Phenomena and mechanisms of crack propagation in glass-ceramics

The influence of the composition, shape and mean size of the crystallites on the mechanical performance of glass-ceramic materials that contain solid wastes originating from petrochemical distillery and steel scrap recycling facilities has been investigated. It was found that the fracture modes depend on the composition and the shape of the separated crystallites. Additionally, it was found that the increase in their mean size reduces crack propagation, while the fracture mode shifts from transgranular to intergranular. The above shift in the mode of fracture is accompanied by a change in the type of the radial cracks, namely from primary radial in the former to secondary radial in the latter case. Taking into account that the fracture mode determines the strength and toughness, by manipulating the functional properties of glass-ceramic materials, their mechanical performance can be tailored according to the requirements of the potential applications. The results of the present study will be used to determine the composition and morphology of the glass-ceramic materials, in order to achieve the desired morphology that will result in the proper mechanical performance.

Journal of Non-crystalline Solids, 2010

The rectangular specimen edges of optical and technical glasses were fractured in the tests with conical indenters and pyramids. It was demonstrated that in the Rockwell indentation, away from those edges a ring crack and a Hertzian cone were formed. But when the indentation point approaches the specimen edge, only fragments of ring cracks develop and chip scars look like a Hertzian quasi-cone. The behavior of glasses was investigated with the edge fracture (EF) test method. It is shown that the fracture resistance of float glass is higher than that of fused silica and other optical glasses, the lowest fracture resistance is displayed by heavy flint. When glass surfaces are indented by a Berkovich indenter, lead glasses exhibit a better fracture resistance. It was revealed that glasses possessed the barrier to the onset of fracture, not typical of conventional brittle materials. The results confirm that the time-and cost-saving EF test method can be quite promising for comparative estimates of the ability of glasses to resist fracture.

Journal of Materials Science, 1976

Flaw size, c, fracture mirror boundaries, r, fracture stress, o, and critical fracture energy were measured for glasses, glass ceramics, and single and polycrystalline ceramics. The relationship or 1/2 = constant was verified for all these materials. The mirror constants, A, in these materials were shown to be directly proportional to the average critical stress intensity factor for crack propagation, Kin. Based on the A --Kic relationship, the outer mirror to flaw size ratio is shown to scatter about a value of 13:1. Thus, the mirror constants were used to predict critical flaw sizes in these materials. The observed flaw sizes in most cases correlated well with those calculated. The cases in which poorer correlation was obtained are those in which flaw sizes were smaller than the grain size, flaws were pores or surrounded by porous regions, or where severe microcracking existed. it is shown that the elastic modulus is proportional to the mirror constant and probably to the critical fracture energy, but that the latter is highly dependent on local microstructure. The smaller inner to outer mirror ratios for polycrystalline ceramics over glasses is attributed to the difference in available paths for crack propagation.

Dental Materials, 2011

Dental ceramics abstract Objective. To determine the slow crack growth (SCG) and Weibull parameters of five dental ceramics: a vitreous porcelain (V), a leucite-based porcelain (D), a leucite-based glassceramic (E1), a lithium disilicate glass-ceramic (E2) and a glass-infiltrated alumina composite (IC).

Journal of Materials Science: Materials in Medicine, 2009

The objective was to compare fracture toughness (K Ic ), stress corrosion susceptibility coefficient (n), and stress intensity factor threshold for crack propagation (K I0 ) of two porcelains [VM7/Vita (V) and d.Sign/ Ivoclar (D)], two glass-ceramics [Empress/Ivolcar (E1) and Empress2/Ivlocar (E2)] and a glass-infiltrated alumina composite [In-Ceram Alumina/Vita (IC)]. Disks were constructed according to each manufacturer's processing method, and polished before induction of cracks by a Vickers indenter. Crack lengths were measured under optical microscopy at times between 0.1 and 100 h. Specimens were stored in artificial saliva at 37°C during the whole experiment. K Ic and n were determined using indentation fracture method. K I0 was determined by plotting log crack velocity versus log K I . Microstructure characterization was carried out under SEM, EDS, X-ray diffraction and X-ray fluorescence. IC and E2 presented higher K Ic and K I0 compared to E1, V, and D. IC presented the highest n value, followed by E2, D, E1, and V in a decreasing order. V and D presented similar K Ic , but porcelain V showed higher K I0 and lower n compared to D. Microstructure features (volume fraction, size, aspect ratio of crystalline phases and chemical composition of glassy matrix) determined K Ic .

Physical Review B, 2008

Recently, the claim was made that cracks in silicate glasses propagate by the nucleation, growth, and coalescence of cavities at crack tips, which is the same way as in metals but at a much smaller scale. This hypothesis for crack growth is based in part on the measurement of surface displacements near the tip of an emerging crack, which is the point at which a crack front intersects the side surface of the specimen. Surface displacements measured by atomic force microscopy were less than theoretically predicted. The difference between the theoretical and experimental displacements was attributed to a plastic zone surrounding the tip of the moving crack. In this paper, we show that the theoretical analysis used earlier was based on an incorrect assumption about the functional dependence of the displacement with distance from the crack tip. We use a full three-dimensional finite element analysis combined with an asymptotic solution of the crack geometry to obtain a solution to the surface displacement problem. We show that the calculated displacements are fully consistent with those experimentally measured by using an atomic force microscope. No divergence from elastic behavior is observed. Our results support the view that crack propagation in glass is entirely brittle. No evidence for plasticity at the crack tips is obtained.

Dental Materials, 2010

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 483-490 a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . i n t l . e l s e v i e r h e a l t h . c o m / j o u r n a l s / d e m a Strength Weibull statistics a b s t r a c t Objectives. Evaluate the flexural strength ( ) and subcritical crack growth (SCG) under cyclic loading of glass-infiltrated alumina-based (IA, In-Ceram Alumina) and zirconia-reinforced (IZ, In-Ceram Zirconia) ceramics, testing the hypothesis that wet environment influences the SCG of both ceramics when submitted to cyclic loading.

Journal of the American Ceramic Society, 2005

The aim of this study was to investigate the influence of the test method on fracture toughness of a dental porcelain and a soda lime glass. Materials and methods: Three methods were used to determine fracture toughness: the indentation strength (IS) by bending, chevron-notched beam (CNB), and the single-edge-notched beam (SENB). In the IS method, the ratio of elastic modulus to hardness (E/H) in the formula was determined by two methods: individual measurement for E and H (IS M) as well as direct estimation from Knoop's indentation method (IS K). The tested materials were a dentin porcelain, a traditional feldspar-based leucite-reinforced glass ceramic (Carrara Vincent), and a soda lime glass. Result: Carrara Vincent showed a higher toughness (P<0.01) than glass with all three test methods. The toughness values manifested significant differences between the methods used (P<0.01). The two-way analysis of variance suggested that the materials tested and the test methods used had interaction effects, which statistically means that differences in materials and methods influenced the comparability of the toughness result. In this study, a first step was made to compare different toughness test methods by testing the toughness of a traditional feldspar-based leucite-reinforced glass ceramic and a soda lime glass that has a homogeneous microstructure. Conclusion: An interaction effect of the method and the material used was shown. As a consequence, none of the methods tested is suitable as a universal fracture toughness test method. Further research is needed to investigate more extensively the influence of material composition on the fracture toughness test methods' comparability. Strength is commonly seen as an important parameter for understanding the clinical performance of dental ceramic restorations as it reflects an important mechanical property. Since the tensile strength of ceramics is much lower than the compressive strength, ceramic restorations often fail in areas of tensile stress. The traditional method for restoration design and material selection is based on attempts to reduce the tensile stresses generated in the structure under load and to select materials with greater strengths than the expected applied stresses [1-4]. Unfortunately, with extremely brittle materials such as ceramics, high strength does not imply a satisfactory fracture resistance [2-4]. Fracture is caused by a propagating crack, which often originates from flaws and extends when the applied stress exceeds a certain threshold. In very brittle materials, this threshold largely depends on the crack tip radius, flaw size, flaw distribution, and fracture toughness. Fracture toughness is one of the most important material properties in fracture mechanics for brittle materials and is assumed to be independent of flaw size, specimen shape, and the stress concentration acting on the surface. Fracture toughness (K Ic) of a brittle material is characterized by a critical level of the stress intensity factor near the crack tip at which a crack will start to propagate. For ceramics that have a primary disadvantage of brittleness and contain many flaws, fracture toughness is, therefore, more elucidating than strength. Obviously, the availability of an accurate method for fracture toughness determination is very important. In the dental literature, greater attention is being drawn to fracture toughness measurements, resulting in an increasing number of publications [5-12]. Various test techniques have been developed for the determination of fracture toughness: the single-edge-notched beam (SENB, by three-or four-point bending) [3, 4, 12] and its derivatives single-edge-precrack-notched beam (SEPB/SEPNB) [4, 7, 13] and single-edge-V-notched beam (SEVNB), [5, 6] chevron-notched (CN) specimen (short rod/bar, three/four-point bending long beam/rod), [8,13-15] doubletorsion, double-cantilever beam (DCB), indentation strength (IS) by bending with Vickers or Knoop indentations, [12, 16] indentation fracture (IF) method, [6, 12, 17] and surface-crack-by-flexure (SCF, also called controlled surface microcrack).[11] In dental ceramics research, IF, SENB, and IS methods have frequently been used for the ranking of fracture toughness. The CN method, which rarely appears in the dental literature for ceramic evaluation, was developed in the late 1970s. It is different from SENB in that the designed crack for extension has the shape of a triangle instead of a rectangle.

Journal of Prosthodontics, 2007

Purpose: The purpose of this study was to test the hypothesis that the increase in fracture toughness of a fluorcanasite-based glass-ceramic is a linear function of crystal volume fraction.

Acta Materialia, 2013

Crack initiation and deformation behaviors of oxide glasses belonging to different chemical systems were studied using the Vickers indentation test. The crack initiation resistance is chiefly governed by the extents to which densification and isochoric shear flow develop in a process zone beneath and within the contact area. Densification is favored in glasses with relatively small Poisson's ratio (m), whereas shear is favored at large m. Glasses were ranged according to their resistance to the formation of corner cracks as follows: Resilient, for 0.15 6 m 6 0.20; Semi-Resilient, for 0.20 6 m 6 0.25; and Easily-Damaged for 0.25 < m < 0.30. Radial-median cracks occur at low load (650 mN) in Easily-Damaged glasses, while cone cracks predominate in Resilient glasses under higher loads. A critical value for m ($0.22 depending on the Young's modulus/hardness ratio) was identified, at which the intensity of the indentation stress field tends to vanish, preventing crack formation on loading, while the driving force on unloading remains very small.

Journal of Materials Science Letters, 1991

Measurements of subcritical crack growth in ceramic materials are often performed with macroscopic cracks. For most polycrystalline ceramic materials it is sufficient to use a narrow saw cut with a width smaller than the process zone ahead of the crack tip. A saw cut of 50/xm width was found to be an adequate starter notch width. In contrast to the polycrystalline materials for glass, a sharp crack is always necessary for fracture toughness measurements as well as for subcritical crack growth measurements. One possibility of introducing sharp macrocracks into fracture mechanical specimens is given by the bridge precracking procedure . This method was used to precrack rectangular bending bars of a borosilicate glass with dimensions: width W = 5 mm, thickness B = 4 mm and length 50 mm. Starting with a 50 N Knoop-indentation crack, straight-edge cracks of depth about 1.5 mm were introduced. These specimens were tested in a static three-point bending arrangement with a roller distance of 40 ram, and the time-dependent displacement 6 at the centre of the specimens was picked up by a Linear Voltage Differential transformer (LVDT). The crack length a was obtained from the elastic compliance C. Any displacement increment A6 is caused by a change of the compliance C which is a direct consequence of a crack extension Aa. It holds that

Journal of Non-Crystalline Solids, 2004

The fracture toughness of several glasses, including soda-lime-silica and aluminosilicate glasses, was measured by both the indentation fracture (IF) and the single-edge notched beam (SENB) techniques. The flow densification mechanism occurring during indentation for silica-rich compositions leads to discrepancies between the two methods. The influence of the indentation load and the post-indentation fatigue duration on the indentation on the crack length and on the fracture toughness measurement were investigated. Fatigue curves (V-K) were obtained directly from indentation experiments and fatigue parameters were derived. The sub-critical crack growth resistance was found to be improved by increasing the silica content. In comparison with the studied oxide glasses, a Y-SiAlON oxynitride glass exhibits much greater fatigue resistance. The fatigue phenomenon has a major effect on the estimation of K C from indentation-cracking measurements.

Frontiers in Materials, 2020

Stable crack growth was measured for nominal dry and water-bearing (6 wt%) soda-lime silicate glasses in double cantilever beam geometry and combined with DMA studies on the effects of dissolved water on internal friction and glass transition, respectively. In vacuum, a decreased slope of logarithmic crack growth velocity versus stress intensity factor is evident for the hydrous glass in line with an increase of β-relaxation intensity indicating more energy dissipation during fracture. Further, inert crack growth in hydrous glass is found to be divided into sections of different slope, which indicates different water related crack propagation mechanism. In ambient air, a largely extended region II is observed for the hydrous glass, which indicates that crack growth is more sensitive to ambient water.

The growth of 3D star-like cracks in a porous borosilicate glass, with a "mirror-mist-hackle" aspect due to the dynamic character of their propagation was induced in cylindrical specimens by "cold-to-hot" thermal shocks inducing triaxial tension. Pores were identified as the crack initiation sites. Thermo-mechanical simulations were done to analyse the stress field. Crack initiation at mid-height, from the center of the specimens was predicted, in accordance with the observations. The pore-induced stress concentration was found to depend on the local stress triaxiality, as well as K I for an annular crack initiated from a pore. INTRODUCTION. Blocks of vitrified nuclear waste for deep underground storage are prepared by pouring a mixture of waste with molten sodium-borosilicate glass into steel canisters. During cooling, sharp temperature gradients produce tri-axial tensile stresses, responsible for multiple cracking. When stored underground for hundreds of years, the canisters might not remain watertight , allowing leaching of the fractured glass by water and the release of radionucleides in the environment. Crack initiation and growth in cylindrical specimens of an inactive analogous glass during "cold-to-hot" thermal shocks that induce a triaxial tension field representative of the loading conditions during glass blocks cooling was thus investigated. EXPERIMENTAL AND NUMERICAL PROCEDURES Experimental procedures. The material investigated is a non-translucent SON68 glass, an inactive analogue of the industrial product, containing mainly SiO 2 (45,5weight %), B 2 O 3 (14%), Na 2 O (9,9%), many other oxides, plus Ruthenium and Palladium particles, to replace some heavy radionucleides. It is flawless, thanks to a slow cooling process, but, like the industrial glass, it contains a distribution of more or less spherical pores, issued from gas bubbles trapped in the solidifying liquid, whose diameter ranges from 100 to 800µm. The thermo-elastic properties were measured between 20°C and the glass-transition temperature, which is 502°C [1]. Relaxation tests have shown that below 350°C, viscous effects are negligible during the short time fracture tests reported below. 36 to 45mm-high and 80mm-high cylinders, 40mm in diameter were machined. These cylinders were equipped with three thermocouples glued in the center of each base and on the side, at mid-height. They were cooled in a freezer during more than 12h and then submitted to

Acta Metallurgica et Materialia, 1995

Depending on the conditions under which an indentation is performed or indented specimen stored, different crack configurations can be obtained. In this work, the influence bf these different configurations on the failure process, strength and fatigue behaviour of soda-lime silicate glass, was investigated. To obtain the different crack geometries, indentations were performed in moist air and deionized water with two different dwell times. In this way, typical "half-penny" and deeper "circular" cracks were obtained. Both as-indented and annealed samples were studied. An extensive fractographical analysis revealed a strong "pinning" effect of lateral cracks on the radial crack propagation for indentations obtained in air with short dwell times. Strength measured both in inert and active environments was shown to be lower for indentations obtained in water with longer dwell times. An analogous trend was observed for the lifetime results obtained in static fatigue. Some discrepancies were observed in time-to-failure predictions for "half-penny" indentation cracks and these were related to the interaction between the lateral and radial cracks.