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  • 11
    Electronic Resource
    Electronic Resource
    Radial Cracks in a Sheet Weakened by a Pressurized Circular Hole (1998)
    Springer
    Meccanica 33 (1998), S. 533-540 
    Details
    ISSN: 1572-9648
    Keywords: Fracture ; Complex analysis ; Damage processes ; Solid mechanics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract When radial cracks spread into a sheet starting from the boundary of a pressurized circular hole, experience shows that only some of the cracks increase in length. In this paper a simple model is proposed to explain the phenomenon. Thus, if the cracks have a length comparable to the hole radius,it is proposed to represent the boundary of the cracked region by ahypocycloid. This enables us to solve the corresponding boundary value problem of plane elasticity. If the intensity of the pressure is known and if the material obeys Griffith's energy fracture criterion, it is deduced,among all hypocycloidal boundaries, the one that is not stable. Limiting the analysis to the initial phase of the crack propagation, where the boundary of the hole may be assumed to be a hypocycloid, the unstable evolution of the system is explained using energetic considerations. Sommario.Semplici osservazioni mostrano che quando delle fessure radiali si propagano in una lastra, partendo dal contorno di un foro circolare in pressione, diminuiscono di numero con l'aumentare della loro lunghezza. In questo lavoro, si propone un semplice modello con cui spiegare il fenomeno. A tale scopo, si considera solo il caso in cui la lunghezza delle fessure sia minore del raggio del foro. Ciò consente di rappresentare il profilo della regione fratturata con un'ipocicloide e di determinare la soluzione esplicita del corrispondente problema di elasticità piana. In questo modo, nota la pressione nel foro e supponendo che il materiale segua il criterio energetico di frattura di Griffith, si individua, fra tutti i contorni ipocicloidali, quello instabile. Limitando l'analisi alla sola fase iniziale della propagazione delle fratture, l'evoluzione del sistema può essere descritta usando semplici considerazioni energetiche.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004378901689
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  • 12
    Electronic Resource
    Electronic Resource
    Bui's path-independent integral in finite elasticity (1995)
    Springer
    Meccanica 30 (1995), S. 139-145 
    Details
    ISSN: 1572-9648
    Keywords: Elastostatics ; Conservation laws ; Fracture ; Mechanics of solids
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Description / Table of Contents: Sommario Un integrale indipendente dal cammino intorno al vertice di una frattura in un campo di deformazione bi-dimensionale è stato stabilito da Bui. Tale integrale èduale all'integrale di Rice, nel senso che si basa sulla densità di energia complementare o degli sforzi. Qui si propone un integrale invariante in un continuo tridimensionale soggetto a deformazioni finite. Si mostra che il risultato di Bui segue come caseo particolare.
    Notes: Abstract A path-independent integral has been stated by Bui in the presence of a straight crack in a two-dimensional deformation field. Such an integral isdual to the Rice integral in the sense that it is based on the complementary stress energy density. Here we establish a boundary-independent integral in finite elasticity from which Bui's result follows as a particular case.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00990452
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  • 13
    Electronic Resource
    Electronic Resource
    A structural model of ceramics: Multiple fracture (1993)
    Springer
    Meccanica 28 (1993), S. 209-215 
    Details
    ISSN: 1572-9648
    Keywords: Ceramics ; Micromechanics ; Fracture
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Description / Table of Contents: Sommario Le modalita' di deformazione di materiali ceramici in presenza di difetti interni c la propagazione di microfratture sono analizzate considerando una griglia periodica di grani esagonali rigidi collegati tra loro mediante elementi elastici. Il modello di materiale ceramico, lo stato di sforzo in vicinanza di una frattura e la discussione di dati sperimentali sono stati presentati nella prima parte dell'articolo [1]. Il modello si basa sull'potesi che la rigidezza dei grani sia decisamente superiore a quella dei legami e chc le cricche si diffondano attraverso i legami medesimi. Per calcolare lo stato di sforzo si utilizza un metodo basato sulle funzioni di Green e sul principio di sovrapposizione. I difetti, rappresentati dai legami interrotti, sono rimpiazzati da dipoli di polarizzazione applicati ai grani adiacenti al difetto. Lo stato di sforzo nel materiale perfetto in presenza di carichi esterni e di dipoli, e nel materiale con microcricche sono considerali uguali. Questa condizione consente di ottenere un sistema linearc di equazioni in funzione di dipoli di polarizzazione e di determinare lo stato di sforzo. Il criterio di Mises e' utilizzato per individuare la rottura dei legami elastici. Vengono presentati, per diverse condizioni di carico, i risultati di simulazioni numeriche relative all'insorgere e alla propagazione di microfratture a partire da un'insieme di difetti.
    Notes: Abstract The processes of deformation of ceramics containing multiple internal defects, and the dissemination of microcracks, are analyzed by considering an infinite periodic grid of underformable hexagonal grains connected with elastic bondings. The model of ceramics and the stress distribution at the vicinity of the local break, together with a discussion of the experimental data, were studied in the first part of the paper [1]. The model is based on the assumption that the strength of grains is higher than that of bondings, and the cracks in the material spread through the bondings. To calculate the stress state of the medium, a numerical method, based on Green's function and the superposition principle, is used. Defects, presented by failed bondings, are placed by polarization dipoles of forces and moments applied to the grains adjacent to the defect. The stress distributions in the perfect medium, loaded by the external loads and polarization dipoles, and in the medium, containing micro-cracking, should be the same. This condition allows us to obtain simultaneous linear equations in terms of polarization dipoles and to determine the stress state of the medium. The failure of bondings is characterized by the von Mises strength criterion. The results of numerical modeling of the origination of a microcrack from a system of defects, and its dissemination for various loading, are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00989123
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  • 14
    Electronic Resource
    Electronic Resource
    Brittle to Quasi-Brittle Transition (1998)
    Springer
    Meccanica 33 (1998), S. 363-379 
    Details
    ISSN: 1572-9648
    Keywords: Fracture ; Damage ; Particle dynamics ; Brittle deformation ; Statistical mechanics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Abstract. The present study focuses on the kinetic and non-deterministic aspects of the brittle to quasi-brittle transition. A solid is approximated by a lattice formed by the interacting ‘continuum particles’ and the evolution of damage is estimated using particle dynamics. The onset of transition is measured by the rate of the change of correlation length. The proposed method is illustrated on the examples of creep rupture, strain localization and dynamic expansion of a circular void in a ‘brittle’ plate. Sommario. Viene posta l'attenzione sugli aspetti cinetici e non deterministici della transizione dal comportamento fragile a quello quasi-fragile. Un solido viene approssimato da un reticolo formato da particelle interagenti e l'evoluzione del danno viene stimata tramite la dinamica delle particelle. L'inizio della transizione viene misurato tramite la variazione della lunghezza di correlazione. Il metodo proposto viene illustrato su esempi di rottura per ‘creep’, localizzazione della deformazione e l'espansione di un foro circolare in una piastra fragile.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004358316631
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  • 15
    Electronic Resource
    Electronic Resource
    Size distribution of fractured areas in one-dimensional systems (1988)
    Springer
    Journal of statistical physics 51 (1988), S. 569-584 
    Details
    ISSN: 1572-9613
    Keywords: Fracture ; size distribution ; power law ; diffusion process ; Hausdorff dimension ; self-similar
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We study a one-dimensional model for fracture, identifying fractured areas with intervals on which a stress fieldξ exceeds a threshold valueΔ. Whenξ is a diffusion process, the cumulative numberN(l) of fractured areas whose length is greater thanl obeys a power lawCl −p asl↓0 with probability one. The exponentp and the constantC are determined. The exponentp agrees with the Hausdorff dimension of the end points of fractured areas, i.e.,ξ −1(Δ). Even ifξ is self-similar with parameterH〉0, i.e.,ξ(cx)−Δ is equivalent toc H {ξ(x)−Δ} for anyc〉0, the exponentp does not depend solely onH;p=λH, whereλɛ(0, 1/H) is another parameter characterizingξ. Non-diffusion processes are given whereN(l) does not follow a power law.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01028473
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  • 16
    Electronic Resource
    Electronic Resource
    Energies of a Kinked Crack Line (1998)
    Springer
    Journal of statistical physics 93 (1998), S. 511-525 
    Details
    ISSN: 1572-9613
    Keywords: Fracture ; creep ; kinks ; thermal fluctuations ; energy barriers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This article finds the energy of slightly kinked crack fronts in order to find the rate at which thermal fluctuations cause cracks to creep in three-dimensional settings.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/B:JOSS.0000033239.22129.c1
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  • 17
    Electronic Resource
    Electronic Resource
    A study of the depth of weathering and its relationship to the mechanical properties of near-surface rocks in the Mojave Desert (1984)
    Springer
    Pure and applied geophysics 122 (1984), S. 425-439 
    Details
    ISSN: 1420-9136
    Keywords: Weathering ; Mojave Desert ; Near-surface rocks ; Faults ; Fracture
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Weathered granite extends 70 m deep at Hi Vista in the arid central Mojave Desert of southern California. The low strength of this granite is due to the alteration of biotite and chlorite montmorillonite. Deep weathering probably occurs in most granites, although we cannot rule out some anomalous mechanisms at Hi Vista. Geophysical instruments set in these slightly altered rocks are limited by the unstable behavior of the rocks. Thus, tectonic signals from instruments placed in shallow boreholes give vague results. Geophysical measurements of these weathered rocks resemble measurements of granitic rocks near major faults. The rheology of the rocks in which instruments are placed limits the useful sensitivity of the instruments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00874609
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  • 18
    Electronic Resource
    Electronic Resource
    Volume changes during fracture and frictional sliding: A review (1978)
    Springer
    Pure and applied geophysics 116 (1978), S. 603-614 
    Details
    ISSN: 1420-9136
    Keywords: Compaction ; Dilatancy ; Fracture ; Friction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary Volume changes in geologic materials have been measured with strain gauges, cantilever displacement gauges, or through observation of either pore or total volume. When porosity is less than 0.05, compaction is small or absent; apart from elastic strains in the minerals, dilatancy predominates, beginning at 50 to 75 percent of the fracture stress difference. When initial porosity exceeds about 0.05, compaction and dilatancy may overlap. The onset of dilatancy has not been identified, but most of the dilatancy occurs within about 10 percent of the fracture stress difference. In low porosity rocks, dilatancy increases initial porosity by a factor of 2 or more; in porous rocks or granular aggregates the increase is only 20 to 50 percent. However, the actual pore volume increase is larger in rocks of high initial porosity. Hence, earthquake precursors which depend on the magnitude of dilatancy should be more pronounced in porous rocks or in fault gouge. In contrast, precursors which are based on fractional changes in some porosity-related property may be more pronounced in rocks of low initial porosity. Future work is particularly needed on constitutive relations suitable for major classes of rocks, on the effects of stress cycling in porous rocks, on the effects of high temperature and pore fluids on dilatancy and compaction, and on the degree of localization of strain prior to fracture.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00876527
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  • 19
    Electronic Resource
    Electronic Resource
    Electrical resistivity changes in rocks during frictional sliding and fracture (1978)
    Springer
    Pure and applied geophysics 116 (1978), S. 717-731 
    Details
    ISSN: 1420-9136
    Keywords: Electrical resistivity ; Fracture ; Friction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Significant changes of electrical resistivity of saturated rocks and water pressure along sliding surface occurred during stick-slips in our direct shear experiment. Two types of changes of electrical resistivity occurred. In the first, resistivity decreased with increasing shear stress, reached minimum together with a sudden release of shear stress and returned to a higher value immediately afterwards. In the second, resistivity again decreased with increasing stress but, in contrast to the first type of changes, it decreased further upon the sudden drop of shear stress. The magnitude and the direction of the changes of water pressure on the sliding surface during stick-slip were not uniform, indicating local variations of surface deformation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00876534
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  • 20
    Electronic Resource
    Electronic Resource
    Seismic velocity changes during fracture and frictional sliding (1978)
    Springer
    Pure and applied geophysics 116 (1978), S. 732-742 
    Details
    ISSN: 1420-9136
    Keywords: Velocity changes ; Fracture ; Friction ; Holography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Fracture and frictional sliding are considered as phenomena involving brittle failure. Brittle failure is preceded by the formation of small (subcritical) cracks. In non-water-saturated rock, the distribution, shape and size of these suberitical cracks determine the change in the physical properties prior to failure. A model is proposed which suggests that the spatial and temporal distribution, shape and size of subcritical cracks within a stressed rock depend upon the rate of deformation and the volatile content. As a rock is stressed beyond about 50 percent of its ultimate failure stress, dilatancy is initiated. With increasing stress a broad zone of cracks develops within the dilatant region. The seismic velocities through this zone decrease markedly and the cracks grow more numerous., changing in size and shape. Before brittle failure of the rock occurs, the subcritical cracks interact, leading to a concentration of the zone. During the stage when the zone narrows, the seismic velocities in crease in the surrounding volume due to local rotation of stresses and consequent closure of some cracks. In most laboratory experiments the stage during which the velocity increases and the now intense deformation zone becomes narrow is very short and difficult to observe experimentally. At very low strain rates and with volatiles present, the crack growth and subsequent interaction lead to the narrowing of the intense deformation zone and therefore to an observable increase in velocity. The above is based upon an interpretation of a number of experiments. Using optical holography we have observed the development and subsequent intensification of a deformation zone. Ultrasonic velocity measurements showed a distinct anomaly (decrease followed by an increase) before failure. The anomaly was only detectable at our lowest experimental strain rates (3×10−8/sec).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00876535
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