NKK (Nippon Kokan) researchers working on a Kumamoto University research team have discovered a new method for estimating stress factors and durability of offshore structures.
Shin-ichi Hirayama, president of NKK America, Inc., said one result of these studies is AWS's design standards (X curve) which estimates the fatigue strength from strain amplitude of stress concentration at the joints.
As this method does not clearly distinguish fatigue failure and crack initiation, the team devised the following procedures:
(1) From the size and shape of welded joints, the fatigue notch factor, Kf, which describes the condition of the welded toe notch, is calculated.
(2) Assuming that the relationship between the local plastic stress of the notch which results in fatigue failure and the nominal strain can be described by Neuber's calculation, replace the Kt (elastic stress concentration factor) by the Kf in the formula.
(3) After obtaining an S-N diagram (maximum stress versus the number of stress cycles applied on a sample material) from fatigue tests on steel materials and deposit metal, a cyclic stress-strain curve is figured from the diagram.
(4) From the above two steps, the local plastic stress and strain of the notch is determined and the fatigue life until crack initiation is calculated.
When advance information is available regarding the size and shape of the welded joints and the fatigue properties and nominal strain material and deposit metal, the fatigue life until crack initiation can be calculated precisely without making troublesome fatigue tests using models.
This new method has been verified through a variety of tests.
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