Fatigue - Keywords Cyclic Process High Cycle Low Cycle Example - - PowerPoint PPT Presentation

Fatigue - Keywords

• Cyclic Process
• High Cycle
• Low Cycle
• Example

– Aeroplane – turbines – Power Plants – Bridges, Flyovers – Cranes

Mechanism

Basquin Equation : Na p = C

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, 104 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: Δℇ𝑞 2 = ℇ𝑔 ′ (2N)c where

Δℇ𝑞 2

is the plastic strain amplitude ℇ𝑔

′ is the fatigue ductility coefficient defind by the strain interceptat 2N=1

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 Δℇ𝑞 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 593oC At lower frequency, there is more chance for oxidation as the member Is exposed to environment for Longer time.

Fatigue life is shortened by

Oxidation

creep

Grain Boundary voids In presence of cyclic loads G.B voids increase and propagation process and hence reduction in life