Strain Controlled Fatigue Testing - Data analysis

[will123123]

I am currently looking at some 7075 T6 strain controlled fatigue test data to obtain the following parameters: -
Cyclic strain hardening exponent
Fatigue strength exponent
Fatigue Ductility exponent
Cyclic strength coefficient
Fatigue strength coefficient
Fatigue Ductility coefficient

Whilst calculating the proportion of plastic strain during the half life cycle I sometimes obtain negative values at lower strain level tests when using initial elastic modulus values. The plastic strain values are more consistent if a value of modulus is taken from the half life cycle data - this 'cyclic elastic modulus' figure is usually higher than the standard elastic modulus value.

However the literature does state that the elastic modulus rather than the 'cyclic eleastic modulus' should be used.

Comments and advice from any one who has undertaken this type of testing would be appreciated.

 

[metengr]

Typically, from what limited exposure I have had over the years and some handy references you can estimate at least four empirical constants - fatigue strength coefficient, the fatigue ductility exponent, the fatigue ductility coefficient and the fatigue strength exponent using monotonic tensile test properties. In other words take a look at the Manson four-point criteria to determine elastic and plastic strain lines for comparison with your actual strain life fatigue test data.

http://www.polishcimac.pl/Papers3/2011/013.pdf

 

[will123123]

Metengr: Thank you for the paper it looks very useful - it will be interesting to compare the parameters I get using this approximation against my fatigue results.

Although I would still be interested in getting any advice from anyone with experience with conducting strain fatigue tests.