Zdeněk Chlup

Strength, elasticity and failure of composites with pyrolyzed matrices based on polymethylsiloxane resins with optimized ratio of D and T components

Two mixtures of T and D siloxane monomeric components labelled as TxDy (molecular ratio x:y equal 3:1 or 4:1) were chosen as matrix precursors for manufacturing Nextel720 reinforced unidirectional composites which, after pyrolysis at 1000 or 1100°C, revealed good endurance in an oxidizing environment up to 1500°C. Vickers hardness of the heat treated (1000–1500°C) samples of pyrolyzed matrices T3D1 and T4D1 are mutually similar (1100–1400 HV0.2) and reach their maximum between 1200–1300°C. Flexural strength of the pyrolyzed composites is 150–170 MPa and 170–250 MPa for T3D1 and T4D1, respectively. After annealing 3 h in air at 1200–1300°C, the strength slightly decreases but similar treatment at 1500°C yields strengths exceeding those of the pyrolyzed material. Shear modulus of the pyrolyzed T4D1 composite is roughly twice that of the T3D1 one (15 GPa vs. 8 GPa) and both increase sharply to 22–25 GPa after annealing at 1500°C, which manifests substantial improvement of the matrix properties. Fracture toughness of the composites, as measured by chevron notch test at RT, 550°C, and 1100°C, yields 4–5 MPa.m−1/2 for T3D1 and 3–4 MPa.m−1/2 for T4D1. For both composite types, the fracture toughness drops by 1 MPa.m−1/2 when measured at 550°C, which can be attributed to suppression of fibre pull-out due to stress state changes caused by the coefficient of thermal expansion (CTE) mismatch. Fracture surfaces generated during flexural tests of the annealed samples reveal decreasing occurrence of pullout towards the highest annealing temperature.

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.

Micromechanical Aspects of Constraint Effect at Brittle Fracture Initiation

Applying the two-parameter fracture mechanics approach to the analysis of failure initiation condition of the three point bend specimens with shallow and deep cracks were tested at various temperatures. Low carbon manganese cast steel was used for the analysis. This steel is tested as one of several candidate materials for containers of spent nuclear fuel. The effect of crack length on the fracture toughness-temperature diagram has been analysed. Although a strong dependence of measured fracture toughness on crack tip constraint was observed no evident differences in fracture morphology have been identified except for quantitative ones. Peculiarities of fracture behaviour in the transition and lower shelf regions of the steel investigated have been explained. The effect of crack tip constraint on brittle fracture characteristics has been quantified by means of the Q-parameter. The role of critical (cleavage) fracture stress in brittle fracture initiation under the influence of crack tip constraint has been analysed

Alumina and Zirconia Based Composites: Part 1 Preparation

Electrophoretic deposition of Al2O3 and ZrO2 powders from isopropanol suspension in the presence of monochloroacetic acid under constant-current conditions was studied. The similar charge and electrophoretic mobility of Al2O3 and ZrO2 in the suspensions was found. Adjusting to properly controlled kinetics of deposition deposits were prepared of pre-defined thicknesses. In view of the negative charge of Al2O3 and ZrO2 particles in the isopropanol suspensions used, the prepared layers were deposited on the anode and thus they were not affected by possible solvent electrolysis, which contributed to their defect-free and low-porosity structure.

Thermal Shock Resistance of Cordierite-Mullite Refractory Composites

The design of composite materials leads to the development of multi-component systems where each constituent has a specific function in the material, from technological and/or application points of view. Examples of such composite systems are the cordierite-mullite refractory materials investigated in this contribution. Two different commercially available compositions were selected for evaluation of the influence of microstructure on fracture behaviour under thermal shock conditions. The materials were exposed to water-quench tests from 1250°C and subsequently the fracture toughness was evaluated using the chevron notched specimen technique. The results were compared to those obtained on as-received materials. Microstructural damage was also studied applying fractographic techniques with the aim to gain knowledge on the thermal shock damage mechanisms acting in the materials.

Fracture Resistance of Cast Ferritic C-MN Steel for Container of Spent Nuclear Fuel

Fracture resistance of cast ferritic steel predetermined for containers of spent nuclear fuel has been evaluated based on sets of different fracture mechanical test specimens and assessment procedures. Standard fracture toughness values were determined from IT SENB specimens and compared with data from pre-cracked CVN specimens (P-CVN). The other parameters that have been in focus of interests were: the effect of metallurgical technology (two melts followed), specimen location (midthickness vs. surface locations) in thick walled plate, the effect of loading rate (here followed on CVN and P-CVN specimens), statistical effects etc

Control of Electrophoretic Deposition Kinetics for Preparation of Laminated Alumina/Zirconia Ceramic Composites

Electrophoretic deposition of Al2O3 and ZrO2 powders from isopropanol suspension in the presence of monochloroacetic acid under constant-current conditions was studied. The deposits were prepared under different electrical conditions of deposition. It was found that the deposits prepared at low current densities (500 µA) contained smaller pores and achieved higher green densities than deposits prepared at high current densities (20 mA). The deposits prepared at low current densities were formed just by part of the particles in the suspension because of decreasing of efficiency of electrophoretic deposition process.

Effect of hybrid polymer coating of Bioglass® foams on mechanical response during tensile loading

A simplified two-dimensional finite elements model was created for a polyvinyl alcohol (PVA) coated Bioglass® strut undergoing tensile stresses (loading mode I). The strengthening contributions due to the infiltration of coating into surface cracks and coatings stiffness were evaluated in terms of stress intensity factor KI and tensile stresses σyy in the proximity of the crack tip. The infiltration of the coating until the crack tip resulted as the most effective criterion for the struts strengthening. Bioglass® based scaffolds were dip coated into PVA and PVA/microfibrillated cellulose (MFC) aqueous solutions and tested in tensile load. Coated samples exhibited remarkably higher tensile strength than non-coated ones, which further raised with the increased amount of MFC. Contact angle θ and linear viscosity η measurements of PVA/MFC solutions showed that MFC caused a reduction in θ and a drastic increase in η, indicating that a balance between these two effects must be achieved.

Role of Pyrolysis Conditions on Fracture Behaviour of Fibre Reinforced Composites

Fracture response of matrix prepared by pyrolysis of polysiloxane resin used for composite reinforced by long fibres was the main goal of this contribution. A set of composites with matrix prepared by partial pyrolysis of polysiloxane resin was studied. An effect of pyrolysis temperature on the composite behaviour and fracture resistance was monitored. An optimal procedure of pyrolysis was established. Heat treatment at 1550°C in air atmosphere was conducted on fully pyrolysed matrix to explore its high temperature potential. Determination of reliable parameters characterising microstructural changes in the matrix by instrumented indentation technique was used. Both optical and scanning electron microscopy was employed in microstructural observations and fracture mechanism qualification. Observation of indents and associated cracking caused by microstructural changes as well as 3D surface reconstruction using confocal microscopy was employed.