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Lecture14 - Fracture Usually undesirable resulting in...

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Chapter 8 - 1 Fracture Usually undesirable resulting in economic loss, injury, or loss of life. Depending upon temperature, strain rate, and microstructure some materials can fracture in either mode Ductile Brittle Substantial plastic deformation Little or no plastic deformation Usually not catastrophic Can be catastrophic Often slow and detectable Very rapid Substantial energy absorbed Very little energy absorbed Not very crack sensitive Crack sensitive Medium-high K IC Low K IC Most metals, alloys, and some polymers Ceramics , cold metals, some polymers
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Chapter 8 - 2 Loading Rate Increased loading rate... -- increases σ y and TS -- decreases % EL Why? An increased rate gives less time for dislocations to move past obstacles. σ ε σ y TS TS larger ε smaller ε
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Chapter 8 - 3 Impact Testing final height initial height Impact loading: -- severe testing case -- makes material more brittle -- decreases toughness Adapted from Fig. 8.12(b), Callister 7e. (Fig. 8.12(b) is adapted from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials , Vol. III, Mechanical Behavior , John Wiley and Sons, Inc. (1965) p. 13.) (Charpy)
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Chapter 8 - 4 Fracture property of the material Both K Ic and impact test determine the fracture properties of materials K Ic - quantitative – specific property of material Impact test – qualitative – not for design, but for comparison Very limited correlation
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Chapter 8 - 5 Impact energy is affected by Yield strength and ductility Increasing the yield strength of a metal by cold work, precipitation strengthening and solid solution strengthening generally decreases the ductility, therefore, decreases the Charpy impact energy (less plastic work can be done before the strain in the plastic zone is sufficient to fracture the test specimen). Notches The notch in the test specimen acts as stress concentrator and decrease the impact energy. Temperature and strain rate Temperature and strain rate effect the behaviour of dislocations. Increasing the yield strength by low temperatures or high strain rates decreases the ductility, and therefore decreases the Charpy impact energy. Fracture mechanism Charpy impact energy is affected by changes in the fracture mechanism. The work of fracture of cleavage is much less than the work of fracture of microvoid coalescence since it involves much less plastic deformation. The change in fracture mechanism therefore causes a sharp ductile to brittle transition in Charpy impact energy.
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Chapter 8 - 6 Increasing temperature...
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