Vladislav Kozák

Fatigue crack initiation in fibre-metal laminate glare 2

Fatigue crack initiation in notched plates made of fibre-metal laminate GLARE 2 was studied under constant stress amplitude loading using light and scanning electron microscope. Fatigue crack initiation mechanism observed in notched laminates was found different from the mechanism typical for monolithic materials. Stress and plastic strain finite element calculation was used to discuss the anomalous fatigue crack initiation in fibre-metal laminates.

Constraint effects at brittle fracture initiation in a cast ferritic steel

Abstract The fracture resistance of a cast low carbon manganese ferritic steel intended for containers for spent nuclear fuel has been analysed by combining several approaches. Based on data from three-point bend specimens with shallow and deep cracks the effect of crack tip constraint at brittle fracture initiation has been followed. Q-parameter was used for the constraint quantification. The crack length effect on the fracture toughness–temperature diagram has been analysed and peculiarities of fracture behaviour in the lower shelf region have been explained. The role of cleavage fracture stress in brittle fracture initiation under the influence of crack tip constraint has been analysed.

Properties of modified polysiloxane based ceramic matrix for long fibre reinforced composite materials

Abstract The main goal of the work was to prepare a cost effective and simple to preform high temperature matrix for composite materials. To fulfil expectations, it was necessary to optimise the design of the composite to have an optimal fibre–matrix interaction. A number of modified polysiloxane resins were studied in various steps of heat treatment. This contribution deals with changes in the behaviour of the matrix as a stay alone material. This knowledge enables the optimisation of composite properties. A fully instrumented indentation technique for the determination of reliable parameters characterising the microstructural changes was used. The fracture behaviour of the prepared composite matrixes was evaluated in terms of indentation cracks. Both optical and scanning electron microscopies were employed in microstructural observations and fracture mechanism qualification.

Shear creep testing of copper single crystals

Abstract The work tests an application of a single-shear creep technique of double-notch specimens of Dupeux and Rouault-Rogez (in Strength of Materials., edited by H. Oikawa et al., The Japan Inst. of Metals, 1994, pp. 681–685) to copper single crystals. Direct measurement of the specimen dimensions, finite element computation and surface metallography confirm that the strain is strongly localized in the sheared zone and the shear strain is pronounced. A creep test of a double-notch specimen is compared with a usual compression creep test. Observation of the structure identifies slip lines of compact and of non-compact slip planes and a complicated dislocation structure with a high dislocation density.

Method of Threshold Stress Determination for a Local Approach to Cleavage Fracture

The contribution is focused on a new methodology description for determination of threshold stress, as the third parameter in Beremin local approach to cleavage fracture that is using three-parameter Weibull statistics. Nature of the methodology lies in tensile testing of rounded notched specimens at liquid nitrogen temperature and corresponding calculations. Reactor pressure vessel steel was chosen as an example for the illustration.

Micro- and Macro-Scale Thermomechanical Modelling of Bulk Deformation in Early-Age Cement-Based Materials

Final user properties of concrete and other silicate composite structures are conditioned by their early-age history. Bulk deformation in early-age cement-based material system, not subjected to external forces, but generating significant stresses and temperature and humidity redistributions, is associated with the internal volume reduction of cement/water mixture during the hydration process. The proposed experimentally supported computational model starts at the micro-scale balance of mass, momentum and energy to and comes to the macroscopic mixture analysis with (at most) 20 primary variable fields, taking into account full coupling between mechanic strains and stresses, thermal and moisture transfer and chemical phenomena, including phase changes. The evolution of all variables is driven by kinetics of cement hydration, characterized by hydration degree as a function of chemical affinity.

Modelling of Fibre-Matrix Interface of Brittle Matrix Long Fibre Composite by Application of Cohesive Zone Method

Ceramic matrix composites reinforced by unidirectional long ceramic fibre are very perspective materials. The only disadvantage of such materials is relatively high brittleness at room temperature. The main micromechanism acting as toughening mechanism is the pull out. There are other mechanisms as crack bridging, crack deflection etc. but the primer mechanism is mentioned pull out which is governed by interface between fibre and matrix. The contribution shows a way how to predict and/or optimise behaviour of composite by application of cohesive zone method using the FEM numerical package Abaqus. The presented results from numerical calculations are compared with experimental data.

Fracture Behaviour of TiAl Intermetalics

The role of microstructural parameters in fracture behaviour of TiAl intermetalics at room and increased temperatures will be in focus of interest. Based on experimental findings sampled in this field and arising from literature knowledge an overview of typical fracture micromechanisms acting in different microstructures of TiAl intermetalics will be presented. Where possible semi-quantitative evaluation of toughening effects will be presented. Tensile properties, flexural strength and fracture toughness have been evaluated mainly for Ti-40Al-2Cr-2Nb-1B and Ti-46Al-0.7Cr-0.1Si-7Nb-0.2Ni alloys. For fracture toughness determination chevron-notch specimen technique have been applied except for standard pre-cracked samples. Positive effect of Nb on fracture resistance has been found; fracture toughness of the high Nb alloy increased at contemporary increase of flexural strength. Fracture surfaces have been evaluated using scanning electron microscopy showing the key role of mechanical twinning in deformation of both alloys followed. As a governing mechanism of crack nucleation microcrack formation at boundaries between mechanical twins and γ grains has been confirmed.

Parameters identification for GTN model and their verification on 42CrMo4 steel

The base of this paper is exact measurement of deformation and fracture material characteristics in laboratory, evaluation of these parameters and their application in models of finite element analysis modelling the fracture behaviour of components with defects. The base of the work is dealing with ductile fracture of forget steel 42CrMo4. R-curve is modelled by 3D FEM using WARP3D and Abaqus. Crack extension is simulated in sense of element extinction algorithms. Determination of micro-mechanical parameters is based on combination of tensile tests and microscopic observation. Input parameters for the next computation and simulation were received on the base of image analysis, namely fN and fo. The possibility of transferring these parameters to another specimen is discussed.

The Transferability Of Fracture Toughness Characteristics From Point A View Of The Integrity Of Components With Defects

In this work, the use of the Weibull stress as a measure of the failure probability of cracked body is tested. Fracture of large engineering structures and conventional safety assessment and integrity of components and structures still remains a top field of research in the experimental and the theoretical fracture mechanics. Weibull stress seems to be a parameter for prediction of cleavage failure of cracked bodies and the study is focused to assess the effects of constraint loss on cleavage fracture toughness (J,). To quantify the relative effects of constraint variation on the cleavage fracture toughness the form of the toughness-scaling model based on the Weibull stress o, is investigated. Local material parameters have been calculated arising from Beremin approach. It is 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. Accepting this approach to the analysis of local criteria for cleavage fracture the location G,, and shape parameters m were calculated using FEM for notched tensile bars having various type of geometry. The aim of the paper can be seen in fracture toughness correction from various specimen geometries to small scale yielding (SSY). The fracture resistance has been assessed using data from static tests of the bend specimens and from the axisymmetric notched tensile specimens. The standard finite element method package Abaqus was applied and the manganese cast steel considered for storage and transport container for spent nuclear fuel (SKODA) was selected as an experimental material.