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Different types of test specimens were applied in the past for the measure-ment of subcritical crack growth in silica. The rather large scatter of crack-growth curves calls for re-analysis of stress intensity factors. In the present note semi-elliptical surface cracks in long cylinders are addressed. In addition the first regular stress term (T-stress) is considered.

The equation between the fracture strength and the crack size published by Griffith in his famous paper in 1921 is not correct. Griffith corrected the equation in a conference volume in 1924 without a sufficient explanation. Afterwards several authors published papers with correct and not correct derivation of the Griffith relation. These papers are critically reviewed and the flaw in the first paper of Griffith is analysed.

The prediction of strength in silica needs knowledge of failure behaviour of defects in the surface region. Pores and cracks are studied by using the Finite Element method. The results for pores are given in terms of stresses, the results for cracks by stress intensity factors.

Strength of silica fibers heat treated in humid air is affected by swelling strains. In the absence of externally applied stresses swelling is isotropic. Under uniaxial stress an anisotropy of swelling strains has to be expected with maximum expansion in stress direction. In order to compute the maximum possible anisotropy, a Finite Element analysis is carried out. Under the assumption of a spherical nano-pore generated by a single bond breaking event, an averaged axial swelling strain was computed that is by a factor of >2 larger than the volume swelling strain.

Water in contact with silica glass surfaces diffuses into the glass, and reacts with the silica network under hydroxyl generation. Clear evidence has been reported in the literature for a volume swelling due to the water uptake and reaction in silica. In the past, the authors showed in a couple of papers the principle effects of volume swelling by hydroxyl generation on mechanics and fracture mechanics properties. From literature data on density as a function of water content a linear dependence of the volume swelling strain with water concentration was established. This dependency will be discussed in the present report including data scatter.

Fracture toughness resistance curves describe a material’s resistance against crack propagation. These curves are often used to characterize biomaterials like bone, nacre or dentin as these materials commonly exhibit a pronounced increase in fracture toughness with crack extension due to co-acting mechanisms as crack bridging, crack deflection and microcracking. The knowledge of appropriate stress intensity factors which depend on sample and crack geometry is essential for determining these curves. For the dental biomaterials enamel and dentin it was observed that under bending and tensile loading crack propagation occurs under certain constant angles to the initial notch direction during testing procedures used for fracture resistance curves determination. For this special crack geometry (kink crack of finite length in a finite body) appropriate geometric function solutions are missing. Hence, we present in this study new mixed-mode stress intensity factors for kink cracks with finite kink length within samples of finite dimensions for two loading cases (tension and bending)which were derived from a combination of mixed-mode stress intensity factors of kink cracks with infinitely small kinks and of slant cracks. These results were further applied to determine fracture resistance curves of enamel and dentin by testing single edge notched bending(SENB) specimens. It was found that kink cracks with finite kink length exhibit identical stress fields as slant cracks as soon as the kink length exceeds 0,15 times the initial straight crack or notch length. The usage of stress intensity factor solutions for infinitely small kink cracks for the determination of dentin fracture resistance curves (as was done by other researchers) leads to an overestimation of dentin’s fracture resistance of up to 30%.

The effect of water soaking on the strength of silica glass is studied. When silica glass is immersed in warm water and held there for an extended period of time, the strength increases over that of freshly damaged glass. The increase in strength is interpreted as the consequence of water diffusion into exposed surfaces of the test specimen, which results in swelling of the glass and shielding of cracks present in the surface of the glass. Experimental results are compared with theoretical predictions.

The effect of water soaking on the strength of silica glass is studied. When silica glass is immersed in warm water and held there for an extended period of time, the strength increases over that of freshly damaged glass. The increase in strength is a consequence of water diffusion into exposed surfaces of the test specimen, which results in swelling of the glass and shielding of cracks present in the surface of the glass. In our first paper on this subject (Report 19 of this series), we considered swelling effects on the inert strength. In the present report, the strength under subcritical crack growth conditions is studied. For tests carried out in humid environment at various loading rates, so-called dynamic strength tests, we could show theoretically that the swelling effect cuased by the reaction of water with silica must result in apparently increased crack-growth exponents. This prediction is in good agreement with results from literature. In our experiments we could show via an evaluation of the crack extension that even in silicone oil environment local subcritical crack growth accurs.

Most ceramics show an increase of the crack growth resistance during crack propagation (R-curve behaviour). Reasons for such behaviour are bridging effects between opposite crack surfaces, phase transformations around the tip of a crack, and development of micro-cracking zones. This booklet predominantly deals with the bridging behaviour and the discussion of the observed effects in terms of the fracture mechanics weight function procedure.

International audience; In this paper, we discuss the effect of water on the strength and static fatigue of silica glass. When a crack is formed in silica glass, the surrounding environment rushes into the crack; water then diffuses from the environment into the newly formed fracture surfaces to generate a zone of swelling around the crack tip. Because the swollen material is constrained from expanding by the surrounding glass, a zone of compressive stress is generated at the fracture surface around the crack tip. The results are similar to those found for transformation toughened zirconium oxide, with the exception that the transformation zone in silica glass grows with time, so that the effect gets progressively stronger. Using diffusion data from the literature, we show that the diffusion of water into silica glass can explain several significant experimental observations: the reported strengthening of silica glass by soaking in water at 88 degrees C; an increase in the slope of dynamic fatigue curve by prior exposure to water at 88 degrees C; the observation of a static fatigue limit in silica glass at very low values of the applied stress-intensity factor; and the observation of crack face displacements caused by water penetration into the glass at the crack tip.

Failure of cracked components is governed by the stresses in the vicinity of the crack tip. The singular stress contribution is characterised by the stress intensity factor K, the first regular stress term is represented by the so-called T-stress. Sufficient information about the stress state is available, if these two parameters are known Results for K and T are compiled in form of figures, tables, and approximate relations.