Papers by Author: Jan Klusák

Abstract: The use of S355 high strength steel in civil engineering to design bridges, its elements or simple engineering parts allows material and economical savings meeting the strict construction requirements. The knowledge of the fatigue resistance of material plays the key role during design and maintenance of the bridge structures. This contribution brings a comparison of the fatigue crack growth resistance of two standard S355 J0 and S355 J2 steel grades. Differences in chemical composition and the texture of material structure could generally play a role in the fatigue crack growth. This study shows that in the case of studied steels the chemical composition has an influence on material fatigue behaviour, whereas the texture of material structure is irrelevant.

Abstract: Fracture behaviour of a crack approaching a bi-material interface is investigated. A three-point bending configuration of a cracked specimen is simulated numerically by means of the finite element method and the interaction between the crack and aggregate is studied. The crack deflection angle is estimated by means of the maximum tangential stress criterion in its classical as well as generalized (multi-parameter) form considering the Williams’ power series with various numbers of the higher-order terms for the tangential stress approximation. The influence of the elastic mismatch and of other parameters on the calculated initial crack propagation angle is discussed.

Abstract: Concrete used in civil structures is usually made on cement-based matrix and natural aggregates (such as sand, gravel, crushed stone, etc.). Ceramic waste aggregate is considered as a perspective replacement of a part of natural aggregate in modern environmentally oriented building material. Concrete with natural aggregate partially replaced by ceramic waste aggregate usually show different mechanical characteristics than ordinary concrete. This paper introduces the pilot study of fatigue parameters of six concrete mixtures with various amount of ceramic waste. The experimentally obtain results are compared and discussed.

Abstract: General Singular Stress Concentrators (GSSCs) which exhibit singular stress concentration are often responsible for crack initiation and thus failure of the component. The GSSC of the type of bonded bi-material junction occurs in a variety of technical applications including but not limited to sharp material inclusions, silicate based composites and electronic components. The GSSC cannot be assessed by means of standard fracture mechanics. Approaches of generalized fracture mechanics require precise description of stress distribution near the stress concentration points. In order to determine the stress field accurately, the paper incorporates the multi-parameter based description.

Abstract: A stress distribution in vicinity of a tip of polygon-like inclusion exhibits a singular stress behaviour. Singular stresses at the tip can be a reason for a crack initiation in composite materials. Knowledge of stress field is necessary condition for reliable assessment of such composites. A stress field near the general singular stress concentrator can be analytically described by means of Muskhelishvili plane elasticity based on complex variable functions. Parameters necessary for the description are the exponents of singularity and Generalized Stress Intensity Factors (GSIFs). The stress field in the closest vicinity of the SMI tip is thus characterized by 1 or 2 singular exponents (1 - λ) where, 0<Re (λ)<1, and corresponding GSIFs that follow from numerical solution. In order to describe stress filed further away from the SMI tip, the non-singular exponents for which 1<Re (λ), and factors corresponding to these non-singular exponents have to be taken into account. Analytical-numerical procedure of determination of stress distribution around a tip of sharp material inclusion is presented. Parameters entering to the procedure are varied and tuned. Thus recommendations are stated in order to gain reliable values of stresses and displacements.

Abstract: The reliability of the conventional fatigue limit estimation of aluminum alloy Al 2024 provided by thermographic measurements according to the Risitano method is investigated in order to check their validity for practical applications. With this aim, an experimental fatigue program on Al 2024 specimens under load control using a stress ratio R = 0.1 is performed at three different frequencies. The fatigue limit methodologies is first determined according to the methodologies proposed by Risitano et al. and Canteli et al., and then compared with that resulting from the conventional Wöhler curve.

Abstract: The domain of the generalized stress intensity factors dominance ahead of the notch tip can be rather small with respect to the length of the perturbing cracks initiated from the tip of the notch. Thus the non-singular terms of the stress asymptotic expansion at the notch tip would play an important role in the notch tip stability. Following the procedures dealing with complex potential theory and path-independent two-state integrals developed for the singular stress analysis of the stress concentrators one can evaluate their magnitude and include them to the energy release rate of the preexisting crack initiated from the notch tip applying the matched asymptotic procedure. The presented analysis should lead to better understanding of the notch stability process and precising of the notch stability criteria.

Abstract: The application of infrared thermography to obtain the external surface temperature during the application of cyclic loading, allows to evaluate the dynamic behavior of an element and to determine the fatigue limit. In this contribution, the reliability of the fatigue limit provided by the Risitano et al. and Canteli et al. methodologies is investigated in order to check their validity for practical applications. With this aim, an experimental program on aluminium specimens (AL 2024) under load control using a stress ratio minus one is performed. The fatigue limit for AL 2024 is derived, first from the Wöhler curve and then, compared with both Risitano et al. and Canteli et al. methodologies.

Abstract: In the range of linear elastic fracture mechanics, the critical loading assessment of structures made of two dissimilar materials is usually based on the assumptions of the prevailing normal mode of loading. However, in engineering practice there are cases of loading and failure close to the shear mode of loading. The aim of the work is to study the stress distribution in the vicinity of a bi-material notch subjected to a combination of normal and shear modes of loading. Then the stability criteria use knowledge of common fracture mechanics properties for normal I and shear II modes of loading. The assessment of crack initiation conditions is shown on a specimen with two different bi-material notches under loading of a varying direction.

Abstract: The assessment of conditions of crack initiation in a tip of a bi-material notch composed of two orthotropic materials is dealt. The assessment of the bi-material orthotropic notch stability criteria based on standard linear elastic fracture mechanics can lead to incorrect results due to a change of fracture mechanics properties. The change of the fracture mechanics properties are taken into account in the discussed stability criterion. It is shown that the criterion of this kind can qualitatively and quantitatively influence the results, and it contributes to more reliable assessment of components with geometrical and/or material discontinuity.