Jakob Kübler

Crack bifurcation features in laminar specimens with fixed total thickness

A method to analyze the crack bifurcation behavior of symmetric two-component layered composites had been developed. With the method it is possible to compute the layer thickness at which cracks bifurcate. The thickness necessary for cracks to bifurcate depends on the elastic constants of the layers, the number of layers, the difference of thermal expansion coefficients and the temperature gradient. Further, the thickness ratios at which crack bifurcation can occur for layers with tensile and compressive residual stresses were computed. In the work the important case of specimens with fixed total thickness is considered. There are some features of crack bifurcation under these conditions. # 2002 Elsevier Science Ltd. All rights reserved.

Failure Behaviour of High Toughness Multi-Layer Si3N4 and Si3N4-TiN Based Laminates

Multi-layer laminates were produced using alternating layers of Si3N4 and Si3N4+TiN. The differences in the coefficient of thermal expansions between the alternating layers lead to residual stresses after cooling. These are compressive in the Si3N4 layers and tensile in the Si3N4+TiN layers. The existence of these stresses in the laminates effect the crack propagation behaviour during failure. Different designs of laminates were produced with external layers under compression and tension exhibiting different failure mechanisms. Facture toughness was measured by SEVNB method. In systems with external layers under compression the measured fracture toughness was up to three times that of Si3N4, i.e. up to 17 MPa m1/2. In systems with external layers under tension during failure the energy absorbing effects of crack deflection and crack bifurcation were obtained. High temperature tests were performed to determine the onset temperature for residual stresses in these laminates. Micro-laminates with compressive layers of only 30 µm thickness with high strength and fracture toughness and were manufactured.

Jacket Material Evaluation for Net’s Wind-and-React Superconductor

The inner poloidal field coils of the Next European Torus (NET) are planned to be built with a Nb3Sn cable-in-conduit conductor. To meet the severe degradation in the superconductivity of Nb3Sn due to strain effects the coils will be produced with the wind-and-react technique. High service loads together with the need for good ageability and weldability limit the number of possible jacket material candidates.

Influence of Aging on the Fracture Toughness of Cryogenic Austenitic Materials, Evaluated by a Simple Test Method

The fracture toughness of heat treated austenitic stainless steel is measured. The data collected using a simple dynamic bend test at 77 K are compared and scaled with values measured on CT specimens at 4 K. It shows that the toughness depends strongly on the temperature and time of the heat treatment and that — as long as there is no need for values according to certain standards — expensive and time consuming 4 K tests on CT specimens can be substituted. The data are presented in three dimensional graphs.

Structural Micro-Layered Ceramics with Surfaces under Tension and Compression with Increasing Apparent Fracture Toughness

Two different designs of high fracture toughness micro-laminate ceramics were produced containing 50 μm thick Si3N4 layers and 100 μm thick Si3N4 + TiN layers. The first design with external tensile layers had a predicted maximum apparent fracture toughness of 10.5 MPa m1/2. The second design with external compressive layers had a predicted maximum apparent fracture toughness of 18.0 MPa m1/2. The fracture toughness of these micro-laminates was tested by the SEVNB method. A stiff testing machine was used to measure the R-curve behavior by observing crack growth in single notched specimens. A soft testing machine was used to measure the R-curve behavior using several specimens with notches at different depths.

Multi-Layer Silicon Nitride Laminates Exhibiting High Fracture Toughness and Crack Deflection

Si3N4-TiN based multi-layer ceramics laminates have been produced. With external compressive layers, laminates with a three-fold increase in KIc over the monolithic ceramics have been realised. When external tensile layers are used in conjunction with thin internal compressive layers, energy absorbing crack deflection and bifurcation processes are observed.

Development of Ceramic Composites for Industrial Wood-Cutting Tools

Industrial wood cutting is a highly demanding application for cutting tool materials as a wedge angle of ~50°, a tip radius in the 1 m range, and high wear, temperature and corrosion resistance are needed. For this application Si3N4 based ceramic matrix composites (CMC) were developed. Cutting tests showed that reinforcing Si3N4 with 30 wt.% SiC gives a good balance between fracture toughness and wear resistance. The use of an yttria / lanthana sintering aid system resulted in a fine-grained microstructure without degrading the fracture toughness. Post heat treatment was essential for the integrity of the cutting edge. Finally, cutting tests proved that the CMC cutting tips had a 3-fold lifetime compared to tungsten carbide.

Analysis of Layered Composites With Crack Deflection Controlled by Layer Thickness

A method to analyse the crack deflection behaviour of symmetric two-component layered composites had been developed. With the method it is possible to compute the layer thickness at which cracks bifurcate. The thickness necessary for cracks to bifurcate depends on the elastic constants of the layers, the number of layers, the difference of thermal expansion coefficients and the temperature gradient. Further, the thickness ratios at which crack bifurcation can occur for layers with tensile and compressive residual stresses were computed.

European Standardization Activities for Advanced Technical Ceramics

A summary of the progress being made in Europe towards the provision of standards for advanced technical ceramics is given. Under CEN Technical Committee 184, working groups have been preparing more than 40 standards in the areas of classification, powders, monolithic ceramics, long-fibre composites and ceramic coatings. The target date for this work was the end of 1992. Considerable progress has been made, and by the end of 1993, most of these should have been published. It is hoped that the formation of this standards base will improve the quality of testing and the reliability of property data on this class of materials, and thus improve confidence in their use as engineering materials.Copyright

Oxidation and Corrosion of Silicon Nitride at 1200°C and 1500°C

The effect of different sintering additives on the high temperature oxidation and corrosion behaviour of Si3N4 ceramics is investigated. A corrosion furnace was set up which allows testing of ceramic discs at test temperatures upto 1500°C. It was possible to conduct tests under ambient oxygen conditions, or with increased water vapour as well as testing with corrosive gases such as HCl or SO2. Si3N4 was prepared with MgO, Al2O3, Y2O3 and Al2O3+Y2O3 sintering additives. These discs were subjected to different oxidation and corrosion test conditions at temperatures of 1200°C and 1500°C for upto 128 h. The effects that the different corrosion enviroments have on the corrosion resistance of the Si3N4 materials are presented.