Papers by Author: Harald Harmuth

Abstract: The work presented here deals with simulation assisted evaluation of fracture testing of ordinary ceramic refractory materials. Two tests are applied. One of them, a wedge splitting test, is already established for this purpose. An inverse evaluation procedure was developed to derive more information from the test results: It enables the simultaneous determination of the specific fracture energy, the tensile strength and the Young’s modulus. Moreover specific fracture energy can also be determined in the case that the test has to be interrupted at some residual load due to relatively low material brittleness. The other test method, a laser irradiation disc test, was developed in order to determine specific fracture energy and tensile strength for fine ceramic refractory materials behaving relatively brittle. From the time elapsed until crack initiation occurs (t₁) and a stable/instable transition of crack propagation takes place (t₂), respectively, the tensile strength and the specific fracture energy are calculated based on a simulation of the mode I fracture behavior which applies the fictitious crack model according to Hillerborg.

Abstract: Simulations presented here aim to show reasons for undesirable product heterogeneity and possible defects. Potential negative influences are for example insufficient deaeration, friction between the bulk material and the mould, deformation of the mould and an inappropriate mould filling. In a first step data were collected from a production process. This includes the load and displacement during moulding and the mass of the mix filled into the mould. For the Finite Element modelling with ABAQUS the brick material was simulated with a Drucker-Prager model including a cap. The results show the influence of the friction between mix and mould on the densities and stresses in the pressed blank.

Abstract: The present work aims the calculation of the mass transfer coefficient in the slag bath area of a gas-stirred steel ladle. The mass transfer in dependence on influencing factors like flow rate and velocity of purging gas was investigated. For this purpose computational fluid dynamics calculations were performed. Due to the fact that during refractory corrosion diffusion-controlled mechanisms take place simulations considering diffusion were performed. From the resultant concentration of the diffusive component in the slag in dependence on time the mass flux is known. With the help of Fick´s 1st law that describes the diffusive mass flux the mass transfer coefficient may be calculated. From a dimensional analysis followed by a parameter study conducted with the help of computational fluid dynamics calculations a relation for the mass transfer in the slag bath area of the gas-stirred steel ladle was deduced.

Abstract: The denomination ‘flexible’ is chosen in the professional jargon of refractories technology for materials able to bear relatively high strains without or with acceptable loss of strength. In many cases this term is equivalent to relatively low brittleness. Characterisation of brittleness based on fracture mechanical investigations may use figures of merit like brittleness numbers, a so called characteristic length or the R’’’’ parameter according to Hasselman. In many cases these figures show that brittleness reduction of refractories is achieved by decrease of strength with at the same time more or less unaffected specific fracture energy. Microscopic investigations of fracture paths aim to exhibit which peculiarities of crack microstructure enable this change of mechanical properties. A microscopical technique developed for this purpose separately evaluates the relative crack lengths along the grain/matrix interface, within the matrix and within the grain. Results obtained for several types of refractories show brittleness decrease is associated by an increase of the relative crack length along the grain/matrix interface and a decrease of transgranular fracture. Prefabricated microcracks and a relatively low grain/matrix bond strength may support this type of crack propagation.

Abstract: A fractographic procedure was developed and applied for ordinary ceramic refractory materials with a rather large size of heterogeneities and defects. It is based on a stereooptical method for generation of digital surface profiles which are evaluated by an autocorrelation function. Furthermore, a lateral correlation length is derived. A group of seven refractory materials was characterised by mechanical and fracture mechanical investigations, and the same specimens had been characterized by the fractographic procedure. Correlations have been tested. The results show a relation between the lateral correlation length and two fracture mechanical characteristics which are significant for the material brittleness and the elastic strain energy stored at maximum load. These relations are contributed to the dependence of the crack path on brittleness. With decreasing brittleness the amount of the crack path proceeding along the grain/matrix boundary increases for the materials investigated.