The Behaviour of High-Speed Impact Tensile Fracture of Screws

Abstract

For the purpose of investigating the intrinsic nature of the impact strength of screws and establishing the ways of designing and utilizing the bolts under impact tensile load, the screw and bolt specimens with or without nuts were subjected to impact tensile load from the nut side, and the behaviours of high-speed impact tensile fracture of screws were examined. The bolt materials used were made of several kinds of carbon steel, stainless steel and IN steel, and the impact velocity ranged from static to 60m/sec.The results are summarized as follows:(1) To examine the high-speed impact tensile characteristics of screws, it is appropriate as the test condition to employ a specimen fitted with a nut.(2) The impact tensile strength of the screws made of materials having large elongation characteristics, such as S10C-D, S15C-D, S20C-D, 13 Cr steel and IN steel, was greater than that of the round bars at the root. The strength is generally comparable, up to the impact velocity of about 10m/sec, to the impact tensile strength of an equivalent round bar with a diameter of de=(pitch diameter of thread+root diameter)/2. However, beyond the impact velocity of 20m/sec, the latter value exceeds the former and the notch effect appears conspicuously.(3) In the case of a material having small elongation characteristics, such as S55C-D (δ3.7: 25%), the static tensile strength corresponds to that of the abovementioned equivalent round bar. Although a considerable dispersion was observed for the impact tensile strength, its mean value seems to correspond to the impact tensile strength of the equivalent round bar. In the case of a high carbon steel (C: 1.1%), the static tensile strength similarly corresponds to that of the equivalent round bar, but in the impact test all the specimens inclusive of those having a root diameter smaller than the diameter of the equivalent round bar showed a break in the screw portion prior to a break in the smooth portion. Therefore, the abovementioned concept of impact tensile strength of the equivalent round bar is not applicable to this case. Accordingly, a material having large elongation characteristics under high-speed deformation is required for impact-proof screws.