Ductile Failure Limit State Design of High Strength Steel Tension Members With Large Stress Concentrations
Authors: Verdult Michiel , Marchee Raymond , Hommel Guus 
Published: November 2011

The ultimate limit state of structural tension members with stress concentrations due to geometrical (non-welding related) stress raisers is investigated. Examples of such members are pad eyes, brackets etc. The influence of the application of high strength steels (up to S690) is taken into account. The focus lies on members with a predominant static loading regime. Such members frequently occur in the marine environment as parts of lifting appliances and handling systems or as a structural detail of equipment foundations, located outside the fatigue-prone regions of the hull girder. Typically, design stresses at the stress concentration approach the yield limit of the material. Common yield criteria cannot be applied to such peak stresses, due to the small margin between design and yield. Usually, the strength integrity is based on the nominal stresses in the critical cross section. Goal of the study is to determine the ductile failure limit with a method suited for design purposes. This would enable an ultimate limit state design approach and improve the structural safety philosophy. Main question is how the post yield behavior up to failure of a notched section is influenced by the stress gradient and the properties of the high strength materials. For this purpose, the applicability of two damage models based on the work of Rice & Tracey [8] (void growth model) and Bonora [1] (damage mechanics) is studied. In combination with elastoplastic finite element analysis these models enable the prediction of local ductile crack initiation. Calculations are performed on slender tensile members with a geometrical stress raiser, assuming a range of structural steel qualities and using a static loading regime. The results are verified using small scale laboratory tests. It is shown that isolated (non-redundant) tensile members with stress raisers feature a static ductile failure mode similar to that of uniform tensile specimen. Their failure loads can be determined as the product of the material’s tensile strength and the net section area, in the same way as for uniformly stressed members. These findings are valid up to S690 materials and clear the path to a safe and sound application of such materials based on an ultimate limit state approach. It was found that the ultimate limit state is governing design for higher strength steel members with a relatively low stress concentration. A severe stress raiser may be beneficial for efficient design of high strength members, since it allows a design stress in the notch up to yield without compromising the safety up to failure. Damage calculations were found superfluous for isolated member ultimate limit state design. Damage results, however, compare well with the failure mode observed. This is useful for the design of highly stressed notches in details which are surrounded by a large main structure, providing a huge reserve strength capacity. For these so-called embedded stress raisers an ultimate load approach is not possible due to the absence of a critical cross section. Damage mechanics can then be applied to determine a failure point in terms of stress and strain, allowing an ultimate limit state design for these stress peaks as well.