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Strain-controlled fatigue tests were conducted at room tem- perature over the range from about 100 cycles to 1,000,000 cycles. Generally, three specimens were tested at each of six to seven strain levels, and all tests were performed using a tri- angular waveform at a constant strain rate of 2% per second.


Specimens with anticipated fatigue lives in excess of 500,000 cycles were switched to stress-controlled testing after 100,000 cycles and thereafter were tested at a con- stant frequency of 15 cycles/second. Tests were continued until a 20% drop in stress range (~ 50% load drop in cast iron) occurred, or until 5,000,000 cycles were reached without failure. Strain and load data were recorded dur- ing the tests.


Fatigue Data Analysis


This section of the paper summarizes the strain-life fatigue data analysis that was conducted in the database develop- ment effort.7


The reader is directed to other references for further details.3,4,7,8


A fundamental step in the strain-life analysis of cyclic property data is the decomposition of the total cyclic strain amplitude (∆εt


plastic strain amplitude (∆εp (Δεe


(∆ε/2) = (∆εe /2)


/2) or (∆ε/2) into its component strains, i.e., /2) and elastic strain amplitude


/2) according to the equation: /2) + (∆εp


Eqn. 1


tude is then given as the difference between the total strain amplitude and the elastic strain amplitude. Figure 3 shows the definition of the stress and strain ranges, which are twice their amplitude counterparts. The plastic strain data deter- mined by this method are used in the calculation of both the strain/stress-life and cyclic stress-strain constants.


Using data obtained from each fatigue specimen at half-life, the cyclic stress-strain constants n’ (cyclic strain harden- ing exponent) and K’ (cyclic strength coefficient) were de- termined by regressing the stress amplitude (∆σ/2) versus plastic strain amplitude (∆εp


∆σ/2 =Κ’(∆εp /2)n’


In practice, the elastic component of strain is determined by dividing the stress amplitude (∆σ/2) at specimen half-life (0.5Nf


) by the elastic modulus (E). The plastic strain ampli-


Where: ∆σ/2 = true stress amplitude 2Nf σ’f


b


as shown in Figure 5; and ∆εp


/2 = ε’f


Where: ∆εp


c = fatigue ductility exponent as shown in Figure 6.


2Nf ε’f


(2Nf )c


/2 = plastic strain amplitude = reversals to failure


= fatigue ductility coefficient


= reversals to failure (1 reversal = ½ cycle) = fatigue strength coefficient = fatigue strength exponent


Eqn. 4


Figure 3. Stable hysteresis loop at half-life, showing the definitions of stress, total strain, elastic strain, and plastic strain ranges from Reference 4.


/2) in logarithmic coordinates.


Figure 4 shows the cyclic stress-strain constants, which are related as follows:


Eqn. 2


The cyclic stress-strain constants K’ and n’ were also calcu- lated directly from the strain-life constants.


The cyclic stress-life and strain-life constants σ’f,


were calculated as follows: ∆σ/2 = σ’f


(2Nf 10 )b b, ε’f, and c Eqn. 3


Figure 4. Logarithmic cyclic stress-plastic strain plot with cyclic stress-strain constants from Reference 4.


International Journal of Metalcasting/Spring 2012


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