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Item type:Publication, Access status: with files , Coupled evaluation of R-curve behaviour and subcritical crack growth: Experimental method and data interpretationThe fracture resistance is a design-relevant and material-specific feature. In ceramic materials, the fracture resistance can increase as a function of the crack length. Such behaviour is beneficial, e.g. in terms of the thermal shock resistance. It is termed R-curve behaviour and reflects the capacity of the material for intrinsic and/or extrinsic toughening during stable crack advance. Another design-relevant material-specific property is the sensitivity towards subcritical crack growth. The latter is a time-dependent phenomenon and commonly estimated by two crack growth parameters. The presented paper aims for a detailed review of sequential fourpoint bending experiments on V-notched beams, which are used to evaluate both phenomena from a single measurement. Three groups of materials (Al2O3, Si3N4, and Lithosil glass) were tested and analysed by this method in order to provide an experimental data background.Publicationtype: Journal Article13 23 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the influence of antioxidants and recycled MgO–C source material on the mechanical properties of carbon-bonded magnesia refractories(Wiley, 2026-04-12); ; ; In the present study, the role of recycled source material and the addition of antioxidants to refractory MgO–C material commonly used in the steel industry were addressed from the perspective of fracture mechanics. Initially, the fracture behaviour of the fine grained MgO–C-matrix was assessed by sequential bending experiments (particle sizes <150 μm). Based on this, coarse grained parts with particle sizes up to 7 mm were prepared and characterised in regard to their high-temperature properties, i.e. the refractoriness under load, the thermal expansion behaviour and the Young's modulus as function of the temperature. Four differently designed batches were tested. One batch, consisting of all fresh source materials (MgO and C), served as reference. Furthermore, the additives Al, SiO₂ and TiO₂ were systematically added, and 50% of the source material was replaced by recycled MgO–C from the industry. During the batch production, environmentally friendly fructose, collagen and lignin served as temporary binding agents. It was found that while the addition of recycled MgO–C had a reducing effect on the mechanical properties, the opposite was observed for the addition of antioxidants.Publicationtype: Journal Article12 15 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Area moment of inertia and other physical quantities for special cross-sectional areas of bend specimens(2026-03-04)With regard to materials in nature having extraordinary properties, one focus of our research is nacre-inspired materials. On the basis of 3D-printing, it was possible to prepare tiny bending bars in order to be tested in three-point bending (3PB). However, their cross sections are not rectangular but have a special geometric shape. Therefore, analytical calculations concerning the area moment of inertia and other physical quantities were performed for the correct analysis of the measured data.Publicationtype: Technical Report44 15 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Operation of bending machine BM2 : user manual(2024-05-21)This is a user manual for a special bending machine developed at the TUHH. It contains a patented process in which the control of stable crack growth in fracture tests has been automated, making the measurement of so-called R-curves much easier and safer. Furthermore, it enables the simultaneous analysis of subcritical crack growth.Publicationtype: Technical Report43 80 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Impact of thermal shock on the R-curve and subcritical damage behaviour of flame-sprayed alumina compoundsAn alumina rich Al2O3–ZrO2–TiO2 compound was used for the preparation of self-supporting parts via the rod flame spraying technology. The as-sprayed material was compared to self-supporting parts, fabricated from commercially available alumina rods. For both materials, the flexural strength, the fracture resistance, and the behaviour under subcritical loading was evaluated and compared, either before as well as after cold thermal shock. While the flexural strength and the subcritical damaging of both materials were basically affected through thermal shock in a similar manner, the Al2O3–ZrO2–TiO2 compound showed an outstanding behaviour in terms of its fracture resistance, which was only barely reduced after thermal shock quenching.Publicationtype: Journal Article13 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , When cortical bone matrix properties are indiscernible between elderly men with and without Type 2 Diabetes, fracture resistance follows suit(Wiley, 2023-12); ; ; Type 2 diabetes mellitus (T2DM) is a metabolic disease affecting bone tissue and leading to increased fracture risk in men and women, independent of bone mineral density (BMD). Thus, bone material quality (i.e., properties that contribute to bone toughness but are not attributed to bone mass or quantity) is suggested to contribute to higher fracture risk in diabetic patients and has been shown to be altered. Fracture toughness properties are assumed to decline with aging and age-related disease, while toughness of human T2DM bone is mostly determined from compression testing of trabecular bone. In this case-control study, we determined fracture resistance in T2DM cortical bone tissue from male individuals in combination with a multiscale approach to assess bone material quality indices. All cortical bone samples stem from male nonosteoporotic individuals and show no significant differences in microstructure in both groups, control and T2DM. Bone material quality analyses reveal that both control and T2DM groups exhibit no significant differences in bone matrix composition assessed with Raman spectroscopy, in BMD distribution determined with quantitative back-scattered electron imaging, and in nanoscale local biomechanical properties assessed via nanoindentation. Finally, notched three-point bending tests revealed that the fracture resistance (measured from the total, elastic, and plastic J-integral) does not significantly differ in T2DM and control group, when both groups exhibit no significant differences in bone microstructure and material quality. This supports recent studies suggesting that not all T2DM patients are affected by a higher fracture risk but that individual risk profiles contribute to fracture susceptibility, which should spur further research on improving bone material quality assessment in vivo and identifying risk factors that increase bone fragility in T2DM. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.Publicationtype: Journal ArticleScopus© Citations 1 31 63 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the fracture behaviour of niobium–alumina castablesThe present study addresses the characterisation of niobium–alumina composites in terms of their fracture behaviour. Fine-grained Nb–Al2O3 composites with a niobium content of 60 vol% and produced from different alumina sources (tabular alumina or white fused alumina) were prepared by castable technique. The sintered castables were tested for their sensitivity towards subcritical crack advance, their R-curve behaviour, and the work of fracture. All properties were derived from slow, cyclic four-point flexure tests at room temperature, which allowed for the initialisation and the manual and automatic control of a stable growing crack. Overall, the investigated composites showed measurable R-curve behaviour and were more sensitive towards subcritical crack damage than pure alumina.Publicationtype: Journal Article32 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Using polystyrene spheres to produce thin, porous interlayers in alumina laminatesPorous alumina layers were produced by colloidal processing of alumina with the addition of 15, 30, and 45 vol% polystyrene spheres (PS) as pore formers. Alumina laminates were designed with dense layers alternated with porous interlayers using 45 vol% of PS spheres and sintered at 1350°C. The layers’ thickness ranged from 2 to 15 μm, with a random distribution of pores. The higher volume fraction of pores tends to decrease the alumina average grain size, but does not influence the final size of bulk and surface pores. The obtained values of hardness and Young's modulus for the porous interlayer are ∼30% of the values obtained for the dense layer. Vickers indentations suggested that crack propagation can be opposed by the porous interlayers. However, values of mechanical strength, fracture toughness (KIC), and work of fracture presented no relevant difference compared to a monolithic reference. R-curves presented a slight increase and KIC a decrease due to crack propagation through the porous interlayers. Although no macrodeviations of the crack path were observed in the fractured surfaces, microdeviations were detected in the interlayer regions.Publicationtype: Journal Article58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Generalized trigonometric power sums covering the full circle(2022-02-21)The analytical calculation of the area moments of inertia used for special mechanical tests in materials science and further generalizations for moments of different orders and broader symmetry properties has led to a new type of trigonometric power sums. The corresponding generalized equations are presented, proven, and their characteristics discussed. Although the power sums have a basic form, their results have quite different properties, dependent on the values of the free parameters used. From these equations, a large variety of power reduction formulas can be derived. This is shown by some examples.Publicationtype: Journal Article84 207 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Life time prediction of self-supporting flame-sprayed alumina-rich coatingsMain objective of the presented research is the life time prediction of self-supporting flame-sprayed Al O - and Al O -ZrO -TiO -materials under constant load. The characteristic life time and its scatter were derived from stable crack growth tests and Weibull-statistics including the four-point-bending and ball-on-three-balls method. The potential life time was estimated in order to assess room-temperature handling and long term storage of self-supporting flame-sprayed alumina components. In terms of flexural strength, energy release, and subcritical crack growth parameters, distinct differences between both materials were shown. In turn, the characteristic life times only barely deviated from each other. From that the conclusion was drawn that life time performance under constant load application is governed by the characteristic flame-spray microstructure. However, advantages in the flame-spray processing of Al O - ZrO -TiO are still given, attributed to its lower melting temperature.Publicationtype: Journal Article54