Showing papers on "Fracture toughness published in 1981"

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements

Abstract: The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: II, Strength Method

Abstract: An examination is made of the sharp-indentation technique of strength-test precracking for toughness evaluation. The experimental approach follows that proposed by other workers but the theoretical analysis contains one vital new feature; the residual-stress term discussed in Part I of this study is now introduced explicitly into the strength formulation. This modification overcomes a major systematic discrepancy evident in the previous models and at the same time, by virtue of attendant changes in the nature of the crack stability prior to attaining a failure configuration, eliminates the need for frac-tographic measurements. Other advantages are also apparent, notably an insensitivity to postindentation radial crack extension. The main disadvantage is that only one result is obtained per specimen. Indentation/strength data from ceramics listed in Part I confirm the essential features of the theory and provide a suitable calibration factor. The method has special application to those materials which do not necessarily produce a well-defined radial crack pattern, in which case an “effective”Kc appropriate to fracture properties at the flaw level is obtained.

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements

Abstract: The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this flrst part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elasticlplastic indentation frac- ture mechanics. It is thereby asserted that the key to the radial crack response lies In the residual component of the contact fkld. This residual term has important implications concerning the crack evolution, including the possibility of postindentation slow growth under environment-sensitive conditions. Frac- tographic observations of cracks in selected "reference" mater- Ys are used to determine the magnitude of this effect and to investigate other potential complications associated with de- partures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely hpie in procedure and economic in its use of material.

Development of a Processing Map for Use in Warm-Forming and Hot-Forming Processes

Abstract: A fracture initiation map is developed which should be useful in fast forming operations at strain rates greater than about 10-3 s-1 at elevated temperatures. Two types of cavitation mechanisms, one pertaining to cavity formation at second phase particles, as in ductile fracture, and the other pertaining to wedge type microcracking at grain boundaries, are considered. In addition, dynamic recrystallization and adiabatic heating effects are considered. When these concepts are applied to aluminum, it is shown that there may be an intermediate region in the strain rate and temperature field in which neither of these mechanisms should operate and within which the material would, therefore, be safe from fracture.

Preparation, microstructure and mechanical properties of dense polycrystalline hydroxy apatite

Abstract: Hydroxy apatite ceramic blocks of varying density have been prepared from a commercial powder. The elastic properties, fracture toughness, strength and sub-critical crack growth of these materials have been investigated. Young's modulus for the nearly fully dense material is 112 GPa while the compressive strength is about 800 MPa. For the same material the strength and fracture toughness under dry conditions are 115 MPa and 1.0 MPa m1/2, respectively. Substantial slow crack growth was found under these conditions. Under wet conditions the values for strength and fracture toughness drop to about 75% of their “dry” values. In this case very serious slow crack growth is present.

Fabrication and characterization of SiC-AIN alloys

Abstract: SiC-AIN alloys were prepared by the carbothermal reduction of silica and alumina, derived from an intimate mixture of silica, aluminium chloride and starch. The resulting single-phase SiC-AIN powder was hot-pressed without additives to a high density. The dense bodies had a fine-grained uniform microstructure. The Young's elastic modulus, microhardness, fracture toughness, thermal expansion and thermal conductivity were measured as functions of composition. The creep behaviour of the SiC-AIN alloy was compared with that of silicon carbide.

The Penny-Shaped Crack and the Plane Strain Crack in an Infinite Body of Power-Law Material

Abstract: A study is carried out of the problem of a penny-shaped crack in an infinite body of power-law material subject to general remote axisymmetric stressing conditions. The plane strain version of the problem is also examined. The material is incompressible and is characterized by small strain deformation theory with a pure power relation between stress and strain. The solutions presented also apply to power-law creeping materials and to a class of strain-rate sensitive hardening materials. Both numerical and analytical procedures are employed to obtain the main results. A perturbation solution obtained by expanding about the trivial state in which the stress is everywhere parallel to the crack leads to simple formulas which are highly accurate even when the remote stress is perpendicular to the crack.

Fracture Toughness of Human Enamel

Abstract: A microindentation technique was employed to estimate the fracture toughness of human enamel. A pattern of increasing fracture toughness values existed from incisal to cervical in incisor enamel. Statistical analysis indicated that the molar enamel was more brittle than either the canine or incisor enamel. It was observed that the cracks emanating from the comers of an indentation propagated preferentially, with the weakest path of fracture usually extending along the cervical-incisal axis.

Static and energetic fracture parameters for rocks and concretes

Abstract: The object of the paper is to determine the fracture toughness parameters K1C,G1C and J1C for some aggregative materials. Values of the J-integral are calculated from load-displacement curves, following the procedure suggested by Begley and Landes for steel alloys. Some recurring experimental incoherences are explained applying Buckingham's Theorem for physical similitude and scale modeling to Fracture Mechanics. Thus a non-dimensional parameter can be defined (the test brittleness number), which governs the fracture-sensitivity phenomenon. The fracture parameters K1C and J1C are connected by a fictitious Young's modulus E*, which is lower than the real modulus E and represents the stiffness of the damaged material near the crack tip before the extension. When the specimen sizes are so small that the material becomes fracture insensitive, then E* appears higher than E.

Effect of Density on Tensile Strength, Fracture Toughness, and Fatigue Crack Propagation Behaviour of Sintered Steel

Abstract: The tensile strength, fatigue crack propagation behaviour, and fracture toughness of a low-alloy sin tered steel were determined for the porosity range 11–17%. Static and cyclic strength were found to increase with density in a non-linear fashion. The pores both exerted a stress-concentrating influence and reduced the load-bearing section. The micromechanism of failure was always ductile fracture in the necks between sintered steel particles. It was concluded that the stress state at the tips of cracks subjected to static or cyclic loading was closer to plane stress than to plane strain. Retardation of fatigue crack propagation appeared to occur due to the blunting action of the pores. The presence of a wear mechanism had little influence upon fatigue crack growth rates. A companion paper (following) attempts to model the static and cyclic behaviour of the steel, based on the known micromechanisms of failure. PM/0172

Increased fracture toughness of ceramics by energy-dissipative mechanisms

Abstract: A theoretical model for the fracture toughness of ceramics is developed which takes into account such energy-dissipative mechanisms as stress-induced microcracking or phase transformation. To establish the general fracture criterion, a Griffith-type energy balance is employed. This energy balance comprises the elastic energy, the fracture surface work consumed in the process zone at the crack tip, the energy dissipated in the dissipation zone and the energy stored by residual stresses. Stress-induced microcracking is considered in more detail. An expression for the dependence of the fracture toughness on the density of microcracks, the amount of residual stresses caused by thermal expansion mismatch between the ceramic matrix and small particles embedded in it and the volume fraction of these particles is derived. The final results are used to state conditions necessary for the fracture toughness to be increased. The theory agrees well with experimental results taken from literature (alumina with zirconia particles).

Stress‐Induced Microcracking at Second‐Phase Inclusions

Abstract: The critical inclusion size for microcracking due to an applied stress for an inclusion that has a residual stress field is estimated using fracture mechanics. In particular, an analysis is presented for both circumferential and radial crack formation at spherical inclusions that have a uniform misfit strain compared to the matrix. It is found that the critical size can be greatly reduced below that for spontaneous microcracking when the applied stress is of the order of the residual stress. It was predicted that the applied stress would cause extensive micro-cracking when the local fracture toughness is low and when the size of the inclusions approaches the critical size for spontaneous microcracking.

A Plastic Fracture Mechanics Prediction of Fracture Instability in a Circumferentially Cracked Pipe in Bending—Part I: J-Integral Analysis

Abstract: A method of evaluating the J-integral for a circumferentially cracked pipe in bending is proposed. The method allows a J-resistance curve to be evaluated directly from the load-displacement record obtained in a pipe fracture experiment. It permits an analysis for fracture instability in a circumferential crack growth using a J-resistance curve and the tearing modulus parameter. The influence of the system compliance on fracture instability is discussed in conjunction with the latter application. The importance of using a J-resistance curve that is consistent with the type of constraint for a given application is emphasized. The possibility of a pipe fracture emanating from a stress corrosion crack in the heat-affected zones of girth-welds in Type 304 stainless steel pipes was investigated. The J-resistance curve was employed. A pipe fracture experiment was performed using a spring-loaded four-point bending system that simulated an 8.8-m long section of unsupported 102-mm-dia pipe. An initial through-wall crack of length equal to 104 mm was used. Fracture instability was predicted to occur between 15.2 and 22.1 mm of stable crack growth at each tip. In the actual experiment, the onset of fracture instability occurred beyond maximum load at an average stable crack growth of 11.7 to 19more » mm at each tip. 24 refs.« less

Mechanical properties of poly(methyl methacrylate) bone cements

Abstract: Samples of low viscosity poly(methyl methacrylate) (PMMA), graphite reinforced PMMA, and graphite reinforced low viscosity PMMA were evaluated for their compression strength and fracture toughness. These results were compared with two currently used plain PMMA bone cements. There were no statistically significant differences in compression strength between the five cements. Graphite reinforcement of plain cement produced a 32% increase in fracture toughness over plain cement. Graphite reinforcement of low viscosity cement also produced a significant increase in toughness (31%) over low viscosity cement with fiber reinforcement. However, low viscosity cement demonstrated significantly less fracture toughness than plain PMMA.

A micromechanistic approach to the warm pre-stressing of ferritic steels

Abstract: Warm pre-stressing of a cracked body can result in an apparent elevation of its fracture toughness at lower testing temperatures. This effect has been observed in mild steel pre-loaded at room temperature and tested at −150°C, when the apparent toughness elevation can be as great as a factor of two. This toughness elevation is produced both when the pre-load is maintained during cooling and when the pre-load is completely removed prior to cooling.

Linear elastic fracture mechanics computations of cracked cylindrical tensioned bodies

Abstract: The variation of the stress intensity factor along the crack front and the strain energy release rate were computed, by the boundary integral equation method, for semi elliptical cracks on the surface of cylindrical bodies strained in tension or in bending. The results were checked by the compliance method. The fracture toughness of high strength steel wires was measured on notched cylindrical specimens. The value which was found to be 80 MPa √m, was used to predict the fracture loads of cracked wires with various crack configurations, and they compared well with the experimental results. The evolution of the cracks during fatigue was followed as a function of the number of cycles and the fatigue life could be predicted making use of Paris law.

On the Relationship Between Stretch Zone Formation and the J Integral for High Strain-Hardening Materials

Abstract: The relationship between stretch zone formation, associated with crack tip blunting, and the J integral was evaluated for materials with high strain-hardening capabilities: Incoloy® 800, Inconel® 600, SA-387 Grade C steel, Type 304 stainless steel, and Alloy A-286. A conventional blunting line commonly used in the construction of a J integral crack growth resistance curve accurately describes stretch zone formation in intermediate and high strength alloys. For low-strength, high strain-hardening materials, however, this conventional blunting line overestimates apparent crack extension associated with stretch zone formation, and it results in an overestimation of ductile fracture toughness values. An alternative blunting line that better describes stretch zone formation in this class of materials is proposed.

Failure processes in particulate filled polypropylene

Abstract: In this paper the common degradation effect of silicon oxide filler on fracture strain and fracture toughness of isotactic polypropylene is investigated by analysing the failure processes in the composite material by microscopic methods. Experiments demonstrate that, although fracture of the polymer regions absorbs considerable energy by plastic deformation, void formation and cracking of the interface between the polymer and the filler usually requires very little energy. These weak interfaces do not resist cracking and are the cause of brittleness in particulate filled systems. The crucial parameters influencing the fracture data of the composite were found to be the volume fraction of the filler and the interfacial adhesion between polymer matrix and particles. As the interfacial fracture energy is usually much smaller than the polymer fracture energy, the composite toughness drops when filler is added. Using a model which describes the individual steps of crack formation and final fracture, an attempt is made to explain the decrease of crack resistance of the polymer matrix with increasing filler fraction and to calculate the fracture energy of the composite by introducing partial values of crack resistance of the matrix and the interface, respectively. In addition, it is discussed how a coarse spherulitic morphology of the matrix, as produced by isothermal crystallization from the melt, can modify this behaviour.

Fracture Toughness of Fused SiO2 and Float Glass at Elevated Temperatures

Abstract: The fracture toughnesses of fused SiO2 and float glass were measured at high temperatures. In both glasses, low-temperature regions of elastic fracture were identified and correlated with the elastic moduli and their temperature dependence. A viscous flow contribution to the fracture toughness was identified in the fused SiO2 at T>800°C. Similar indications of viscous flow were also noted in the float glass, although at much lower temperatures.

Crack-Blunting Mechanisms in Impact Tests on Polymers

Abstract: Some impact tests are described in which G c , the fracture toughness, of three polymers was determined over a range of strain rates and tempera­tures. It was found that at high strain rates there was a substantial variation of G c with rate. When this was investigated as a function of load­ing times, it was observed that the most likely explanation was in terms of notch tip blunting induced by adiabatic heating at the crack tip. A model of this process is presented which provides a good description of the observations and suggests some useful guidelines for practical impact testing.

J andG canalysis of the tearing of a highly ductile polymer

Abstract: The use of a conventionalJ analysis to describe the ductile tearing of thin low density polyethylene sheet is described. This is a measure of the total energy required to cause fracture. The use of the current energy release rate to determine the local dissipation rate is then given and it is shown that an initiation (plane strain) and reasonably constant propagation (plane stress) values are obtained.

Criteria for Fracture Initiation at Hydrides in Zirconium-2.5 Pct Niobium Alloy

Abstract: In many ductile commercial alloys, fracture is initiated at second phase particles. In this work, the initiation process in Zr-2.5 pct Nb pressure tube alloy is examined. In particular, the conditions for fracture of a hydride platelet in a zirconium matrix are sought, by testing tensile specimens containing hydrides oriented with the normals of the platelet parallel to the tensile axis. Acoustic emission is monitored to signal the fracture event. It is concluded that some plastic deformation must precede hydride fracture and that fracture is encouraged by a triaxial stress state. The fracture mechanism appears to be one of slip-induced crack nucleation in the hydrides with the critical event being the growth of this crack to a critical size.