Zdeněk Knésl

Fatigue Parameters of Cement-Based Composites with Various Types of Fibres

The paper introduces the basic fracture mechanics parameters of advanced building material – cement-based composites with various types of fibres, prepared as high performance concrete/mortar developed by ZPSV, a.s. company for production of thin-walled panels/elements. To this end three-point bend specimens with starting notch were prepared and tested under static (load–deflection diagram, effective fracture toughness) and cyclic loading (fatigue parameter – Wöhler curve). The experimentally obtained results of cement-based composites are compared and the suitability of these types composites for its application are discussed.

The Effect of the Singularity Induced by the Free Surface on Fatigue Crack Growth in Thin Structures

In many industrial applications is necessary to predict fatigue lifetime of thin structures, where the stress field near the crack front have a real three-dimensional nature. Due to the existence of vertex singularity in the point where the crack front touching free surface, crack propagation in 3D structures cannot be reduced to a series of plane strain or plane stress problems along the crack front edge. The paper describes the influence of vertex singularity on the distribution of the stresses around the crack front for three-dimensional body. The distribution of the stress singularity through the thickness of the specimen gives us indication of the crack behavior in thin structures. The estimation of the thickness of the specimen where the change of singularity plays an important role on fatigue crack growth rate (in dependence on Poisson’s ratio) is carried out. The results contribute to a better understanding of the crack behavior in thin structures, and can help to more reliable estimates of their residual fatigue life.

Crack Behaviour in Polymeric Composites: The Influence of Particle Shape

In this paper polymeric particulate composites are studied (especially polypropylene (PP) matrix stuffed by rigid mineral fillers). Presently, polymeric particulate composites are frequently used in many engineering applications. The composite was modeled as a three-phase continuum – matrix, interphase and particle. The properties of the particles (size, shape) have a significant effect on the global behaviour of the composite. On the basis of fracture mechanics methodology the interaction of micro-crack propagation in the matrix filled by rigid particles covered by the interphase was analyzed. The effect of the composite structure on their mechanical properties is studied here from the theoretical point of view.

Optimized design of a high pressure compound vessel by fem

Abstract The problem of safety optimization of the high pressure compound vessel is presented. The safety for both assembly and working states is considered. Two different objective functions are taken into account, corresponding to two methods of loading from actual stress state to the limiting brittle fracture state. For the vessel of the given type consisting of n parts (the matrix and n−1 steel rings), the number of design parameters is 2n−1. Design parameters were preoptimized on the cylindrical compound vessel model. Because of the cost-effectiveness of finite element calculations, only two variables having the greatest influence on the matrix safety were taken into account. These were the radius and the overlap of the first ring. For optimization the variable metric method is used.

Sensitivity of Fatigue Crack Growth Data to Specimen Geometry

The formulations of fatigue crack growth prediction are still mostly based on phenomenological models. A commonly used formula in the field of high cycle fatigue is the Paris- Erdogan law. For given experimental conditions (such as temperature, stress ratio or environmental conditions) the parameters C and m have to be experimentally determined and considered as material constants. Thus, for a given material, the fatigue crack growth rate (FCGR) depends only on the applied range of the stress intensity factor. In a threshold region a significant shift in the data of the fatigue crack propagation rate can be observed. The shift is induced by different test specimen geometry. To analyses it the authors will present their own laboratory fatigue crack growth rate test data measured on two different specimens with different levels of constraint and for different steels. It is demonstrated that fatigue characteristics (i.e. C, m and Kth) obtained from different specimen geometries are not only properties of the materials but depends on the specimen geometry.

A mixed numerical-analytical approach to strength optimization of an axisymmetric compound vessel

Abstract The optimal design of an axisymmetrical compound vessel is realized in two computational steps. In the first step, the transformation into the equivalent cylindrical compound vessel is carried out using the finite element method. From analytical relations based on Lames analytical solution the optimal number of components n and optimal radii r j , of cylindrical contact surfaces are calculated. In the second computational step, the optimal overlaps Δr j , of the real axisymmetrical vessel are calculated. An iterative numerical method using the design differences of maximal effective stresses δσ ef ( i , j ) calculated by the finite element method has been developed for this purpose.

Structural Changes, Evolution of Damage Parameters and Crack Propagation Behaviour in Welded Plastic Pipes

The microstructure as well as the local mechanical and fracture behaviour of welded joints in plastic pipes made form polyethylene and material zones outside of the welded joints have been analysed using recording microhardness testing, laser extensometry and crack resistance curve tests. In has been found that the mechanical basic properties and damage kinetics are clearly depending on the welding parameters and additional notching.

Transverse Cracking of Layered Structures: Evaluation of Fatigue Crack Propagation

The objective of this paper is to investigate the transverse fatigue cracking of layered structures. To this end a fatigue crack propagation rate for a crack growing perpendicularly through the interfaces between individual layers of the structure is estimated. The suggested procedure takes into account the influence of different material properties of the layers on the stress field around the fatigue crack and the change of the stress singularity exponent in the special case of a crack touching the material interface. The assumptions of linear elastic fracture mechanics and elastic behaviour of the composite are considered. It is shown that the existence of the interfaces can have a negative influence on the residual fatigue life of layered structures.

A Bi-Material Wedge – a Model for the Prediction of Failure Initiation at Shape and Material Discontinuities

Geometrical and material discontinuities in constructions lead to singular stress concentrations and consequently to a crack initiation. The model of a bi-material wedge makes it possible to analyse such construction points to assess their stability. The presented approach is based on the knowledge of the strain energy density factor distribution in the concentrator vicinity.

Basic Modes of Crack Propagation through the Interface in a Polymer Layered Structure

The multi-layered structure components of dissimilar materials are used in many engineering applications to protect the base structure from outer damage. Typical examples of coated structures are pipes with the dimensional addition of protective layers (multi-layer pipes). The purpose of this development is to protect the main (functional) part of the pipes from damages (e.g. surface scratches, internal crack propagation). In the contribution the attention is paid to cracks existing and propagating in inner protective layer. In many cases the cracks are stopped at the interface between protective and functional layers. The important task is to decide if they penetrate further through the interface in the bulk material and thus cause the failure of the system. The critical stress for the crack propagation through the interface depends on the relation between bulk and coating materials as well as on the crack geometry. All these important topics are taken into account and their effect on crack propagation is broadly discussed.