Page 94 - Biaxial Multiaxial Fatigue and Fracture

Micro-Cmck Growth Behavior in Weldments of a Nickel-Base Superalloy Under ... 79

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Number of cycles to failure Nf
Fig. 21. Relationship between maximum principal strain range and failure cycle.
strain were good parameters for evaluating the crack growth rate, because of the reduction in
stress intensity around the crack tip due to the effect of the X-like crack shape and the
minimum principal strain parallel to the crack, as mentioned above.
Fatigue strength reduction in a weldment is generally evaluated based on the failure life of
standard specimens. In the case of a weldment of Hastelloy-X, fatigue life reduction can be
explained by the grain boundary cracks induced by oxidation in the early stage of life.
Therefore the fracture mechanics approach is suitable for rationalizing the evaluation of fatigue
life reduction. The principal stress and strain are appropriate parameters for taking into account
the stress multiaxiality because the crack growth dominates the fatigue life in both base metal
and weldment.

SUMMARY
We investigated crack growth behaviors in weldments of Hastelloy-X under biaxial low-cycle
fatigue in order to improve the life assessment techniques for high-temperature components.
From the results, the following conclusions were, obtained.
(1) The reduction in the fatigue life of a welded specimen is about 112 and the fatigue strength
reduction factor is about 1.4. Failure lives in the torsional fatigue tests were about twice
those in the uniaxial test.
(2) In low-cycle fatigue tests of welded specimens, 0.5-1.0 mm long cracks were observed in
the early stages of life. These cracks initiated at grain boundaries and the oxidation of the
grain boundary affected the cracking.
(3) In torsional fatigue tests, the crack growth rate in the early stage of life was smaller than in
uniaxial tests. This is because of the reduction in stiffness around the crack due to the effect
of the X-like crack shape and the minimum principal strain being parallel to the crack.
(4) Crack-growth rates in weldments were almost the same as in base metal so fatigue life
reduction is considered to be due to the initiated crack lengths. The cyclic J-integral range
based on principal stress and strain is a suitable parameter for evaluating crack growth rates
in biaxial stress states.