We attempt the identifícation, study and modeling of possible sources of size effects in concrete structures acting both separately and together. We are particularly motivated by the interplay of several identified scaling lengths stemming from the material, boundary conditions and geometry. We model the well published results of direct tensile tests of dog-bone specimens with rotating boundary conditions using methods of stochastic nonlinear fracture mechanics. Firstly, we model the specimens using microplane material law to show that a large portion of the dependence of
nominal strength on structural size can be explained deterministically. However, it is clear that more sources of size effect play a part, and we consider two of them. Namely, we model local material strength using an autocorrelated random field attempting
to capture a statistical part of the complex size effect, scatter inclusive. Next to it, the strength drop noticeable with small specimens, which was obtained in the experiments is explained by the presence of a weak surface layer of constant thickness (caused e.g. by drying, surface damage, aggregate size limitation at the
boundary, or other irregularities). All three named sources (deterministic-energetic, statistical size effects, and the weak layer effect) are believed to be the sources most contributing to the observed strength size effect; the model combining all of them is
capable of reproducing the measured data. The computational approach represents a marriage of advanced computational nonlinear fracture mechanics with simulation techniques for random fields representing spatially varying material properties. Using a numerical example, we document how different sources of size effects detrimental to strength can interact and result in relatively complex quasibrittle failure processes. The presented study documents the well known fact that the experimental determination of material parameters (needed for the rational and safe design of structures) is very difficult for quasibrittle materials such as concrete. and Obsahuje seznam literatury
The paper presents stochastic discrete simulations of concrete fracture behavior. The spacial material randomness of local material properties is introduced into a discrete lattice-particle model via an autocorrelated random field generated by the Karhunen-Loève expansion method. The stochastic discrete model is emploeyd to simulate failure of the three-point-bent beams with and without a central notch.. The effect of spatial randomness on the peak load and energy dissipation is studied. and Obsahuje seznam literatury
The pullout response of a short fiber embedded in matrix depends on both the bond between the two materials and on the inclination angle and embedded length of the fiber. Fibers placed and oriented randomly in 3D space bridge matrix cracks with certain inclination angles and embedded lengths. With a pullout law available in analytical form, a statistical description of the force per fiber depending on crack opening can be evaluated for a uniformly loaded crack bridge in a short fiber reinforced composite by integrating the powers of all possible fiber responses multiplied by their probabilities of occurrence. This information is utilized to probabilistically evaluate the crack bridging force by computing the sum of a random number of random contributions; the random number of contributions to be summed is the number of bridging fibers, and the independent random contributions are the single fiber responses. and Obsahuje seznam literatury