Petr Skalka

Plasma-sprayed thermal barrier coatings: numerical study on damage localization and evolution

Thermal barrier coatings (TBCs) are advanced material systems used to enhance performance and in-service life of components operated at high temperatures in gas turbines and other power-generation devices. Because of complexity, numerical methods became important tools both for design of these coatings and for in-service life estimations and optimization. In this contribution, two main features that affect the TBCs’ performance, namely the roughness of the bond coat and the microstructure of the ceramic top coat, are discussed based on Finite Element Method (FEM) and Finite Element Microstructure MEshfree (FEMME) simulations that were used to calculate stresses and assess damage within the coating. Roughness data obtained from plasma-sprayed CoNiCrAlY + YSZ coated samples are supplemented to discuss assumptions and results of employed numerical models.

Applicability of the Critical Energy Release Rate for Predicting the Growth of a Crack in Nanoscale Materials Applying the Strain Gradient Elasticity Theory

The paper investigates the limits of applicability of the critical energy release rate for predicting the growth of a crack in nanoscale materials applying the strain gradient elasticity theory (SGET) capable to capture size effects, nonlocal material point interactions and surface effects in the form of (phenomenological) higher-order stress/strain gradients.

Prediction of the Critical Energy Release Rate of Nanostructured Solids Using the Laplacian Version of the Strain Gradient Elasticity Theory

The aim of the paper is quantify the material length scale parameter of the simplified form of the strain gradient elasticity theory (SGET) using first principles density-functional theory (DFT). The single material length scale parameter l is extracted from phonon-dispersions generated by DFT calculations and, for comparison, by adjusting the analytical SGET solution for the displacement field near the screw dislocation with the DFT calculations of this field. The obtained results are further used in the SGET modeling of cracked nanopanel formed by the single tungsten crystal where due to size effects and nonlocal material point interactions the classical fracture mechanics breaks down.

Influence of the Ceramic Foam Structure Irregularity on the Tensile Response

To better understand response or fracture conditions of the ceramic foam materials to the mechanical loading, a finite element (FE) analysis of these structures has to be employed. The cellular structure of foams can be modelled either using a detailed realistic FE model based on the computer tomography scans or by using of simplified, beam element based, models. Nevertheless a main drawback of the realistic foam modelling consists in its high demandingness on computational resources. Therefore, simplified models are welcome substitutions (at least for analysis of the global mechanical foam response). The regular foam structure, based e.g. on Kelvin cells, is simple from the modelling point of view, but it doesn´t exactly capture the fully random character of the real foam structures and corresponding response to the external load. Definition of the random beam foam structure (respecting the real cell shapes and their distribution within volume), can thus improve this deficiency. The main aim of this work is thus to compare these different modelling approaches and quantify the influence of the foam irregularity on the response of ceramic foams to external (tensile) loading for various model sizes.

Computational Analysis of Crack Bridging in Bioglass®-Based Porous Scaffolds by Using Polymer Coatings

The main drawback still impairing the use of bioactive glasses in load-bearing applications is their intrinsic brittleness. The addition of coating constituted by polyvinyl alcohol (PVA) and microfibrillated cellulose (MFC) PVA/MFC led to a 10 fold increase of compressive strength and a 20 fold increase of tensile strength in comparison with non-coated scaffolds. Crack bridging by polymer coating was identified by fractographic observations as a main toughening mechanism. In this contribution a detailed computational analysis of crack bridging due to coating film fibrils is presented and an improvement of fracture resistance of coated scaffolds is explained.