Fracture of Ceramics Danzer, R.; Lube, T.; Supancic, P. ...
Advanced engineering materials,
April, 2008, Volume:
10, Issue:
4
Journal Article
Peer reviewed
The paper reviews the current state of art of the fracture of brittle ceramic materials. Typical loading situations (thermal shock, contact damage) are analysed and the resulting fracture modes are ...discussed. In focus of the paper are the brittle fracture and the resulting probabilistic aspects. The delayed failure of brittle materials (sub critical crack growth and cyclic fatigue) is also discussed.
The paper reviews the current state of art of the fracture of brittle ceramic materials. Typical loading situations (thermal shock, contact damage) are analysed and the resulting fracture modes are discussed. In focus of the paper are the brittle fracture and the resulting probabilistic aspects. The delayed failure of brittle materials (sub critical crack growth and cyclic fatigue) is also discussed.
It is claimed in almost every experimental work on ceramics that the strength is Weibull distributed. The literature demonstrates that this is not valid in any case, but it is up to now the backbone ...in the design of brittle components. An overview on situations that deviate from Weibulls statistics is presented (multi-modal flaw distribution,
R-curves, etc.). It is also shown that testing specimens with different volumes may help to understand the real strength distribution.
Inaccuracies that arise from using the Weibull’s theory are presented. Monte Carlo simulations on the basis of the standardised testing procedure (30 specimens) clearly reveal that these deviations can be hardly detected on the basis of small samples.
The biaxial strength of Low Temperature Co-fired Ceramics was determined using the ball-on-three-balls test in several environments (dry oil, air and water). Subcritical crack growth phenomenon was ...observed, activated by the prolonged presence of humidity at the specimen surface. Whereas high strength values were reached during high-rate testing in dry oil, up to a 50% lower strength was measured on specimens immersed in water tested for longer periods. Experiments in a relative dry environment have shown for the first time evidence of two different crack growth mechanisms in this material. A model has been implemented to interpret the experimental results.
The strength of materials suffering from subcritical crack growth (SCCG) can be affected by the loading conditions and environment. This crack growth can also occur during the measurement of the ...material’s fracture toughness, KIc, yielding lower measured (apparent) values. In this work the KIc measurements of two different brittle materials was compared in different environments (i.e., water, ambient air and argon) using three testing methods: (i) Single Edge V-Notch Beam, (ii) Chevron Notch Beam, and (iii) Ball-on-three-Balls-KIc. A strong effect of the environment was found on the measured KIc values using all three methods. Low “KIc values” were measured in water, i.e., ∼1.3–1.7MPam1/2 depending on the material, whereas higher “toughness values” were obtained in argon, i.e., ∼2.0–2.3MPam1/2. This is explained with a model accounting for the SCCG during the fracture toughness experiment. Recommendations on the assessment of fracture toughness in brittle materials are given.
Heterochromatin Protein 1 (HP1) is a structural component of silent chromatin at telomeres and centromeres. Euchromatic genes repositioned near heterochromatin by chromosomal rearrangements are ...typically silenced in an HP1-dependent manner. Silencing is thought to involve the spreading of heterochromatin proteins over the rearranged genes. HP1 associates with centric heterochromatin through an interaction with methylated lysine 9 of histone H3, a modification generated by SU(VAR)3-9. The current model for spreading of silent chromatin involves HP1-dependent recruitment of SU(VAR)3-9, resulting in the methylation of adjacent nucleosomes and association of HP1 along the chromatin fiber. To address mechanisms of silent chromatin formation and spreading, HP1 was fused to the DNA-binding domain of the E. coli lacI repressor and expressed in Drosophila melanogaster stocks carrying heat shock reporter genes positioned 1.9 and 3.7 kb downstream of lac operator repeats. Association of lacI-HP1 with the repeats resulted in silencing of both reporter genes and correlated with a closed chromatin structure consisting of regularly spaced nucleosomes, similar to that observed in centric heterochromatin. Chromatin immunoprecipitation experiments demonstrated that HP1 spread bi-directionally from the tethering site and associated with the silenced reporter transgenes. To examine mechanisms of spreading, the effects of a mutation in Su(var)3-9 were investigated. Silencing was minimally affected at 1.9 kb, but eliminated at 3.7 kb, suggesting that HP1-mediated silencing can operate in a SU(VAR)3-9-independent and -dependent manner.
Heterochromatin protein 1 (HP1) is a conserved non-histone chromosomal protein enriched in heterochromatin. On Drosophila polytene chromosomes, HP1 localizes to centric and telomeric regions, along ...the fourth chromosome, and to specific sites within euchromatin. HP1 associates with centric regions through an interaction with methylated lysine nine of histone H3, a modification generated by the histone methyltransferase SU(VAR)3-9. This association correlates with a closed chromatin configuration and silencing of euchromatic genes positioned near heterochromatin. To determine whether HP1 is sufficient to nucleate the formation of silent chromatin at non-centric locations, HP1 was tethered to sites within euchromatic regions of Drosophila chromosomes. At 25 out of 26 sites tested, tethered HP1 caused silencing of a nearby reporter gene. The site that did not support silencing was upstream of an active gene, suggesting that the local chromatin environment did not support the formation of silent chromatin. Silencing correlated with the formation of ectopic fibers between the site of tethered HP1 and other chromosomal sites, some containing HP1. The ability of HP1 to bring distant chromosomal sites into proximity with each other suggests a mechanism for chromatin packaging. Silencing was not dependent on SU(VAR)3-9 dosage, suggesting a bypass of the requirement for histone methylation.
Low temperature co-fired ceramics (LTCCs) are multilayered ceramic based components, which can be used as high precision electronic devices in highly loaded environments. In many applications, LTCC ...end components are exposed to mechanical stresses, which may yield different types of failure coming from different locations, thus decreasing the mechanical reliability of the device. The aim of this work is to assess the mechanical strength of LTCC parts and investigate the influence of the metal internal structure (supporting the maximum load) on the local fracture response. Strength of different positions (e.g. near vias, metal-pads, ceramic layers) has been measured under biaxial loading and compared with a reference bulk LTCC. The strength results were interpreted in the framework of Weibull theory. Fractographic analyses revealed a significant effect of the first metallisation layer below the tensile surface on the strength reliability of the structure, which should be considered to optimise LTCC designs.
For the hot rolling of wire, tools are nowadays made of cemented carbides. In service, these rollers suffer primarily from wear. Due to the properties of ceramics, improvements in tool behaviour ...could be expected. In a recent paper the thermal and mechanical stresses in silicon nitride rollers were analysed. In field tests – when rolling materials with high-deformation resistance – cracks developed in the ceramic rollers, which grew for a long-time period before large parts of the rollers broke apart. In more moderate conditions the rollers operated safely.
In this paper a FE model is used to analyse the in-service behaviour of cracks in the silicon nitride rollers. For the observed crack path the stress intensity factor of the cracks is determined using the weight function method. It increases up to a crack depth of around 0.35
mm and then decreases again with increasing crack depth. This explains the observed pop-in-type growth of cracks after an overload. Depending on the rolled materials, the popped in cracks have a length between approximately 0.35 and 1.2
mm. The further growth of the cracks to a length of several millimetres, which is caused by a fatigue growth mechanism, needs several thousand additional revolutions.
A procedure to predict the fracture toughness of a ceramic multilayer composite made of different phases is presented. The procedure requires experiments to measure the intrinsic fracture toughness ...of the phases and to determine the required material data. The numerical modeling includes a conventional finite element stress analysis and the calculation of the crack driving force based on the concept of configurational (material) forces. The procedure yields the fracture toughness of the composite as a function of the crack length. A bending bar consisting of layers made of alumina and an alumina–zirconia composite is investigated. The bar has a crack perpendicular to the interfaces. The spatial variations of both the thermal residual stresses and the elastic modulus induce shielding and anti-shielding effects on the crack, which are quantified. The numerically predicted fracture toughness is compared with the experimental values.
Ceramic varistors are electronic components which have a sharp change (over several orders of magnitude) of their electrical resistance at a well-defined voltage (switching voltage). Starting at low ...voltages, the resistance is large. But by exceeding the switching voltage the resistance drops dramatically and the component acts as a good conductor. Mounted parallel to a consumer, they are used as protection devices against over voltage loading. In the technical routine yearly billions of varistors are used to protect power lines, transformation stations, electronic devices, microelectronic systems or even LEDs.
In service large temperature differences may come into existence, which cause severe mechanical stresses that even may destroy the components. The basic principles of the varistor behaviour are explained and examples of mechanical failure are discussed.
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