by Levin, Valery M.; Alvarez-Tostado, Juan M.

The influence of applied stresses and internal fluid pressure on deformation of a flat ellipsoidal inhomogeneity in a homogeneous poroelastic medium is studied. The inhomogeneity is filled with a porous material having a skeleton much softer than the corresponding to the surrounding medium (crack-like inclusion). The result of the calculation of stress and strain fields inside such inclusion is presented. Explicit formulas for the inclusion aspect ratio changing are obtained. They are relevant for velocities of seismic waves in porous rocks (Toksöz et al, 1976) their permeability (Gibson & Toksöz, 1990) as functions of confining and pore pressures. Results are compared with the ones available in literature.

DOI: 10.1007/s10704-006-8378-1
Print publication date: 5/1/2006
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by Chiang, Chun-Ron

The thermal stress induced in a spherical inclusion by the difference of the thermal expansion coefficiences of the inclusion and its embedding matrix is considered. Both the inclusion and the matrix are assumed to be of cubic symmetry. Eshelby’s equivalent inclusion method is used to solve the problem. A smple expression for the determination of thermal mismatch stress is thus derived. Some numerical examples are provided.

DOI: 10.1007/s10704-006-8377-2
Print publication date: 5/1/2006
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by Nilsson, Fred

The double cantilever beam specimen for fracture testing was investigated for large displacement conditions. J-expressions were derived for arbitrary loading of the beam-ends. As special cases two different loadings, transverse force and bending moment were studied. Explicit relations for use in experimental situations were derived.

DOI: 10.1007/s10704-006-8376-3
Print publication date: 5/1/2006
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by Askes, Harm; Aifantis, Elias C.

Partly in response to a communication recently published in this journal on apparent inconsistencies between certain continuum and atomistic formulations of gradient elasticity (Yang and Guo 2005), we further elaborate on this issue in view of results and works not known or not cited in the aforementioned communication. In particular, we unify the concepts and motivations of two different formats of gradient elasticity. The first format was motivated for use in statics and aims at removing strain singularities. The second format was motivated for use in dynamics and aims at describing wave dispersion. We suggest here an alternative format of gradient elasticity that is dispersive, while its static version is identical to the first format mentioned above. Also, procedures are outlined by which the higher-order coefficients can be related to micro-structural properties. Finally, solution methods are described for static and dynamic analysis.

DOI: 10.1007/s10704-006-8375-4
Print publication date: 5/1/2006
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by Zhou, Fenghua; Molinari, Jean-François; Ramesh, K. T.

We present a fundamental investigation of the influence of material and structural parameters on the mechanics of fragmentation of brittle materials. First, we conduct a theoretical analysis (similar to Drugan’s single wave problem, Drugan, W.J. (2001), Journal of Mechanical and Physics Solids
49, 1181–1208.) and obtain closed form solutions for a problem coupling stress wave propagation and single cohesive crack growth. Expressions for a characteristic fragment size s
0 and a characteristic strain-rate $${\dot\varepsilon}_0$$ are given. Next, we use a numerical approach to analyze a realistic fragmentation process that involves multiple crack interactions. The average fragment size s is calculated for a wide variety of strain-rates $${\dot \varepsilon}$$ and a broad range of material parameters. Finally, we derive an empirical function that relates the normalized fragment size s/s
0 to the normalized strain-rate $${\dot \varepsilon}/\dot{\varepsilon}_0$$ and that fits all of the numerical results with a single master curve.

DOI: 10.1007/s10704-006-7135-9
Print publication date: 5/1/2006
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by Nairn, J. A.

Prior methods for calculating energy release rate in cracked laminates were extended to account for heterogeneous laminates and residual stresses. The method is to partition the crack tip stresses into local bending moments and normal forces. A general equation is then given for the total energy release rate in terms of the crack-tip moments and forces and the temperature difference experienced by the laminate. The analysis method is illustrated by several example test geometries. The examples were verified by comparison to numerical calculations. The residual stress term in the total energy release rate equation was found to be essentially exact in all example calculations.

DOI: 10.1007/s10704-006-0044-0
Print publication date: 5/1/2006
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by Cao, Yuguang; Tanaka, Kiyoshi

In order to investigate the causes of material fracture, a new method is proposed that uses only the fracture surfaces for determining the fracture parameters in terms of the Cract-tip opening angle (CTOA) and the J integral. This method is based on the principle of fracture-surface topography analysis (FRASTA). In FRASTA, the fracture surfaces are scanned by laser microscope and the elevation data is recorded. Based on this recorded elevation data, the J integral can be calculated. The J integral calculated by the new method agrees well with that calculated by the elastic compliance method. FRASTA allows easy determination of the crack opening deformation (CTOA and COA) and the variation in CTOA and COA through specimen thickness.

DOI: 10.1007/s10704-006-0042-2
Print publication date: 5/1/2006
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by Szekrényes, András

The prestressed end-notched flexure fracture specimen is developed in the present work, which combines the traditional double-cantilever beam and the end-notched flexure specimens in a very simple way. The most important features of the new beam-like specimen are that it is able to provide any combination of the modes I and II strain energy release rates and it may be performed by using a simple three-point bending fixture. The mode-I part of the strain energy release rate is fixed by inserting a steel roller, which causes a fixed crack opening displacement. The mode-II part of the energy release rate is provided by the external load. A simple closed-form solution using beam theory is developed for the energy release rates of the new configuration. The applicability and the limitations of the novel configuration are demonstrated using unidirectional glass/polyester composite specimens. If only propagation onset is involved then the prestressed end-notched flexure specimen can be used to obtain the fracture criterion of transparent composite materials in a very simple way.

DOI: 10.1007/s10704-006-0043-1
Print publication date: 5/1/2006
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by Hillig, William B.

The often cited “Charles–Hillig” model of delayed failure assumes that corrosive attack on existing flaws in glass is controlled by interactions involving the moisture in the environment, which leads to time and stress dependent failure. The analysis by Inglis of the stress multiplication at a rounded end of a flaw is combined with stress-dependent chemical kinetics and thermodynamics. Failure occurs when the tip stress reaches the ultimate strength of the glass. The model provides a physical interpretation of the empirical “Universal Fatigue Law.” When sufficient data is available, it also provides an algorithm that allows a precise determination of the minimum stress needed to induce time-dependent failure. This model is compared with the competing LEFM model.

DOI: 10.1007/s10704-006-0025-3
Print publication date: 5/1/2006
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by Ožbolt, J.; Rah, K. K.; Meštrović, D.

Three different effects control the influence of the loading rate on structural response: creep of bulk material, rate dependency of growing microcracks and structural inertia. The first effect is important only at extremely slow loading rates whereas the second and third effects dominate at higher loading rates. In the present paper, a rate sensitive model, which is based on the energy activation theory of bond rupture, and its implementation into the microplane model for concrete are discussed. It is first demonstrated that the model realistically predicts the influence of the loading rate on the uniaxial compressive behaviour of concrete. The rate sensitive microplane model is then applied in a 3D finite element analysis of the pull-out of headed stud anchors from a concrete block. In the study, the influence of the loading rate on the pull-out capacity and on the size effect is investigated. To investigate the importance of the rate of the growing microcracks and the influence of structural inertia, static and dynamic analyses were carried out. The results show that with an increase of the loading rate the pull-out resistance increases. For moderate loading rates, the rate of the microcrack growth controls the structural response and the results of static and dynamic analysis are similar. For very higher loading rates, however, the structural inertia dominates. The influence of structural inertia increases with the increase of the embedment depth. It is shown that for moderately high-loading rates the size effect becomes stronger when the loading rate increases. However, for very high-loading rate the size effect on the nominal pull-out strength vanishes and the nominal resistance increases with an increase of the embedment depth. This is due to the effect of structural inertia.

DOI: 10.1007/s10704-006-0041-3
Print publication date: 5/1/2006
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