The railroad was nationalised by the Prussian state railways in 1852 and Wöhler's growing reputation led to his appointment by the Prussian Ministry of Commerce to investigate the causes of fracture in railroad axles, work that was to occupy Wöhler over the next two decades. The recognition of his keen administration and technical leadership resulted in his appointment as director of the newly formed Imperial Railways in Alsace-Lorraine in 1874, based at the board's headquarters in Straßburg, a post he held until his retirement in 1889.
Drawing of a fatigue failure in an axle, sketched by Joseph Glynn following the Versailles accident, 18425S-N curve of aluminium alloy
His work on fatigue marks the first systematic investigation of S-N curves, also known as Wöhler curves, to characterise the fatigue behaviour of materials.[3] Such curves can be used to minimise the problem of fatigue by lowering the stress at critical points in a component. Wöhler showed clearly that fatigue occurs by crack growth from surface defects until the product can no longer support the applied load. The history of a fracture can be understood from a study of the fracture surface. He developed apparatus for repeated loading of railway axles, mainly because many accidents were caused by sudden fatigue fracture. The presentation of his work at the Paris Exposition in 1867 brought it to a wide international audience.[4][5]
^Smith, R. A.; Hillmansen, S. (2004). "A brief historical overview of the fatigue of railway axles". Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit. 218 (4): 267–77. doi:10.1243/0954409043125932. S2CID109401654.
^Wöhler, A. (1855). "Theorie rechteckiger eiserner Brückenbalken mit Gitterwänden und mit Blechwänden". Zeitschrift für Bauwesen. 5: 121–166.
^Wöhler, A. (1870). "Über die Festigkeitsversuche mit Eisen und Stahl". Zeitschrift für Bauwesen. 20: 73–106.
^"Wöhler's experiments on the strength of metals". Engineering. 4: 160–161. 1867.
