PSI - Issue 42

6

Yahar Sarafraz et al. / Procedia Structural Integrity 42 (2022) 125–132 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

130

The bores were generally oversized along the cross direction (thickness direction) (Fig. 5a), where lack of material facilitated the deflection and ovality of the bore wall by expansion through the tool feed (D fd,max > d fd ). This oversizing of the friction drilled bores in the thickness direction led to an incomplete forming of the thread profiles in comparison to formed thread along the longitudinal direction as can be observed in Fig. 5b. The measured oversizing of the bores and threads were compared with each other according to manufacturing process parameters (Fig. 6). An increase in friction drill diameter for 0.1 mm increases the dimension of the bore, which raises the volume of the formed thread as is presented in Fig. 6a and b. Although the friction drilling by an adapted tool speed decreased the oversizing of the bores compared with friction drilling by a constant speed, the formed thread shows a higher inner volume, justifying less deformation of the thread profiles by thread forming process.

Fig 6. Measured volume V fd and maximum diameter D fd.max of (a) friction drilled bores and (b) formed threads by CT scan b a

The maximum achieved force and strain up to failure from conducted tensile and load increase is presented in Fig 7.

Fig 7. Maximum achieved force and strain measured from conducted (a) tensile and (b) load increase fatigue tests (extensometer gauge length = 25 mm)

An increase in friction drill diameter (pilot bore) of the manufactured threads led to a decrease in static and dynamic strength, justifying in incompletely forming of the thread profiles (crest and tip) and an oversized nut diameter. This oversizing reduces the shear area of the internal thread by engaging with the steel thread rod in the conducted tensile and fatigue tests. The formed M6 thread, which was manufactured with adapted friction drill speeds shows a better dynamic property compared to manufactured thread with constant friction drill speeds (Fig 7b). This trend was not observed in the results from tensile tests. However, the lower achieved elongation at break for adapted speed in comparison to the constant speed indicated a lower nut dilation under tension. Nut dilation moves the nut radially,