Engineering Design
STRAIN LIFE THEORY OF FATIGUE FAILURE (see section 7-2, text book)
The SAE Fatigue Design and Evaluation Steering Committee, in a report released in 1975 indicates, that the life in reversals to failure is related to the strain amplitude.
The total strain is the sum of the elastic and plastic components. Therefore the total strain amplitude is
The relationship between fatigue life and total strain (Manson-Coffin equation) is
s’F -The Fatigue strength co-efficient is the true stress corresponding to fracture in one reversal. (Tensile testing)
e’F – The Fatigue ductility co-efficient is the true strain corresponding to fracture in one reversal.
b- Fatigue strength exponent is the slope of the elastic-strain line and is the power to which the life 2N must be raised to be proportional to the true-stress amplitude
c- Fatigue ductility exponent is the slope of the plastic-strain line and is the power to which the life 2N must be raised to be proportional to the true plastic-strain amplitude.
The theory or equation can be used for obtaining the fatigue life of a part when the strain and other cyclic characteristics are given, however it is of little use to the designer. More over it is necessary to compound several idealizations and so some uncertainties will exists in the results. This theory is more applicable to low cycle fatigue
STRESS LIFE RELATIONS-THE S-N DIAGRAM
To determine the strength of materials under the action of fatigue loads, test specimens are subjected to repeated or varying forces of specified magnitude while the cycles or stress reversals are counted to destruction. The most widely used fatigue-testing device is the R.R Moore high-speed rotating beam machine. This machine subjects the specimen to pure bending (no transverse shear). Tests on several specimens are conducted under identical conditions with varying levels of stress amplitude. The results are plotted as an S-N diagram (Fig 7-6) usually on semi-log or on log-log paper.
