Domagoj Rubeša

Finite element modelling of the electrical impulse induced fracture of a high voltage varistor

In testing and in service, varistors are subjected to very short (μs range) high current pulses. Due to the inertia effects that appear on rapid Joule heating dynamic stress waves are generated, which can cause brittle failure. An analytical solution for the one-dimensional case was presented recently by Vojta and Clarke. In this work a full three-dimensional analysis of an axisymmetrical varistor has been performed using Finite Element Simulation. The reflections of the stress waves from the bases and the shell of a varistor and their interference are analyzed. The resulting stress field and its development with time is much more complex than in the 1D case. The aspect ratio of the varistor has been shown to have a strong influence on the amplitude of the mechanical stresses and can be varied to minimize the maximum stress level reached. Damping has been considered but found to be negligible in realistic cases.

Fracture toughness, strength and thermal shock behaviour of bulk plasma sprayed alumina— effects of heat treatment

Bulk plasma spraying Al2O3 produces a material with a defect rich splat-internal microstructure and a non-equilibrium phase composition which are both subject to extensive change by heat treatment. In the present work heat treatment induced changes in the strength, fracture and thermoshock characteristics of this material have been investigated and related to microstructural development. It is found that the as-sprayed intrasplat defect structure provides low energy paths for crack propagation. This results in low toughness and strength but excellent thermoshock resistance. Heat treatment increases overall porosity, but heals the defect structure and increases splat integrity, leading to a large increase in strength, accentuated R-curve behaviour and a disproportionate increase in toughness, but also a reduction in thermoshock resistance.

Failure Analysis of Si3N4 Rolls for Wire Hot Rolling by Numerical Simulation of Thermal and Mechanical Stresses

Silicon nitride rolls for wire hot rolling have been tested in the rolling mill. After short employment for processing of ultra-high strength materials, cracks appeared in the roll calibre, which limited further application of the rolls. In order to find out possible causes for the damage, relevant mechanical properties of the roll have been determined and thermally and mechanically induced stresses were assessed by numerical simulation. It has been found that thermal stresses are of little relevance, whereas mechanically induced stresses are high enough to enhance subcritical crack growth resulting in the development of the macroscopic cracks, so that the observed damage of the rolls could have been adequately explained. One further result of the analysis was that flaws induced by grinding are decisive for the service time of ceramic rolls, so that special attention has to be paid to the machining of the roll calibre.

Main features of designing with brittle materials

Brittle material behavior and mode of failure are contrasted with those characteristics of ductile materials. The stochastic nature of brittle fracture, which results from the random occurrence of fracture-initiating microstructural imperfections, necessitates a probabilistic fracture mechanics approach to design with brittle materials. It is also clearly shown which main properties of brittle materials have to be optimized to improve the reliability of mechanically loaded components made of brittle materials. Important features of designing with brittle materials are elucidated and illustrated by an exemplary design calculation of a ceramic disc spring. It is shown how even environmentally induced subcritical crack growth, characteristic of ceramic materials, can be adequately accounted for in the assessment of reliability.

Silicon Nitride Tools for Hot Rolling of High-Alloyed Steel and Superalloy Wires

For hot rolling wires of high-alloyed steels or superalloys tools are nowadays made of ce¬mented carbides. In service they suffer from roughening of the surfaces and severe wear, which de¬teriorates the surface quality of the wires and restricts the lifetime of the tool. Due to their high hard¬ness and good high-temperature properties, improvements in tool behaviour can be expected by the use of silicon nitride tools. Experiments with several types of rollers were performed in commercial rolling mills. At modest and medium severe loaded positions (e.g. in the case of guidance rolls) silicon nitride rolls show superior performance to conventional steel or cemented carbide rolls. At the most severe loaded positions silicon nitride rolls were also superior to conventional rolls when rolling high strength steel wires. But for rolling superalloy wires, cracks, which limited further applications of the rolls, appeared in the roll surface profile (calibre). Cracks in the surface of the rollers are in general caused by Hertzian contact stresses, which can reach several hundred MPa. These cracks come into existence if a limiting load is exceeded. Then small flaws can quickly extend to a length of more then one millimetre, and then they stop again (pop in behaviour). Popped in cracks can slowly extend by cyclic fatigue up to a length where breaking out of large fragments of the rollers occurs. The critical load depends on the flow curve of the rolled materials and on the design of the rolls. For the analysed design it is exceeded when rolling superalloy wires, but it is not exceeded when rolling materials having a lower flow curve.

An analysis of application of modified Jominy-test in simulation of cold work tool steels quenching

The possibility of application of modified Jominy-test in computer simulation of quenching of cold work tool steels has been investigated. Because of high hardenability of cold work tool steels there are limits in application of original Jominy-specimen in simulation of quenching of steels. The modified Jominy-test was designed for prediction of hardenability of cold work tool steels. The characteristic cooling time, relevant for results of quenching, was predicted by computer simulation of quenching of both of JM ® -specimen and of cylindrical specimen. Modified Jominy-test can be applied in simulation of quenching of high hardenability steel more successfully than by original Jominy-test.

Die maximale Geschwindigkeit, auf die ein Körper durch eine Feder beschleunigt werden kann

ZusammenfassungFedern werden in der Technik häufig als Auswerfer verwendet, wobei oft eine möglichst hohe Auswerfgeschwindigkeit erwünscht ist. Diese Geschwindigkeit ist aber grundsätzlich begrenzt. Sie hängt einerseits vom entsprechenden Festigkeitskennwert den elastischen Eigenschaften und der Dichte des Federwerkstoffes und andererseits von der Gestalt der Feder ab. Dies wird am Beispiel zylindrischer Schraubenfedern dargelegt. Die Grenzgeschwindigkeit, auf die ein Körper durch eine Schraubenfeder beschleunigt werden kann, wird analytisch berechent. Bei gleichen Federabmessungen erweisen sich mehrere Nichtmetalle, z.B. Verbundwerkstoffe, Elastomere und verschiedene Hochleistungskeramiken als interessante Alternative zum Federstahl.

Lifetime Prediction in the Creep-Fatigue Interaction Regime Applying the SRM Rule Combined with an Appropriate Constitutive Model

The application of any conventional lifetime prediction method requires some knowledge about the response of a material to the imposed loading. The stress-strain response should preferably be computationally simulated by an appropriate constitutive model of material behaviour, instead of carrying out the corresponding tests.