Showing papers on "Stress concentration published in 1994"

A review of crack closure, fatigue crack threshold and related phenomena

Abstract: Published fatigue crack closure mechanisms are reviewed in the context of the following five ideas that have been developed over the past twenty years to explain the closure effects on near-threshold crack growth behaviour: (1) oxide, (2) asperity, (3) plasticity, (4) phase transformation and (5) viscous fluids. The first three have been considered as more important than the last two. Our analysis indicates that (a) there can be no contribution from plasticity to crack closure, (b) the crack closure contribution can be significant only if it is closed fully, which is reflected as an infinite slope in the load-displacement curve, (c) formation of oxide asperities from a fretting action is a random process and not a deterministic one, and therefore cannot explain the deterministic behaviour of the effect of load ratio on the threshold, and (d) the closure contribution from the asperities resulting from oxides or corrosion products,or surface roughness, it is less than 20% of what has been deduced based on the change in the slope of the load-displacement curves. Thus the analyses show that crack closure can exist, but its magnitude is either small or negligible. The critical evaluation of the literature data on (1) the threshold stress intensity variation with load ratio on many materials, and (2) and examination of the experimentally observed load-displacement curves confirm the above conclusions. Hence to rationalize the observed variations in the fatigue crack threshold with load ratio, we have proposed a new model. It postulates a requirement for two critical stress intensity parameters, namely ΔK th ∗ and K max ∗ , that must be satisfied simultaneously as the crack tip driving forces for fatigue crack growth. This requirement is fundamental to fatigue, since an unambiguous description of cyclic loads requires two independent load parameters. Several experimental results from the literature are presented in support of this postulation. Using these two critical parameters, the entire functional relationship between Kmax, ΔKth and R is explained without invoking an entrinsic factor, namely crack closure. In addition, for a given material and its crack tip environment, a unique relationship between ΔKth and Kmax exists that is independent of test methods used in determining thresholds. Finally, because of this two parameter requirement, all fatigue crack growth data need to be represented in terms of three-dimensional plots involving da/dN, ΔK and Kmax. For a two-dimensional representation, the data need to be transformed correctly, defining the net driving force involving both ΔK and Kmax parameters. The concepts presented are independent of whether crack closure exists or not, or even whether cracks exist or not.

Handbook of fatigue crack propagation in metallic structures

Abstract: Volume 1. Introductory Section. Failure criteria for anisotropic bodies (P.S. Theocaris). Introduction to fracture mechanics of fatigue (H. Kitagawa). Numerical methods for fracture parameters calculation (G.J. Tsamasphyros). Fatigue Behaviour of Metallic Materials. Fatigue of steels for concrete reinforcement and cables (M. Elices et al). Fatigue crack growth and crack shielding in a Fe-C-Cu sintered steel (Y-W. Mai et al). Fatigue and fracture properties of aerospace aluminium alloys (R.J.H. Wanhill). Fatigue crack propagation in titanium alloys (J.K. Gregory). Theoretical Models and Numerical Methods. Mechanical model for fatigue crack propagation in metals (X.-L. Zheng). Application of the three-dimensional boundary element method to quasistatic and fatigue crack propagation (M.H. Aliabadi, Y. Mi). Method of damage mechanics for prediction of structure member fatigue lives (X. Zhang et al.). Stochastic fatigue crack propagation (J.H. Yoon, Y.S. Yang). A fracture mechanics approach to the optimum design of cracked structures under cyclic loading (Z. Knesl). Fundamental Aspects of Fatigue Crack Propagation Phenomenon. Stable and unstable fatigue crack propagation in metals (V.T. Troshchenko). Fatigue crack growth from stress concentrations and fatigue life prediction in notched components (C.S. Shin). Propagation of surface cracks under cyclic loading (A. Carpinteri). Growth behaviour of small fatigue cracks and relating problems (H. Nisitani et al). Analytical and experimental study of crack closure behaviour (D.-h. Chen). Studies of fatigue crack closure (D. Francois). Fatigue threshold of metallic materials - a review (A. Hadrboletz et al). Mechanics of fatigue crack growth as a synthesis of micro-and macro-mechanics of fracture (V.V. Bolotin). Random material non-homogeneity effects on fatigue crack growth (K. Dolinski). Volume 2. Influence of Loading Conditions. Fatigue crack growth under variable amplitude loading (J. Dominguez). Mixed mode fatigue crack propagation (L.P. Pook). Numerical and. experimental study of mixed mode fatigue crack propagation (A.S. Kobayashi, M. Ramulu). Crack growth behaviour under repeated impact load conditions (T. Tanaka et al). Influence of Environmental Conditions. Influence of ambient atmosphere on fatigue crack growth behaviour of metals (J. Petit et al). Influence of hydrogen-containing environments on fatigue crack extension resistance of metals (V.V Panasyuk et al.). Fatigue crack propagation in aqueous environments (Y. Nakai). Application of fatigue crack growth data to low cycle fatigue at high temperature (L. Remy). Creep-fatigue interaction under high-temperature conditions (R. Ohtani, T. Kitamura). Fatigue crack propagation in metals at low temperatures (X.-L. Zheng, B.-T. Lu). (Part contents).

Stress perturbations associated with active faults penetrated by boreholes: Possible evidence for near-complete stress drop and a new technique for stress magnitude measurement

Abstract: Detailed studies of stress-induced wellbore breakouts in wells drilled through active faults reveal stress field discontinuities that are apparently associated with recent fault movements. These discontinuities are expressed as localized rotations in wellbore breakout orientation in the vicinity of the fault penetrated by the borehole. This phenomenon is observed in a variety of tectonic environments and rock types. Utilizing cases where relatively complete knowledge of the horizontal principal stresses is available from in situ measurements, we use three-dimensional dislocation modeling to demonstrate that these discontinuities can be explained as the superposition of a reference stress state and a perturbation caused by movement on preexisting faults. Case studies from normal, strike-slip and reverse faulting stress states indicate that nearly complete stress drop is required to match the observed breakout orientation anomalies. Hydraulic fracturing data independently confirm the occurrence of near-complete stress drop on some faults penetrated by drilling. Modeling of the observed interactions between breakouts and fractures can also be used to obtain information about the magnitude of in situ stress.

Some general properties of stress-driven surface evolution in a heteroepitaxial thin film structure

Abstract: Heteroepitaxial thin films are subjected to very large stresses, typically in the giga-Pascal range, due to lattice mismatch. Their applications in microelectronic devices have drawn a growing research effort which involves, in part, a better understanding of the processes by which defects such as cracks and dislocations are nucleated in a thin layer structure. One of the possible defect mechanisms is related to a stress-induced morphological instability which tends to roughen the film surface by mass diffusion during film growth or annealing. A major objective of this paper is to show that a few invariance properties of the strain energy density and the chemical potential in a heteroepitaxial structure can be utilized to obtain some valuable information concerning the equilibrium profile, the stress concentration and the formation of cusp-like stress singularities without having to resort to full-scale numerical studies. The process of cusp-formation requires a critical film thickness Hcr which is independent of the Matthews critical thickness herfor misfit dislocation formation by propagation of threading dislocations within the film. Calculations show that Hcr is significantly larger than hcr for a wide range of misfit strain, suggesting that nucleation of threading dislocations via cusp-formation could be critical in the overall process of strain relaxation in this type of film. Finally, a “phase” diagram is constructed to categorize important consequences of the surface evolution based on the instability wavelength and average film thickness.

Numerical simulation of fracture set formation: A fracture mechanics model consistent with experimental observations

Abstract: A physically based model for the evolution of a single set of planar, parallel fractures subject to a constant remote stress is presented. The model simulates the mechanical interaction between fractures using a recently developed approximation technique for stress analysis in elastic solids with many fractures. A comparison between experimental and numerical results shows that the model can accurately simulate the development of experimentally generated fracture sets. Once the flaw geometry is specified, only one parameter controls the geometric evolution of the fracture set. This parameter, the velocity exponent, relates fracture propagation velocity to stress concentration at the fracture tip. Monte Carlo sensitivity analyses suggest that this parameter also controls the extent to which fracture growth is concentrated within zones or clusters. Similar analyses suggest that the extent of fracture clustering is less sensitive to the flaw density.

Stress Concentrations in Laminated Composites

Abstract: From the Author's PrefaceThe objective of this book is to provide a thorough and systematic study of the problem of laminated composites containing stress concentrations. Stress concentrations are introduced in laminated plates in the forms of circular holes, elliptical openings and straight cracks. These forms of cutouts have many practical applications, and are familiar to most engineers. Stress concentrations exist in all known structural components. Stress concentrations have great practical importance because they are normally the cause of failure. In addition to stress analyses of laminated composites, we need more fundamental understanding of the failure mode, the failure criterion, the effects on global laminate response, and the design of composites in the presence of stress concentrations. In this book, all the subjects studied are closely related to the problem of stress concentrations in laminated composites . . . . All the models are verified with many experimental results. The underlying objective of this comprehensive study is to give the readers an in-depth and thorough understanding of the problem of stress concentrations in composites. This book is the first to address the problem of laminated composites containing stress concentrations in a systematic way.

Particle size, volume fraction and matrix strength effects on fatigue behavior and particle fracture in 2124 aluminum-SiCp composites

Abstract: The effects of particle size, volume fraction and matrix strength on the stress-controlled axial fatigue behavior and the probability of particle fracture were evaluated for 2124 aluminum alloy reinforced with SiC particles. Average particle sizes of 2, 5, 9 and 20/~m and volume fractions of 0.10, 0.20 and 0.35 were examined for four different microstructural conditions. Tensile and yield strengths and fatigue life were substantially higher in the reinforced alloys. Strength and fatigue life increased as reinforcement particle size decreased and volume fraction loading increased. The frequency of particle fracture during crack propagation was found to be dependent on matrix strength, particle size and volume fraction and on maximum crack tip stress intensity. Particle fracture can be rationalized, phenomenologically, by the application of modified process zone models, originally derived for static fracture processes, and weakest link statistics which account for the dependence of matrix yield strength and flow behavior and particle strength on the probability of particle fracture during monotonic fracture and fatigue crack propagation.

An investigation of the role of a liquid phase in AlCuMg metal matrix composites exhibiting high strain rate superplasticity

Abstract: Tensile experiments were performed under conditions of true constant strain rate on three AlCuMg metal matrix composites containing 20 vol.% of either β-Si3N4 whiskers or α-Si3N4 particulates with diameters of 0.2 or 1.0 μm, respectively. The tests were conducted at temperatures from 758 to 833 K and at strain rates up to 102 s−1. The results show that each composite exhibits a strain rate sensitivity of >0.3 at strain rates in the vicinity of ∼10−1 −1 s−1 and with associated superplastic elongation of >200% or even up to >800% for the composite with particulates having a diameter of 1.0 μm. It is concluded that high ductilities are achieved at effective testing temperatures above the solidus temperature of the matrix alloy but the results are not consistent with a rheological model based on deformation in a semi-solid matrix. It is suggested instead that the presence of a liquid phase at the interfaces between the reinforcement and the matrix serves to relieve the stress concentrations due to sliding and thereby to limit the development of internal cavitation.

A fatigue design parameter for spot welds

Abstract: — Mode I and mode II stress intensity factors for two half-spaces connected by a circular patch were used to develop a mixed-mode stress intensity factor (termed the stress index Ki) which can correlate the fatigue life of all spot weld geometries, base metals, and specimen dimensions. Empirical corrections were applied to Broek's equivalent stress intensity factor (Klq) to account for the weldment geometry (sheet thickness, nugget diameter, specimen width) and the effect of mean stress. The final expression, (Ki), is a measure of the notch-root stress field in the location where crack initiation and early crack growth occur. The stress index (ki) should be a useful tool for spot-weld fatigue design.

Near surface microstructures developing under large sliding loads

Abstract: The subsurface zones of copper developed during the application of large sliding loads were observed using TEM and SEM. Differences in microstructural development as a function of load and sliding velocity are assessed. The observed microstructural changes, such as the development of a dislocation substructure and a mechanically mixed layer, are used to estimate the stress and strain-state of the near surface zone during sliding. These estimates of local stress and strain were compared to the applied stresses to show that large stress concentrations develop at and below a sliding interface. Thus, the stresses which develop locally within the near surface zone can be many times larger than those predicted from the applied load and the friction coefficient. It is postulated that these stress concentrations arise from two sources: (1) asperity interactions and (2) local and momentary bonding between the two surfaces. These results are compared to various friction models.

A review of modelling small-crack behavior and fatigue-life predictions for aluminum alloys

Abstract: The small-crack effect, where small fatigue cracks grow faster and at lower stress-intensity factors than large cracks, has been found to be significant for many materials and loading conditions. In this paper, plasticity effects and crack-closure modelling of small fatigue cracks are reviewed. A crack-closure model with a cyclic-plastic zone-corrected effective stress-intensity factor range (related to the cyclic J-integral) and microstructural data on crack-initiation sites were used to calculate small-crack growth rates and fatigue lives for unnotched and notched specimens made of two aluminum alloys. The crack-closure transient from the plastic wake was shown to be the dominant cause of the small-crack effect and plasticity effects on the cyclic-plastic zone-corrected stress-intensity factor range were negligible except at extremely high stress levels. Small-crack growth rates and fatigue lives under both constant-amplitude and spectrum loading from tests and analyses agreed well.

Superplastic deformation mechanism accommodated by the liquid phase in metal matrix composites

Abstract: The accommodation processes for superplastic metal matrix composites are required to release the stress concentration near matrix-reinforcement interfaces because of stress concentration caused by interfacial sliding. An optimum superplastic elongation was found at the temperature where local melting of interfaces was confirmed by in situ transmission electron microscopy observation. It is postulated that superplastic flow in metal matrix composites is controlled by a grain-boundary sliding mechanism accommodated with relaxing the stress concentration by an isolated liquid phase at matrix-reinforcement interfaces.

Microstructural development during fatigue of copper foils 20–100 μm thick

Abstract: Fatigue tests were conducted on metallic copper thin sheets of 100 μm and 20 μm in the traction-traction mode. Adequate specimens, suitable grips and a high frequency fatigue apparatus were designed and constructed. The development of dislocation structure during fatigue tests was shown by transmission electron microscopy. Upon cycling we observed the formation of an equiaxed cellular microstructure which gradually, owing to the stress concentration, transforms to a duplex microstructure consisting of elongated grain regions and cellular regions. Finally the elongated regions transform into persistent slip bands. Crack initiation occurs at stress concentration sites at the specimen surfaces but do not appear to be the essential cause of fatigue failure. For samples with grain sizes comparable with the sample thickness, thinner specimens show higher cycle numbers at the same stress amplitude.

Laser shock peened rotor components for turbomachinery

Abstract: Turbomachinery rotor components having a metallic body with at least a portion of the body subject to a tensile stress field due to centrifugal and thermal growth induced forces generated by the rotor when the rotor is rotating, at least one stress riser in the stress field portion that causes stress concentration when the rotor is rotating, and at least one region of the component around the stress riser having deep compressive residual stresses imparted by laser shock peening (LSP).

Statistical investigation of the behaviour of small cracks and fatigue life in carbon steels with different ferrite grain sizes

Abstract: — In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propagation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks.

Ultrasonic evaluation of elastic parameters of sintered powder compacts

Abstract: The variation of elastic moduli, M, of sintered powder compacts with porosity, p, has been analysed in terms of an equation M = M 0 (1−p)n, where M 0 is the elastic modulus of non-porous material and n is a constant. The variation of ultrasonic velocities has also been described in terms of a similar equation derived from the relations given by physical acoustics theory. It has been shown that the parameter n is related to a stress concentration factor around pores in the material and is dependent on pore geometry and its orientation in the material. The observed variation in moduli and velocities with porosity has been compared with the theoretically predicted values based on self-consistent oblate spheroidal theory.