Effect of Form and Size of Backing Strip on the Bending Fatigue Strength of Butt Welded Joint

Since a fatigue crack of the butt welded joint with a backing strip initiates at the toe of root, fatigue strength of the joint may be strongly influenced by the form and size of the backing strip. Therefore, reversed bending tests were carried out to clarify the relation between stress concentration and fatigue strength of the butt welded joint with a backing strip, by varying the form. and size of the backing strip.In a notched specimen, when a non-propagating crack exists at the root of a notch whose radius is smaller than the root radius of a notch at the branch point (the critical point where a non-propagating crack appears), the fatigue limit of the notched specimen can be estimated from the maximum stress at the root of a notch with no crack developing, whose radius is equal to that at the branch point.As the butt welded joint with a backing strip can be considered as a kind of sharp notches, the fatigue limit of the joint is estimated in a similar manner as in the case of a sharp notch.The specimens used are the simulated specimens cut out from a SS41 plate.The main results obtained are as follows;(1) As the thickness of a backing strip or the root opening decreases, fatigue strength of the butt welded joint increases.(2) In the butt welded joint with a backing strip, the existence of reinforcement increases fatigue strength of the joint.(3) When the notch radius at the toe of root is smaller than the critical radius for the existence of a nonpropagating crack, the fatigue limits of the joint are almost the same under the condition of constant thickness of the specimen and backing strip.