PSI - Issue 5

Zuheir Barsoum et al. / Procedia Structural Integrity 5 (2017) 1401–1408 Zuheir Barsoum / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

Structural integrity is important in many industrial sectors where welding is a primary technique for joining. In numerous structures welds are identified as critical sections and prone to mechanical failures. A common failure mode identified in metals is ductile failure which utilizes excessive plastic deformation which is used in designing structures in vehicle industry e.g. cranes, spreaders, and Haulers etc. It is also used for designing cabin of the drivers in construction equipment etc. to protect drivers in an unexpected roll over collisions. To successfully implement HSS in mechanical structures it is important that different parts are joined together without defects in the weld and therefore achieving sufficient static and fatigue strengths. The speed of developing new steels is much higher than the speed of developing new filler materials of higher strength. As the yield strength of steel reaches 960MPa, it becomes difficult to find filler materials of similar strength which is one of the hurdles in utilizing higher strength steels as base materials. Due to melting of the steel, microstructural changes occur near the weld which results in the development of HAZ. For steels of higher yield strength especially greater than 500MPa this zone is obvious and can result in lower hardness than the base material and weld metal. This zone might limit the global strength of the joint and the strength of base material will not be utilized to full capacity. In this way, the target of higher pay load capacity might not be achieved. Fig. 1 shows the development in strength of filler materials and HSS. The comparison is only based on the yield strengths of the available steels and filler materials. Filler materials of similar yield strength are rarely available as the yield strength of base material reaches 960 MPa. This make steels of yield strength greater than 700MPa to be welded with under-matching filler i.e. yield strength of filler material is lower than the yield strength of base material. Therefore, the static strength of the joint is limited to the strength of filler material. Using steel of yield strength greater than 700MPa in such situation might not be an appropriate choice if the weld is not properly designed.

Fig. 1. Development of HSS and filler materials

Various design methodologies for welded joints subjected to predominant static loading are given in Eurocode 3 (2011) and AWS D1.1 (2010). These are valid for welds in steel up to yield strength 700MPa. Darrel et al (1989) investigated the ultimate strength of partially penetrated groove welds in two different mild steels and five weld metal penetration ratios (20-50%). They developed a method and proposed equations for predicting the ultimate tensile strength of welded joint. Lesik et al (1990) developed expressions for predicting ultimate load and deformation capacities in fillet welded connections. Satoh et al (1975) studied the influence of under-matching filler on the strength of weld in HT 80 structural steel. They found that the strength of the weld reaches the strength of base metal as the width of soft layer i.e. the undermatching weld is decreased. A reasonable under-matching strength was found not less than 90% of the base metal. In Björk et al (2012), Collin et al (2005) and Rafael et al (2009), the validity of design rules in Eurocode 3 for HSS are investigated. Under-matching filler is observed to increase the ductility of the joint and increase in the strength of filler material increases the global strength of the joint. This study investigates the influence of the mismatch in the yield strength of the filler material and the welds penetration depth on the ultimate strength capacity and failure modes of butt and fillet welded high strength steels in the grade range of 350 – 960 MPa. The load carrying capacities of these mentioned joints are evaluated with experiments and compared with the estimations by finite element analysis (FEA), and design rules in Eurocode 3 and