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The Influence of Specimen Thickness on Crack Initiation in the Tip of General Singular Stress Concentrator
Kopp, Dalibor ; Řoutil,, Ladislav (referee) ; Malíková, Lucie (referee) ; Klusák, Jan (advisor)
Geometrical discontinuities, like sharp notches, appear in constructions and engineering structures and lead to stress concentrations. These technical objects are very dangerous due to the fact that they reduce the structural conformity and can lead to crack initiation. Technical objects are not always designed as homogenous bodies but can consist of two or more materials with sharp notches on the interface of these materials. The influence of free surface on crack initiation conditions is studied and assessed by means of 3D model of sharp and bi-material notches with finite thickness. Stress fields around the singular stress concentrators are calculated with finite element method and the results are evaluated by means of criterion of critical quantity. This approach is easy applicable and can be used in combination with the knowledge of basic material properties and results of finite element analysis of the assessed notches. In order to estimate weather crack will initiate from the middle of the observed notched specimen or from its free surface, the value of averaged critical applied stress was introduced. With this value it’s possible to determine the location of crack initiation thru the sample thickness. Thru the ratio of values of critical applied stress in the middle and on the free surface of the observed specimen it’s possible to quantify the influence of the free surface on the location of crack initiation. With the use of this approach it’s shown, that the location of crack initiation depends on more parameters, loading direction, the notch opening angle and the sample thickness. In case of bi-material notches it depends also on the ratio of young modulus.
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Application of pedestrian evacuation and fire spreading models for specific buildings safety improvement
Okřinová, Petra ; Hromada, Martin (referee) ; Dvořák, Zdeněk (referee) ; Apeltauer, Tomáš (advisor)
The current trend in the construction industry is construction “without limits”. In projects and implementations, we encounter non-traditional combinations of operational solutions, geometry and materials, which are still approached mainly using the classic standards from the fire safety point of view. The latter does not allow to comprehensively cover all aspects of these specific buildings without a greater degree of subjective simplification. This raises the question of whether, even in these cases, it is still appropriate and safe to use only a standards approach, or whether buildings deviating from traditional construction should be assessed differently – for example, by so-called fire analysis using numerical models. Currently, users still have very little information about the use of these models, and even in the Czech legislation, there is no document recommending how to proceed when using them for the purposes of building fire safety solutions. Numerical models of evacuation and fire analysis are thus currently the most effective tool for impact analysis and preliminary verification of the effectiveness of proposed safety measures. Therefore, it is necessary to deal with their validation and use, with the goal of innovating existing approaches and standardizing numerical models for fire safety assessment of buildings. Furthermore, it is necessary to compare their possibilities and limitations concerning ČSN and to put together a comprehensive information background as a basis for their more active implementation in practice. The presented work thus focuses on the above-mentioned issue and demonstrates its use of application examples of real specific buildings and a set of compiled studies. The work also points out specific shortcomings and simplifications of standard approach methods, based on which verified procedures and recommendations for the incorporation and use of numerical models in fire safety practice are then established.
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The Influence of Specimen Thickness on Crack Initiation in the Tip of General Singular Stress Concentrator
Kopp, Dalibor ; Řoutil,, Ladislav (referee) ; Malíková, Lucie (referee) ; Klusák, Jan (advisor)
Geometrical discontinuities, like sharp notches, appear in constructions and engineering structures and lead to stress concentrations. These technical objects are very dangerous due to the fact that they reduce the structural conformity and can lead to crack initiation. Technical objects are not always designed as homogenous bodies but can consist of two or more materials with sharp notches on the interface of these materials. The influence of free surface on crack initiation conditions is studied and assessed by means of 3D model of sharp and bi-material notches with finite thickness. Stress fields around the singular stress concentrators are calculated with finite element method and the results are evaluated by means of criterion of critical quantity. This approach is easy applicable and can be used in combination with the knowledge of basic material properties and results of finite element analysis of the assessed notches. In order to estimate weather crack will initiate from the middle of the observed notched specimen or from its free surface, the value of averaged critical applied stress was introduced. With this value it’s possible to determine the location of crack initiation thru the sample thickness. Thru the ratio of values of critical applied stress in the middle and on the free surface of the observed specimen it’s possible to quantify the influence of the free surface on the location of crack initiation. With the use of this approach it’s shown, that the location of crack initiation depends on more parameters, loading direction, the notch opening angle and the sample thickness. In case of bi-material notches it depends also on the ratio of young modulus.
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