Materials Science, Vol. 39, No. 1, 2003

EFFECT OF WATER ON THE IMPACT STRENGTH OF GLASS PLATES WITH ERODED SURFACES

S. Benbahouche, F. Roumili, and R. Zegadi

Introduction

Historic. There are many useful research works on reactions between glass and aqueous solutions [1–8].After 1970, the research works based on the results of contemporary studies began to appear giving the real situ-ation on the surface and, thus, leading to the understanding of the surface phenomena [9–18].

Reactions with Water

If a glass reacts with water or an aqueous solution, then the chemical changes in the solution occur on thesurface and depend on the chemical composition of the glass, the pH value and temperature of the solution, andthe time of immersion [18].

The initial stage of the chemical reactions is the ionic exchange between the molecule of water and a sili-

con–oxygen bond. This exchange decreases the number of H+ ions of water which become alkali and provokethe process of glass dissolution.

This chemical interaction proceeds in three stages:

1. A molecule of water slides into a crack and is absorbed at its mouth. Electrons of the atom of oxygen inthis molecule pass to a vacant electron orbit of the atom of silicon and one of the atoms of hydrogen in the mole-cule of water is attracted by one of the atoms of oxygen in the silicon–oxygen chain.

2. Formation of these new bonds weakens the original silicon–oxygen bonds.

3. The atom of hydrogen is completely linked with the atom of oxygen of the silicate system and the sili-con–oxygen link breaks (see Fig. 1) [18], [20].

The effect of water on the static fatigue of glass structures is ambiguous: if the glass is loaded, its strengthconsiderably decreases and the fatigue fracture is accelerated. However, if the unloaded glass is removed fromwater and subjected to tests, then we observe an insignificant increase in its strength [20].

Silicate glasses age as a result of humidity absorption and, thus, the following hydrogen reaction takesplace:

H2 O + Si – O – Si → 2 Si – OH.

This chemical reaction forms a fine layer of glass on the surface called silica gel.

Departement d’Optique et de Mécanique de Précision, Faculté des Sciences de l’Ingénieur, Université Ferhat ABBAS; Sétif, Algeria.Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 39, No. 1, pp. 124–126, January–February, 2003. Original article sub-mitted May 3, 2001.

148 1068–820X/03/3901–0148 $25.00 © 2003 Plenum Publishing Corporation

EFFECT OF WATER ON THE IMPACT STRENGTH OF GLASS PLATES WITH ERODED SURFACES 149

Fig. 1. Explanation of the static fatigue of glass: (a) two-dimensional model of a crack induced by water in silica glass, (b) uniform

attack of cracks in the absence of stresses, (c) preferential attack at the bottom of cracks under tensile stresses [22].

Testing Method

In the test, a steel ball falls upon the glass plate supported along the perimeter by a layer of plastic materialto absorb the shock.

The height h from which the ball falls on the same specimen increases with steps P until the specimenfails. The energy of its fracture (J) is given by the formula

Er = m g h, (1)

where m is the mass of the ball (kg), h is the maximum height of falling (m), and g is the gravitational acce-

leration ( m / sec2 ).

The impact strength is characterized either by the maximum height of the ball or by the level of stresses forwhich the specimen suffers fracture. The indicated level is specified by the formula [21]

σr = 18E E

Vr [MPa], (2)

where E is Young’s modulus (MPa) and V is the volume of the specimen ( m3 ). For a plate, V = l

2 e (m3),

where l is the perimeter of the support of the plate (m) and e is the thickness of the specimen (m).

Materials and Experimental Procedures

For our study, we used a soda-lime glass plate with Young’s modulus equal to 72 GPa, Poison’s ratio equalto 0.22, and the following chemical composition: 71.5 wt.% Si O2 , 13.7 wt.% Na2 O, 7.9 wt.% Ca O, 4.2 wt.%Mg O, 1.3 wt.% Al2 O3 , 0.9 wt.% K2 O, and 0.25 wt.% S O3 ; balance 0.25 wt.%.

All specimens (100 x 100 x 4 mm) were prepared from the same plate. The surface of the specimens wasdamaged by a certain amount sand with known grain size freely falling from a fixed height [22]. The ability ofsand grains to create microcracks on the specimen surface is responsible for its deterioration by gravitation.

The experimental study included:

— Measuring of the roughness criteria prior to and after immersion with a Hommel-test-type TODC pro-filometer. Soda-lime glass plates inclined at an angle of 45° were bombarded with sand (200 g). The

150 S. BENBAHOUCHE, F. ROUMILI, AND R. ZEGADI

size of sand grains was selected by sifting and measured with a NEOPHOT-21 optical microscope. Itwas equal to (0.456 ± 0.065) mm. The height of falling of sand was equal to 1 m with a constant de-bit of 1.66 g / sec.

Fig. 2. Impact strength as a function of immersion time for different temperatures.

Table 1. Ball-Shock Strength of Glass Specimens After Degradation and Immersion in Water

t, min 30 60 90 120

σr , MPa 93.8 ± 4.1 94.7 ± 5.7 95.0 ± 6.3 95.3 ± 4.5

T = 50°C δ σr , % 0.3 1.2 1.5 1.9

Nsr 5 6 6 7

σr , MPa 95.2 ± 5.2 95.5 ± 6.3 95.6 ± 7.8 95.8 ± 6.4

T = 70°C δ σr , % 1.7 2.1 2.2 2.4

Nsr 5 6 7 7

σr , MPa 95.6 ± 5.4 95.8 ± 6.4 96.1 ± 5.3 96.3 ± 4.2

T = 90°C δ σr , % 2.2 2.4 2.7 2.9

Nsr 5 6 7 8

— The specimens were immersed in a bath of water with different temperatures (50°C, 70°C, 90°C) fordifferent times (30, 60, 90 and 120 min). The bath of water 90 × 60 × 40 cm in size was equippedwith 03 thermoplungers, a thermostat, a thermometer, and an agitator aimed at leveling the tempera-ture inside the bath. The pH value of water was equal to 6.5.

— The impact strength was determined by ball shocks in the course of laboratory tests with the followingparameters: steps of the height of falling P = 30 mm and the mass of the ball m = 48 g.

EFFECT OF WATER ON THE IMPACT STRENGTH OF GLASS PLATES WITH ERODED SURFACES 151

Results and Discussion

The impact strength prior to degradation σ0 = (95.2 ± 6.6) MPa, the number of repeated shocks Nsr 0 = 6,

the arithmetic mean of the parameter of roughness R a0 = (13 ± 1) nm, and the maximum roughness Rmax 0 =(57 ± 6) nm.

After degradation and prior to the immersion, we measured the impact strength of the specimens σbef =(93.5 ± 5.2) MPa by shocking these specimens with balls for the number of repeated shocks Nsr = 7.

Roughness. The values of roughness were measured in the upper part of the specimen along a straight linedirected from the side of the specimen to its center with steps following the increments of the height of falling ofthe ball of 10 mm. The arithmetic mean of the surface roughness after the immersion varied within the range be-tween 0.3 and 0.1 µm.

We note that the surface roughness of the specimen insignificantly varies as a function of immersion timeand temperature, which can be explained by the reaction of water.

At the same time, for T = 90°C and two immersion times t = 90 min and t = 120 min, we get nearly thesame values of roughness. This can be explained by the total equalization of corrosion on the surface of the spe-cimen. Hence, the time and temperature required to have almost uniform corrosion of the specimen surface areequal to 120 min and 90°C, respectively.

Impact Strength. It follows from Table 1 and Fig. 2 that the impact strength σr and the number of repeatedshock Nsr insignificantly increase with temperature and immersion time. This is explained by the fact that wa-ter attacks the surface of immersed specimens and remains in microcracks. As a result, the microcracks becomemore rounded with time and at higher temperatures.

CONCLUSIONS

1. In the present work, we study the influence of water attack for different times and at different tempera-tures on the impact strength of the soda-lime glass surface eroded by sand.

2. The relative increment of the mechanical strength of the immersed specimens varies from 0.29% to2.87% as the immersion time and temperature increase. The period of time and temperature required for the al-most uniform corrosion of the specimen surface are as follows: t = 120 mm and T = 90°C.

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