Fatigue - Keywords • Cyclic Process • High Cycle • Low Cycle • Example – Aeroplane – turbines – Power Plants – Bridges, Flyovers – Cranes
Basquin Equation : N a p = C Mechanism
Appearance of Fracture Surface • A smooth surface due to rubbing action on the crack propagating – cleavage fracture • Rough Region – area where it is failed in ductile manner. • Fracture is indicated by a series of beach marks or Stirations – there number can give the count of No. of Cycles to failure. • Failure usually occurs at sharp edges, corners – pits the points of Stress concentrations.
Low cycle fatigue Low cycle fatigue is actually more severe. The main definition of LCF is not that the material fails here at lesser number of cycles, 10 4 or so, but it is because of the fact that the stress change in each cycle is so severe that each cycle is represented by a definite plastic deformation. This is shown by Coffin Manson Relationship as given below: Δℇ 𝑞 ′ (2N) c 2 = ℇ 𝑔 Δℇ 𝑞 where is the plastic strain amplitude 2 ′ is the fatigue ductility coefficient defind by the strain interceptat 2N=1 ℇ 𝑔 2N – number of strain reversed to failure ( I cycle = 2 reversal) c- fatigue ductility exponent which varies between 0.5-0.7 A smaller value of C, results in larger value of fatigue life The plot between Δℇ 𝑞 vs 2N is known Coffin Manson Plot
High Temperature Fatigue in Corrosive Environments
Effect of Temperature on Fatigue strength
Effect of Frequency
Effect of Frequency of Loading On the extent of oxide formation Frequencies are : 5.6, 1 and 0.22 cpm And the test is at 593 o C At lower frequency, there is more chance for oxidation as the member Is exposed to environment for Longer time.
Fatigue life is shortened by creep Oxidation Grain Boundary voids In presence of cyclic loads G.B voids increase and propagation process and hence reduction in life
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