Fracture toughness of glasses as measured by the SCF and SEPB methods

The fracture toughness, K_Ic, of six glasses was measured by the surface crack in flexure (SCF) and single-edged precracked beam (SEPB) methods. Results depended upon the loading rate as well as the test environment. Environmentally-assisted slow crack growth affects the results for tests done in air. Dry nitrogen testing is preferred. Crack healing may be a severe complicating factor with precracked flexure bar type specimens if the specimens are unloaded between the precracking and final fracture test. Success in K_Ic testing depends to a large degree on upon the ability to make good precracks.     

Oxidation effects on toughness and slow crack growth in polycrystalline graphites

The fracture toughness (K_Ic) and slow crack growth behavior of four fine-grained polycrystalline graphites, oxidized to 5, 10 and 20% weight losses, were measured in air at room temperature. Exponential decreases in the elastic moduli as well as decreases in the fracture surface energy contributed to lowering K_Ic. Oxidation generally shifted the stress intensity-crack velocity (K_I-V_I) diagram to lower stress intensity values, and decreased the slope, or N-value. Scanning electron microscope fractography revealed that a combination of filler particle and binder phase degradation with increasing oxidation was responsible for the decreased toughness and changes in the (K_I-V_I) characteristics. Oxidation conditions were shown to significantly affect the magnitude of decreases in the physical, elastic and mechanical properties of these graphites.     

Comparison of fracture resistance as measured by the indentation fracture method and fracture toughness determined by the single-edge-precracked beam technique using silicon nitrides with different microstructures

Because of the industrial need for an assessment of fracture resistance, K_R from small ceramic parts, K_R of Si₃N₄ ceramics has been measured by the indentation fracture (IF) method using representative formulae to evaluate the compatibility with the fracture toughness, K_Ic determined from the single-edge-precracked beam (SEPB) technique. K_R of the fine Si₃N₄ showed little dependence on the crack length, whereas the samples with coarse microstructures exhibited a rising R-curve behavior. The IF equation which gave the nearest value to K_Ic from SEPB was different depending on the microstructures. The assessment of fracture resistance with Miyoshi's equation was considered to be preferable for the flat R-curve behavior. By contrast, in the case of the rising R-curve behavior, it was revealed that the relationship between the IF and SEPB values was difficult to explain unless the effective crack extension against K_Ic for SEPB was clarified.     

A new method for the preparation of ultra-sharp V-notches to measure fracture toughness in ceramics

The preparation of an ultra-sharp V-notch on structural ceramics is a prerequisite for reliable fracture toughness assessment. Femtosecond lasers are used to cut ultra-sharp V-notch on the Si₃N₄ and Al₂O₃ ceramics for fracture toughness testing by single-edge V-notched beam method. The radius of the V-notch tip is smaller than 0.5 μm. The K_Ic of Si₃N₄ and Al₂O₃ ceramics determined by this method is much close to the actual fracture toughness. This method exhibits obvious advantage in good reproducibility, high accuracy and precision for reliable fracture toughness testing of structural ceramics.     

Relationship between fracture toughness determined by surface crack in flexure and fracture resistance measured by indentation fracture for silicon nitride ceramics with various microstructures

The reliability of the Vickers indentation fracture (IF) method for various types of silicon nitride (Si₃N₄) ceramics was assessed by comparing the fracture resistance, K_R obtained from the IF test with the fracture toughness, K_Ic from the surface crack in flexure (SCF) technique in the same crack depth region. The K_R of a fine-grained and equiaxed Si₃N₄ matched with the K_Ic from the SCF test when Miyoshi's equation was used, while the K_Ic of a bearing-grade Si₃N₄ was found to lie between K_R values calculated with Niihara's equation (higher side) and Miyoshi's equations (lower side). In the case of coarse Si₃N₄ with elongated grains, the K_R determined using Niihara's equation gave the best fit with K_Ic. The inconsistent outcomes were explained by the probable mechanisms, indicating that the K_R from the IF test cannot be correlated directly with the K_Ic unless the effective crack length for the IF test was clarified.     

Fracture mechanics studies on concrete compounds

An experimental and analytical investigation was conducted to determine fracture mechanics characteristics of hardened cement paste, aggregates and aggregate-cement paste interfaces. For this the fracture toughness K_Ic was determined on wedge loaded CT-specimens. It was found that hardened cement paste, aggregates and interfaces exhibit unique K_Ic values which are independent of the initial crack length. In additional test series the ductility of various model concretes tested in flexure was determined. The ductility depends primarily on the fracture toughness toughness of the aggregate-hardened cement paste interfaces and is less affected by the fracture toughness of the hardened cement paste.     

Effect of second phase addition of zirconia on the mechanical response of textured alumina ceramics

The effect of second phase addition of zirconia on the mechanical response of textured alumina was analysed. Highly textured monolithic tape-casted alumina was obtained through templated grain growth. Compositions containing 1, 2, 5 and 10 vol% of (i) non-stabilised and (ii) 3 mol% yttria-stabilised zirconia, respectively, were investigated. XRD analyses revealed that the texture degree decreased with increasing second phase content. Microstructural analysis showed zirconia grains inside the textured alumina grains for contents ≤ 5 vol%, affecting the mode of fracture. Fracture toughness of textured alumina significantly decreased with the addition of a second phase. In the case of non-stabilised zirconia, the constraint of the alumina matrix and the small grain size led to a lower fracture toughness in comparison to monolithic textured alumina (K_Ic = 5.1 MPa m^1/2). The fracture toughness of textured alumina with 3 mol% yttria-stabilised zirconia was comparable to equiaxed alumina, independent of the content ratio (K_Ic = 3.5 MPa m^1/2).     

Toughness measurement on ball specimens. Part II: Experimental procedure and measurement uncertainties

The “Surface Crack in Flexure” method is widely used for fracture toughness (K_Ic) determination of ceramics. In part I of the paper we developed the theoretical fundamentals to apply this procedure to ceramic balls by using the stress application as developed for the so-called “Notched ball test”. The new test (SCF-NB) can be used to test spherical components without the need to cut out special specimens such as bending bars. In this work the practical part is presented including suggestions for crack introduction and specimen preparation and possible measurement errors are discussed. It is concluded that a measurement error less than ±5% is possible.Experiments on balls and bars made from the same silicon nitride ceramic indicate that SCF-NB delivers the same K_Ic-values as standardised measurements on bars. Additionally, K_Ic-values obtained for silicon carbide, alumina and zirconia ceramics are presented. They coincide with K_Ic-data from the literature.     

Fracture properties of a rigid polyurethane foam over a range of densities

Single edge notch fracture tests have been carried out on rigid polyurethane foam in the density range 32 to 360 kg/m³. Fracture properties were characterized in terms of the fracture toughness parameter (K_Ic), the critical strain energy release rate (G_Ic) and crack opening displacement (c.o.d.). The values of crack opening displacement lead to a proposed mechanism of crack propagation in foams of density greater than about 140 kg/m³.     

Relationships between fracture toughness,Y2O3 fraction and phases content in modern dental Yttria-doped zirconias

The relationship between fracture toughness and Yttria content in modern zirconia ceramics was revised. For that purpose, we evaluated here 10 modern Y₂O₃-stabilized zirconia (YSZ) materials currently used in biomedical applications, namely prosthetic and implant dentistry. The most relevant range between 2-5 mol% Y₂O₃ was addressed by selecting from conventional opaque 3 mol% YSZ up to more translucent compositions (4−5 mol% YSZs). A technical 2YSZ was used to extend the range of our evaluation. The bulk mol% Y₂O₃ concentration was measured by X-Ray Fluorescence Spectroscopy. Phase quantification by Rietveld refinement considered two tetragonal phases or an additional cubic phase. A first-account of the fracture toughness (K_Ic) of the pre-sintered blocks is given, which amounted to 0.4 – 0.7 MPa√m. In the fully-densified state, an inverse power-law behavior was obtained between K_Ic and bulk mol% Y₂O₃ content, whether using only our measurements or including literature data, challenging some established relationships. A linear relationship between K_Ic and the fraction of the transformable t-phase was established within the range of 30–70 vol%.     

Fracture toughness of zirconia from a shallow notch produced by ultra-short pulsed laser ablation

Fracture toughness of submicron grain size tetragonal zirconia polycrystals doped with 3 mol% yttria (3Y-TZP) is measured by the single edge V-notch beam (SEVNB) method from a shallow sharp notch produced by ultra-short pulsed femtolaser ablation (UPLA) on the surface of a bending bar. It is shown that the radius of the notch tip achieved is in the submicron range and the damaged volume in front of the notch tip is characterized by using focus ion beam milling and scanning electron microscopy. It consists of a narrow fully microcracked region less than ∼4 μm wide and ∼15 μm deep in front of the notch. If the extension of this region and the length of the notch are used in the determination of the fracture toughness (K_Ic) in the four bending test, the values obtained for submicron grain size 3Y-TZP are in agreement those obtained by using very sharp cracks. It is concluded that the SEVNB testing method with a sharp notch induced by UPLA may be used for K_Ic testing of submicron grain size ceramics.     

The fracture toughness of natural fibre- and glass fibre-reinforced SMC

The fracture toughness properties, in terms of stress intensity factor K_Ic and strain energy release rate G_Ic, of hemp fibre mat-reinforced sheet moulding compound (H-SMC) are measured using the compact tension (CT) method and compared with those of glass fibre-reinforced SMC (G-SMC). Three material parameters were considered for composite optimisation: fibre volume fraction, CaCO₃ filler content and hemp fibre surface treatments using either alkaline, silane or a combination of these two treatments. The highest fracture toughness for H-SMC composites was obtained at a fibre loading of around 30?vol.-%, while it was also shown that the fracture toughness properties of H-SMC are sensitive to mineral filler content. Surface treatment of the hemp fibres using a combined alkaline-silane treatment resulted in a significant improvement in fracture toughness of H-SMC composites. Optimised H-SMC composites exhibited fracture toughness properties similar to those of G-SMC at fibre contents of 20?vol.-%, with K_Ic values of around 6?MPa.m^?1/2.     

Effect of fiber reinforcement on the tensile, fracture and thermal properties of syntactic foam

This paper examines the effect of the fiber content and fiber length on tensile, fracture and thermal properties of syntactic foam. Results showed that a hybrid structure demonstrates a significant increase in the ultimate tensile strength, σ_uts, and Young's modulus, E, with increasing fiber loading. Interestingly, the fracture toughness, K_Ic, and energy release rate, G_Ic, increased by 95% and 90%, respectively, upon introduction of 3 wt% short carbon fibers in syntactic foam, indicating the potent toughening potential for short carbon fibers in syntactic foam systems. SEM and OM studies identified the presence of several toughening mechanisms. An estimate of the contribution from each toughening mechanism by composite theory and fractography revealed that the specific energy required to create new surfaces was enhanced by the presence of fibers and was the main contributor to the toughness of the short fiber reinforced syntactic foam.     

Deformation and fracture behaviour of a rubber-toughened epoxy: 1. Microstructure and fracture studies

The microstructure and fracture behaviour of an unmodified and a rubber-modified epoxy have been studied. Values of the stress intensity factor, K_Ic, at the onset of crack growth, the type of crack growth, and the detailed nature of the associated fracture surfaces have been ascertained. Both materials exhibit essentially the same types of crack growth but the values of K_Ic for the rubber-modified material were usually significantly higher than those for the nmodified epoxy. The mechanisms for this increased toughness have been considered and a mechanism that accounts for all the observed characteristics has been proposed.     

Fracture resistance estimation of elastic ceramics in edge flaking: EF baseline

The fracture resistance of single-phase oxide and nonoxide ceramics was studied in flaking the specimen edge with a Rockwell indenter (EF test method) and in bar flexure (SEVNB method). The fracture resistance F_R and fracture toughness K_Ic are shown to be proportional, the plot with the F_R–K_Ic coordinates is termed the EF base diagram, in which the EF baseline is constructed. It was revealed that the fracture resistance of ceramics was not influenced by chip scar shapes on the specimen edge. The data points for inelastic ceramics with a lower EF barrier to the onset of fracture lay below the EF baseline in the base diagram, while the data points for glasses and ceramics with a higher barrier were located above it.The EF test method is appropriate for comparative evaluation of fracture resistance of ceramics and verification of estimates obtained by other methods.     

Effect of molecular orientation on the fracture toughness of thermoplastics

An account is given of fracture toughness experiments on injection moulded plaques of a propylene homopolymer and two different molecular weight grades of poly(methyl methacrylate). The study was concerned essentially with the behaviour of large, centrally notched, plaques since this represents a step towards component testing. However, in order to establish this point, small specimens machined from the plaques were also tested. Analysis of behaviour is by linear elastic fracture mechanics. The study showed that the fracture toughness parameter, K_IC, was not a material constant. Variations in fracture toughness are attributed to molecular orientation produced during the injection moulding process. As expected, the propylene homopolymer was tougher than the PMMA. However, a potentially ductile material can still fail in a brittle manner if a large enough specimen is tested. Overall, this study has emphasized that tests on complete mouldings can provide a valuable means of assessing potential practical performance, and also that data obtained on small specimens can be misleading with respect to the performance of large components.     

Plane-strain fracture toughness of epoxies at different loading rates

The plane-strain fracture toughness of two common epoxy systems of different ductility were determined at different loading rates, according to ASTM E 399 for metallic materials. The ASTM standard was applicable, but underestimated slightly the specimen thickness required for K_Ic. K_Ic decreased with an increasing loading rate and with an increasing yield stress. The fracture surfaces were all very smooth as long as plane-strain conditions prevailed.     

Plane strain fracture toughness testing of high density polyethylene

The concepts of linear elastic fracture mechanics (LEFM) are applied to three grades of high density polyethylene in an attempt to determine their fracture behavior in terms of a linear elastic fracture toughness, K_c. The effect of specimen size (thickness and width), crack length and the mode of loading on K_c has been investigated in order to determine the plane strain fracture toughness, K_Ic, of these materials. The effect of temperature (between +23 and ?180°C) on their fracture behavior has also been investigated and compared in terms of their plane strain fracture toughness values.     

Effect of boiling water aging on strength and fracture properties of chopped strand mat–polyester laminates

Four types of unsaturated polyesters and chopped strand mat laminates prepared from these resins were examined for their durability after immersion in boiling water. The boiling water aging of fiber-reinforced plastics (FRP) by use of a pressure cooker was found to be 3–4 times faster than boiling water immersion at atmospheric pressure. The weight gain of neat resins increased, and their tensile strength decreased with an increase in immersion time. No variation in the plane strain fracture toughness, K_Ic of the neat resins was observed after boiling water immersion. The elastic–plastic fracture toughness J_Ic of the immersed FRP was measured by a partial unloading procedure. Although J_Ic showed considerable scatter, on the whole, J_Ic of FRP was found to be independent of the four types of matrix resins and also was not effected by immersion in boiling water at 116°C for 24 h.     

The fracture toughness of inorganic glasses

Measuring the fracture toughness (K_Ic) of glasses still remains a difficult task, raising experimental and theoretical problems as well. The available methods to estimate K_Ic are reviewed, with emphasis on their respective advantages and drawbacks. In view of our current understanding, this analysis gives precedence to the SEPB method. The ultimate glass strength, the critical flaw size, and the indentation load for the onset of crack initiation are discussed, in the light of the fundamentals of fracture mechanics and classical background regarding the mechanics of brittle materials. Analytical expressions were further proposed to predict the fracture energy and fracture toughness of glasses from different chemical systems from their nominal compositions. The theoretical values were compared with the experimental ones, as obtained by self‐consistent methods when available. The agreement observed in most cases suggests that measured K_Ic values correspond to the crack propagation regime (as opposed to the crack initiation threshold), and supports previous investigations in glasses and ceramics, which showed that a crack tip is nearly atomically sharp in these materials (but for metallic glasses). Some ideas to design tougher glasses are finally presented.