by Dvorak, George J.

DOI: 10.1007/s10704-006-7350-4
Print publication date: 1/1/2006
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by Fedele, R.; Maier, G.; Miller, B.

In several existing dams alcali–silica reaction (ASR) during several decades of service life, or diffused micro-cracking (due to concrete ageing and/or past extreme loads, such as earthquakes) give rise to deterioration of concrete stiffness and to correlated reduction of its strength. An inverse methodology is presented herein apt to identify damage in concrete dams on the basis of hydrostatic loading, measurements by traditional monitoring instruments, such as pendulums and collimators, and artificial neural networks trained by means of finite-element simulations. The arch-gravity dam referred to in this study is sub-divided into homogeneous zones, to which a constant Young modulus is attributed as unknown parameter which quantifies possible damage. These elastic moduli are estimated on the basis of pseudo-experimental data and identification procedures. After a suitable ‘training’ process, artificial neural networks (ANNs) are employed for numerical solutions of the inverse problem, and their potentialities and limitations are examined to the present purposes. In particular, they turn out to be robust and practically useful in the presence of information which are scarce quantitatively (few available measurements) and/or qualitatively (large noise-to-signal ratio).

DOI: 10.1007/s10704-006-6582-7
Print publication date: 1/1/2006
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by Dvorak, George J.; Suvorov, Alexander P.

A novel procedure is proposed for evaluation of stress intensity factors of planar Mode III shear cracks perpendicular to a nearby interface between two isotropic elastic solids. Shear cracks traversing a flat layer bonded to two different elastic solids are also analyzed. The method is based on superposition of singular near tip stress and displacement fields generated by both the main crack and certain image cracks. Both the main and the image cracks are loaded by self-equilibrating shear tractions of different magnitude, such that matching parts of the said fields are made to satisfy traction and displacement continuity conditions at the interface. Selected comparisons with results obtained by different methods show good agreement. Applications of the method to other crack problems are discussed.

DOI: 10.1007/s10704-006-6636-x
Print publication date: 1/1/2006
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by Christensen, Richard; Miyano, Yasushi

Consistent with viscoelastic behavior, a power law form in terms of the stress intensity factor is used to specify crack kinetics (growth rate) in the central crack problem under Mode I conditions. The crack growth rate is integrated to obtain the crack size and thereby the stress intensity factor as a function of time. The crack is allowed to grow in a controlled, load dependent manner until it reaches the size at which it becomes unstable. The corresponding time at which this occurs is taken as the lifetime of the material under the specified load history. The special cases of constant load (creep rupture) and constant strain rate to failure are found to have a very simple relationship with each other. This lifetime relationship is verified through the comparison with corresponding data upon a polymeric composite. Finally the creep rupture case is generalized to a probabilistic formalism. The theoretically predicted lifetime distribution functions are verified with data, also upon a polymeric composite. Possible extension of the entire formalism to cyclic fatigue in metals is discussed.

DOI: 10.1007/s10704-005-9062-6
Print publication date: 1/1/2006
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by Kumar, R. S.; Wang, A. -J.; Mcdowell, D. L.

Understanding the effects of microstructure variability on fatigue resistance is a key to selection and design of materials for fatigue applications. The traditional empirical approach rooted in experiments is being increasingly combined with systematic computational modeling. This work is concerned with demonstrating the feasibility of linking effects of microstructure variability on cyclic plasticity at the scale of intrinsic microstructure of a single crystal nickel-base superalloy. The precipitate and the matrix phases of the alloy are modeled explicitly using a physically based crystal viscoplasticity constitutive framework with appropriate scale and spacing effects to reflect dislocation–precipitate interactions. The model is implemented as a user material subroutine within a finite element code. Various realizations of different microstructures are generated using a constrained Poisson point process. Statistical volume elements (SVEs) with random-periodic boundary conditions are simulated under fully reversed cyclic loading at 650°C. Primary cooling γ′ precipitate size and volume fraction are considered in terms of their effects on the macroscopic stress–strain response and on distributed cyclic microplasticity within the SVE. To compare various microstructures in terms of probability of fatigue crack formation, an appropriate nonlocal measure of cyclic plastic shear strain range is proposed based on percolation of cyclic microplasticity at the scale of the SVE.

DOI: 10.1007/s10704-005-3149-y
Print publication date: 1/1/2006
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by Weertman, Johannes

Maximum shear stress trajectories are obtained for the mode I and for the mode II crack in an isotropic elastic solid in plane strain.

DOI: 10.1007/s10704-005-3078-9
Print publication date: 1/1/2006
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by Ladevèze, Pierre; Lubineau, Gilles; Violeau, David

A new computational damage micromodel for laminates, which takes into account classical experimental micro- and macro-observations for various stacking sequences, is described. The first computational examples are shown.

DOI: 10.1007/s10704-005-3077-x
Print publication date: 1/1/2006
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by Gutiérrez, Miguel A.

The direct differentiation method is applied to the estimation of statistical size effect behaviour in quasi-brittle solids. The scale factor is included in the finite element model and the autocorrelation function. Particular attention is paid to the proper differentiation of the Nataf transformation, which has been chosen to convert the basic random variables into a set of uncorrelated, standard normal variables. The predictive possibilities of the presented algorithm provide a valuable insight in the actual mechanisms responsible for failure. It can be evaluated to what extent the scale factor sensitivity of the failure probability is influenced by the phenomena related to the material disorder or the deterministic size effect.

DOI: 10.1007/s10704-005-3076-y
Print publication date: 1/1/2006
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by Rabczuk, T.; Belytschko, T.

Particle methods for modeling reinforced concrete are described. The reinforcements are modeled by finite elements and are coupled to the particle method by Lagrange multipliers. The method is applicable to nonlinear problems, problems with moderate to severe cracking and deformable interfaces. Applications to the static response of reinforced concrete structures where the concrete is discretized with particles and the reinforcement with elements are described. The method is also tested for several static problems where no relative displacements between the concrete and the reinforcement are allowed.

DOI: 10.1007/s10704-005-3075-z
Print publication date: 1/1/2006
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by Kulkarni, Salil S.; Sun, L.; Moran, B.; Krishnaswamy, S.; Achenbach, J. D.

A probabilistic method to predict macrocrack initiation due to fatigue damage is presented in this paper. Acoustic non-linearity is used to quantify pre-macrocrack initiation damage. This data is then used in a probabilistic analysis of fatigue damage. The probabilistic fatigue damage analysis consists of a suitably chosen damage evolution equation to model accumulated damage coupled with a procedure to calculate the probability of macrocrack initiation. The probability of macrocrack initiation is evaluated using the Monte Carlo Method with Importance Sampling. Numerical results for the probabilistic assessment of fatigue damage for a sample problem are presented and compared with experimental results.

DOI: 10.1007/s10704-005-3074-0
Print publication date: 1/1/2006
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