Crack growth, including fatigue limit and crack arrest, have been investigated for glasses of the systems xNa20-11AI203-(89-x)SiO 2, xNa20-(100-x)Si02 and xNa20-7CaO-(93-x)Si02 in water as well as in acid and alkaline solutions. From studies of the dependence of the crack arrest on the alkali content of the glass, the kind of alkali (K § Na § Li § the pH of the corrosive medium, the ageing time and ageing loading in conjunction with measuring the alkali leaching behaviour, the basic mechanism of crack arrest and fatigue limit can be concluded. Owing to load-and medium-dependent diffusion processes, a crackgrowth retarding leached layer at the crack is generated with modified strength and crack growth properties compared to the bulk properties. In high alkali-containing glasses the process is additionally stimulated by stresses produced in the leached layer at the crack tip and at the crack surfaces.

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E WELCOME the experimental results in the paper by W Han and Tomozawa' concerning the important influence of the corrosion processes on the strength behavior of glass. Our results have confirmed2-6 that the leached layer generated by corrosive attack plays a central role in the mechanisms determining the strength. However, in our experience, the results limited to the strength of soda-lime glass and other commercial glasses are not sufficient to decide whether the crack-tip blunting model or the residual stress model represents the real mechanism. Corrosion processes on glass are so various that studies of strength and fatigue as well as crack growth have to be conducted in media with different pH values on many model glasses. Several glass systems must be used to separate the effects of different mechanisms. This comment presents four concerns in detail.(1) We agree that the change of strength by aging is based on the generation of leached layers in the crack-tip region (without fixing the special crack-tip model) because the correlation to the leaching process is evident2 and crack length variation is not ~bservable.~ However, there is not only strength R. E. Newnham-contributing editor increase but also strength reduction. A spectacular example of this is how the dependence of inert strength on aging time is completely changed by increasing the temperature from 23" to 80°C (Fig. 1). The strength reduction due to stress generation in the layer becomes so strong that the specimen fails without any external load. If the leaching process plays the dominant role, the maximum attainable enhancement of the strength by aging must be dependent on the alkali content in the same way for different series of glasses. As first shown in Fig. 2, there is the same relative strength rate rising up linearly to about 24 mol% NazO. Above that level, the relative strength of the borosilicate glass increases continuously, whereas the relative strength of the alumosilicate glass decreases (stress generation due to structural modification by water incorporation) .'(2) We can confirm the observed change of dynamic fatigue curves by aging (Fig. ll in Ref. l), but more interesting effects occur by extending the range of the stressing rate by 5 orders of magnitude to smaller rates. Figure 3 demonstrates the combined effect of stress generation and network dissolution in the range of failure time above 10 min. In this range of low stressing rates, the fatigue curves are also influenced for glasses which do not show network dissolution as the dominant mechanism. Therefore, a substantial increase of the fatigue limit has been observed for different kinds of mechanical damage in the following order: emery paper, grit blasting, and Vickers pyramid indentati~n.~ The data show that both residual contact stresses' and leaching might affect strength 3oNa20 -ltA124 -59Si0, aging in water 0 20 40 60 80 100 aging time t(h) Fig. 1. Superposition of the processes which cause either an enhancement or a reduction of the inert strength on highly alkali-co...