Page 261 - Biaxial Multiaxial Fatigue and Fracture

The Multiaxial Fatigue Strength of Specimens Containing Small Defects 245

shear stress state and extended the use of the & parameter to mixed-mode threshold
problems. In addition, Nadot et al. [19] have discussed the extension of Dang Van’s multiaxial
fatigue criterion [20] to the defect problem by using the & parameter. Beretta and
Murakami [21,22] used numerical analysis to calculate the stress intensity factor (SIF) for a
three-dimensional mode I crack emanating from a drilled hole or a hemispherical pit under a
biaxial stress state. By comparing with the previous experimental data [ 171, they concluded that
the value of SIF at the tip of a crack emanating from a defect determined the fatigue strength of
a specimen which contained a small defect above the critical size subjected to combined
stresses. The present author [23] subsequently proposed a new criterion for fatigue failure
which was also based upon the SIF. This criterion was expressed in the form of an equation
which, by including within the criterion the && parameter model, provided a unified
method for predicting the fatigue strength of a metal specimen containing a small defect. The
applicability of the method was investigated with an annealed steel E231 and nodular cast irons
[23,24]. The essence of this approach will be presented in the present paper.
The principal objective of this study is to determine the generality of the author’s predictive
method [23] with additional experimental newly obtained data. In the present study the relation
between the fatigue strengths of smooth specimens and specimens containing defects in multi-
axial fatigue will also be discussed.

BACKGROUND FOR THE PREDICTION OF THE MULTIAXIAL FATIGUE STRENGTH

The &¶meter model

Murakami and Endo [4] have shown that the maximum value of the SIF, Klmax, at the crack
front of a variety of geometrically different types of surface cracks can be determined within an
accuracy of 10% as a function of & , where the area is the area of a defect or a crack
projected onto the plane normal to the maximum tensile stress, see Fig. 1. The expression for
KI,,, as a function of area (Poisson’s ratio of 0.3) is:

\
Maximum tensile stress direction
Fig. 1. Definition of area.