PSI - Issue 42

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

131

7

which decreases the depth of the thread engagement. This reduction sinks the thread stripping area and strength, consequently. To better compare the failure mode for both manufacturing methods (adapted and constant speeds), the fractured threads by tensile tests were scanned as illustrated in Fig 8. The manufactured threads with adapted friction drill speeds showed less dilation and the crack was initiated at 4 th turns and propagated radially toward the outer wall, when manufactured thread with constant speed indicated thread stripping with a crack initiation and propagation in axial direction.

Fig 8. CT scans of the fractured tensile specimens, manufactured with (a) constant and (b) adapted friction drill speed

To understand the thread behavior under cyclic loads, the change in deformation-induced temperature and potential (ACPD) were measured as NDT-monitoring for the conducted fatigue tests with a constant amplitude.

Fig 9. (a) Deformation-induced changes in plastic strain amplitude, temperature and ACPD potential vs. number of cycles (constant speed), (b) failure by fatigue lifetime

According to the measured change in deformation-induced temperature and potential, the lifetime could be divided into four steps. Inadequate contact at the beginning due to high roughness on threads flank increases resistance, which increases the deformation-induced temperature and potential. After about N = 10 3 cycles the contact surface is increased by eliminating the roughness tips, which decreases the resistance and potential. In the third step, the contact surface is decreased due to nut dilation and bending, which results in a higher resistance and potential. Finally, the potential is increased with a vibration in trend, implying a stepwise crack initiation and propagation in thread profiles. 4. Conclusions Dimension-based failure analysis of the formed M6 internal indicated that increasing the friction drilled diameter by