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The Weibull Stress Parameters Calibration upon the Toughness Scaling Model between Cracks Having Different Constraint
Kozák, Vladislav ; Vlček, Libor
An extensive investigation has been carried out on the sensitivity parameters determination describing the fracture behaviour of body with crack with respect to the character change of true stress-strain curve with dominant region of Lueders deformation.Consequently,its convenient substitution has been tested.This paper presents the consideration on the change judgement of J-integral and constraint as the base parameters of two-parameters fracture mechanics.The Weibull stress model for cleavage fracture of cast steel requires calibration of two micromechanics parameters (m,ó u).Weibull stress seems to be a parameter for prediction of cleavage failure of cracked bodies and the study is focused on the assessment of the effects of constraint loss on cleavage fracture toughness (Jc).To quantify the effects of constraint variation on the cleavage fracture toughness the form of the toughness-scaling model based on the Weibull stress ó w is investigated.Local material parameters have been calculated arising from Beremin approach and calibration is based on various approaches.Methods are based on weakest link assumption and incremental fracture probability,which depends not only on the maximum principal stress,but also on the equivalent plastic strain.The fracture resistance has been assessed using data from static tests of there point bend specimens.
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Micromechanical aspects of constraint effect at brittle fracture initiation
Chlup, Zdeněk ; Dlouhý, Ivo
Cast manganese steel predetermined for container casks of spent nuclear fuel has been tested extensively from the point of view of resistance against brittle fracture. For the fracture toughness evaluation the three-point bend specimens with different crack length (varying a/W ratio > 0,5; 0,2; 0,1) and with the same ligament below the crack tip have been used. Stress distributions below crack tip have been calculated by finite elements method (ABAQUS code) for each specimen geometry separately. Modification has been introduced into calculation taking into account specific material behaviour.
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Analýza algoritmů výpočtu T napětí s ohledem na vliv vyšších členů Williamsova rozvoje
Gröger, Roman
A comparison between a couple of difference methods for quantification of T stress is presented. The stress distribution around a crack tip is expressed using the first four terms of Williams eigenfunction expansion for specimens corresponding to low constraint (SECT) as well as three-point bending specimens with high constraint. The dominant regions are quantified by accounting the different number of stress terms. An alternative method for calculation of T stress using the conservation integrals is used to compare the obtained results with the products of more accurate integral method. The appropriate ratios of r/a, on which the difference methods give the reasonable results, are outlined in conclusion.
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Two-parameter characterisation of a crack growing in the vicinity of the material interface
Gröger, Roman ; Knésl, Zdeněk
A crack propagating in the vicinity of the material interface is analyzed. The problem is solved in the range of two-parameter linear elastic fracture mechanics. The aim of the contribution is to assess the influence of T stress on the direction of a crack growing to the interface. The interface is modelled for different material properties characterised by the Dundur's composite parameters. First the effect of the interface on T stress value is studied and values of KI, KII and T stress are estimated. Consequently the two-parameter modification of MTS criterion and Sih's SED criterion is used for prediction of a crack propagation direction. Both these approaches are compared. It is concluded that the negative values of T stress stabilize the crack propagation perpendicularly to the interface.
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Management tools utilization for disclosing firm's constraints
Grégr, Jiří ; Chocholatý, Drahomír (advisor) ; Volráb, Ondřej (referee)
This diploma thesis is focused on the use of management tools and practices specialized in finding organizations constraints that limit their performance and make it impossible to effectively achieve their goal. The tools can be divided into static ones that describe relationships between causes and their effects using causal relations and dynamic ones that are oriented on cyclical relations that appear in periods. Although those dynamic tools are not suitable enough for validating causal relations they can better visualize these relations in time. Both of the tools are based on the principle that the user has to change the way of thinking when looking for undesirable effects in the system. The theoretical part of the thesis is extended by Greiner growth model that describes growth phases of organizations and typical problems that appear in each phase. Thus this model can be used to help uncover causes of organizations constraints. Practical part of the thesis is focused on application of the tools and on finding the main constraint in a small software company. It is divided into a few parts: company description, proposal of requirement collection and its implementation, description of relationships between the company and external systems, proposal of an improvement of a notation that is used for diagramming current state of problems in a company, diagramming the current reality tree and finding the main constraint. The contribution of the thesis lies in depicting advantages and disadvantages of the tools used for finding company constraints and in application of one of the tools including the proposal of improvement. The conclusion includes recommendations for improvement implementation that help to eliminate main company constraint and increase the system performance.
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