Stress analysis of load conditions in a pre-stressed AISI 304L bolt by high-energy-dispersive X-ray diffraction measurement and FEA

This paper investigates the multiaxial stress state in an AISI 304L finish-rolled bolt of a gradually pre-stressed M8 bolt-nut connection in the first load-bearing thread using high-energy-dispersive X-ray diffraction (EDXRD). This thread area is crucial for the structural integrity of the connection due to the high stress resulting from the pre-stressing force and tightening torque. The EDXRD technique enables precise recording of deep-seated stress distributions in several directions. A single measurement, based on the fundamental equation of X-ray stress analysis, allows the determination of the difference between axial and radial components. The experimental data are compared with finite element analysis (FEA) results at characteristic points to validate the accuracy and validity of the simulation. The investigation reveals a complex stress distribution in the first load-bearing thread, caused by bending moments in the flanks and the notch effect at the thread root. The stress difference shows significant changes in sign in this area. Deviations between experimental and simulated results occur in certain load situations and are associated with manufacturing-related residual stresses and simplified model assumptions in the simulation. The comparison of experimental and FEA results provides valuable insights into the stress behaviour of bolted connections and serves as a basis for optimising future design drafts. The results highlight the importance of accurately analysing the stress state in the first load-bearing thread to prevent overload-induced failures.