The original bolt phenomenon (Virgin Bolt Phenomenon) was created by Rolf Steinbock in an early study of the Superbolt stretcher. To determine the preload force value, tighten the Superbolt stretcher to stretch the corresponding test screw and measure the total length before and after the test (using Hooke's Law to calculate the load). The inconsistent results in the test caused the icon data to form a strange curve that could not be explained. When the screw was re-measured after releasing the test load, he observed a peculiar phenomenon that the test screw did not always return to its original length, resulting in a large error (one of which reached 34% in one test).
What is puzzling is that this phenomenon only occurs during the first test. After this test, the screw can be restored to the newly established base length. This "permanent stretching" on the surface is not due to material plastic deformation, as the test is performed at yield values. Further testing has shown that this is a problem that occurs when pre-heat treated materials are processed into studs. This phenomenon was not observed in the post-heat treated bolts. This means that the heat treatment causes the bolt rod to quickly harden outside, causing the core to undergo intense compression and to bring the grains together. Stripping the outer layer during manufacturing releases this pressure. The tensile load applied to the exterior of the finished screw effectively repositions the grain (as if the chain was unwound).
Since the test is performed on the original machined piles during the test or on-site repair, the screw must be stretched once and the new base length must be re-established to ensure the correct value. The original bolt phenomenon is not well known, but it is an important variable when properly evaluating tensile measurements and must be taken into account.
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