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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Fatigue & fracture of engineering materials & structures 26 (2003), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Natural fatigue crack formation and growth were studied in notched Al–Cu alloy coupons through high-resolution SEM fractography. The experiments were conducted under programmed loading conditions designed to induce microscopic marking of the crack formation and growth process under varying stress ratio and closure-free crack tip conditions. Control experiments were performed by switching between an air and vacuum environment. In air, varying the stress ratio from 0.74 down to 0.64 retards crack growth by up to a factor of five. This ‘closure-free’ stress ratio history effect totally disappears in vacuum, suggesting a significant environmental influence on stress ratio and its history. Crack-tip stress state appears to moderate environmental action, revealing a potential mechanism sensitive to residual stress. Consequently, crack closure, residual stress and crack front and plane orientation are identified as major load interaction mechanisms whose synergistic action controls fatigue under variable amplitude loading. The study also appears to suggest that as a consequence of the crack seeking the path of least resistance, load-sequence sensitive crack plane and front orientation may only induce retardation effects.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Fatigue & fracture of engineering materials & structures 25 (2002), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Fractures from tests on 2014-T6511 and 2024-T3 test coupons under specially designed programmed loading reveal voids with distinct fatigue markings. These ‘fatigue voids’ appear to form as a consequence of the separation of noncoherent secondary particulates from the matrix in early fatigue. The process of their formation is through the initiation, growth and coalescence of multiple interfacial cracks around the particulate. Such voids become visible on the fatigue fracture surface if and when the crack front advances through them. In vacuum, each fatigue void is the potential initiator of an embedded penny-shaped crack. The one closest to the specimen surface is likely to become the dominant crack, indicating that fatigue voids appear to be the likely origins of the dominant crack in vacuum. In air, the dominant crack forms at the notch surface and grows much faster, giving less opportunity for multiple internal cracks to spawn off from the innumerable internal fatigue-voids. Thus in air, fatigue voids do not appear to affect the fatigue process at low and intermediate growth rates. At high crack growth rates involving considerable crack tip shear, slip planes with particulate concentration offer the path of least resistance. This explains the increasing density of fatigue voids with growth rate. Very high growth rates signal the onset of a quasi-static crack growth component that manifests itself through growing clusters of microvoid coalescence associated with static fracture. Fatigue voids are likely to form in other Al-alloys with secondary noncoherent particulates. They have nothing in common with microvoids associated with ductile fracture.
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  • 3
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Results of an experimental estimation of the crack closure stress, using electron fractography and laser interferometry for CT specimens of an aluminium-copper alloy are presented in this paper. Crack closure stress was estimated at stress ratios of 0.1, 0.3, 0.5 and 0.7 using programmed load sequences. In addition, the crack closure stress was estimated for a simple random load sequence. Scanning and transmission electron fractography provided a similar estimate of crack closure, with the later technique providing better quality fractographs. Laser-interferometry based estimates were closer to fractographic estimates when the crack-tip is very close to the micro-indent axis. In addition, laser-interferometry based estimates of the crack opening stress for a random load sequence, confirmed the possibility of a cycle-by-cycle variation of the crack opening stress.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A numerical investigation of the fatigue crack closure phenomenon has been performed by an elastic-plastic finite element analysis. Computer software was developed to consider many aspects affecting plasticity-induced crack closure. Linear and power-law hardening models are considered in the finite element analysis. The paper presents results from the study carried out on compact tension (CT) coupons at various crack lengths corresponding to different loading conditions. Finally the results of the analysis are compared with the experimental estimates of fatigue crack closure levels obtained from laser interferometry, scanning-electron and transmission-electron fractography presented in Part I of this paper on identical specimens.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    International journal of fracture 19 (1982), S. 115-129 
    ISSN: 1573-2673
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Résumé On a étudié des géométries d'éprouvettes d'essai de propagation de fissure en forme d'anneau circulaire comportant des fissures simples radiales et soumises à des charges diamétralement opposées de traction et de compression. On montre que lorsque les rapports entre le diamètre intérieur et le diamètre extérieur est inférieur ou égal à 1/2, les facteurs d'intensité de contrainte de mode I pour la géométrie en anneau sont essentiellement proportionnelles à la seule charge sur une gamme de longueurs de fissure non dimensionnelle. Ces éprouvettes seraient utiles dans des essais comportant des études de croissance de fissure paramétrique, lorsque la variation du facteur d'intensité de contrainte relative à une charge appliquée de manière constante doit être éliminée.
    Notes: Abstract Circular ring-shaped crack-propagation test-specimen geometries containing single radial cracks have been analyzed, with diametrically opposite tensile and compressive loads applied. It is shown that for innerradius-to-ounter-radius ratios of one-half and less, mode I stress-intensity factors for the ring geometry are essentially proportional to only the load over a range of non-dimensional crack lengths. These specimens should be useful in experiments involving parametric crack-growth studies where the variation of the stress-intensity factor for a constant applied load is to be eliminated.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    International journal of fracture 14 (1978), S. 527-541 
    ISSN: 1573-2673
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Résumé On présente une solution par équation intégrale du problème bidimensionnel d'une fissure libre de contrainte dans un milieu limité, linéairement élastique et isotrope soumis à des forces coplanaires. Une série d'équations couplées comportant des intégrales sur les frontières extérieures et sur la ligne de fissure est obtenue par la solution simultanée d'un problème de fissuration dans un milieu non cerné (problème perturbé) et un problème dit uniforme dans lequel une région dépourvue de défaut présente le même frontière extérieure que le milieu qui contient la fissure. La solution pour le problème perturbé est donné sous une forme quadratique sous forme de la dérivée du déplacement normal de la surface de la fissure. La solution pour le problème uniforme est donné par une série d'équations intégrales aux limites. Le caractère de la singularité du champ de contraintes aux extrémités de la fissure est fourni par la solution du problème perturbé. En vue d'une évaluation numérique, la série d'équations intégrales couplées fait l'objet d'une approximation à l'aide d'une série d'équations algébriques linéaires simultanées. Comme les facteurs d'intensité de contrainte et les déplacements de la surface de la fissure sont incorporés dans les équations intégrales, les valeurs de ces quantités sont obtenues directement à partir des solutions numériques. Plusieurs problèmes sont présentés à titre d'exemple en vue de déterminer la versatilité et la précision de cette approche.
    Notes: Abstract An integral-equation solution for the two dimensional problem of a stress-free crack in a bounded, linearly elastic, isotropic medium subjected to in-plane forces is presented. A set of coupled equations, involving integrals over the outer boundary and the line of the crack, is obtained by the simultaneous solution of a crack problem in an unbounded medium-the perturbed problem-and a problem in an unflawed region having the same outer boundary as the medium containing the crack-the equable problem. The solution for the perturbed problem is given in quadrature form in terms of the derivative of the normal displacement of the crack surface. The solution for the equable problem is given by a set of boundary integral equations. The character of the stress-field singularity at the crack tips is provided by the solution for the perturbed problem. For a numerical evaluation the set of coupled integral equations is approximated by a set of simultaneous, linear algebraic equations. Since the stress intensity factors and the crack surface displacements are incorporated into the integral equations, the values of these quantities are obtained directly from the numerical solution. Several sample problems are presented in order to determine the versatility and accuracy of this approach.
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  • 7
    Publication Date: 1978-10-01
    Print ISSN: 0376-9429
    Electronic ISSN: 1573-2673
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Published by Springer
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  • 8
    Publication Date: 1982-06-01
    Print ISSN: 0376-9429
    Electronic ISSN: 1573-2673
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Published by Springer
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  • 9
    Publication Date: 1997-07-01
    Print ISSN: 0376-9429
    Electronic ISSN: 1573-2673
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Published by Springer
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  • 10
    Publication Date: 1997-07-01
    Print ISSN: 0376-9429
    Electronic ISSN: 1573-2673
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Published by Springer
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