E F F E C T OF A R T I F I C I A L D E F E C T S ON S T R E N G T H OF GLASS

O. A. T r o i t s k i i UDC 666.11.01 : 539.4

E. D. Shchukin and colleagues [1] investigated the action of defects in glass created artificially in its surface layer with a diamond pyramid or corundum needle, and imitating corresponding real defects in glass, whichlower its strength.

This work continues the investigation of the effects of the same type of defects on the strength of glass, and in it special attention is paid to the reciprocal orientation of defects and the influence of the atmosphere.

Specimens of ordinary object glass measuring 25 x 25 x 1.7 ram, subjected to annealing and preliminary strong etching (to remove the defective surface layer 100-120/1 thick), with pricks applied to the center with a diamond pyramid, were placed on the annular support. The strength of the glass was assessed [2] as the magnitude of the strains at the moment of destruction on the lower surface of the specimens containing the defects*.

Figures 1-3 give the basic results of the investigation showing the relationship between the strength of the glass and the distance between the defects, their orientation and numbers. The type and form of defects in all ex- periments were kept constant; this is an impression from a four-face pyramid obtained on the PMT-3 apparatus. The depth of the defect and the resulting danger was due to the load on the indenter in the process of applying the impressions. Each point on the graphs is the mean of 30 measurements. In all experiments, we maintained the fol- lowing sequence of operation. The specimens were heated in low vacuum conditions at 200* for 2 h. The batches of glasses, consisting of 30 specimens, were subjected to strong etching with hydrochloric acid, then they were placed in a dessicator, whence they were extracted one at a time for applying the indentations, and tested for strength, The time between the etching and indentation with the diamond pyramid and strength tests was governed by technical processes, and for the first specimens was 2-3 rain, and for the latter about 1 h. To reduce the possible damage to the glass from the test equipment during measurements of strength, we used soft gaskets, and the place of contact of the specimens with the test components was additionally protected with a lacquer film.

Figure 1 gives the values for the strength of the specimens of the original glass with two indentations made by the diamond pyramid, orientated relative to each other by diagonals, with different loads on the indenter, de- pending on the distance between the impressions, measured in units of length of the diagonals of the impression d. The vertical lines are the mean square deviations of the various measurements; the striated region is the measured strength of the original glass. As seen from the diagram, with increase in the distance between the defects, as a re- sult of reducing the reciprocity between them, the strength of the glass increases, and with small loads on the in- denter (5 g), the values tot the strength of the glass without the impressions. Hence, we can make the important conclusion, that the real defects remaining in the "nondefective" glass, and determining its resistance to destruction, are equivalent in their action to impressions with small loads on the indenter. The mean-square deviations for the specimens with a load of 20 g also enter the region of the originaI nondefective glassi however, only with large dis- tances between the indentations (200 d). Consequently, to some degree it would be justifiable to consider that these defects also give a certain equivalent, characteristic, real defect in the glass, namely, a tentative, nonrelated,

dangerous defect.

Figure 2 gives the values for strength of specimens of original glass with 2 indentations from the pyramid, ap- plied at distances of 20 d, in the case of orientation of the impressions, on the sides and diagonals with different loads P on the indenter (h is the depth of the imPression). As Fig. 2 shows, with diagonal impressions we get an ad- ditional reduction in the strength of the specimens, which is due, in addition to the reinforcement of the effect of

* The fact that the values for strength of the glass given in this article are lower than the maximum level (200- 300 kg /mm 2) obtained F. F. Vitman for window glass (see F. F. Vitman, et al., FTT, 1965, Vol. 7, No. 9), in this case is not important, since we are talking not about the level of the strength, but of the new method of studying the strength of glass.

Institute of Solid State Physics, Academy of Sciences of the USSR. Translated from Steklo i Keramika,

Vol. 23, No. 8, pp. 5-7, August, 1966.

406

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10 2 20 200 d

Distance between defeem, S

Fig. 1. 1) Load on indenter 200 g; 2) same 100; 3) 50 g; 4) 20 g;5) 5 g.

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Fig. 2. 1) Impressions with the sides of diamond pyramid; 2) same, diag- onals.

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No. of defects N

Fig. 3. 1) Load on indenter 100 g; 2) same 50g; 3) 2 0 g ; 4 ) 5g .

geometrical concentrations of strains and expansion of the zone of action of microcracks in the angles of the im- pressions, also to the reinforcement in the role of local microheterogeneities during the plastic deformation in the glass.

Figure 3 shows the relationship between the strength of the glass and the number of orientated defects intro- duced in the central part of the specimens in the square zone at a distance of 200 d from each other (the accidental nonrelated and independent defect). As seen from this diagram, the maximum fail in strength due to defects, oc- curs with the appearance of the first impressions. This is in complete agreement with previously published work [1] on the strength of glass.

On the parts of the glasses where the defects were applied by the standard method (9 impressions applied in the square zone at a distance of 200 d from each other, with loads on the indenter of 5 and 100 g) directly before testing at the site of the impressions we introduced a droplet of water or a solution of water glass. The presence of the film of water on the surface of the glass always reduced the strength. Our experiments showed that the surface- active medium (water) reduces the strength of the specimens to 32%0 compared with glasses containing defects but not having the films of water (in the case of impressions obtained with loads of 100 g; with small loads on the in- denter the refractory was much weaker). This phenomenon, reduction in strength of up to 20%o, was observed with the introduction of a solution of water.

The results obtained are explained by the adsorption-reduction in the strength of glass in conditions of addi- tional adsorption of atoms and molecules from solutions on the developed surfaces of the boundary in the bond with the newly introduced defects. The resulting reduction in free surface energy of the body reduces the work of defor- mation of the glass, and reduces the measured s~ength of the material. In a somewhat alternative arrangement, the question of the stated defect can be considered as proof of the enhanced interaction of defects, due to surface-active media.

Some of the specimens containing the same number of standard defects, before testing for strength, were sub- jected to etching: weak 1.5%o solution of HF for a period of 1-1.5 rain, and strong 10%o solution of HF to remove the layer, exceeding in thickness the depth of the defect application. (If after basic etching, which was done for all specimens, we etched 100-120~:, then after repeated strong etching we removed a layer 80-90 /l,) The table gives the values for the strengtl~ of nondefective glass; glass containing 9 impressions at different loads on the indenter; and glass containing impressions and subjected to weak etching; finally, glass containing impressions and subjected to strong etching. The tests showed that with weak etching, the strength of the specimens was depressed compared with the nonetched specimens (but also possessing defects) by 25-30%, while with strong etching, on the other hand, there was a sharp increase in strength, of 250-300%.

407

Strength of glass having 9 impressions,kg/mm 2

without repeat etching after weak etching after strong etching Smength of strongly etched nondefective

kg/mm2 load, g glass,

5 50 5 50 5 50

46- 52 43 23 32 26 48 52

Special attention should be given m the reduction, revealed in our experiment, in the strength of defective glass, as a result of the action of light etching (the increase of the strength of glass after strong etching has been known for a long time).

As follows from the table, light etching reduces the strength only in specimens with indentations obtained with low loads on the indenter. In the case of large loads (50 g and above) any etching gives rise to an increase in the strength of the specimens. With low loads on the indenter, in the angles of the impressions we did not detect visible microcracks, but as a rule they were present with large loads. If we consider this fact, then the action of light etch- ing should be connected, not with the geometrical concentration of strains, introduced by the impressions, but in the main with the local microheterogeneities of the plastic deformation of the glass. Consequently, the introduction of artificial defects in combination with light etching can be shown to be an effective method of investigating the mi- croheterogeneity of plastic deformation in glass.

Reviewing the results of the experiments with simple glass, we should mention that the method of applying artificial microscopic defects permits us to model the action of real defects on the strength of glass. Quantitative investigation of this type of defect, especially with different types of etching, and in the presence of surface-active media, creates the prerequisite for carrying out the well-known analogy with defects in periodic structures, and plays the role of local deformation and strains in glass, and contributes to an understanding of the mechanism of deform- ation of glass.

1.

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LITERATURE CITED

E. D. Shehakin, et al., DAN SSSR, 160, No. 5 (1965).

Manual of Glass Production, edited by I. I. Kitaigorodskii and S. I. Silvestrovich, Gosstroiizdat (1963).

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