Showing papers on "Fracture toughness published in 1969"

Unified Theory of Thermal Shock Fracture Initiation and Crack Propagation in Brittle Ceramics

Abstract: A fracture-mechanical theory is presented for crack propagation in brittle ceramics subjected to thermal shock. The criteria of crack stability are derived for a brittle solid uniformly cooled with triaxially constrained external boundaries. Thermal stress crack instability occurs between two values of critical crack length. For short initial crack length, crack propagation occurs kinetically, with the total area of crack propagation proportional to the factor St2 (1-2v)/EG, where St is tensile strength, v is Poisson's ratio, E is Young's modulus, and G is surface fracture energy. Under these conditions the newly formed crack is subcritical and requires a finite increase in temperature difference before propagation will proceed. For long initial crack length, crack propagation occurs in a quasi-static manner and can be minimized by maximizing the thermal stress crack stability parameter Rst= [G/α2E]1/2, where α is the coefficient of thermal expansion. For heterogeneous brittle solids, such as porous refractories, the concept of an “effective flaw length” is introduced and illustrated on the basis of experimental data in the literature. The relative change in strength of a brittle solid as a function of increasing severity of thermal shock is estimated. Good qualitative agreement with literature data is found.

Fracture Surface Energy of Glass

Abstract: Fracture surface energies of six glasses were measured using the double-cantilever cleavage technique. Values ranged from 3.5 to 5.3 J/m2 depending on the chemical composition of the glass and the temperature of the test. The fracture surface energy increased with decreasing temperature and increasing Young's modulus; however, exceptions to this behavior were noted. The magnitude of the values obtained is discussed with respect to the theoretical strength of glass and possible irreversible effects at the crack tip such as stress corrosion and plastic deformation are considered.

Crazing, yielding, and fracture of polymers. I. Ductile brittle transition in polycarbonate

Abstract: Most thermoplastics far below their glass transition give a brittle fracture when de-formed in uniaxial tension. Bisphenol-A polycarbonates are an exception and deform in a ductile manner. However, it has been observed in Izod impact studies of notched samples that the mode of failure changes from a ductile to a brittle fracture on annealing samples below Tg. It has been found that, when notched samples are stressed, a Griffith type flaw is formed under the notch. The criterion for the ductile brittle transition is evaluated in terms of σG (the stress required to propagate the Griffith flaw), and σy, the yield stress for the polymer. It has been found that the density and yield stress for the samples annealed at various temperatures are dependent upon previous thermal history and in particular on the molecular weiAght. On the basis of these measurements, it is concluded that many of the so-called anomalous effects observed with polycarbonate can be explained.

On the Fracture Energy, Rehealing Velocity and Refracture Energy of Cast Epoxy Resin

Abstract: The fracture energy of cast epoxy resin was measured by a novel method in the presence of various agents. The crack spontaneously rehealed under heating to above the heat distortion temperature to give a fresh joint of about the same fracture energy as the virgin material.

Fracture Toughness of Portland Cement Concretes

Abstract: FRACTURE TOUGHNESS IS A MATERIAL PROPERTY DETERMINED BY EVALUATING THE STRESS INTENSITY FACTOR AT THE ONSET OF RAPID, UNSTABLE CRACK PROPAGATION. THE STRESS INTENSITY FACTOR DENOTES THE ELASTIC STRESS AND DISPLACEMENT FIELDS IN THE REGION OF THE CRACK TIP OF A HOMOGENEOUS MATERIAL. WHEN THE CONCRETE IS ANALYZED AS A HOMOGENEOUS MATERIAL AN EFFECTIVE FRACTURE TOUGHNESS IS OBTAINED. AN EXPERIMENTAL INVESTIGATION WAS CONDUCTED TO DETERMINE THE EFFECTS OF SEVERAL CONCRETE PARAMETERS ON THE FRACTURE TOUGHNESS OF CONCRETE. /A/RRL/

Fatigue crack propagation and strain cycling properties

Abstract: Fatigue crack propagation caused by cumulative damage due to strain cycling at crack tip, relating resistance to cyclic ductility

The brittle fracture of alumina below 1000°C

Abstract: Alumina is normally regarded as truly brittle below ∼ 1000°C. Contrary to this view, it is argued that plastic deformation at the crack tip plays a necessary part in the fracture. This is used to explain the minimum observed at 250°C in the fracture stress of polycrystalline plates containing drilled cracks and in the stress intensity required for crack-branching. Other details of the fracture of single crystal and polycrystalline alumina, including changes in the proportion of intergranular and transgranular fracture with temperature, crack length and crack-branching, are also explained by the involvement of plastic deformation.

Plane strain fracture toughness.

Abstract: The general concept of the K I c plane strain fracture toughness, due to G. R. Irwin, is reviewed, and the current (tentative) standard procedure for determination of K I c with crack-notched bend specimens is discussed in detail. The standard procedure constitutes an operational definition of K I c which is somewhat arbitrary, but no more so than is the definition of offset yield strength. Experimental data for high-strength steels, titanium, and aluminum alloys validate the standard procedure for determination of K I c , and confirm that it is reasonable to regard K I c as an inherent material property which controls fracture under conditions of sufficiently high constraint. However, the required specimen dimensions increase in proportion to the square of the ratio of K I c to yield strength, and impractically large specimens may be required for tough, low-strength materials. The standard test procedure for bend specimens can be adapted to other types of specimens with the aid of information given in an appendix.

The application of fracture mechanics to concrete and the measurement of fracture toughness

Abstract: The failure of concrete in uniaxial compression and tension is explained in terms of the energy concepts of fracture mechanics, and the difficulties associated with the measurement and evaluation of concrete “fracture toughness” are discussed. Basic assumptions relating to slow crack growth, stress concentration because of notch and value of E to be adopted for the calculation of the fracture toughness parameters are proposed. The effect of these assumptions on the values of critical strainenergy release rate and critical stress intensity factor are reported together with the trend of values with changes in concrete properties.

The brittle fracture of [100] axis tungsten single crystals

Abstract: Cleavage cracks have been introduced into 〈100〉 axis single crystals of tungsten by a modification of the spark discharge technique used by Hull, Beardmore and Valintine (1965). It has been possible to insert cracks in specimens at 77°K and to subsequently test these specimens in tension at 77°K without an intermediate rise in temperature. Various other test conditions have also been used. From the values of fracture stress and crack length obtained, values of the effective surface energy, γ, have been determined. In analysing the results, account has been taken of the shape of the spark crack and specimen geometry and dimensions using theoretical analyses of the stress concentrations associated with cracks. Both the temperature of crack initiation and the temperature of crack propagation were found to affect γ. For specimens in which both initiation and propagation of the cracks took place at 77°K, a value of γ = 1700 ± 300 ergs/cm2 was obtained.

The Role of the Strain Hardening Exponent in Stress Corrosion Cracking of a High Strength Steel

Abstract: Stress corrosion testing of a vanadium modification of 4340 alloy was conducted using precracked specimens in a cantilever beam apparatus. Plastic zones at the crack tip were determined using optical interference measurements, and the effect of these plastic zones on the fracture paths was demonstrated. Crack propagation rates were measured for steels of varying yield strengths (140–205 ksi). Electron fractographs and photomicrographs associated with the fracture through plastic zone are presented. Crack propagation rates were shown to vary inversely with the yield strength of the steel with the overall failure time being related to both the propagation rate and the fracture toughness of the material.

Temperature dependence of yield and fracture in polymethylmethacrylate

Abstract: The effect of temperature on the deformation and fracture of polymethylmethacrylate has been determined by constant strain rate compressive and tensile tests from 78°K to 350°K. In compression, the yield stress increased continuously with decreasing temperature and, except at 78°K, substantial macroscopic plastic flow occurred at all temperatures. At 78°K, only a small amount of plastic flow occurred before fracture. A similar type of temperature dependence was found in tension between 230° and 350°K for both the yield and fracture stresses, but the tensile yield stress was always lower than the compressive yield stress by about 1500 p.s.i. In this temperature range, fracture is controlled by macroscopic yielding in that defects (crazes) produced during the yielding process serve as crack nuclei. It is deduced that the temperature dependence of fracture in this temperature range is a reflection of the temperature dependence of the yield stress. Below about 200°K, PMMA fractures in tension with li...

Origin of Water‐Induced Toughening in MgO Crystals

Abstract: The toughening effect of water on MgO, as measured by the increase in apparent surface energy of a double cantilever beam specimen, is shown to derive directly from the increased roughness of the fracture surfaces when a crack propagates in water Dimethyl formamide, a medium reported to produce a larger Rebinder effect (surface softening) in MgO than water, has no effect on the fracture surface or fracture toughness of MgO

A relation between crack surface displacements and the strain energy release rate

Abstract: The relationship between the strain energy release rate, G, and the displacements of the surfaces of an extending crack in an elastic, tensile member is examined It is shown that G can be expressed in terms of the volume of the deformed crack provided that any stresses applied to crack surfaces are uniform This form is especially useful for superposition applications as it depends linearly on displacements The strain energy release rate is calculated from crack volumes for a crack in an infinite sheet and for two cases of a crack in an infinite solid: (a) a penny-shaped crack subjected to internal pressure as well as axial stress, and (b) an elliptical crack loaded by axial stress The importance of the shape of the propagating crack is demonstrated by the elliptical crack by considering various shapes for the propagating crack such as preferential propagation along a diameter or propagation as an ellipse of invariant shape A discussion of the distinction between a fracture criterion based on the strain energy release rate and one based on the stress intensity factor is presented

Coupling of fracture mechanics and transition temperature approaches to fracture-safe design,

Abstract: : The effect of mechanical constraint on the transition temperature features for steels of thick section is examined in the framework of linear elastic fracture mechanics and tests and the Dynamic Tear (DT) test. Steels of fracture toughness and yield strength levels representative of the A533-B type exhibit a transition to high fracture toughness in thick sections. The distinguishing feature of thick sections is a size effect involving a shift of the transition temperature midpoint on the order of 60F. The DT test defines a limiting transition temperature region, which can be divided into three parts: a low temperature regime in which plane strain fracture mechanics is applicable, a higher temperature regime which is beyond the limits of fracture mechanics definition and is relatable to crack arrest conditions, and a final regime involving large plastic deformation in association with the fracture. (Author)