Showing papers on "Crack closure published in 1978"
400 citations
TL;DR: In this article, the deformation field consistent with a Prandtl stress distribution travelling with an advancing plane-strain crack is analyzed and the functional form of the near tip crack profile in an elastic-plastic solid.
Abstract: Analysis of the deformation field consistent with a Prandtl stress distribution travelling with an advancing plane-strain crack reveals the functional form of the near tip crack profile in an elastic-plastic solid. The crack opening δ is shown to have the form δ ∼ r In (const./r) at a distance r from the tip. This observation coupled with data generated from finite element investigations of growing cracks in small-scale yielding permits the construction of a relation characterizing the deformation at an extending crack tip. A ductile crack-growth criterion consisting of the attainment of a critical opening at a small characteristic material distance from the tip is adopted. Predictions of the stability of a growing crack for both small-scale yielding specimens and those subject to general yielding are discussed.
284 citations
TL;DR: In this paper, a simple model was proposed which qualitatively accounts for all observed notch phenomena including non-propagating cracks and size effects, and quantitatively predicts the experimental fatigue failure limit of sharply notched specimens.
267 citations
TL;DR: In this paper, the authors used the acoustic emission technique to monitor fatigue crack growth, and attributed the peak load emissions to both crack extension and deformation and fracture events occurring within the crack tip plastic zone, and these emissions have been correlated with the energy released during crack growth.
240 citations
190 citations
TL;DR: In this paper, the authors considered the deformation of nonelliptical thin cracks in a loaded elastic material, where the cracks considered are two-dimensional with nonblunted, tapered ends such that opposite faces are tangent to each other at points of contact.
Abstract: Dislocation theory is used to study the deformation of nonelliptical thin cracks in a loaded elastic material. The cracks considered are two-dimensional with nonblunted, tapered ends such that opposite faces are tangent to each other at points of contact. Under compression the cracks shorten by closing near the crack tips, the proportion of crack surface area in contact becoming gradually larger. Some cracks make contact between the crack tips, becoming multiple cracks. Normal stresses on the crack surface vary rapidly over the closed portions from zero near the open surfaces to a peak value at the original crack tip. Stresses remain finite everywhere. At a given load the effective rock compressibility due to arbitrarily shaped, tapered cracks depends only on crack length, giving results identical to a distribution of elliptical cracks of the same lengths. However, at different loads the varying length causes the modulus to vary. As a result, interpretation of features like porosity and modulus under varying applied stress will depend on the specific crack model chosen. In particular, a single aspect ratio of a simply tapered crack yields the same nonlinear effect as a flat distribution of elliptical cracks. Consequently, estimates of crack spectra from nonlinear strain data are totally nonunique.
168 citations
TL;DR: In this article, the combined effect of load biaxiality and crack orientation on K l, K II, maximum shear, anile of initial crack extension, elastic strain energy density, as well as on the local strain energy rate is made explicit.
162 citations
TL;DR: In this paper, field equations of nonlocal elasticity are solved to determine the state of stress in the neighborhood of a line crack in an elastic plate subject to a uniform shear at the surface of the crack tip.
Abstract: Field equations of nonlocal elasticity are solved to determine the state of stress in the neighborhood of a line crack in an elastic plate subject to a uniform shear at the surface of the crack tip. A fracture criterion based on the maximum shear stress gives the critical value of the applied shear for which the crack becomes unstable. Cohesive stress necessary to break the atomic bonds is calculated for brittle materials.
148 citations
TL;DR: In this article, the details of crack nucleation phenomena in the high strain fatigue of OFHC copper have been studied by optical microscopy and two-beam interferometry, and the plastic instability mechanism of cracks has been ruled out in polycrystalline metals in favor of one which takes into account the detailed crystallographic aspects of slip in adjoining grains.
135 citations
TL;DR: In this article, a unified picture of fracture mechanics is proposed for small scale yielding conditions (a small plastic zone at a crack tip) for two-dimensional cracks, where the stress singularity at the crack tip can break the solid apart and create surfaces of energy.
133 citations
TL;DR: In this paper, a series of amine-cured epoxy resin adhesives in both the bulk material and adhesive joints has been studied as a function of the formulation of the resins and the conditions of testing using a linear elastic fracture mechanics approach.
TL;DR: In this article, a cracked adhesive-bonded DCB-type fracture was analyzed using a hybrid stress model finite element analysis which incorporated an advanced crack tip element, and the results were compared to monolithic systems with regard to the stress intensity factor and the localization of the singular stress domain associated with the crack tip.
Abstract: The problem of a cracked adhesive bonded DCB-type fracture specimen has been analyzed using a hybrid stress model finite element analysis which incorporates an advanced crack tip element. Stresses in the near and far fields have been studied as a function of adherend/adhesive modulus ratio and adhesive thickness. The results are compared to monolithic systems with regard to the stress intensity factor and the localization of the singular stress domain associated with the crack tip.
TL;DR: In this article, field equations of nonlocal elasticity are solved to determine the state of stress in a plate with a line crack subject to a constant anti-plane shear, and it is found that no stress singularity is present at the crack tip.
TL;DR: In this paper, the authors measured the fatigue crack growth rate (FCGR) of Alloy 718 on CT type specimens at 298 and 823 K and found that a substantial increase in FCGR occurred at low stress intensity levels, as the temperature was increased from 298 to 823 k and as the frequency was decreased at 823k.
Abstract: The fatigue crack growth rate (FCGR) of Alloy 718 was measured on CT type specimens at 298 and 823 K. At 823 K, the influence of frequency was studied in the range between 5 – 10-3 Hz and 20 Hz, using a sinusoidal wave form signal. A substantial increase in FCGR occurred, particularly at low stress intensity levels, as the temperature was increased from 298 to 823 K and as the frequency was decreased at 823 K. At elevated temperature, the effect of cyclic stress wave form was equally investigated, using triangular and square wave form signals producing the same frequency of 5.10-2 Hz. The triangular load led to higher FCGR than the square wave form. In addition the hold time of 10 s both at the maximum and the minimum load associated with the square load had no significant effect on the FCGR. Electron microscopy was used to observe the substructures that developed ahead of fatigue cracks. These observations showed that in certain circumstances plastic deformation proceeded by the propagation of planar bands which were identified as twins. At room temperature, twinning was found to be abundant only in the threshold regime. At 823 K, twinning was observed in the domain of higher FCGR, particularly at low frequencies. Fractography was carried out to study the micromechanisms of crack propagation. At 823 K. intergranular cracking occurred as the frequency was decreased. The comparison between the substructures formed in low cycle fatigue and those associated with the plastic zones of propagating cracks is made. The importance of planar deformation and twinning on intergranular cracking and on the acceleration of FCGR when the loading rate is decreased at 823 K, is discussed.
TL;DR: The mechanisms by which cracks extend from flaws in brittle materials subjected to compressive loads are presented in this paper, where they are restricted to single-phase, dense materials in order to provide a model amenable to analysis and experimental confirmation.
TL;DR: In this article, the authors show that at 1 atm pressure and temperatures up to 900°C, Westerly granite and two diabases (Frederick MD and Pageland SC) produce cracks with relatively low crack closure pressures.
TL;DR: In this article, it is proposed that internal compressive stresses as induced by a positive local volume change can retard crack growth, and measurements of fatigue crack growth rates of specially designed austenitic alloys with test temperature T and martensite temperature Ms are used to test this hypothesis.
TL;DR: In this article, an automated test system utilizing computer control was developed to obtain crack growth rate data down to the fatigue crack growth threshold with a decreasing stress intensity technique and compact type specimens.
Abstract: An automated test system utilizing computer control was developed to obtain crack growth rate data down to the fatigue crack growth threshold with a decreasing stress intensity technique and compact type specimens. The starting stress intensity range ΔK0 was chosen to yield crack growth rates in the range of 2.54 × 10−8 m/cycle (10−6 in./cycle) and subsequent values of ΔK are controlled to the equation ΔK = ΔK0 exp [C(a − a0)] (a0 and a are the initial and instantaneous crack lengths and C is a test variable). Crack length is continuously monitored by using the elastic compliance technique, thereby enabling ΔK to be decreased continuously. Comparison crack growth data were also obtained by the more conventional constant load amplitude or K-increasing method. Excellent agreement was observed between data obtained from the two procedures for a Society of Automotive Engineers 1045 steel at load ratios R of 0.1 and 0.5, an A356-T6 sand-cast aluminum alloy at load ratios of 0.1 and 0.8, and a 2219-T851 aluminum alloy at a load ratio of 0.1.
TL;DR: In this paper, the authors used computer simulation methods to estimate the fatigue crack initiation life of butt welds using strain-controlled fatigue concepts, and found that the initiation life was very sensitive to changes in Kf, but rather insensitive to strength level.
Abstract: The fatigue crack initiation life (cycles to obtain a 0.25-mm fatigue crack) was estimated for butt welds using strain-controlled fatigue concepts. Key developments which facilitated these estimates were the assumption of(K f ) m a x conditions (the largest value of Kf possible for a given weld shape), the use of computer simulation methods which modeled cyclic hardening and softening as well as mean stress relaxation effects, and the use of the fatigue properties of the actual weld zone in which the initial notch was located. The initiation life was found to be very sensitive to changes in Kf, but rather insensitive to strength level. The importance of residual stresses and mean stress varied with material as did the fraction of total life devoted to crack initiation. Mild steel (ASTM Specification for Structural Steel (A 36)) high strength, low alloy (ASTM Specification for High-Yield Strength, Quenched and Tempered Alloy Steel Plate, Suitable for Welding (A 514)) steel and aluminum alloy welds were considered.
TL;DR: In this article, the authors investigated the effect of load biaxiality on the crack-tip region and showed that the impact of load applied parallel to the plane of the crack appears entirely in the second terms of the series representations for local stress and displacement.
TL;DR: In this article, the authors studied the fracture behavior of an epoxy resin by a method which involves the pressurization of an internal circular crack and derived fracture energy as a function of crack velocity.
Abstract: The fracture behaviour of an epoxy resin has been studied by a method which involves the pressurization of an internal circular crack. The method can be used to study both cohesive fracture and the adhesive failure of an interface. Plane strain conditions are assured because the crack does not intersect a free surface and (for adhesive failure) shrinkage stresses are eliminated as a crack driving force. Using high speed photography, the dependence of crack speed on critical pressure and specimen geometry was determined. An elastic analysis permits the derivation of fracture energy as a function of crack velocity. Fracture energy values lay between 100 and 200 Jm−3 at 35° C with a peak at a crack velocity of 37 m sec−1.
TL;DR: In this article, the authors determined fatigue crack growth rates for Ti-6Al-4V alloys with respective oxygen contents of 0.06, 0.18 and 0.20 wt pct.
Abstract: Fatigue crack growth rates have been determined for beta-annealed Ti-6Al-4V alloys with respective oxygen contents of 0.06, 0.11, 0.18 and 0.20 wt pct. For each of these alloys, transitional crack growth behavior has been observed which appears to correlate with a critical value of the reversed plastic zone size,viz the Widmanstatten packet size. Moreover, growth rates below transitional levels order in terms of packet size, with lower growth rates associated with larger packets. The present results suggest that intersti-tial oxygen content and prior beta grain size significantly affect fatigue crack growth rates through control of the Widmanstatten packet size.
TL;DR: In this paper, a critical review of available information and models for crack propagation at elevated temperatures is provided, where the authors divide the behavior of crack propagation into brittle, ductile and plane stress conditions.
Abstract: The prediction of crack propagation rates at elevated temperatures is important and this paper provides a critical review of available information and models for behaviour. For simplicity, behaviour is divided into three situations. At one extreme, a brittle situation may exist in which the material is brittle and the degree of constraint high, so that substantially plane strain conditions exist and stress redistribution at the crack tip is small; in this situation, the fracture is a local crack tip event and the stress intensity may be of use in correlaiting creep crack propagation data. At the other extreme, ductile behaviour may result if the material is ductile and the constraint is low with plane stress conditions prevailing; in this case, stresses at and near the crack tip will redistribute quickly down to more even values and conventional creep analysis techniques using, say, the reference stress will be most useful, particularly for estimating times to rupture. It is postulated that there ...
TL;DR: In this article, a theory of intergranular creep crack growth in brittle materials has been developed, which is analogous to those developed previously for the growth of r-type voids in grain boundaries.
TL;DR: In this article, a mechanism of retardation is proposed, deriving from metallographic observations, for aluminum alloys, based on the rate of overload and its intensity, but other factors such as the number of overloads and the frequency of the test are considered.
TL;DR: In this article, a model for SLC has been proposed based on hydride precipitation at the crack tip and subsequent crack propagation by creep, cleavage, and Jor interfacial separation at the hydridesJmatrix interface.
Abstract: Sustained-load cracking (SLC) characteristics of Ti-6A1-4V are significantly influenced by 1) exposure temperature, 2) hydrogen content, and 3) basal plane crystallographic texture. The stress intensity applied to precracked specimens did not play a major role in affecting crack initiation or crack growth rate except that there would appear to be a required minimum level. Increasing hydrogen content raises the temperature at which SLC occurs and increases crack growth rate. A model for SLC has been proposed based on hydride precipitation at the crack tip and subsequent crack propagation by creep, cleavage, andJor interfacial separation at the hydrideJmatrix interface.
TL;DR: In this paper, the authors investigated the closure phenomena of fatigue cracks with a 1 mm gage length extensometer over the range of stress ratio, R, from −1 to 0.8.
Abstract: The closure phenomena of fatigue cracks were investigated with a 1 mm gage length extensometer over the range of stress ratio, R, from −1 to 0.8. Plate specimens with a center slot of HT80 steel and SUS304 stainless steel were fatigued under push-pull loading, and the crack propagation rate, da/dn, was measured. The stress ratio, R, was found to influence da/dn in both materials. The crack opening stress intensity factor, K
op, was determined from the relationship between the crack tip extensometer displacement and the load. The effective stress intensity range ratio, U(=ΔKeff/ΔK), decreases with the decrease of the stress intensity amplitude, ΔK/2. As for the data which show the crack closure phenomena (R≦0.4), the relationship between log(da/dn) and log(ΔK
eff/2) falls on a straight line near the stress intensity threshold level. For R=0.8 where the crack tip is fully open over the whole range of loading, the data show a discrepancy from the same line. The strain at the crack tip was also measured with the Moire fringe multiplication method. A large amount of plastic strain at the crack tip was observed even below the crack opening load for R=−1 in HT80 steel. These phenomena show that fatigue damage still exists when the crack is closed. These also show that the crack closure cannot fully account for the effect of R on da/dn.
TL;DR: In this paper, the authors compared the strain intensity factor for both linear elastic and high strain fatigue crack growth tests in cast Cr-Mo-V steels at 550 °C and found that there was a good correlation of rates in air and under vacuum in terms of strain intensity factors in conditions where there was no significant creep damage.
TL;DR: In this article, the fracture is related to a shear mechanism across the colonies from the initiation stage through the overload fracture, and the density of the shear bands increased with increasing stress intensity.
Abstract: In order to better understand the large scatter in the fatigue results associated with β-annealed microstructures of α+ β titanium alloys, the fatigue crack initiation and propagation behavior of thin center notched Ti-11 specimens with a large colony α platelet microstructure was investigated. Colonies with a mean intercept diam of greater than 1 mm were grown in 2 mm thick specimens by means of a vacuum β-annealing process. This enabled crack path morphologies and crack propagation rates to be determined within single colonies by means of optical microscopy on the polished and etched surfaces. The results showed that the fracture is related to a shear mechanism across the colonies from the initiation stage through the overload fracture. Intense shear bands were observed ahead of and in the same direction as the propagating cracks. The density of the shear bands increased with increasing stress intensity. Since the colonies are randomly oriented, the fatigue cracks propagated at various angles with respect to the tensile axis. The crack propagation rate across a single colony is no faster than the propagation rate in the equiaxed α+ β microstructure of the same material. However, cracks were halted at the colony boundaries and forced to reinitiate through a cycle consuming process into the next colony. It is mainly this reinitiation process and microstructurally dependent growth which are responsible for the slower crack growth rates and large scatter band obtained for the β-annealed microstructures when compared to the α+ β microstructures. It is suggested that by reducing the colony size the crack growth rates will be reduced and the fatigue scatter band will be narrowed.
TL;DR: In this article, an effective engineering solution for the type of cracks that have been discovered in boiling water Reactor (BWR) plants was first developed, based upon a simple net section flow stress criterion.