Page 346 - Marine Structural Design

322 Part III Fatigue and Fracture

= S, = Sf(2N,)” (1 6.4)
2
where,
A€
2 = Elastic strain amplitude
2
E = Modulus of elasticity
Sa = Stress amplitude
S’, = Fatigue strength coefficient, defined by the stress intercept at one load reversal
(2N, =1)
N, = Cycles to failure
2N, = Number of load reversals to failure
b = Fatigue strength exponent
The plastic component of strain is described by the Manson-Coffin relation (Manson, 1964
and Coffin, 1959):

(16.5)
where,
AS
- = Plastic strain amplitude
2
E; = Fatigue ductility coefficient defined by the strain intercept at one load reversal
(2N, =1)
2N, = Total strain reversals to failure

C = Fatigue ductility exponent, a material property in the range of -0.5 to -0.7
Manson suggested that the fatigue resistance of a material subjected to a given strain range
could be estimated by superposition of the elastic and plastic strain components. Therefore, by
combining Eqs. (16.4) and (16.5), the total strain amplitude may be given by
AS A& LIE S‘
-=-I+- =1(2N,)b+&f(2Nf)‘ (1 6.6)
2 2 2 B