Key Engineering Materials Vols. 592-593

Abstract: The present study is on the concept of modeling of heterogeneous materials by means of Wang tilings. The central idea is to store a microstructural information within a finite set of Wang Tiles, which allow for reconstructing heterogeneity patterns of random media in planar domains of arbitrary sizes. The particular objective of presented work is our automatic tile set designer in conjunction with stochastic tiling synthesis algorithm. The proposed methodology is demonstrated on different examples. The proximity of synthesized microstructures to reference media is explored by statistical descriptors and discussed in terms of parasitic spatial orientation orders that may occur.

Abstract: Tungsten carbide-cobalt (WC/Co) is a two phase material composed of two distinct interpenetrating phases having different mechanical properties. The hard and brittle WC phase behaves elastically whereas the Co phase demonstrates an elasto-plastic response. In order to predict the global behavior of the material from its microscopic constitution, the representative volume element (RVE) model has to satisfy basic requirement of a certain size. Moreover the homogenized response of the RVE has to be characterized with respect to the macroscopic mechanical property of interest. Taking into consideration such phenomenon the current study investigates the size of 3D RVEs that are adequate to reflect the global elasto-plastic response and the fracture (damage) energy dissipation. The results of the homogenized elasto-plastic responses were compared to a macroscopic experimental stress-strain curve and the fracture energy dissipation was validated by a convergence study. It has been observed from the numerical simulation that, despite the randomness of the structure, the development of volume averaged elastic potential, plastic and as well as the fracture energy dissipation would stabilize with the increasing size of the RVE.

Abstract: Mechanical behaviour of concrete structures and their durability are conditioned by the reduction of cracking due to plastic and drying shrinkage and of permeability of concrete. This can be done using small metal particles, typically short steel fibres, randomly distributed in the concrete matrix. Since the technological requirements to the preparation and early-age treatment of such mixtures are rather demanding, some reliable a posteriori validation of expected solid material structure is needed. As alternatives to destructive testing, various low-invasive radiographic, electromagnetic, etc. approaches have been developed in the last decade. This paper demonstrates an original magnetic approach to the identification of volume fraction, macroscopic (in) homogeneity and orientation of particles in the matrix, using the Hall effect and the properties of solutions of the Laplace equation, as well as an advanced computational homogenization approach, coupled with the least-square based optimization technique.

Abstract: On the basis of the uniaxial compression tests realized in the temperature range from 800 °C to 1200 °C and at strain rates from 0.05 s^-1 to 30 s^-1 the value of the activation energy at hot forming of coarse-grained as-cast iron aluminide Fe40at.%Al-Zr-B was calculated as Q = 502 kJ·mol^-1. With use of this material constant, relatively precise mathematical description of the maximum flow stress and the corresponding strain in dependence on the Zener-Hollomon parameter was possible. As was discovered by metallographical analysis, at high values of the Zener-Hollomon parameter slight decrease in flow stress occurs even without significant course of dynamic recrystallization.

Abstract: This paper presents a methodology suitable for estimation of residual lifetime of polymer pipes. The linear and non-linear distribution of residual stresses in the pipe wall is studied using FEM analysis. The approximate relation for the stress intensity factor calculation is presented. It is shown that the presented relation gives a stress intensity factor similar to the FEM analysis for the linear or non-linear distribution of residual stress. The suggested procedure produces a slightly conservative lifetime estimation where the accuracy of the procedure increases with increasing ratio between applied internal pressure level to residual stress level. The accuracy of the residual stress estimation and corresponding stress intensity factor is discussed and a final recommendation for lifetime determination based on simplified methodology is given.

Abstract: The problem of crack path stability along the interface between two orthotropic elastically dissimilar materials under the presence of in-plane residual stresses is analyzed using the concept of Finite Fracture Mechanics and matched asymptotic procedure. An energy based fracture criterion is introduced for this problem and it is investigated whether and how is the criterion for the prediction of crack kinking from the interface affected by residual stresses. The complex stress intensity factor and the T-stress characterizing the stress state at the crack tip are calculated both for the thermal (residual stresses) and mechanical loading using the two-state integral. The matched asymptotic procedure together with FEM is used to derive the change of the potential energy induced by the crack growth by crack increment of finite length.

Abstract: In this paper the mechanical heterogeneity of welded joints was studied. The experimental results show that the mechanical properties and fracture toughness vary significantly in zones of welded joint. To estimate the fracture parameter J-integral a numerical finite-element model based on the random variation of yield strength was proposed.

Abstract: The contribution is focused on estimation of a critical value of generalized stress intensity factor for crack propagation from sharp V-notches. Stress distribution around the tip of the V-notch is described on the base of generalized linear elastic fracture mechanics, because V-notch is a singular stress concentrator with stress singularity exponent different from 0.5 (depending on V-notch opening angle). Then also stability criteria based on strain energy density factor and average critical stress are generalized for the stress singularity different from 0.5. Using FE analysis the critical stresses for crack initiation was estimated and compared with experimental data from the literature.

Abstract: Strength characteristics of an elastic plane weakened by round holes arranged in a quadratic order are considered. The stress concentrations in the three different hole lattices are studied in conditions of the uniaxial tension/compression with different directions (0°÷45°, it is enough, because the lattices are symmetrical). The minimal and maximal concentrations values are calculated; the stress fields are considered in different lattices. It is shown that in the particular conditions of fracture at compression can occur in the inner zone of the material rather than at the hole contours. It is demonstrated that in dense lattices a power-like type of the concentrations dependencies on the structural parameter takes place; the parameter is a ratio of the holes interval to the hole radius. Next, the modes of initiation of fracture in elastic perforated plates (lattices) under external uniaxial compression are considered. Special attention is paid to the internal mode of initiation (i.e., to the case of fracture initiation inside the material, and not on the contours of the holes). The parametric region of the internal mode is studied, it depends on the angle of the applied external loads and the structural lattice parameter; the size of the region (extreme values) is calculated. A particular point of the region is found; it is shown that at this point there are three possibilities of fracture initiation: two positions at the hole contour, and one position inside of material. Keywords: elasticity, perforated plates, lattice of circular holes, fracture initiation, uniaxial compression.

Abstract: The fracture-mechanical parameter values of concrete, a quasi-brittle composite material, are determined via evaluation of records of experiments on specimens with stress concentrators. One of the fracture models applicable to concrete is the double-K model. This model combines the concept of cohesive forces acting on the effective crack increment with a criterion based on the stress intensity factor. The outputs of the model are critical crack tip opening displacement and fracture toughness values, including the initiation stress intensity factor value corresponding to the beginning of stable crack propagation. Outputs of three-point bending fracture tests of fibre reinforced concrete obtained using double-K fracture model are presented in this paper. The main aim is the determination and comparison of the above-mentioned parameter values of two types of the composite both without and with polypropylene fibres. Both types of tested composite had the same basic matrix consisting of cement, sand and water. In one case, gravel was used for normal weight concrete, in the other case lightweight aggregates were used for lightweight concrete. Both types of the testing matrix were designed with a similar value of compressive strength. Concretes were reinforced by spread polypropylene fibres of three lengths. There were made eight sets of testing concrete specimens: without fibres, and with fibres of 19 mm, 38 mm and 54 mm length. Dosage of fibres was 9 kg/m³ in all six cases.