Dr.-Ing. Julian Unglaub
Beethovenstraße 51
38106 Braunschweig
Telephone: 0531/391-3375
Research project:
Influence of residual stresses on the fatigue strength of large bolts
Bolts as joints elements are subject to aging. Cyclic loading can lead to fatigue failure in the thread.
The manufacturing process (rolled, cut, quenched and tempered, galvanized) influences the fatigue strength. When the thread is rolled, compressive residual stresses are applied to the root of the thread, and the material is subjected to local strain hardening.
The present dissertation examines residual stresses induced by the thread forming process on the component rolled-after-heat-treatment HT-bolt as well as their effects on fatigue strength. The fatigue data and findings are used to formulate a local simulation concept.
For the first time, a comprehensive database of Wöhler curves on large rolled-after-heat-treatment and rolled-after-heat-treatment, hot-dip galvanized M 36 and M 48 HT bolts of strength class 10.9 is compiled: If the HT bolts are hot-dip galvanized, the fatigue life is significantly reduced. However, it exceeds the fatigue life of rolled-before-heat-treatment, hot-dip galvanized bolts.
Fatigue tests show that the VDI 2230 can reflect the manufacturing influence for large (> M 30) HT-bolts. The 20 % reduction in fatigue life for hot-dip galvanizing, which so far has only been investigated on rolled-before-heat-treatment bolts, can also be applied to large, rolled-after-heat-treatment HT bolts. The fatigue detail 50 of DIN-EN-1993-1-9 is a conservative classification of the fatigue problem.
In-depth residual stress paths of unloaded and cyclically loaded rolled-before-heat-treatment, hot-dip galvanized M 24 HT bolts and rolled-after-heat-treatment, hot-dip galvanized M 36 HT bolts are determined by neutron diffraction. Measurements on cyclically loaded M24 HT bolts show cyclic softening. The maximum of compressive residual stresses is reached close to surface. For the M36 HT bolt, the maximum of compressive residual stresses is reached below the surface.
The simulation of the forming process achieves good agreement of the thread geometry. The residual stress path corresponds qualitatively to the measured ones. The material model according to Chaboche overestimates the plastic comparative strain in the thread base.
According to the proposed simulation concept a case study is performed for a rolled-after-heat-treatment M48 HT bolt. Modelling problems are analyzed and requirements for a better consideration of residual stresses are formulated.
Publications within the framework of the RTG
Doctoral thesis:
Unglaub, J.: Einfluss von Eigenspannungen auf die Ermüdungsfestigkeit großer Schrauben. Diss. TU-Braunschweig, 2019.
Publications in peer-reviewed scientific journals / Book chapter
J. Unglaub and K. Thiele: Stahlbau 2.0 - Selective-Blended-Learning im Ingenieurstudium. In: Kauffeld, S., Othmer, J. (Hg.): Innovative Lehre. Heidelberg: Springer.
J. Müller, M. Grabowski, C. Müller, J. Hensel, J. Unglaub, H. Kloft, K. Thiele and K. Dilger. Design and parameter identification of Wire and Arc Additively Manufactured (WAAM) steel bars for use in construction. In: Metals 2019, 9(7), S. 725ff, 2019.
K. Ritter, J. Unglaub and K. Thiele. Untersuchung der Mikroschädigung in Baustahl S355 mit ESPI. In: Bautechnik 2020, 97(3), S. 180-187.
Conference contribution with publication in conference proceedings:
J. Unglaub, M. Reininghaus and K. Thiele. The fatigue behaviour of bolts with large diameters under overloading. The Eighth International Conference on Low Cycle Fatigue (LCF8) 27.- 29.06.2017 Dresden (2017).
J. Unglaub, J. Hensel, R. Wimpory, T. Nitschke-Pagel, K. Thiele and K. Dilger. Analysis of residual stress state in deep-rolled HT-Bolts. European Conference on Residual Stresses - ECRS10 - KU Leuven 2018 (2018).
J. Unglaub and K. Thiele. Fatigue Behavior of Large Rolled-After Heat-Treatment and Hot-Dip Galvanized HT Bolts. Proceedings of the ASME 2019 38th International. Conference on Ocean, Offshore and Arctic Engineering OMAE2019. June 9-14, 2019, Glasgow, Scotland, 2019.
J. Unglaub, H. Jahns and K. Thiele. Finite Element Analysis of Residual Stresses in Large Cold-Rolled Threads. XV International Conference on Computational Plasticity - COMPLAS 2019, September 3-5, 2019, Barcelona, Spain, 2019, S.479–488.
Grabowski, M., Ritter, K., Unglaub, J., Thiele, K.: Detektion von Mikrorissen in zyklisch beanspruchtem Baustahl mittels Dehnungsfeldmessung. Detection of micro-cracks in cyclically loaded structural steel by means of full field strain measurements. Werkstoffprüfung 2019 - Werkstoffe und Bauteile auf dem Prüfstand" der DGM (Deutsche Gesellschaft für Materialkunde) in Neu-Ulm vom 3.-4.12.2019 (2019).
Other publications:
J. Unglaub, M. Reininghaus, K. Thiele. Zur Ermüdungsfestigkeit von feuerverzinkten Zugstäben mit Endgewinden. In: Stahlbau 84 (8), S. 584-588, 2015.
Additional literature:
J. Unglaub, M. Reininghaus and K. Thiele. Betriebsfestigkeit von Zugstäben mit Endgewinden. 19. Dast-Forschungskolloquium , S. 154-157, 2014.
T. Wagner, J. Unglaub and K. Thiele. Ansatz zur Risserkennung in Stahlblechen anhand der Verzerrung von Lamb-Wellen. Bautechnik, 91.6. S. 425-429, 2014.