Analysis of thermal shock in brittle materials

University dissertation from Stockholm : Materialvetenskap

Abstract: This thesis concerns thermal shock damage of ceramicmaterials. More specifically the work is focused on theoreticaland practical aspects of a new technique for determining thethermal shock resistance of brittle materials. The principalidea of this Indentation-Quench Test is to study the growth ofVickers indentation cracks after exposing them to a transientthermal load (thermal shock) obtained by rapid cooling(quenching).It is established that relatively light quenches causestable crack growth whereas more severe quenches result inunstable growth. A model is derived that clarifies that theregime with stable growth is due to the combination of thermalstress from the quenching and residual stress from the indent.Based on the model an equation for the transition to unstablecrack growth is derived. This equation can be used forprediction of thermal shock resistance and it is shown thatfracture toughness is of great importance. Another valuable useof the equation is for estimating unknown parameters, such asthe surface heat transfer coefficient. It is also demonstratedhow crack growth data obtained in the stable regime can be usedfor making a rough estimate of the thermal stress pattern oncomponents.Thermal conductivity is evaluated for alumina siliconcarbide whisker composites and a new procedure for evaluatingthe thermal conductivity of the whiskers is suggested.A significant part of the thesis focuses on changes in crackshape and fracture mode when indentation cracks are subjectedto thermal stress. Crack shapes in cross-section show that thecrack growth rate is higher in the surface direction comparedto the downward direction. To simplify estimation of the crackdepth, an algorithm is suggested which reveals the crack depthfrom surface crack length data. The fracture mode in aluminavaries from intergranular to transgranular and it is suggestedthat the stress intensity at the crack tip is one importantparameter that determines the fracture path. When the stressintensity is close to the fracture toughness of the materialthe fracture path is predominantly intergranular, while thetransgranular contribution increases for higher levels ofstress intensity.All of the major aspects of the Indentation-Quench Test havebeen thoroughly explored. Due to the use of artificialprecracks and the presence of residual stress the test has manyadvantages compared to other methods. Practical parameters forthe test are well established and firmly linked to models whichidentify the influence of various physical parameters. Thisleads to a reliable test that can be extremely suitable toevaluate thermal shock resistance of materials and can be usedto map thermal shock effects in components.Keywords:Thermal shock, thermal stress, stressintensity, Vickers indentation, alumina, silicon carbidewhisker reinforced alumina, silicon nitride, high speed steel,water quench, thermal conductivity, fracture toughness, crackshape, intergranular, transgranular,

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