Ivo Dlouhý

Toughened and machinable glass matrix composites reinforced with graphene and graphene-oxide nano platelets

Abstract The processing conditions for preparing well dispersed silica–graphene nanoplatelets and silica–graphene oxide nanoplatelets (GONP) composites were optimized using powder and colloidal processing routes. Fully dense silica–GONP composites with up to 2.5 vol% loading were consolidated using spark plasma sintering. The GONP aligned perpendicularly to the applied pressure during sintering. The fracture toughness of the composites increased linearly with increasing concentration of GONP and reached a value of ∼0.9 MPa m1/2 for 2.5 vol% loading. Various toughening mechanisms including GONP necking, GONP pull-out, crack bridging, crack deflection and crack branching were observed. GONP decreased the hardness and brittleness index (BI) of the composites by ∼30 and ∼50% respectively. The decrease in BI makes silica–GONP composites machinable compared to pure silica. When compared to silica–Carbon nanotube composites, silica–GONP composites show better process-ability and enhanced mechanical properties.

Reliability of the chevron-notch technique for fracture toughness determination in glass

The applicability of the chevron-notch (CN) three-point bend test fordetermining the fracture toughness (KIc) of borosilicate glass was studied. As-receivedspecimens with straight sharp notch were also tested in three-point bending for comparisonpurposes. It was found that both chevron and straight notched specimen geometries wereequally suitable for the fracture toughness determination in terms of average values anddata variability according to Weibull and Student’s t statistics. Testing with CN geometryshowed that annealing of specimens at a temperature (550oC) higher than the glasstransformation temperature led to a lower value of KIc. The difference in mean fracturetoughness values (0.75 and 0.64 MPam1/2 before and after annealing, respectively), wasascribed to the different level of compressive residual stresses in the samples.

Transferability of fracture mechanical characteristics

Preface. Address List of Authors. SKODA Cast Testing and Licensing Requirements J. Brynda, et al. Mechanical Properties of Cast Mn Steel after Intercritical Heat Treatment and Microalloying L. Kraus, et al. IAEA Requirements for Cast Testing and Short Summary of Developments at GNB H.P. Winkler. Fracture Resistance of Cast Ferritic C-Mn Steel for Container of Spent Nuclear Fuel I. Dlouhy, et al. Micromechanical Aspects of Constraint Effect at Brittle Fracture Initiation Z. Chlup, I. Dlouhy. Problems of Q-parameter Calculation L. Vlcek, et al. Construction of J-Q Locus for Material of Reactor Pressure Vessel WWER 440 in the Ductile-Brittle Transition Region D. Lauerova, et al. The Use of the Local Approach for Brittle Fracture Prediction V. Kozak, A. Janik. On the Application of the Beremin Model for Predicting the Brittle Fracture Resistance G.B. Lenkey, et al. Physical Fundamentals of Local Approach to Analysis of Cleavage Fracture S. Kotrechko. Probability of Brittle Fracture in Low Alloy Steels B. Strnadel, I. Dlouhy. Damage Mechanisms and Local Approach to Fracture C. Berdin, P. Hausild. Damage Mechanisms and Local Approach to fracture Part II: Brittle fracture prediction in the ductile to brittle transition P. Hausild, C. Berdin. Toughness Scaling Model Applications I. Dlouhy, et al. Relation of Fracture Energy Measured on Sub-sized Charpy Specimens to Standard Charpy and Fracture Toughness H.J. Schindler, P. Bertschinger. Master Curve Methodology and Data Transfer from Small on Standard Specimens M. Holzmann, et al. Master Curve Validity for Dynamic Fracture Toughness Characteristics I. Dlouhy, et al. Evaluation of Strain Rate Effects on Transition Behaviour by Applying the Master Curve Methodology J. Kohout, et al. Dynamic FractureToughness Determination of Large SENB Specimens G.B. Lenkey. Fracture Toughness Behaviour of Thick-Walled Nodular Cast Iron at Elevated Loading Rates K. Muller, et al. Finite Element Modelling of Charpy Impact Testing G.B. Lenkey, et al. Notch Sensitivity Analysis on Fracture Toughness G. Pluvinage, A. Dhiab. Quantification of Notch Effects -- in Brittle-Ductile Transition L. Toth. Brittle Mixed Mode Fracture I+II: Emanating From Notches. -- Equivalent Notch Stress Intensity Factor --H. El Minor, et al. Influence of Thermal Stresses on Operational Life of NPV welds S. Vejvoda, et al. Index.

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.

Boron nitride nanosheets reinforced glass matrix composites

The effect of reinforcing boron nitride nanosheets (BNNSs) on the mechanical properties of an amorphous borosilicate glass (BS) matrix was studied. The BNNSs were prepared using liquid exfoliation method and characterised by transmission electron microscopy, scanning electron microscopy and X-ray diffraction (XRD) analysis. The average length was ∼0.5 μm, and thickness of the nanosheets was between 4 and 30 layers. These BNNSs were used to prepare BS-BNNS composite with different loading concentrations of 1, 2.5 and 5 mass-% (i.e. 1.395, 3.705 and 7.32 vol.-%). Spark plasma sintering (SPS) was used to densify these composites to avoid structural damages to the BNNSs and/or crystallisation within the composite sample during high temperature processing. The BNNSs were found to be evenly distributed in the composites matrix and were found to be aligned in an orientation perpendicular to the direction of the applied force in SPS. The mechanical properties including fracture toughness, flexural strength and elastic modulus were measured. Both fracture toughness and flexural strength increased linearly with increasing concentration of BNNSs in BS glass. There was an enhancement of ∼45% in the fracture toughness (1.10 MPa.m1/2) as well as flexural strength (118.82 MPa) with the addition of only 5 mass-% loading of BNNSs compared to BS glass (0.76 MPa.m1/2; 82.16 MPa). The toughening mechanisms developed in the composites because of the reinforcement of BNNSs were thoroughly investigated.

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

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 Toughness and Work of Fracture of SiC-Fibre Reinforced Glass Matrix Composite

The applicability of the chevron-notch technique for changes monitoring in fracture behaviour of fibre/glass matrix composites has been investigated. A commercial SiC-fibre reinforced glass matrix composite was aged in argon at temperatures in the range 500–700 °C for duration of up to 1000 hrs. The mechanical properties of aged samples were evaluated at room temperature by using four-point flexure strength and three-point flexure chevron notch techniques. For identification of the unstable fracture onset an acoustic emission technique was applied. The fracture toughness values in the range 19–26 MPam1/2 were little affected by the ageing conditions except for the most severe ageing conditions. The procedure for the work of fracture determination and its dependence on ageing condition is presented.

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

The role of microstructure in brittle fracture behaviour of low alloy tempered bainitic steel

The validity of various micromechanistic models for the cleavage initiation in tempered bainite have been tested using two quite different model bainitic microstructures. The microscopic cleavage fracture stress (CFS) was measured for both microstructures using Charpy V-notch type specimens. Various micromechanistic models were applied to experimental results to explain the metallurgical nature of CFS. The validity and applicability of these models for a particular bainitic microstructure have been discussed.