Fracture Resistance of Ceramics upon Edge Chipping

Modern methods of determining fracture resistance are analyzed. The necessity of developing a crucially new method based on edge chipping of a brittle material is shown. The results of experimental studies are presented. The applicability of the method to the comparative fracture resistance evaluation of ceramics is substantiated.     

Fracture Resistance of Brittle Materials Under Local Loading by Scratching to Edge Chipping. Part 2. The S+EF Relation for Linear Elastic Ceramics

Strength of Materials - Experimental studies to evaluate the fracture resistance of brittle materials by Rockwell scratching of the specimen surface to its edge chipping (S+EF method) are...     

Fracture resistance of technical and optical glasses: edge flaking of specimens

We present the results of fracture resistance tests for technical and optical glasses performed by indentation flaking of the rectangular specimen edge. The features of crack propagation in these glasses are analyzed during chipping fracture caused by indentation with blunt and sharp diamond indenters. By comparative analysis of the values for resistance to edge flaking and those for fracture resistance, new data on the mechanical behavior of the above glasses in fracture are obtained. In particular, it is shown that these glasses, along with ceramic materials used to produce armor, have an enhanced resistance to the onset of fracture.     

Local Fracture of Nanolaminates: Edge Chipping Test

Strength of Materials - The fracture resistance of domestic and foreign nanolaminate ceramics was studied by the EF (edge fracture) method. This method provides for indentor chipping of rectangular...     

On the mechanics of edge chipping from spherical indentation

Edge chipping is a basic failure mode in brittle materials which is dictated by a wealth of material and geometric variables. Here we examine the effect of indenter bluntness on chipping load and chip dimensions. Soda-lime glass and YTZP plates are subject to surface-normal loading near an edge by a W/C ball or a Vickers tool. The ball radius r is varied from 0.2 to 8.7 mm while the indent distance h is varied from several millimeters down to a few microns. Although cone cracks are a common feature under spherical indentation, the chipping event is dominated by median-radial cracks formed under the contact. The fracture behavior is characterized by a “large” indent distance regime where the median cracks progress stably up to chipping and a “small” one where they grow unstably to form a chip once initiated. Closed-form relations for chipping load P _F under spherical indentation is developed with the aid of the test data and non-dimensional arguments. While in the “large” distance regime P _F is proportional to h ^3/2 irrespective of tool bluntness, in the “small” regime P _F is proportional to r ^1/2 h ^3/4. Interestingly, the chip dimensions are virtually independent of ball radius, varying linearly with h. Beyond relevance to structural integrity, the chipping test facilitates a simple means for determining fracture toughness K _C as well as the load needed to initiate median cracks in opaque brittle materials. An attempt is made to extend the static analysis to low-velocity impact. The results show that the damage formed during the fracture process has a major influence on dynamic chipping.     

The dentin-enamel junction and the fracture of human teeth

The dentin-enamel junction (DEJ), which is the interfacial region between the dentin and outer enamel coating in teeth, is known for its unique biomechanical properties that provide a crack-arrest barrier for flaws formed in the brittle enamel1. In this work, we re-examine how cracks propagate in the proximity of the DEJ, and specifically quantify, using interfacial fracture mechanics, the fracture toughness of the DEJ region. Careful observation of crack penetration through the interface and the new estimate of the DEJ toughness ( approximately 5 to 10 times higher than enamel but approximately 75% lower than dentin) shed new light on the mechanism of crack arrest. We conclude that the critical role of this region, in preventing cracks formed in enamel from traversing the interface and causing catastrophic tooth fractures, is not associated with the crack-arrest capabilities of the interface itself; rather, cracks tend to penetrate the (optical) DEJ and arrest when they enter the tougher mantle dentin adjacent to the interface due to the development of crack-tip shielding from uncracked-ligament bridging.     

Investigation on impact toughness and wear resistance of nanocomposite Al2O3/WC-Co hammer bit teeth

The material of the hammer bit tooth is cemented carbide YG8, whose impact toughness and wear resistance are insufficient, leading to tooth fracture and wear. In this paper, cemented carbide YG8 teeth and nanocomposite teeth doped with Al₂O₃ nanoparticles and VC inhibitors were fabricated through a series of processes. Cemented carbide YG8 teeth act as comparison. Two kinds of teeth were analyzed and compared using the scanning electron microscopy (SEM) and the MLD-10 dynamic impact-wear tester. SEM micrographs of samples show that Al₂O₃/WC-Co nanocomposite grains are finer and more uniform, which can improve impact-wear resistance. Impact-wear tests indicate that nanocomposite teeth had 3 to 5 times higher impact toughness and wear resistance than YG8 teeth. The fracture toughness can be significantly improved by adding the proper amount of Al₂O₃ nanoparticles. Based on fracture morphology analysis, it is found that the fracture of the YG8 tooth under impact-wear condition is brittle fracture and the fracture of the nanocomposite tooth is ductile fracture.     

Micro-tensile strength of sound primary second molar dentin

Although biomechanical properties of dentin are important factors to dentin bonding, as well as for understanding caries, cervical erosion/abfraction, and tooth fracture, limited information for primary teeth has been reported. This study evaluated the micro-tensile strength (MTS) of sound primary second molar dentin with an originally designed system that we have developed. Twenty-seven dumbbell-shaped specimens were prepared from eight teeth. The MTS of the dentin beneath the occlusal surface was measured and fractured dentin surfaces were observed using SEM. Data was analyzed using ANOVA subsequent to Fisher's PLSD at p < 0.05. The novel jig system used in this study allowed symmetric dumbbell-shaped and uniformly sized specimens. The mean (standard deviation) MTS of all the specimens was 38.2 (15.9) MPa. The mean MTS of the specimens sectioned from the central area (46.5 MPa) was significantly higher than those of the specimens that were sectioned from the most mesial (31.1 MPa) and distal (27.8 MPa) sides of the teeth. Sound primary second molar occlusal dentin showed regional variations in tensile strength. This might influence the prognosis of dental restorations.     

Failure analysis of exit edges in ceramic machining using finite element analysis

Edge chipping is a mode of surface damage in ceramic machining. Its presence not only compromises dimensional and geometric accuracy, but also causes possible failure of the ceramic components during service. In this paper, a finite element analysis is used to investigate the size of exit edge chipping as related to the loading conditions and intrinsic flaws of materials. A local remeshing algorithm was implemented to accommodate the curved crack trajectory in forming an exit edge chip based on the theory of fracture mechanics. Stress intensity factors at the crack tips were calculated for a variety of loading conditions and cracks presented at the critical locations. Machining experiments were performed on glass-ceramics to validate the FEA results. A close correspondence between the predicted and measured width of exit edge chipping gives the confidence of using this proposed finite element method in selecting machining conditions and material properties to minimizing machining damage.     

Fracture initiation in elastic brittle materials having non-linear fracture envelopes

A theory is outlined for determining the initiation of fracture and initial fracture propagation in elastic brittle materials having non-linear Mohr fracture envelopes. This theory is applied to a specific boundary value problem, i.e. a truncated quarter plane with arbitrary traction distribution on the truncated boundary and varying confining pressure. This problem simulates the chipping phase of the penetration of a wedge shaped tool into an elastic brittle material. Numerical results are obtained for two rock materials, Blair dolomite and quartzite. Results indicate that for increasing confining pressure, a limit condition is reached for both fracture initiation location and force. This limit location is closer to the boundary than the fracture initiation points at lower confining pressures, indicating smaller chips. It is also found that initial fracture propagation is less clearly defined at higher confining pressures. Both of these results have been observed experimentally.     

Edge chipping of borosilicate glass by low velocity impact of spherical indenters

The edge chipping of borosilicate glass by the low velocity impact of freely falling steel and ceramic balls was investigated experimentally. The maximum impact forces were recorded using a force sensor, and comparisons were made between edge chipping by impact and quasi-static loading. In general, edge chipping by impact loading required lower forces than by quasi-static loading. This was attributed to the reduced constraint in the impact tests which created a bending moment at crack tips causing cracks to deviate toward the edge and form chips.     

Deformation behavior of root dentin under Sjögren's syndrome

Microstructure and deformation behavior under uniaxial compression of human root dentin cut from injured premolars (Sjögren's syndrome) are studied. Optical metallography in transmission light is the technique for microstructure examination. There is sclerosis dentin near the root channel of a Sjögren's tooth. Root dentin of injured teeth is high elastic and plastic hard tissue, whose mechanical properties, including fracture mechanism, are close to the normal root dentin. This may be explained by the fact that samples for tests contain small quantity of the sclerosis tissue. The roots of a Sjögren's teeth can be used as a base for standing of implants after endodontic treatment.     

Study of Portland cement fracture surfaces by scanning electron microscopy techniques

An extensive scanning electron microscopy study was carried out with respect to the fracture surfaces of Portland cement hydrated for various times. It is shown that the two major products of hydration are calcium silicate hydrate spherulites, which consist of radiating fibres and calcium hydroxide platelets. These fibres bond with one another to hold the spherulites together. The volume between the spherulites consists of calcium hydroxide platelets. The fracture is frequently found to be across the weakly bonded basal planes of the calcium hydroxide, and is believed to limit the strength of the Portland cement.     

Analyses of the failures on shear cutting blades after trimming of ultra high-strength steel

Damages on shear cutting blades were analyzed after 50,000 strokes of trimming on an ultra high-strength steel sheet. Traditional D2 alloy and an advanced one (Cr08H) based on the composition of 1C-8Cr were quenched from 1030 °C, tempered at 180 °C and submitted to the shear cutting test. Cr08H had lower hardness, a smaller volume fraction of M₇C₃ carbides while it contained a larger volume fraction of retained austenite. And these resulted in more scratches and rounded edges because of degraded resistance to wear and local plastic deformation. In spite of higher impact toughness, Cr08H exhibited inferior resistance to chipping which was the consequence of localized brittle fracture. It could be concluded that this was caused by more transformation of austenite as well as by insufficiently hardened matrix, both of which were attributed to inappropriate conditions of the heat treatment.     

On lateral cracks in glass

The importance of lateral cracks in solid particle erosion of brittle materials has been confirmed as a result of a large number of previous investigations in this area. Even though the underlying mechanism of steady-state erosion of a brittle material is the formation and growth of lateral cracks, the surface morphology of the eroded material does not readily indicate this aspect. This has precipitated the need for a study of single impact events. This study concentrates on lateral cracks in glass produced by solid particle impacts. Single impacts are studied in terms of lateral crack extensions and their probability of chipping at two angles of impact of 20 and 90°. Comparisons between these two sets of data were made at the same normal component of velocity to clearly bring out behavioural differences at the two angles o of impact. Steady-state erosion results are then interpreted in terms of the above results obtained from a study of single impacts. There appears to be marked agreement between these results and experimental observations. The same trend was observed in strength degradation measurements. Increased chipping and lateral crack extensions in the 20° impact situation has been explained in terms of linear elastic fracture mechanics, as opposed to the plastic deformation mechanism proposed earlier. The importance of single impacts in the study of steady-state erosion of brittle materials by solid particle impact is well demonstrated by this study.[/p]     

Edge chipping of brittle materials: effect of side-wall inclination and loading angle

An earlier analysis of chipping fracture in brittle solids is here extended to include the case of blocks with inclined side faces and non-normal contact loading. The simple relation P _F = β K _c h ^3/2 for the critical chipping load P _F in terms of indent location h and material toughness K _c is preserved, with angular coordinates simply incorporated into the β coefficient. Chipping fracture tests using a Vickers indenter near the edges of glass blocks with non-orthogonal faces is used to validate the analysis. Implications of the results in relation to practical engineering, biomechanical and anthropological structures are indicated. An erratum to this article can be found at     

An extended maximum tangential strain energy density criterion considering T‐stress for combined mode I–III brittle fracture

The maximum tangential strain energy density (MTSED) criterion was modified by taking the influences of stress intensity factors and T‐stress into consideration for combined mode I–III brittle fracture. Furthermore, the Poisson's ratio and T‐stress influencing the fracture characteristics of cracked components were discussed by using the extended MTSED criterion. Moreover, the predicted values of this extended MTSED criterion and some testing results were comparatively analysed. The results indicate that the Poisson's ratio and T‐stress have no impact on the out‐of‐plane initiation angle; however, their effects on fracture resistance ratios are significant especially for pure mode III. A positive T‐stress increases the fracture resistance ratio, and it is opposite for a negative T‐stress. The predicted values calculated by the extended MTSED criterion agree very well with the testing data obtained with edge‐notched disc bend samples especially for pure mode III case.     

An analysis of Chipping in Brittle Materials

The chipping process in a brittle material subjected to a uniformly applied edge load has been investigated. The present analysis extends earlier work by recognizing that as the chip is formed it may bend and change the loading at the crack tip. This geometry change introduces a nonlinear effect and has significant influence on the phenomenon. The nonlinear effect was demonstrated by incorporating it into an analytical model for a crack propagating along an interface parallel to the free surface. A finite-element analysis was then conducted to examine the crack trajectory formed in a homogeneous material. This numerical analysis showed that the crack reaches a maximum depth, and then deviates back to the free surface to form a spall. The form of this trajectory results from the additional bending moment acting at the crack tip induced by the bending of the chip and the consequent displacement of the applied load. The length of the spall was found to be approximately proportional to the square root of êd^5/2/K_IC, where K_IC is the fracture toughness of the material, ê is the appropriate modulus of the material, and d is the depth over which the edge load is applied. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterisation of the scratch resistance of annealed and tempered architectural glass

The brittle nature of glass prevented real structural applications of glass in architecture up to the 1990s. As a result of intense research, nowadays tempered architectural soda-lime-silica (SLS) glasses are particular capable for modern architectural, structural and solar applications due to their superior mechanical strength and resistance to thermal break. The problem for the application of tempered glass in civil engineering is the sensitivity to scratching and the visibility of the scratches on the surface. To analyse the sensitivity to scratching for commercial annealed and thermally tempered glass scratch tests were performed with the Universal Surface Tester. The tests have shown that the crack width strongly depends on the indenter geometry, the used normal load and the type of glass (annealed or tempered glass). Macroscopically visible chipping of glass fragments at the scratch faces widens scratch tracks from typically 20-30 μm to 200-300 μm. Blunt indenters were identified to cause chipping at lower normal forces than sharp indenters and tempered glass was found to be more sensible for chipping. Contrary, the indentation depth and permanent deformation did not significantly change between annealed and tempered glass.     

Tangential strain‐based criteria for mixed‐mode I/II fracture toughness of cement concrete

Experimental and theoretical works are performed on the mixed‐mode I/II brittle fracture of cement concrete tested by edge cracked semicircular bend specimens. Theoretical background of the traditional fracture criteria including strain energy density, maximum tangential stress, and maximum tangential strain (MTSN) are introduced. The ability of each fracture criterion in prediction of the fracture test data is investigated. The comparison between the evaluations by the traditional criteria and the experimental data shows that none of them are capable of successfully estimating the fracture resistance of cement concrete. An enhanced version of the MTSN criterion is then employed to predict the test data. It is demonstrated that the extended MTSN criterion can successfully predict the test data in a higher accuracy than traditional criteria.