BAUSCHINGER EFFECT IN HIGH-STRENGTH TENSILE BOLTS

High-strength tensile bolts of class 8.8 were produced without final heat treatment. Shank-threaded bolts with shanks larger than 65 mm showed inadmissibly large elongations after being tested in proof-load tensile tests according to DIN-ISO 898-1. The reason could be plastic compression applied during fabrication in combination with tensile stresses during testing (Bauschinger effect). For a better understanding of this phenomenon different types of steel were tested in combined compression-tension tests. It was discovered that plastic compression of 1 - 1.5 % can lead to a maximum decrease in yield strength of 50 %. The mathematical relation between decrease and compression rate can be described by a simple power law. As a consequence bolts of this size need an additional heat treatment. In case of being integrated in some other part of production no additional costs occur. Wholly threaded bolts and shank-threaded bolts with shanks smaller than 65 mm can be used without final heat treatment.