19fastfracture

19fastfracture - AAE 352 1 AAE 352 Lecture 19 Fast fracture...

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Unformatted text preview: AAE 352 1 AAE 352 Lecture 19 Fast fracture and fatigue Fatigue failures account for 80% of stress failures AAE 352 2 Take-aways • Materials with cracks or other flaws will fail at a lower stress than the yield stress • A material property called fracture toughness relates “critical” crack size and stress to cause “fast fracture,” the propagation of a small crack to failure • Fatigue analysis considers how fast a small crack grows to critical size • At Purdue there is a course AAE554 devoted entirely to fatigue and fracture AAE 352 3 Fatigue is prevalent in all kinds of engineering AAE 352 4 New terms • Toughness-the ability of a material to absorb energy and plastically deform before fracture • Fatigue – two stage process-crack initiation, crack growth-annual cost in U.S. said to be 3% of GNP • Fracture toughness-the ability of a material containing a crack to resist fracture-it is a material property called K 1c • Crack propagation • Brittle • Stress intensity factor AAE 352 5 Stress-strain refresher AAE 352 6 Resilience = area under linear stress-strain curve Young’s modulus=slope of stress-strain curve Yield stress=entry into plastic zone Toughness – the area underneath the entire stress-strain curve AAE 352 7 The ability of a metal to deform plastically and to absorb energy in the process before fracture. Increasing strength usually leads to decreased toughness Brittle/ductile comparison - Brittle materials have high strength but are sensitive to small flaws AAE 352 8 Information and insights...
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19fastfracture - AAE 352 1 AAE 352 Lecture 19 Fast fracture...

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