Failure Analysis of Breakaway Couplings

摘要

As part of a lighting pole installation, typically used only on high speed roads to reduce the effect of impacts, four breakaway coupling are employed between the pole and the concrete base in the ground. Internally threaded, they screw onto four threaded steel rods, one at each corner of the concrete base. Threaded rods pass through holes in the pole base and screw into the opposite end of the coupling. This coupling system has several aims. Firstly, minimize damage to vehicles and occupants as little energy is transferred into the vehicle. Secondly, rapid pole replacement can be achieved with no base work required. Thirdly, the time spent at the work site is minimal which increases the safety for the highway workforce. Fourthly, poles can be reset in winter. If the threaded rods or the base area are damaged, or if the base is moved, the base must be adjusted or replaced. This is difficult when the ground is frozen. As the behavior of the poles directly reflects the performance of the breakaway coupling, this study to assess breakaway couplings was conducted. The purpose was to determine why breakaway couplings fail when not under impact conditions, why they do not fail after impact and why do they damage the base concurrent with the pole falling during impact in some instances. A field examination was combined with mechanical property testing and metallography in order to understand the processes occurring during service that may affect performance.