Failure of solids
Level: MSc and 3rd-year UG students (MTRM025/MAT501)
To provide the student with a basic understanding of the failure of materials. Failure mechanisms are studied together with the theoretical background and their use in design.
Students will be able to: recognise the difference between ductile and brittle failure and the factors influencing each type of failure; use fracture mechanics concepts to solving simple fracture problems. Interpret standard fracture test data; use simple laws to predict fatigue behaviour and perform simple calculations of component lifetimes; recognise the conditions under which creep deformation is important; use simple laws to predict creep behaviour and creep lifetime.
1. Elasticity and plasticity: General multi-axial stress state, 3D Hooke’s law, Principal stress, Hydrostatic stress, Deviatoric stress, Mohr’s circle, Strain energy, Tresca yield criteria, Von Mises yield criteria (distortion energy / J2-flow theory).
2. Morphological aspects of fracture: Ductile and brittle fracture. Morphological examples of different fracture types. Fracture transitions. Fracture mechanics. Thermodynamic concepts and generalised energy criterion. Griffith’s equation. Fracture energy and crack extension force. Practical application of the compliance method. Stress distribution at the tip of a crack. Stress intensity factor and its use in design and failure prediction. Influence of a plastic zone at the tip of a crack. The critical crack tip opening displacement and J-integral concepts.
3. Development of tough materials: Toughness and influence of microstructure. Micro-mechanics of fracture and crack resistance concept. Fractography.
4. Fatigue: Definitions of fatigue parameters. Fatigue tests and presentation of fatigue data. Cyclic hardening and softening. Fatigue crack nucleation and propagation. Empirical laws of fatigue failure and lifetime prediction. Influence of environment on fatigue properties. Design of fatigue resistant materials and need for NDT.
5. Creep: Phenomenological aspects of creep and definitions of creep parameters. Creep tests and presentation of creep data. Theories of creep and application to different materials. Creep fracture. Use of deformation mechanism maps. Development of creep resistant materials.
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