Showing papers on "Fracture toughness published in 1970"

Stress Corrosion and Static Fatigue of Glass

Abstract: Stress corrosion cracking of six glasses was studied using fracture mechanics techniques. Crack velocities in water were measured as a function of applied stress intensity factor and temperature, and apparent activation energies for crack motion were obtained. Data were consistent with the universal fatigue curve for static fatigue of glass, which depended on glass composition. Of the glasses tested, silica glass was most resistant to static fatigue, followed by the low-alkali aluminosilicate and borosilicate glasses. Sodium was detrimental to stress corrosion resistance. The crack velocity data could be explained by the Charles and Hillig theory of stress corrosion. It is probable that stress corrosion of glass is normally caused and controlled by a chemical reaction between the glass and water.

The interaction of a crack front with a second-phase dispersion

Abstract: Observations are presented showing that a crack front increases its length by changing its shape when it interacts with two or more inhomogeneities in a brittle material. These observations are presented for both cleavage and conchoidal types of fracture. Based on these observations and the concept that a crack front possesses a line energy, an expression for the fracture energy is obtained. This expression shows that the fracture energy should increase as the distance between the dispersed inhomogeneities is decreased. Using the Griffith fracture criterion, this result is discussed for the case of strengthening a brittle material.

Fracture of Bone

Abstract: Fracture of bone is investigated by measuring the energy required to propagate a crack and by observing, microscopically, the mechanism of fracture. At low strain rates bone displays very high resistance to crack propagation comparable even to some metallic materials. The type of fracture is similar to fiber‐reinforced composite materials.

Theoretical estimation of fracture toughness of fibrous composites

Abstract: A method of estimating the fracture surface energy of fibre-reinforced materials is discussed. The surface work is shown to increase with increasing fibre content, strength and diameter, and decrease with increasing fibre modulus and matrix flow stress (or hardness). Relatively short fibres should be used if high toughness is required, and the maximum toughness that can be achieved is limited by the amount of crack opening that can be permitted. Under certain conditions, incorporation of fibres into a material can lead to embrittlement.

The fracture toughness of composites reinforced with weakened fibres

Abstract: Fibre fractures which occur near, but not at, the plane of matrix failure in a composite, lead to fibre pull-out during fracture. Energy absorbed in this process contributes directly to the work of fracture and hence to the toughness of the composite. Factors which determine the mean length of fibre pulled out during fracture are discussed for the case of composites reinforced with continuous fibres having variously spaced points of weakness. The presence of such weak points also affects the strength of the composite, but not all composites of the same strength have the same toughness. The greatest toughness for a given strength is always found in composites reinforced with discontinuous fibres.

Fatigue crack propagation in polymeric materials

Abstract: In order to gain a better understanding of matrix-controlled fatigue failure processes in non-metallic materials a series of fatigue tests were performed on several different polymer materials representing different classes of mechanical response Fatigue crack propagation rates between 5×10−6 in cycle−1 (127 nm cycle−1) and 4×10−4 in cycle−1 (10 300 nm cycle−1) were measured in nylon, polycarbonate, ABS resin, low-density polyethylene and polymethyl methacrylate A strong correlation was found between the fatigue crack propagation rate and the stress intensity factor range prevailing at the advancing crack tip Whereas metals exhibit comparable fatigue growth rates for a given stress intensity range when normalised with respect to their static elastic modulus, the polymer materials exhibited a 1300-fold difference in crack growth rate for a given normalised stress intensity range This observation dramatically illustrates the importance of understanding molecular motion and energy dissipation processes in polymer materials as related to their chemistry and architecture The relative behaviour of the different polymer materials could be generally correlated with their reported damping characteristics

The Theory of Brittle Fracture Initiation under Triaxial Stress Conditions—I

Abstract: Summary A calculation has been carried out of the conditions under which crack propagation is initiated in an isotropic, brittle solid of the Griffith type when the solid is subjected to a homogeneous, uniform but otherwise arbitrary triaxial system of stresses at infinity. The cracks have been taken to be flattened ellipsoidal cavities. The results are in broad agreement with results obtained in the two-dimensional treatment of the problem, and with experimental observations. Tensile and shear fracture is possible and the Mohr envelope is of parabolic form. Fracture initiation is shown to be independent of the intermediate principal stress.

Crack growth under creep conditions.

Abstract: IN recent years there has been a growing awareness that the techniques of linear elastic fracture mechanics provide a powerful method for quantifying the behaviour of engineering components containing crack-like defects. In the fast fracture of high strength, low toughness materials1 and in fatigue crack growth2, it is now established that data from laboratory tests can be confidently applied to predict the integrity of a structure or crack extension in service using sharp crack stress intensity factors. More recently, a study of fatigue crack initiation from sharp notches in mild steel3 has shown that the number of cycles to initiate a crack can also be calculated using stress intensity factors. In components which operate at elevated temperatures there is a need to develop methods which describe the growth of crack-like defects in creed conditions. This is particularly relevant to welded joints where defects can be evident after stress relief and assessments are required of fitness for use.

The effect of heat treatment on the fracture toughness and subcritical crack growth characteristics of a 350-grade maraging steel

Abstract: The heat treatment response of a 350-grade maraging steel, with the nominal composition 18.5 Ni, 12 Co, 4.6 Mo, 1.4 Ti, balance Fe, has been determined in billet and bar form. When aged at temperatures below 900°F, the material was very susceptible to subcritical crack growth, and premature brittle fracture occurred in unnotched tension specimens loaded at a slow strain rate in laboratory air. Fracture mechanics was used to interpret this behavior. The introduction of reverted austenite significantly decreased the strength level but had little effect on fracture toughness. The resistance to brittle fracture of this material is contrasted with that of high-strength steels currently used by the airplane industry.

On the location of fracture in brittle solids-I: Due to Static Loading

Abstract: It is shown that the Weibull treatment of brittle strength may be extended to predict the distribution of the location of fracture in brittle solids. Conversely, observations of fracture location enable conclusions to be drawn about fracture loads. Illustrations are given for the three-point bending of beams and the ring-crack of plates by spherical indenters. In the latter case the problem of multiple cracking is also considered.RésuméOn montre que la procédure suggérée par Weibull pour traiter le problème de la résistance à la rupture fragile peut être étendue à la prédiction de la localisation de la rupture dans les solides fragiles. Inversément, connaissant l'emplacement de la rupture, il est possible de tirer des conclusions sur les charges qui ont engendré celle-ci. Le raisonnement est illustré par le traitement de deux exemples: la poutre fléchie sur deux appuis par une charge concentrée, et la fissuration annulaire d'une plaque de verre sous l'effet d'une bille d'indentation. Dans ce dernier cas, on considère également le problème de la polyfissuration.ZusammenfassungEs wird gezeigt, daß das Weibull-Verfahren zur Behandlung von Problemen der Sprödbruchfestigkeit auf die Vorausbestimmung der Stelle an welcher der Bruch in spröden Wertstoffen auftritt, erweitert werden kann. Umgekehrt, ist die Bruchstelle bekannt, so ist es möglich Schlüsse über die Beanspruchung zu ziehen, welche den Bruch bewirkt hat. Diese Überlegungen werden an Hand von zwei Beispielen erläutert. Das erste bezieht sich auf den Fall eines an beiden Enden aufliegenden Trägers welcher einer Punktlast unterworfen ist, das zweite, auf den Fall von Ringrißbildung einer Platte unter Einwirkung einer Kugel. Für den letzten Fall wird auch das Problem der Mehrrißbildung in Betracht gezogen.

High temperature-high strain rate fracture of inconel 600

Abstract: The hot fracture of Inconel 600 has been studied over the temperature range from 800° to 2000°F using a hot torsion tester that is capable of superimposing either axial tensile or compressive stresses on the torsional shearing stresses. Microscopic studies of fracture initiation have been made over the entire temperature region. From 800° to 1200°F fracture initiates at inclusions and propagates by transgranular shear. In the temperature region of minimum ductility, 1300° to 1500°F, fracture initiates at grain boundaries and propagates readily in an intergranular manner. At 1600°F and above, fracture initiates easily at grain boundaries, but because recrys-tallization intervenes crack propagation is difficult and strain to fracture is high. Microcracks initiate at the peak in the torque-twist curve. The higher the temperature the smaller is the strain at which fracture initiates. Correlations have been found between the stress state and the shearing strain at crack initiation and total fracture strain. These correlations show the strong influence of a compressive normal stress on retarding crack initiation and resisting crack propagation.

Dependence on strain rate of the nucleation of cracks in polystyrene at 293°K

Abstract: The processes associated with the deformation and fracture of polystyrene tested in uniaxial tension have been studied over a range of strain rates from 1.4 × 10−2 to 4.3 × 10−7 sec−1 and at constant stresses between 4.1 and 2.9 kg/mm2. The effect of strain rate on the surface craze distribution prior to fracture, the fracture stress, the mechanism of nucleation of cracks, and the nature of fracture surfaces associated with slow and fast crack propagation have been determined. The changes in fracture surface appearance have been studied using optical and stereoscan microscopy. The observations are consistent with the model presented in a previous paper. Fracture is preceded by craze formation, cavitation in the craze, coalescence of cavities to form large planar cavities which propagate slowly until a critical stage is reached at which fast crack propagation occurs. The effect of changes of strain rate and material variables on these processes is discussed.

Fracture Properties of Polyphenylene Oxide Composites

Abstract: The mechanical properties of glass bead, randomly oriented short glass fiber and randomly oriented short graphite fiber polyphenylene oxide composites were investigated. Fracture toughness measurements were made using double edge-notched tensile specimens and tensile strength, tensile strain and initial elastic modulus were measured in standard ASTM tensile tests. It was found that the fracture toughness was relatively independent of filler types and geometry (and nearly equal to that of the unfilled polymer) when no polymer-filler adhesion was present. When adhesion was promoted by surface treatment of the reinforcement, the fracture toughness of the treated fiber composites was greater than that of the untreated composites, while that of the treated particulate composites was below that of the untreated materials. The fracture toughness was also relatively independent of fiber concentration, but decreased sharply with increased glass bead concentration. The elastic moduli of the composites vari...

Brittle fracture of rocks under impulse loads

Abstract: Brittle fracture of Tennessee Marble, Charcoal Granite and sandstone was studied using the Hopkinson split bar method. The study of the actual fracture behaviour of the rock was possible as no transducers were attached to the specimens themselves. The degree of brittleness of rock was determined by using the attenuation of the first wavefront in a fracturing specimen. There existed a maximum limit to the stress amplitude which could be transmitted through a rock specimen. Fracture is initiated in rock at the same stress under both static and dynamic loads.

The Brittle Fracture of Amorphous Thermoplastic Polymers

Abstract: The brittle fracture properties of polyphenylene oxide, polysulfone, polycarbonate, and poly(methyl methacrylate) thermoplastic polymers were investigated over a wide range of temperatures Fracture energy measurements were made using double edge-notched tensile samples Tensile strength, tensile strain, and initial elastic modulus were measured for calculation of the fracture energy and further analysis of the polymer behavior It was found that mechanical transitions in the tensile properties corresponded reasonably well with transitions in the fracture energy in the temperature range investigated Fracture surface photographs permitted visual analysis of the fracture process It was found that the roughest fracture surface corresponded to the maximum in the fracture energy for a given polymer A theory for prediction of polymer tensile yield strain is presented, based on the volume dilation concept The implications of this theory are discussed in terms of the crack tip flow process leading to brittle fracture

Plastic Deformation and the Fracture Surface Energy of Sodium Chloride

Abstract: A correlation between plastic deformation at crack tips in sodium chloride and the measured value of the fracture surface energy is presented. Plastic deformation can either aid or hinder crack growth, depending on the mode of deformation at the crack tip. If plane-stress deformation occurs, crack motion is hindered by step formation, dislocation generation, and plastic blunting of the crack tip. If plane-strain deformation occurs, crack motion is aided by stress fields that arise from the deformation. The specific surface free energy of sodium chloride, {100} plane, is estimated to be less than 0.37 J/m2.

Observations on the Stress Corrosion Crack Propagation Characteristics of High Strength Steels

Abstract: The relationship between stress corrosion crack velocity and crack-tip stress intensity is discussed. In most high strength steels, there is a wide range of stress intensity over which crack velocity is essentially constant. Methods of estimating this velocity are described. Values for a variety of high strength steels are presented and the effects of metallurgical variables are indicated. Implications with regard to testing procedure, crack morphology, and service performance are outlined.

Fracture mechanics guidelines for aircraft structural applications

Abstract: : The document provides the guidelines, limitations, and modifications required to perform a structural, fracture analysis using Griffith-Irwin, fracture mechanics principles. It serves as an introduction to fracture mechanics for those personnel who are concerned with fracture strength estimates for aerospace structural applications. Illustrations and hypothetical examples are included which show how engineering solutions for critical crack size and fracture stress may be made. The critical stress intensity (fracture toughness) concept is used as a basic factor for the fracture analysis of materials. For most crack situations, a stress intensity factor can be computed which can be related to critical conditions and estimates made of critical crack lengths, stresses, and crack propagation behaviors. To provide a complete and accurate fracture analysis, the user is encouraged to become familiar with all aspects of the analysis and its limitations.

On continuum models of ductile fracture.

Abstract: Several fracture criteria are reviewed with respect to ductile fracture. It is suggested that both critical crack-tip displacement, 2Vc*, and critical fracture strain,∃*, criteria may describe the fracture of a ductile second phase rod in a ductile matrix. As a first approximation, this is experimentally verified by observations of ductile stainless steel fibres fracturing in an age-hardened aluminium matrix. For 0.05, 0.10 and 0.20 volume fraction composites, the average fracture strains are calculated to be 1.15 as compared to a measured average of 0.93 while the average critical crack-tip displacement is calculated to be 0.50 mm as compared to an “observed” average of 0.40 mm. The statistical variation in the fracture strain was not sufficiently small to allow any choice between these proposed criteria. In fact, both the experimental and theoretical evidence point to the equivalency of these criteria as given by 2Vc*=π/*∃* where /* is the microstructural unit in front of the crack over which the strain is greater than or equal to∃*.