Papers by Author: Pavel Hutař

Abstract: While mixed-mode loading conditions became hot topic recently, it is still quite unexplored area, even in case of metals. In case of polymers, this is even more complex problem, thanks to their higher sensitivity on higher temperatures connected with friction. Since polymer materials are very popular, this topic is of high interest. One of the best experimental specimens for achieving mixed mode loading conditions is the CRB specimen, as it provides clear mixed-mode I + III conditions under tension + torsion. Furthermore, for lifetime predictions it is necessary to calculate some material constants based on measured crack growth rates. However, these crack growth rates are not as easily and accurately measured on CRB specimens, as on CT specimens. Therefore, the main focus of this study is to observe difference between lifetime predictions based on CT and CRB data and possible application of CT data for CRB specimens.

Abstract: This paper presents a finite element analysis of a pre-cracked freestanding gold thin film subjected to bulge test. These tests were conducted in order to determine the elasto-plastic properties and fracture toughness of the gold films. For the experimental tests, a pre-crack was introduced in the center of the film by focused ion beam (FIB) milling with a length of 10 and a width of 100nm. For the numerical fracture analysis, the problem was divided into two stages; the first stage was the development of the numerical model on the whole film without pre-crack (elasto-plastic analysis) and the second one was performed on a film portion that included the pre-crack (sub-modeling stage). Three different notches (rounded, sharp and V-sharp) were applied to calculate the stress intensity factor around the crack tip using path independent J-integral. The obtained results show that the load-deflection curves for non-cracked and pre-cracked film reproduced the experiments using the calculated elasto-plastic properties. This indicates that the proposed models presented a good correlation and robustness. Additionally, fracture toughness values were calculated between 0.288 and 0.303with J-integral values 1.037 J/m² (elastic) and 1.136 J/m² (elasto-plastic) which correspond with other calculations available in the literature.

Abstract: Outstanding durability of plastic pressure and non-pressure pipes can cause difficulties, when a reasonable lifetime estimation is needed. It is impossible to prove the lifetime by testing, but there is a method of calculation that can provide a certain idea about the expected lifetime. The lifetime estimation is based on the assumption that the failure occurs as a result of the slow crack growth mechanism and it is calculated using the linear elastic fracture mechanics approach. Numerical simulations of crack growth in the pipe are necessary for this calculation. These simulations must consider various effects that can play a role in the lifetime. This paper deals with the lifetime calculations of a pressure and a non-pressure corrugated pipe considering the soil loads acting on pipes when they are buried. In the simulation of the pressure pipe, a combination of loads is applied that consists of internal pressure, residual stress and the soil loads. The influence of the loads is discussed. The non-pressure corrugated pipe is loaded by the soil loads only.

Abstract: As polymer parts are made by injection molding, most defects are usually caused by the shrinkage of material during solidification. These defects are then source of cracks in these parts. Present contribution is dealing with central cracked compression loaded polymer cylinder. In order to describe crack behavior under operational load, parametrical finite element model was developed. As results of this study stress intensity factors for central crack with different crack lengths are determined to describe crack behavior.

Abstract: Mixed mode loaded fatigue specimens are not used very frequently. The main reason is that there are still very few qualitative results, which would help with better understanding of how the mode II and mode III are affecting the overall crack propagation. On the other hand, there is considerable number of parts loaded in mixed mode. Studying fatigue failure of roller bearing elements made of polymer material was a motivation to design a new experimental specimen for study of fatigue behaviour of the material loaded in mixed mode. The contribution presents developed FEM model of such specimen with focus on determination of fracture mechanics parameters of propagating cracks.

Abstract: A micro-crack propagation in particulate ceramic based composite was studied using finite element method (FEM). Subcritical crack growth (SCG) was numerically simulated under complex load conditions (mechanical loading and loading by internal residual stresses). The effect of residual stresses on the crack propagation was studied. Two-dimensional computational model of particulate ceramic composite with material properties corresponding to low temperature co-fired ceramics (LTCC) was developed. The results indicate that the presence of residual stresses significantly reduces values of stress intensity factor in the vicinity of composite surface and the direction of residual stresses around the particles contributes to the micro-crack deflection from the particles. The time to failure of the composite under mechanical loading was determined. Results obtained contribute to a better understanding of the role of residual stresses during micro-crack propagation in ceramic particulate composites.

Abstract: A crack propagation and fracture behaviour of particulate ceramic composite were investigated. Influence of 3D shape of particles on the crack propagation was studied together with influence of the presence of residual stresses, which are developed inside the composite during manufacturing process. Finite element (FE) method was used for numerical simulation of propagating crack in the composite. Basic numerical models of low-temperature co-fired ceramics (LTCC) with alumina particles homogenously dispersed in the glass matrix were developed. Volume fraction of alumina phase was 20vol.%, which is typical amount for LTCC. The results show that existence of residual stresses retards the crack propagating under conditions of sub-critical crack growth (SCG). Presented results contribute to a better understanding of the role of residual stresses in particulate ceramic composites.

Abstract: The study deals with the interaction of creep and high cycle fatigue of cast polycrystalline nickel-based superalloy IN 713LC at high temperatures. Previous works indicated that creep lifetime of superalloy structures was un-affected or even slightly increased in the cases with superimposed vibrations. The reason for this behaviour was not well described up to now. Therefore, set of fatigue tests was conducted at high mean stresses level to observe this phenomenon. The mean stress was kept constant while the stress amplitudes were selected in order to measure wide range of conditions from pure creep to pure fatigue. Fractographic analysis by scanning electron microscopy (SEM) was done with the aim to identify governing damage mechanisms for particular test conditions as a preliminary evaluation of conducted tests.

Abstract: Railway axles are subjected to cyclic amplitude loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect in railway axle is often required. Because of different operation regimes of trains (fast/slow ride, ride on straight track, on curved track, over switches etc.) the load amplitude of axle is not constant. The variability of load is defined by a load spectrum, which is determined experimentally by measuring of load in service conditions. Even though the load spectrum is measured on several hundreds or thousands of operation kilometres, the railway axles are in operation much longer time (often tens of years). Therefore, some load amplitudes higher than ones measured in the test can occur during a long-term axle service. The contribution presented deals with the effect of extension of load spectrum by rare high load amplitudes, which can occur during long-term operation, on residual fatigue lifetime of railway axles.

Abstract: The contribution deals with the issue of residual stresses in particulate ceramic composites used in microelectronics. Residual stresses are developed in the composite due to cooling during manufacturing process. Different coefficients of thermal expansions of particles and matrix cause important residual stresses in the composite influencing their mechanical behaviour. The main aim of the paper is to determine influence of particle properties on magnitude and distribution of residual stresses in the composite matrix. Three dimensional numerical model was developed and finite element method (FEM) was used for numerical simulations. Results obtained contribute to a better understanding of residual stresses distribution and fracture processes in the studied type of composite.