exam 3 notes - Failure Chapter 9 Brittle fractureCleavage-...

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Failure Chapter 9 Brittle fracture- Cleavage- when cracks propagate along specific crystallographic planes Transgranular (transcrystalline)- fracture passes through the grains. Intergranular- then cracks propagate along grain boundaries Fracture Mechanics- Stress raisers- imperfections in a solid that cause measured fractured strength to be lowered More significant in brittle materials rather than ductile Fracture toughness (K) - measure of a materials resistance to brittle fracture when a crack is present, brittle materials have lower K values than ductile materials. K decreases with increasing strain rate, and decreasing temp. Plain strain- when thickness of specimen doesn’t matter, I.E. the thickness is much greater than crack dimensions NDT(non-destructive test) techniques detect cracks, leaks, etc. Brittle fracture of ceramics Static fatigue/delayed fracture- slow propagation of cracks Origin—mirror region where the crack is accelerating to its critical velocity---- mist----hackle Fracture of Polymers Crazing- when very small holes form instead of a line crack, eventually holes combine and form a crack Impact Fracture Testing Charpy, Izod- both measure impact energy (notch toughness).draw pg 311 Ductile-to-Brittle transition- occurs with decreasing temp. during transition features of both exist up looking a broken specimen, fibrous or dull indicates ductile, granular = brittle. Usually a range of temps. Most common type are low-strength BCC steels Fatigue- when stresses lower than yield strength are repeated over and over again until failure Cyclic stresses- reversed stress cycle- when goes from max tensile to max compressive Repeated stress cycle- when it goes to max tensile and back to zero/max C back to zero. Fatigue limit- for some ferrous and titanium alloys it is the limiting stress level, where below it fatigue failure will not occur. No such thing for nonferrous alloys Fatigue strength- the stress level at which failure will occur for some specified # of cycles Fatigue life- number of cycles to cause failure at a specified stress level. Fatigue in polymers Similar to metals, sometimes failure occurs due to localized heating & softening during testing Crack Initiation and Propagation Beachmarks and striations, both are markings of crack propagation from fatigue, beachmarks you can see with unaided eye, striations are microscopic, neither appear at rapid failure areas Factors that affect fatigue life Mean stress, surface effects, design factors (fillets, sharp corners, etc), surface treatments (finishes, etc). Case Hardening- carburizing or nitriding steel alloys, increases hardness and fatigue life Environmental Effects Thermal fatigue- occurs at elevated temps. Caused by fluctuating thermal stresses, expansion, contraction etc. Corrosion Fatigue Creep- constant load/stress, slow time-dependent deformation Primary/transient creep- decreasing rate due to strain hardening Secondary/steady state creep- linear plot, rate constant Tertiary creep- rate increases to failure
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exam 3 notes - Failure Chapter 9 Brittle fractureCleavage-...

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