Issue 33

D. G. Hattingh et alii, Frattura ed Integrità Strutturale, 33 (2015) 382-389; DOI: 10.3221/IGF-ESIS.33.42 384 applications as the pin must be retracted over a minimum distance or at a rate that prevents the formation of defects. Fig. 2 shows the final tool being developed and optimised with a 10 mm shoulder and threaded pin configuration. Control of this additional fixture was integrated with that of the I-STIR platform. Via an iterative optimisation process the optimum values for the manufacturing process variables were determined. This allowed FS welds of high quality to be made in small diameter aluminium tubes. Fig. 3 shows a typical Al 6082-T6 specimen manufactured by using this innovative FS welding technology. The FS welded tubular samples used to generate the necessary experimental results had outer nominal diameter equal to 38 mm and inner nominal diameter to 31 mm. Both the static and fatigue results were determined by testing samples in the as-welded condition. This new FS welding technology was developed and optimised at the Nelson Mandela Metropolitan University (NMMU), South Africa. B R =0, R=-1 N f =1664764 cycles to failure B R = 3 , R=-1,  =0° N f =369237 cycles to failure B R =1, R=-1,  =0° N f =650684 cycles to failure B R = 3 , R=-1,  =90° N f =173954 cycles to failure B R =0, R=0 N f =1071840 cycles to failure B R = 3 , R=0,  =0° N f =501988 cycles to failure B R =1, R=0,  =0° N f =857370 cycles to failure B R = 3 , R=0,  =90° N f =224230 cycles to failure Figure 4 : Examples of the observed macroscopic cracking behavior under biaxial fatigue loading (B R =  nom,a /  nom,a , R=  nom,min /  nom,max =  nom,min /  nom,max ,  = out-of-phase angle). E XPERIMENTAL DETAILS ince welded joints’ mechanical properties are a suitable indicator of the overall weld quality, several static tests were run by testing the manufactured tubular joints under pure tension as well as under pure torsion. Irrespective of the type of applied loading, cracks were seen to initiate at the tip of the undercut resulting from the FS welding process. As a further check of the tensile strength, the axial strength was also determined at the University of Plymouth, UK, via microtensile quasi-flat specimens tested by using a Gatan Microtest 2000EW device. Such a systematic experimental investigation resulted in an average value of the ultimate tensile strength for FS welded Al 6082-T6 equal to 152 MPa, the axial static strength of the parent material being equal to 303 MPa. This allowed us to prove that the average efficiency of the manufactured welded joints (which is defined as the ratio of weld over parent plate tensile strength) approaches 0.5; this value compares well with the figure of 0.49 which is usually reported for FS welds in 3 mm thick plates of 6082-T6 [8]. The average static strength under torsion was seen to be equal to about 120 MPa. The axial tests were performed at the University of Ferrara, Italy, by using an MTS 810 Mod. 318.25 servo-hydraulic axial testing machine. The samples were tested under a load ratio (R=  nom,min /  nom,max ) equal to 0.1 as well as equal to -1. The fatigue behaviour of the Al 6082-T6 FS welded specimens under biaxial loading was investigated at the University of Sheffield, UK, by using a SCHENCK servo-hydraulic axial/torsional testing machine equipped with two MTS hydraulic grips. Both the torsional and the biaxial tests were carried out under nominal load ratios equal to -1 and 0. The force/moment controlled tests were run under in-phase and 90° out-of-phase constant amplitude sinusoidal load histories. The pictures seen in Fig. 4 show some examples of the typical cracking behaviours displayed by the Al 6082-T6 FS welded joints tested under biaxial loading. S