; especially as to whether or not the fatigue behaviour of glass specimens with well-defined surface damage can be predicted from macroscopic crack growth data by means of the fracture mechanical model, if the specimens are submitted to corrosive or heat treatment after the damage has been applied but before the fracture test was performed. The results show that the correlation between macroscopic crack growth and fatigue behaviour of such specimens is only of restricted validity. Differences become obvious in the range of the long-term strength (which is of special practical interest) as the relative fatigue limit and the slope n1 in region 1 are increased. This means that the tendency to fatigue is reduced through the additional treatment.
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    Effect of corrosive media on crack growth of model glasses and commercial silicate glasses
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Gehrke, Eckhard; Ullner, Christian; Hähnert, Manfred
    The crack growth on silicate glasses of the systems x Na2O - (100-x) SiO2, x Na2O - 11 Al2O3 - (89-x) SiO2, x Na2O - 11 B2O3 - (89-x) SiO2, x Na2O - 7 CaO - (93-x) SiO2 and x Na2O - 17 CaO - (83-x) SiO2 as well as on two commercial glasses (soda-lime-silica and sodium borosilicate glass) in water, acids and bases has been studied. In alkali oxide containing silicate glasses the crack-growth mechanism is based on a stress-stimulated splitting of siloxane bridges due to attack of both hydroxide ions (dominant in NaOH) and molecular water (dominant in LiOH). Real network dissolution with a rate like the crack-growth velocity (occurring in hydrofluoric acid or in water on extremely leachable glasses) leads to a plateau-like region of the crack growth. The threshold of crack growth in alkali-containing glasses is caused by alkali leaching forming a surface-near glass layer with modified mechanical properties within the cracks. Therefore, this threshold is enhanced with increasing pH value of the medium. The two commercial multicomponent glasses behave in a similar way like the investigated model glasses. The crack-growth curves indicate that the failure risk decreases in the order base, water, acid as well as with increasing concentrations of base or acid.