# Resonant Fatigue Tests on Drill Pipe Connections with Different Geometries and Sizes
## 5 Conclusions  
In the present paper, a resonant test bench is presented along with four series of tests with different connection geometries and sizes. This test rig is based on the principle of resonance, which is quite efficient in enhancing the working test frequency and avoiding a heavy structure to produce a static load with a closed path. Several aspects of the testing procedure are presented in detail in the first part of the paper. Accurate testing requires a calibration of the strain gauges, and this can be performed statically with a dedicated procedure and an actuator with a calibrated load cell. However, attention must be paid to the actual diameters of the sections to which the strain gauges are attached. The external diameter can be easily measured with a common caliper, while a remotely adjustable comparator was devised and produced for the internal diameter measurement. After using this tool, the actual bending modulus can be accurately evaluated at the calibration stage. A specific control system was also designed and implemented by creating and interfacing different electronic devices, and then to have a closed-loop control on either the stress amplitude (at the strain gauge positions) or the laser vibration amplitude. The control in terms of the stress amplitude was obviously preferential after the accurate calibration previously mentioned. However, the control on the displacement amplitude, which was  
Figure 14: Bending moment with corresponding beam theory maximum nominal amplitude bending stress of: (**a**) Connection with 6-5/8\(\times\)0.750” pipe body, Z140, (**b**) Wired Connection with 5\(\times\)0.374” pipe body and (**c**) Wired Connection with 5\(\times\)0.374” pipe body—True dimensions. measured by contactless laser sensors, was assumed whenever the strain gauge experienced damage, which can be attributed to the fatigue itself of the strain gauge grid, being the strain amplitude quite large during these tests. The fatigue results highlighted that three out of four connection drill pipes failed at the FET of the threaded zone, whereas just one failed at the pipe body. This comparison highlighted the great role of the section modulus of bending in driving the fatigue failure; in fact, for the wired connection samples, which presented a massive section at the connection zone, the failure was located at the pipe body. A more comprehensive study of the connection zone could be performed, in future work, by means of FE analyses and eventually involving the modeling of the cyclic plastic behavior of the steel, in particular at the thread root of the FET. After a material calibration, for example, by following the procedure proposed in ref. [56] to find the Chaboche kinematic hardening rule constants, the possible thread root relaxation can be modeled, and investigated in terms of the applied Make-Up torque preload, to obtain a more direct comparison between the loading condition at the pin FET and the pipe body. Conceptualization, C.S., L.R. and A.B. ; methodology, C.S., L.R. and A.B. ; software, A.B. ; validation, C.S., L.R. and T.I. ; formal analysis, C.S. and L.R. ; investigation, C.S., L.R. and A.B. ; resources, C.S., L.R., A.B. and T.I. ; data curation, C.S., L.R. and A.B. ; writing--original draft preparation, C.S., L.R. and T.I. ; writing--review and editing, C.S., L.R. and T.I. ; visualization, C.S. and L.R. ; supervision, T.I. ; project administration, C.S., L.R. and T.I. ; funding acquisition, T.I. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Data are contained within the article. The authors declare no conflict of interest. The following abbreviations are used in this manuscript: