Effects of moisture-assisted slow crack growth on ceramic strength

An investigation was made of strength degradation caused by moisture-assisted slow crack growth of surface flaws in two dense polycrystalline ceramics. Specimen sizes were varied. The observed strength degradation of each ceramic was predictable usingK _IC as a material property in fracture-mechanics relations, suggesting that the only material variable involved was critical flaw size. The strength of one of the ceramics in water decreased significantly with increased specimen size, but its Weibull modulus was essentially unaffected by specimen size and slow crack growth.     

Application of artificial neural network for predicting plain strain fracture toughness using tensile test results

A back‐propagation neural network was applied to predicting the K_IC values using tensile material data and investigating the effects of crack plane orientation and temperature. The 595 K_IC data of structural steels were used for training and testing the neural network model. In the trained neural network model, yield stress has relatively the most effect on K_IC value among tensile material properties and K_IC value was more sensitive to K_IC test temperature than to crack plane orientation valid in the range of material data covered in this study. The performance of the trained artificial neural network (ANN) was evaluated by comparing output of the ANN with results of a conventional least squares fit to an assumed shape. The conventional linear or nonlinear least squares fitting methods gave very poor fitting results but the results predicted by the trained neural network were considerably satisfactory. This study shows that the neural network can be a good tool to predict K_IC values according to the variation of the temperature and the crack plane orientation using tensile test results.     

Flaw growth in a polycrystalline lithium-aluminium-silicate glass ceramic

The growth of indentation-produced controlled flaws in a polycrystalline lithium-aluminium-silicate glass ceramic has been studied, over a wide range of temperatures and strain rates. Significant scatter in the fracture stress at elevated temperatures suggests that the extent of slow crack growth is highly sensitive to microstructural details. The initial flaw shape is important inK _IC determination. Up to 1000° C the fracture toughness,K _IC, is essentially strain-rate insensitive. The value ofK _IC decreases with temperature beyond 850° C. Intergranular cavity formation is suggested as the reason. Crack blunting by diffusive crack healing probably occurs at high temperatures. Also, intergranular slow crack growth occurs essentially under Mode I loading.     

Carbon nanotubes: do they toughen brittle matrices?

The development of a model CNT-brittle matrix composite system, based on SiO₂ glass containing well-dispersed CNTs at up to 15 wt%, allows a direct assessment of the effect of the nanoscale filler on fracture toughness (K _IC). Samples were prepared by colloidal heterocoagulation followed by spark plasma sintering. Detailed K _IC measurements, using both indentation and notched beam techniques, show a linear improvement with CNT content, with up to a twofold increase of fracture toughness at maximum loading. The results from the two methods used in this study show equivalent trends but differing absolute values; the relative merits of these two approaches to measuring nanocomposite toughness are compared. Possible toughening mechanisms associated with CNT pull-out, crack bridging, and crack deflection are identified, and discussed quantitatively, drawing on conventional short-fibre composite theory and the potential effects of scaling fibre diameter.     

On the statistical approach to fracture toughness variations with specimen size in wood

The application of Weibull statistics to predictions of specimen size effects on K_IC in wood is discussed and found to be inappropriate. Plane stress conditions induce high energy absorbing microstructural deformation mechanisms and lead to crack front tunnelling under both static and fatigue loading.     

Development and mechanical properties of metal–particulate glass matrix composites from recycled glasses

The great number of glasses available from recycling activity and vitrification treatment of industrial wastes leads to the need for new applications, with the development of new materials, such as low-cost composite materials from a powder technology route. In the present work a variety of recycled glasses is investigated, in order to obtain aluminium reinforced glass matrix composites via cold-pressing and viscous flow sintering. A good compatibility between lead silicate glasses from cathode ray tubes dismantling and aluminium reinforcement is found to be effective. Composites exhibiting good mechanical properties were developed from these materials. A particular attention was due to fracture toughness (K_IC) determination. The absolute K_IC of glass matrix composites value remains low, but a notable increment in relation to unreinforced matrix is observed.     

A fractographic criterion for subcritical crack-growth boundaries in hot-pressed alumina

The percent intergranular fracture (PIF) was measured along radii extending from fracture origins in hot-pressed alumina specimens, fractured at various loading rates and temperatures, and plotted versus estimates of stress intensity factors (K _I) at the various crack lengths. Minima in PIF occur at values ofK _I that are close to the critical stress intensity factors (K _IC) for cleavage on various crystal lattice planes in sapphire. The subcritical crack-growth boundary (K _I=K _IC of the polycrystalline material) occurs near the primary minimum in PIF suggesting that this minimum can be used as a criterion for locating this boundary. In addition, it was noted that the polycrystallineK _IC (4.2 MPa m^1/2) is very close to theK _IC for fracture on {¯1 ¯1 2 6} planes which is 4.3 MPa m^1/2. These observations suggest that critical crack growth begins when increased fracture energy can no longer be absorbed by cleavage on these planes. There is a secondary minimum atK _I>K _IC that appears to be associated with theK _IC necessary for fracture on combinations of planes selected by the fracture as alternatives to the high fracture-toughness basal plane.     

Toughness of glass fibres reinforced glass-ionomer cements

Two fracture toughness parameters, the critical stress intensity factor, K _c and the work of fracture, W _f have been used to characterise the toughness of conventional and resin-modified glass-ionomer cements reinforced with glass fibres. The critical stress intensity factor was determined from the peak load, and the work of fracture was determined as the energy required to extend an introduced crack through the respective glass ionomers. For both materials, crack propagation became more stable as the weight fraction of glass fibres was increased. Additionally, when the weight percent of glass fibres was increased the work of fracture increased. Fibre bridging at the crack tip resulted in the increase in the work of fracture. As the percentage weight of fibres was increased, the critical stress intensity factor decreased proportionally to the increase in porosity.     

Untersuchungen zur Volumenabhängigkeit von KIC-Daten mit Hilfe einer modifizierten Weibullstatistik

A Modified Weibull Statistics of Ceramic K_IC Data An explanation is given for the standard deviation of K_IC-data and for the dependence of K_IC values measured with ceramic materials on the specimen volume (specimen thickness). The probability distribution of measured K_IC-data is described by a modified Weibull theory. The theory is verified by experimental data using specimens with different thickness and a through the thickness notch. The experimental results are in good agreement with the theory.     

Fracture toughness and crack-growth measurements in GRP

Critical crack tip stress intensity factor (K _c) measurements were made for polyester resin reinforced with glass chopped strand mat (CSM) and woven roving fabric (WRF). Specimen thickness and initial crack length were varied for centre notched (CN) 100 mm wide specimens. Some specimens were saturated by immersion in water under pressure. K _c was negligibly affected by specimen thickness and it was concluded that plane strain conditions are not achieved in laminates of normal thickness. Scatter can be reduced by adjusting results to a standard glass content and K _c varies continuously with crack length. The CSM experiments were extended to 915 mm wide specimens which failed at very low nett section stresses but there may be a region in which K _c is roughly constant relative to crack length. In WRF specimens, however, it is the nett section stress which is constant at a value substantially below the UTS. Fatigue crack-growth studies were carried out on CN specimens. The Paris law adequately describes crack growth in CSM specimens at low rates of growth but Forman's law is better at high rates of growth. Neither law is valid for WRF material when dry but the behaviour changes after saturation with water. The crack-growth resistance of both materials is severely reduced by saturation with water.     

Evaluation by indentation of fracture toughness of ceramic materials

A transition fracture mode from Palmqvist to median has been observed in a number of ceramic materials. A new expression to determine the fracture toughness (K _IC) by indentation is presented. The K _IC values calculated by this formula are independent of the crack profile (median or Palmqvist) and of the applied load. This formula has been obtained by modifying the universal curve of Evans and Charles to incorporate Palmqvist and median cracks over a wide range of loads in the case of brittle materials with different mechanical properties (elastic properties: E, v, K _IC).     

Determination of subcritical crack growth on glass in water from lifetime measurements on Knoop-cracked specimens

The crack growth behaviour of glass in water is investigated by use of lifetime measurements carried out with Knoop-damaged specimens in four-point bending tests. The method is outlined in detail and the crack growth law - exhibiting a threshold stress intensity factor K_ch = 0.35 MPam^1/2 - is determined. The initial and final crack size data as well as the fracture toughness are given by Weibull representations.     

A fractographic criterion for subcritical crack growth boundaries at internal fracture origins in hot pressed silicon nitride

Using the elliptic integral method, stress intensity factors (K _I) were estimated at boundaries defined by fracture features observed at various distances from internal fracture origins in H.P. silicon nitride. The fracture origins are surrounded by regions of transgranular fracture. At the outer boundaries of these regionsK _I is less thanK _IC showing that these are regions of subcritical crack growth. Regions of hummocks and depressions were observed surrounding the regions of transgranular fracture.K _I was calculated at the elliptical boundary determined by the outer edge of the nearest of these features to the fracture origin. At this boundary,K _I K _IC. Therefore, these features can be used to locate the subcritical crack growth boundary.     

Crack-size dependence of fracture toughness in transformation-toughened ceramics

Fracture toughness (K _IC) has been determined for Y₂O₃-partially stabilized zirconia, Y₂O₃-partially stabilized hafnia, CaO-partially stabilized zirconia and Al₂O₃+ZrO₂ composites. It is shown thatK _IC determined using the identation technique may not yield a unique number but may depend upon the crack size (C) (on the indent load). The slope ofK _IC againstC ^1/2 yields the magnitude of the surface stress created by the tetragonal monoclinic transition on the surface induced by grinding.K _IC determined using the double cantilever beam (DCB) technique, on the other hand, is shown to be independent of crack length.     

FATIGUE CRACK PROPAGATION CHARACTERISTICS OF GLASS FIBRE REINFORCED POLYCARBONATE IN HOT DISTILLED WATER

The fatigue crack propagation behaviour of polycarbonate and glass fibre reinforced polycarbonate was studied in hot distilled water. The effects of temperature, distilled water and glass fibre content on fatigue crack growth rate were determined. In distilled water at 333 K, the fatigue crack growth rate decreased with increasing glass fibre content. A melting was observed of the adhesive bond between the glass fibres and the matrix. It was evident that the fracture morphology closely relates to the fatigue crack growth rate which depends on the ΔK parameter rather than the K_max parameter when the materials are tested in hot distilled water.     

Micromechanics of crack nucleation during indentations

An analysis for the nucleation of microcracks from the inhomogeneous flow lines in soda-lime glass under Vickers indentations is considered. The minimum loads for crack nucleation are shown to depend on the hardness,H, and the critical stress intensity factor,K _IC. Unlike the Lawn and Evans analysis, the present model does not require the presence of any fortuitous flaws of critical dimensions in the material, since the flaws are nucleated by the deformation in the deformed zone.     

Crack resistance curves of alumina and zirconia at room temperature

Ceramic three-point bend specimens were pre-cracked in a displacement-controlled test in air at room temperature to form sharp cracks of different lengths. Critical stress intensity factors (K _IC were then measured as a function of sharp crack length in a fast-fracture, load-controlled test. Crack resistance curves (K _IC against crack length) were determined for three commercially pure aluminas of different grain size, a debased alumina containing a glassy phase, and a partially stabilized zirconia (PSZ) material. The crack resistance curves proved to be flat for the finer-grained and the debased alumina. A steeply rising crack resistance curve was, however, observed for a pure coarse-grained alumina material which is explained by friction effects of the cracked microstructure behind the measured crack front. The effect is influenced by the test procedure itself. Though crack branching takes place the crack resistance curve of PSZ is completely flat, which is attributed to fast fracture testing where only the most dangerous flaw is activated.     

Effects of thickness and environmental temperature on fracture behaviour of polyetherimide (PEI)

The fracture behaviour of a polyetherimide (PEI) thermoplastic polymer was studied using compact tension (CT) specimens with a special emphasis on effects of specimen thickness and testing temperatures on the plane strain fracture toughness. The results show that the valid fracture toughness of the critical stress intensity factor, K _IC, and strain energy release rate, G _IC, is independent of the specimen thickness when it is larger than 5 mm at ambient temperature. On the other hand, the fracture toughness is relatively sensitive to testing temperatures. The K _IC value remains almost constant, 3.5 MPa in a temperature range from 25 to 130°C, but the G _IC value slightly increases due to the decrease in Young's modulus and yield stress with increasing temperature. The temperature dependence of the fracture toughness, G _IC, was explained in terms of a plastic deformation zone around the crack tip and fracture surface morphology. It was identified that the larger plastic zone and extensive plastic deformation in the crack initiation region were associated with the enhanced G _IC at elevated temperatures.     

Assessment of the fracture toughness of cast steels

Estimates of the fracture toughness in terms of the critical stress intensity factorsK _C andK _IC are made for a 1Cr steel, a 1/2Cr-1/2Mo-1/4V steel, a 1 1/2Mn-Ni-Cr-Mo steel and a 1 1/2 Ni-Cr-Mo steel all in cast form. The methods used are linear elastic fracture mechanics,J-integral and crack opening displacement methods. The last two methods are applied in combination with an electrical potential method to detect the initiation of fracture.     

Lifetime prediction of brittle materials having spatial variations in fracture properties K IC and v versus K 1

The lifetimes of brittle materials under conditions of subcritical crack growth are analysed to indicate the effects of spatial variations of K _IC and v versus K _I and methods for studying the variations are suggested.