共查询到20条相似文献,搜索用时 15 毫秒
1.
Unified Theory of Thermal Shock Fracture Initiation and Crack Propagation in Brittle Ceramics 总被引:15,自引:0,他引:15
D. P. H. HASSELMAN 《Journal of the American Ceramic Society》1969,52(11):600-604
A fracture-mechanical theory is presented for crack propagation in brittle ceramics subjected to thermal shock. The criteria of crack stability are derived for a brittle solid uniformly cooled with triaxially constrained external boundaries. Thermal stress crack instability occurs between two values of critical crack length. For short initial crack length, crack propagation occurs kinetically, with the total area of crack propagation proportional to the factor St 2 (1-2 v )/EG, where St is tensile strength, v is Poisson's ratio, E is Young's modulus, and G is surface fracture energy. Under these conditions the newly formed crack is subcritical and requires a finite increase in temperature difference before propagation will proceed. For long initial crack length, crack propagation occurs in a quasi-static manner and can be minimized by maximizing the thermal stress crack stability parameter Rst = [G/α2 E ]1/2 , where α is the coefficient of thermal expansion. For heterogeneous brittle solids, such as porous refractories, the concept of an "effective flaw length" is introduced and illustrated on the basis of experimental data in the literature. The relative change in strength of a brittle solid as a function of increasing severity of thermal shock is estimated. Good qualitative agreement with literature data is found. 相似文献
2.
Multiple cracking due to pseudo strain–hardening of fiber-reinforced brittle matrix composites contributes significantly to enhanced composite strain capacity as well as toughness. This unique phenomenon is true for both continuous aligned and discontinuous random fiber composites. Theoretical formulation of multiple crack spacing is reviewed in this paper. In particular, a snubbing effect of fiber pullout and a bundling effect of fiber strands on crack spacing are discussed. Taking account of these effects, expressed via the snubbing coefficient, f , and effective bond strength, τeff , a better spacing prediction has been obtained for a discontinuous random glass-strand-reinforced cement composite. 相似文献
3.
Rajendra K. Bordia Brian J. Daigleish Panos G. Charalambides Anthony G. Evans 《Journal of the American Ceramic Society》1991,74(11):2776-2780
Results of four-point bend tests on notched beams of a laminated unidirectional fiber-reinforced glass matrix composite are presented. The failure sequence has been established through in situ examination. The dominant damage mode is a mixed-mode, split crack that runs parallel to the predominant fiber directions. The crack interacts with and crosses over imperfectly aligned fibers. The resulting bridging tractions are sufficient to cause the critical strain energy release rate to increase substantially as the crack extends. Several other damage modes are also observed. These include mode I (tensile) matrix cracks bridged by fibers, mode II (shear) cracks, and compressive damage at the loading points. 相似文献
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Silicate glass rods containing Vickers indentations in the subthreshold-fracture region can show dramatic strength losses on brief immersion in acid solutions. The effect is most significant for "normal" glasses, notably soda-lime glass, and for HF-based acids. After extended immersion the strength begins to recover, ultimately surpassing the original (pre-immersion) level, as glass dissolution processes become dominant. The weakening effect is interpreted in terms of chemically enhanced residual-contact stress effects. 相似文献
6.
Crack Spacing in Brittle Films on Elastic Substrates 总被引:2,自引:0,他引:2
M. D. Thouless 《Journal of the American Ceramic Society》1990,73(7):2144-2146
An approximate, analytical solution is derived for the minimum spacing that can exist between a series of parallel cracks propagating in a thin film. The analysis is pertinent to the specific case of a film and substrate having identical elastic properties. In the absence of plasticity, only three parameters govern the minimum crack spacing. The cracks are predicted to be more closely spaced if the film stress or the film thickness is increased, or if the fracture toughness of the film is decreased. 相似文献
7.
Crack Initiation in Unidirectional Brittle-Matrix Composites 总被引:1,自引:0,他引:1
This paper describes the initiation of matrix cracking for glass and glass-ceramic matrix composites reinforced with small-diameter silicon carbide and carbon fibers under uniaxial tensile loading. Acoustic emission, replication, and optical microscopy in conjunction with stress-strain curves are employed to detect the initiation of matrix cracking. The proportional limit of the stress-strain curve is found to overestimate the initiation of matrix cracking in the material systems studied. The matrix cracking iniates at axial strains from 0.07% to 0.15%. The ACK model overestimates the initiation of the matrix cracking for the material systems studied in this paper. 相似文献
8.
A pure Mode I fracture specimen and test procedure has been developed which provides extended, stable, through-thickness crack growth in ceramics and other brittle, nonmetallic materials. Fixed displacement loading, applied at the crack mouth, promotes stable crack extension by reducing the stored elastic strain energy. Extremely fine control of applied displacements is achieved by utilizing the Poisson expansion of a compressively loaded cylindrical pin. Stable cracks have been successfully grown in soda-lime glass and monolithic Al2 O3 for lengths in excess of 20 mm without uncontrollable catastrophic failure. 相似文献
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Rollie E. Dutton Nicholas J. Pagano Ran Y. Kim 《Journal of the American Ceramic Society》1996,79(4):865-872
The initiation of matrix microcracking was investigated in unidirectional glass matrix composites having controlled fiber spacing. Observations were taken from composites consisting of regular arrays of TiB2 -coated SIGMA 1240 and carbon-coated SCS-6 monofilament SiC fibers in a series of borosilicate glasses. The thermal expansion mismatch between the fibers and glass matrix was varied such that the resulting radial stresses after processing ranged from tensile to compressive. The glass strongly bonds to the TiB2 -coated SIGMA 1240 fiber but weakly bonds to the carbon coating of the SCS-6 fiber, allowing the investigation of the effects of bonding at the fiber/matrix interface. The observed crack initiation stresses of the various composites are compared to predictions based on a previously developed semiempirical model and used to study the influence of the volume fraction of fibers, residual stress state and interface strength. 相似文献
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Janet B. Davis Annika Kristoffersson Elis Carlström William J. Clegg 《Journal of the American Ceramic Society》2000,83(10):2369-2374
A simple and general method for making ceramic laminates with porous crack-deflecting interlayers is demonstrated. Both the strong laminae and the porous interlayers are made by tape casting suitable slurries using the same ceramic powders. Porosity is introduced into the interlayer by adding starch particles to the slurry. The effects of the starch on the burn-out and sintering behavior of the laminates has been fully described. The influence of resulting porosity on the ability of an interlayer to deflect a growing crack and to remain stable on prolonged heating has also been investigated. For the pore morphologies studied here, it is the volume fraction of pores that controls whether crack deflection takes place. Using existing mechanics solutions, a simple theoretical criterion for this volume fraction of porosity is given and is consistent with all the experimental observations. 相似文献
13.
The bowing of cracks between obstacles has been proposed as a mechanism for increasing the .fracture toughness of brittle materials. An analytical expression is presented based on previous theoretical studies that can be used to predict the increase in fracture toughness. The approach incorporates the effect of obstacle failure and the assumptions needed to compare the predictions with experimental data. 相似文献
14.
Indentation crack initiation and propagation in tempered glass surfaces are examined and the results compared to those in annealed and in ion-exchange-strengthened glasses. The presence of surface compression due to tempering inhibits median crack initiation during the loading cycle of the indentation and depresses the radial crack initiation load during unloading. However, the extent of lateral cracking is enhanced in tempered glass surfaces. In situ crack propagation experiments reveal that the compressive stress tends to weakly stabilize crack extension prior to failure. The degree of crack stabilization is considerably lower than expected from a theoretical analysis. 相似文献
15.
Andrew A. Wereszczak Kurt E. Johanns Osama M. Jadaan 《Journal of the American Ceramic Society》2009,92(8):1788-1795
The use of Hertzian indentation to measure ring crack initiation force (RCIF) distributions in four hot-pressed silicon carbide (SiC) ceramics is described. Three diamond indenter diameters were used with each SiC; the RCIF in each test was identified with the aid of an acoustic emission system; and two-parameter Weibull RCIF distributions were determined for all 12 combinations. RCIF testing was found to be an effective discriminator of contact damage initiation and response. It consistently produced the same ranking of RCIF between the four SiCs, with all three different indenter diameters, which is noteworthy because Knoop hardness and fracture toughness measurements were only subtly different or equivalent for the four SiCs. However, because RCIF, like hardness, is a characteristic response of a target material to an applied indentation condition (e.g., a function of indenter diameter) and not a material property, the implications and possible limitations should be acknowledged when using RCIF to discriminate the target material response. 相似文献
16.
A cleavage technique was used to measure the surface free energy and the grain boundary energy for twist boundaries of varying twist angles in potassium chloride. The results for the surface energy and its temperature dependence are in good agreement with theory and experiment. Based in part on this agreement and the measured misfit orientation dependence, the grain boundary energy values obtained are concluded to be reasonable. 相似文献
17.
Hidehiro Kishimoto Akira Ueno Atsushi Matsunaga Takuya Kondo 《Journal of the American Ceramic Society》1998,81(1):55-60
Crack-initiation sites and crack-propagation rates of small cracks in smooth specimens of alumina with two grain sizes have been studied. The principal results that have been obtained are as follows: (i) in most cases, the crack-initiation life comprises a large portion of the fatigue life, (ii) small cracks that are initiated in smooth specimens propagate with stress intensity factors that are much lower than the apparent threshold stress intensity factors ( K Imax ) of artificial cracks that are 200 µm in length, and (iii) the critical crack lengths beyond which the crack-propagation rate is described uniquely by K Imax are 120 µm for fine-grained alumina and 250 µm for coarse-grained alumina. 相似文献
18.
The load relaxation technique was applied to double- cantilever-beam specimens for the determination of diagrams of the stress intensity factor (K1 ) vs crack velocity (V) for soda-lime glass. The effect of machine compliance was explicitly incorporated into the velocity equation. The data were in good agreement with published data from different techniques. It was shown that a hard machine (small compliance) is best suited for the load-relaxation technique. The use of a soft machine (high compliance) will lead to catastrophic failure during load relaxation. A single-edge notched-beam specimen in bending is unsuitable for the relaxation technique since catastrophic failure will occur during relaxation regardless of machine compliance. 相似文献
19.
Characterizing Brittle Fracture by Modeling Crack Deflection Angles from the Microstructure
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Lukas Porz Sai Wei Jiamin Zhao Eric A. Patterson Bin Liu 《Journal of the American Ceramic Society》2015,98(12):3690-3698
This study introduces a simple analytical model for fracture toughness to bridge the length scales from grain size to bulk thickness by assembling a virtual crack path from the angles recorded on an unfractured microstructure, which is a great challenge in fracture mechanics due to the high geometrical complexity. Good agreement is found between a crack deflection angle distribution measured from 5764 crack segments and the prediction by the model and the possible influence of residual stress is quasi quantitatively discussed. A total of 7.4% of the crack segments observed acted as crack bridges, while 7.3% was predicted by the model. A quantification of how high an angle needs to be to turn crack deflection into crack bridging is given. The ratio of fracture toughness from grain boundary to grain, G1c(gb)/G1c(g), was measured indirectly from all samples to be between 0.3 and 0.35. 相似文献
20.
Critical load characteristics for the indentation-induced median crack in ternary lead-zinc-borate and commercial silicate glasses were investigated. The critical load is directly proportional to the fracture toughness/Vickers microhardness term, supporting the Lawn-Evans model; however, the magnitude of the load is considerably greater than that estiated from the model. The relation of critical load to deformation/fracture properties is discussed in terms of the stress field around the indent area and fractography. 相似文献