Strength Degradation of Thermally Tempered Glass Plates

An earlier theory of contact-induced strength degradation of brittle materials is extended to include plates in residual surface compression. The scale of the strength-controlling flaw is predicted by indentation fracture mechanics, with the modifying effect of the residual field incorporated into both indentation and strength equations. Experimental verification of the predictions is obtained from diamond-pyramid indentation tests on thermally tempered glass plates. As with untempered plates, the theory accounts for the load dependence of the strength loss; it also explains the insensitivity of the degradation characteristics to initial flaw distribution and identifies toughness as the controlling material parameter. Most significant, however, is the demonstration that surface strengthening can produce dramatic improvements in degradation resistance. The possibility of obtaining all parameters necessary for a complete degradation analysis of a given tempered inaterial entirely by routine indentation/strength testing is discussed.     

Strength Degradation of Glass Impacted with Sharp Particles: I, Annealed Surfaces

The strength characteristics of annealed brittle surfaces impacted with sharp particles were studied. A theory of the degradation process is constructed in three steps: (1) A sharp particle delivers an impulsive load to the target surface via a plastic contact; (2) the contact loading initiates and propagates median cracks in the surface; (3) the cracks thus induced reduce the strength of the material. Static indentation tests provide essential contact parameters for the degradation equations, thereby allowing for prediction of strengths under in-service conditions. Strength tests on soda-lime glass laths impacted with SIC grit confirm basic predictions of the theory. Higher toughness and lower hardness are the main material requirements for improved resistance to degradation. Initial flaw population in the target surface and projectile geometry are not important factors in the damage process. The study shows that impact energy is the important service variable in determining the extent of strength loss.     

A simple method of short‐term mechanical reliability prediction for ceramics in reactive environments

A method is demonstrated for prediction of ceramic reliability limited by fracture in reactive environments. The simplicity of the method relies on the ceramic displaying ideal indentation‐strength behavior and on the applied loading leading to short‐term power‐law crack velocity behavior. Reactive failure strength measurements, treating flaw size as a variable and failure time as a constraint in a “pass‐fail” test, enable reliability, including both intrinsic and contact flaws, to be predicted. Little to no computation is required.     

Statistical Determination of Surface Flaw Density in Brittle Materials

The density of surface flaws in brittle materials can be determined from the loads in many different kinds of tests. A general method is derived and applied to indentation experiments on crown, plate, window, lead, and commercial heat-resistant and heat-absorbing glasses. Knowledge of flaw density is important for estimating size effect, and differences in flaw density indicate the possibility of significant improvements in strength by eliminating a few major sources of flaws.     

Multi‐Scale Effects in the Strength of Ceramics

Multiple length‐scale effects are demonstrated in indentation‐strength measurements of a range of ceramic materials under inert and reactive conditions. Meso‐scale effects associated with flaw disruption by lateral cracking at large indentation loads are shown to increase strengths above the ideal indentation response. Micro‐scale effects associated with toughening by microstructural restraints at small indentation loads are shown to decrease strengths below the ideal response. A combined meso‐micro‐scale analysis is developed that describes ceramic inert strength behaviors over the complete indentation flaw size range. Nano‐scale effects associated with chemical equilibria and crack velocity thresholds are shown to lead to invariant minimum strengths at slow applied stressing rates under reactive conditions. A combined meso‐micro‐nano‐scale analysis is developed that describes the full range of reactive and inert strength behaviors as a function of indentation load and applied stressing rate. Applications of the multi‐scale analysis are demonstrated for materials design, materials selection, toughness determination, crack velocity determination, bond rupture parameter determination, and prediction of reactive strengths. The measurements and analysis provide strong support for the existence of sharp crack tips in ceramics such that the nano‐scale mechanisms of discrete bond rupture are separate from the larger scale crack driving force mechanics characterized by continuum‐based stress‐intensity factors.     

Effect of Flaw State on the Strength of Brittle Coatings on Soft Substrates

A study is made of the role of flaw state on the strength properties of brittle ceramic coating layers bonded to soft polycarbonate substrates. We introduce Vickers radial cracks at prescribed loads into the coating undersurfaces prior to bonding to control the sizes and locations of the starting flaws. A spherical indenter is then loaded on the top bilayer surfaces, directly above the Vickers indentation sites, subjecting the radial cracks to flexural tensile stress. Radial crack responses are monitored in situ , using a camera located below the transparent substrate. Critical loads to cause radial crack instability, and ensuing growth of the arrested cracks, are recorded. Conventional biaxial flexure tests on corresponding monolith coating materials provide a baseline for data comparison. Relative to the monolith flexure specimens, the bilayers show higher strengths, the more so the larger the flaw, indicating enhanced flaw tolerance. A simple fracture mechanics analysis of the radial crack evolution in the concentrated-load field, with due account for distribution of flexural tensile stresses at the coating undersurface, is unable to account completely for the enhanced bilayer strengths for the larger Vickers flaws. It is hypothesized that the epoxy used to bond the bilayer components enters the cracks, causing crack-wall adherence and providing an increased resistance to radial crack instability. The fracture mechanics are nevertheless able to account for the arrest and subsequent stable extension of the radial cracks beyond the critical loads once this extraneous adherence has been overcome.     

Revealing crack profiles in polycrystalline tetragonal zirconia by ageing

Exposure to hot water vapour is shown to be useful for staining indentation crack profiles in doped zirconia polycrystals. This is illustrated here in 3Y-TZP with two different grain sizes, for which Vickers indentation cracks are of Palmqvist type, as well as in 3Y-TZP with 2.5 wt.% cerium oxide, for which indentation cracks are half-penny. The crack profile is clearly revealed on the fracture surface after biaxial flexural testing in all the specimens previously exposed to hot water vapour. The contrast in 3Y-TZP is induced by t–m transformation caused by hydrothermal degradation, which induces an intergranular fracture zone in front of the initial position of the indentation crack tip. The biaxial strength and apparent fracture toughness of 3Y-TZP increase substantially with the time of exposure at a rate that depends on the grain size. On the contrary, in 3Y-TZP doped with ceria no signal of t–m transformation is found and the flexure biaxial stress remains practically constant, but the initial position of the indentation crack is also clearly revealed by an intergranular fracture zone in front of the initial position of the crack tip. In this case, this is associated to environmentally assisted slow crack growth under the indentation residual stress during exposure to hot water vapour in autoclave.     

Effect of Moisture on the High-Temperature Stability of Unidirectionally Solidified Al2O3/YAG Eutectic Composites

The corrosion resistance of a unidirectionally solidified alumina/yttrium aluminum garnet (Al₂O₃/YAG) eutectic mixture was investigated at high temperature. Samples were exposed to high temperature (1200°–1800°C) in different atmospheres, which included argon, argon/water vapor, air, and air/water vapor. The most important microstructural changes occurred at the interface between the YAG and the Al₂O₃. Those changes consisted of localized thermal grooving, especially when the corrosive atmosphere contained water vapor. The samples exhibited significant weight loss at high temperature (1800°C) after 20 h of exposure. The calculated volume gain that was induced by the increased surface relief was low and limited, except when the corrosive atmosphere contained air, which indicated that the presence of air (particularly oxygen) induced a more-active corrosion process. On the other hand, no change in the flexural strength was observed, even after 100 h at 1800°C in a humid atmosphere, because of the cross-linked structure of the composite, which limited propagation of the groove.     

Fracture failure processes in polymers. I: Mechanical tests and results

Tensile tests and Izod impact tests were carried out on eight thermoplastics and an epoxy. The results are compared with shear strengths determined previously, and it is shown that for seven of the polymers, the nominal tensile strength is equal to the yield strength and is approximately the same as the shear strength measured by the punch test. (The true ultimate tensile strength is not always a useful concept because the more ductile polymers are radically changed during the tensile testing process.) In shear tests the shear strains can be extremely high before eventual failure. Because of this the Iosipescu test appears to be unsuitable for measuring the ultimate shear strengths of polymers. Classical shear sliding of failure appears to be rare, and instead most of the polymers appear to be failing in tension when tested in the punch test. (Shear is equivalent to tension at +45° and compression at ?45° to the shear directions.) The polymers with shear Strengths that are significantly less than the tensile strengths are brittle ones, and these differences are probably due to flaw statistics. Thus the idea first advanced to explain unduly high interface shear strengths in centrosymmetric systems, i.e., that most polymers ultimately fail in tension when tested in shear, appears to be validated.     

Strength Degradation of Brittle Surfaces: Blunt Indenters

Indentation fracture mechanics is used to develop a theoretical basis for predetermining the strength properties of brittle surfaces in prospective contact situations. Indenters are classified as blunt or sharp; only the first is considered in the present work. The classical Hertzian cone crack conveniently models the fracture damage incurred by the surface in this class of indentation event. Significant degradation is predicted at a critical contact load; when the load is increased beyond this critical level, further degradation occurs at a relatively slight rate. Bend tests on abraded glass slabs confirm the essential features of the theoretical predictions. The controlling variables in the degradation process, notably starting flaw size and in-denter radius, are investigated systematically. An indication is also given as to optimization of material parameters.     

Biaxial Fracture Studies of a Glass-Ceramic

Uniaxial and equibiaxial tensile strengths are reported for glass-ceramic specimens exhibiting strength isotropy. Uniaxial strengths were determined in both 3- and 4-point bend tests. Stressing rate was a controlled variable and the tests were conducted in both dry N₂ and water environments, to provide data for different conditions of slow crack growth. There was little or no difference between strengths in the 3- and 4-point bend tests, indicating the absence of a size effect. In comparable tests, the strength under equibiaxial tension was greater than under uniaxial tension. The biaxial strengthening became less pronounced with increased slow crack growth during testing. A maximum observed biaxial-to-uniaxial strength ratio of 1.21 resulted from ball-on-ring and uniaxial bend tests in dry N₂. These results are attributed to differences in flaw severity between biaxial and uniaxial stressing. The decreasing strength ratios can be explained by a change in configuration of critical flaws due to slow crack growth. Numerical calculations, using a simplified two-dimensional model of the strength-controlling flaw, supported this hypothesis. The calculations also indicate that the slow crack growth exponent is smaller for the biaxial tension mode of stressing than for the uniaxial mode.     

Surface Energetics and Adhesion

The relationships between surface energetics and adhesion are critically reviewed. New data that confirm such relationships, for peel tests as well as lap shear tests, are presented. The effect of hydrothermal aging of aluminum surfaces on surface energetics can be used to predict degradation in bond strength. The mechanism of failure for elastic adhesives (such as Scotch ® tape) in peel tests may be essentially the same as for more brittle adhesives (such as epoxies) in lap shear tests. This mechanism may involve brittle fracture that forms a critical flaw at the adherend-adhesive interface (on a microscopic level), followed by crack propagation which then may include considerable elastic and plastic deformation. The locus of propagation (fractography) is generally not (but may be) relevant to the problem of how to remedy mechanical weakness in an adhesive joint, since the local region of critical flaw formation rather than the general surface area determines the joint strength.     

Effect of Contact Residual Stress Relaxation on Fracture Strength of Indented Soda-Lime Glass

The strength of Vickers-indented soda-lime glass measured in air at room temperature steadily increases with time after indentation, whereas optical retardation steadily decreases in the same interval. Annealing after indentation causes further strength increase and retardation decrease. The results are consistent with Marshall and Lawn's treatment of the slow crack growth of indentation flaws driven by the combined influence of residual contact stress and applied stress. Post-indentation strengthening of indentation flaws can be explained without recourse to flaw blunting.     

Strength and Fatigue Properties of Optical Glass Fibers Containing Microindentation Flaws

The inert strength and dynamic fatigue properties of fused-silica optical fibers are studied using subthreshold indentation flaws, i.e., flaws without radial cracks. These subthreshold properties differ from those obtained in comparative tests on silica rods containing postthreshold indentation flaws in three major respects: (1) the inert strengths are significantly higher than predicted by extrapolation of the postthreshold data; (2) the slopes of the dynamic fatigue plots are likewise greater, indicating a greater susceptibility of the subthreshold flaws to chemical kinetic effects; and (3) the scatter in strengths is wider. These trends reflect the change in mechanical response reported for optical fibers with "natural" flaw populations in going from ordinary to ultra-high-strength regions. Direct observations of the indentation sites up to the point of failure indicate that the property differences can be interpreted in terms of a transition from propagation-controlled to initiation-controlled fracture instabilities at reduced contact loads. The subthreshold instability condition is modeled qualitatively as a two-step, deformation-fracture process, with strong emphasis on the importance of residual stress fields in parametric evaluations. The relevance of the results to the practical issue of fiber reliability, most notably in connection with the potential dangers of using macroscopic crack velocity data to predict long-lifetime characteristics, is addressed.     

Toughening of a Cordierite Glass–Ceramic by Compressive Surface Layers

An indentation fracture mechanics analysis is developed to characterize the toughening effects of a compressive surface layer in brittle materials. The analysis is used to describe the enhanced toughness of cordierite glass–ceramic laminate composites, in which thermal-expansion mismatch effects induced uniform stress in the exterior layers of the symmetric exterior:interior:exterior structures. Interpretation of indentation crack length and inert strength tests via the analysis shows that cracks can be viewed as experiencing discrete regions of decreasing stabilization on propagation from small cracks and complete containment within the compressive layer to large cracks and partial extension into the compensating tensile interior. The observations are described using a stress-intensity factor for circular cracks in linear stress fields that includes different base and surface values for extended cracks. Deconvolution of inert strength data for the model cordierite system studied suggested an increase in toughness from 1.4 MPa·m^1/2 for the base material to a peak of about 5 MPa·m^1/2 for a 1:18:1 composite structure, with attendant increases in strength and flaw tolerance.     

Sea-Salt Corrosion and Strength of a Sintered α-Silicon Carbide

A commercially available, sintered silicon carbide was exposed to a temperature of 982°C for up to 50 h in a burner rig pressurized to 500 kPa. Synthetic sea salt added to the flame (5 ppm) resulted in the deposition of sodium sulfate and formation of a sodium magnesium silicate corrosion product. A 16% reduction in room-temperature strength occurred after 5 h of exposure; this reduction was due to the formation of surface pits. Exposure for longer times resulted in continued strength reduction, up to 56% at 25 h. Samples exposed for 50 h were so degraded that mechanical tests could not be conducted. The strength after 25 h of exposure to a salt concentration of 2 ppm was similar to the as-received strength, whereas exposures to 10 ppm of salt resulted in strengths similar to that observed with 5 ppm of salt.     

Effects of Sodium Silicate Exposure at High Temperature on Sintered α-Silicon Carbide and Siliconized Silicon Carbide

Sintered α-silicon carbide and siliconized silicon carbide were exposed to combustion off-gas containing sodium silicate vapors and particulates in a combustion test facility for 24 to 373 h at 900° to 1050°C. Degradation was evaluated by measuring dimensional changes, by measuring loss in strength due to changes in flaw population, and by evaluating surface corrosion morphology. It is suggested that passive oxidation and dissolution of the silica oxidation scale play an important role in the corrosion process. These mechanisms were enhanced by the continuous removal and replenishment of corrosive material by the high-velocity gas. These degradation phenomena caused surface pitting and an approximately 50% reduction in strength for both materials after long-term exposure (>100 h). Morphological evaluation suggested that the grain boundaries in the α-silicon carbide were oxidized more rapidly than the grains, while for the case of the siliconized silicon carbide the silicon phase was oxidized rapidly along with preferential oxidation of the silicon carbide grains parallel to the {0001} plains.     

Biaxial and Uniaxial Data for Statistical Comparisons of a Ceramic's Strength

The uniaxial and equibiaxial tensile strengths of a brittle material were measured in bending. Equibiaxial tension was attained by concentric ring loading of disks and uniaxial tension by four-point line loading of plates. The two specimen designs give equal volumes, surface areas, and stress gradients. Ground surfaces and lapped surfaces were tested. The equibiaxial tensile strength of a dense alumina was lower than the uniaxial tensile strengths for both ground and lapped surfaces, 8.5 and 8.1%, respectively. The Batdorf theory of flaw statistics, in which biaxial tensile strengths can be predicted from the statistical distribution of uniaxial tensile strength measurements, agreed with the data.     

Predictability of the Mechanical Properties of Inclusion-Containing Ceramics

The incorporation of inclusions into a brittle matrix can change its properties significantly. The quantitative change of strength, toughness, and retained strength after severe thermal shock and Vickers indentation damage in so-called "duplex ceramics" can be reasonably well predicted by relating these properties to the empirically derived internal stress-intensity factor K ₁. In the present paper the relationship between K ₁ and three other recently measured properties—flaw resistance behavior, K _R-curve behavior, and Stress–Strain behavior—is investigated. In addition, the strength-degrading influence of pressurized inclusions observed in "duplex ceramics" is compared to the strength reduction and enhancement reported by other authors for composites containing a glass matrix and glass and alumina inclusions. An attempt is made to identify the important parameters which control the strength development in duplex structures.