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As discussed previously, the total strain is the sum of the elastic and plastic strains. In terms of strain amplitude,


∆ε/2 = ∆εe /2 + ∆εp /2


The elastic term can be written as: ∆εe


/2 = ∆σ/2E


We can now state this in terms of reversals to failure: ∆εe


/2 = (σ’f /2 = ε’f / E) x (2Nf


The plastic term is: ∆εp


(2Nf )c


The total strain can now be rewritten: ∆ε/2 = (σ’f


/ E) x (2Nf )b + ε’f (2Nf )c )b Eqn. 5 Eqn. 6 Eqn. 7 Eqn. 8 Eqn. 9


This last equation is the basis of the strain-life method and is termed the strain-life relation, as shown in Figure 7.


life that is the boundary between low and high cycle fatigue. By replacing Nt


It is sometimes useful to calculate the transition fa- tigue life (Nt


for elastic strain amplitude and plastic strain ampli- tude, the following equation is obtained:


), which is one definition for the fatigue for Nf


in the above equations Nt = ½ (ε’f E / σ’f ) 1/(b-c) Eqn. 10


The cyclic stress-strain constants K’ and n’ were also calculated directly from the strain-life and stress-life constants as follows:


K’ n’


= σ’f = b/c


/(ε’ f)n’ Thus, the database contains K’ and n’ constants that


were calculated by two methods, i.e., direct log-lin- ear regression of the stress and plastic strain ampli- tudes and simple manipulation with the above two equations. Only the K’ and n’ data obtained from the regression analyses will be discussed in this paper.


Results and Discussion Database Contents


The database has a directory structure that is sum- marized in the report, and other computer files. A typical database page for the composition and mi- crostructure austempered ductile iron is shown in Figure 8. A typical database page with monotonic properties, strain-life fatigue constants and curves


International Journal of Metalcasting/Spring 2012


Figure 6. Logarithmic plastic strain amplitude versus reversals plot from Reference 4.


Figure 5. Logarithmic stress amplitude versus reversals plot from Reference 4.


is shown in Figure 9. These database pages show average values, but the raw fatigue data and micrographs are also contained in the database. The turquoise-shaded regions are contributed datasets.


Figure 7. Logarithmic total strain amplitude versus reversals plot from Reference 4, showing the addition of the logarithmic elastic strain and plastic strain versus reversals lines.


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