Fastener True Elongation Threshold as a Function of Thread Length and Tensile Strength 2001-01-3826
In the last years, new and precise tightening methods have been introduced in the automotive industry with the purpose of reaching high-precision and low-scatter assembly force. In general, these methods are based on measurement of the angular displacement of a bolt head or nut taking into consideration a fixed reference.
The attainment of maximum precision assembly x pre-load is reached at time fasteners are tightened in the elastic-plastic zone, i.e. under permanent deformation. In some applications fasteners are tightened more than once thus resulting in increase in the plastic deformation that add up after each tightening.
The purpose of present study is to demonstrate behavior (plastic deformation in 8.8 and 10.9 high strength bolts) whenever bolts are submitted to assembly forces up to 100% of their capability to generate assembly x pre-load, using these results to establish bolting parameters and adequate number of reutilization.
A number of force-torque-angle-elongation curves have been constructed using fasteners manufactured in low alloy steel whose non-engaged thread length has varied from 10 to 100 mm and M8 and M12 bolts size. The tempering temperature was 500 °C or higher.
Along of this study empirical mathematical expressions have been developed to determine the fastener plastic deformation capability versus unengaged thread length (free thread) ratio and tensile strength. Also it was obtained an equation that relates linear plastic deformation to the fastener permanent deformation angle.