Helical-Thread Elastoplastic Finite Element Analysis of Plastic-Region Tightening of Aluminum Bolts with Torque–Angle–Preload Tests

Aluminum bolts for electric-vehicle housings often have to be tightened beyond their yield point, where the preload–angle behavior depends on the bolt and the clamped members together. A finite element (FE) procedure that resolves the thread load along the helix during plastic-region tightening of a prototype aluminum bolt is compared with torque–angle–preload tests of the bolt in steel nuts and with the VDI 2230 combined-stress relation, using elastoplastic helical-thread analyses of aluminum and steel bolts, clamped plates and nuts. With the yield-detection algorithm of the tests applied to the model, the preload at the 50% gradient criterion (19.5 versus 18.4–18.6 kN) and the plateau preload (21.6 versus 21.2–21.4 kN) agreed within 6%; the angle to yield differed by a factor of three because the rig was three times more compliant, and an equivalent spring with the compliance inferred from the tests brought it to 237° against 210–220°. For thread friction coefficients of 0.1–0.3 the yield preload followed the VDI 2230 relation: the force at yield is set by the bolt and its thread friction, the angle to yield by the joint. Bolt plasticity equalized the load shares of the helical turns to 17–19% while the plastic strain concentrated in the first two threads; in an axial-tension mesh study a 2 × 2 refinement left the load shares and the plastically strained fraction practically unchanged but raised the maximum plastic strain by one third to two thirds. A tightening criterion for aluminum bolts should therefore specify the force or torque at yield and a bounded angle beyond it.