Recently research has been initiated on fatigue improvement effects of high frequency peening methods. Experimental results show that the fatigue strength of butt welds, welded transverse and longitudinal stiffeners can be highly improved. In this paper design concepts for the determination of the fatigue resistance of such posttreated welds are presented. Local approaches are adopted in order to consider the geometric improvement of the weld toe. Further, the application of the peening methods is simulated and the resulting stress field is incorporated in the model. As another simple method a local concept is presented, which calculates the local fatigue strength also considering the local hardness and residual stress field.

INTRODUCTION

Designing fatigue loaded structures like offshore structures, the fatigue strength becomes often decisive. Therefore, an improvement of the fatigue strength can yield to more economic design. Investigations carried out in the last decades prove that some weld improvement methods may raise the fatigue strength of welded details significantly. Recently, research has been initiated on the effects of high frequency hammer peening methods on the fatigue strength. Laser measurements of the weld seam show that these methods increase the overall weld toe radii, so that the notch stresses are reduced. Furthermore the fatigue resistance is enhanced by an increase of the surface hardness and also by induced compressive residual stresses. Measurements verify inherent compressive stresses at least up to a depth of 1 mm with values up to the yield strength. The experimental results prove that the fatigue strength can be highly increased compared to as welded details combined with a reduced slope of the SN-curve. The improvement of the fatigue strength can be defined by new SN-curves. Extensive experimental investigations become necessary to derive these SN-curves for various weld details. Therefore, alternative local concepts are in demand which consider for fatigue improvement effects and can be applied to various details.

Keywords:
improvement factor, stress approach, compressive residual stress, calculation, ummenhofer, compressive stress, residual stress, recommendation, sncurve, weld toe
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2007. The International Society of Offshore and Polar Engineers
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