Fracture mechanics oxidation mechanisms

In this chapter, we have sought to provide a state-of-the-art review of the mechanics and micromechanisms of high temperature crack growth in ceramics and discontinuously reinforced ceramic composites. Because of the limited amount of experimental data available in the literature which pertains primarily to oxide cermics and SiC reinforcements, the discussions of crack growth rates and fracture mechanisms have centered around alumina ceramics, with and without SiC reinforcements. However, the generality of the mechan-... 

The processes at the crack tip of a notched specimen in a stress cracking liquid under mechanical stresses and possibly elevated temperatures can be regarded as local aging phenomena. Fracture mechanics, on the one hand, is a very sensitive method of detecting a polymer s behavior with regard to these influences. On the other hand, as demonstrated in the previous examples, FCP measurements can distinguish between small material differences. As depicted in Fig. 21, PE-HD shows the different behavior in static fracture mechanics tests with respect to water at a temperature of 80 °C for two different stabilizers. Stabilizer 2 leads to higher K values at comparable da/dt values and is therefore more appropriate for hot-water applications when oxidation processes also have to be taken into account [88]. 

The mechanical behaviour of oxide scales has been investigated by a complex of appropriate techniques (cf. for example [12]), where the attention has been focused on the analysis of the stress development in the scale, as well as the measurement of the scale adherence. In the case of weak scale adherence, spontaneous scale failure is often observed during oxidation or cooling of specimens. For systematic investigations of the fracture-mechanical properties of oxide scales, scale failure is induced by a controlled loading of the scale which is produced by an appropriate deformation of the whole specimen. 

In the following, a brief mechanical analysis is given which is to provide a first qualitative understanding of the relationship between the crack patterns and the fracture-mechanical oxide properties. It will be pointed out that appropriate scale loading generates crack patterns which characterise either the tensile strength or the fracture toughness of the oxide scale. 

Figure 8 from (Jochum 2013) shows the behavior in cutting of brittle hard materials, namely Circonium oxide and Yttrium oxide as it is used for dental implants. The picture shows ductile material behavior due to high compressive stresses and thus ploughing in the upper scratch. In the lower scratch an intercrystalline fracture is shown, which is due to the interaction between grain and material possibly as shear fracture, mechanism similar to the one pointed out by Kragelski in Fig. 6. 

Wei Wei, G. X., Sue, H.-J. Fracture mechanisms in preformed polyphenylene oxide particle-modified bismaleimide resins. J. Appl. Polym. Sci. 74 (1999) 2539-2545. 

Parallel to this, special expertise is being built up on HTR fiiel and HTR reactor vessels. Experiments are being performed on oxidation and interaction with fission products of SiC as well as the stability of UOj/UCj mixtures. The pressure vessel stress assessment code ISAAC is being equipped with a fracture mechanics and creep assessment feamre for high temperatures, to be able to design a licensable HTR pressure vessel in the near future. 

The purpose of surface preparation is to remove contamination and weak surface layers, to change the substrate surface geometry, and/or introduce new chemical groups to provide, at least in the case of metals, an oxide layer more receptive to the adhesive. An appreciation of the effects of pretreatments may be gained from surface analytical or mechanical test techniques. Experimental assessments of the effects of surface pretreatment, even when using appropriate mechanical tests, are of limited value unless environmental exposure is included. Self-stressed fracture mechanical cleavage specimens, as discussed in Chapter 4 and in the texts edited by Kinloch(2,5) for example, are therefore referred to wherever possible. 

A large number of test results indicate that supported metal catalysts and metal-oxides have properties similar to that of brittle materials. That is to say, basically plastic deformation does not occm before the materials fracture under external stress, and the fracture is instantaneous without any obvious forewarning. Solid catalysts are highly porous with crystal boundaries and a lot of defects, such as dislocation or discontinuities arising from the presence of promoters. They are similar to cracks, as defined in fracture mechanics, where fracture stresses are focused, eventually leading to breakup of the catalyst particles. ... 

Another basic major advantage is that the cyclic-fatigue fracture-mechanics data may be gathered in a relatively short time-period, but may be applied to other designs of bonded joints and components, whose lifetime may then be predicted over a far longer time-span. Obviously, the fracture-mechanics tests need to be conducted under similar test conditions and environments as the joints, or components, whose service-life is to be predicted. This is important since the fracture-mechanics test specimens do need to exhibit a similar mechanism and locus of failure (e.g. cohesively through the adhesive layer, or interfacially between the adhesive and substrate, or through the oxide layer on the metallic substrate, etc.) as observed in the joints, or components, whose lifetime is to be ranked and predicted. 

In products with a large wall thickness, the influence of weathering is often limited to a surface layer, either due to limited oxygen diffusion or to limited UV penetration. The depth of this layer may be small ( 0.5 mm) compared with the whole wall thickness ( 4 mm) but it can causes brittle fracture. Mechanical behavior depends on the oxidation (degradation) profile, a critical degradation profile accounted for the failure of samples, that is time of failure. Tensile tests were performed on films microtomed from the samples [39]. The density of weathered samples can increase because of chemicrystallization, increase in polar groups, oxygen uptake or the loss in volatile products. The decrease in the nominal strain corresponded to an increase in the vinyl index and in the density. The carbonyl index showed too much scatter. 

Experimental data for the mechanical and physical properties of the selected graphite candidates (IG-110, IG-430, NBG-18, and NBG-25) were produced. In addition, the fracture and oxidation behaviors were estimated. To understand the radiation effects in nuclear-grade graphite, an atomistic stmctural change in IG-110 irradiated with 3 MeV H" " and gamma—irradiation effects were characterized (Kim et al., 2009b Hong et al., 2012 Corwin et al., 2008). 

No crack growth rate data under well controlled fracture mechanics and electrochemical conditions are currently available for any alloy in supercritical aqueous solutions. Indeed, it may well be that under conditions where the Chemical Oxidation (CO) mechanism dominates (low density) the electrochemistry may not be important, but that deeds to be demonstrated. At high density (p > 0.06 g/cm ) general corrosion occurs via an Electrochemical Oxidation (EO) mechanism and it is likely that SCC also will be an electrochemical phenomenoa... 

Xu R, Fan XL, Zhang WX, Wang Tf. Interfacial fracture mechanism associated with mixed oxides growth in thermal barrier coating system. Surface and Coatings Technology. 2014 253 139-147. 

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