After several years' use, an eyebolt suffered brittle fracture in the first turn of the thread. The fracture started at the notch at the root of the thread. Neither localized material defect nor an old crack were present. The investigation showed that instead of the specified steel quality St 37-2 N, a steel with about 0.5% C had been used. The microstructure with the coarse ferrite network indicated that the forged eye bolt had been normalized either at too high a temperature or not at all. In any case the anneal at 900 deg C produced a considerably more finely grained structure. In addition, the nature of the fracture and the results of the notched bar impact tests showed that in spite of the high C-content, the eye bolt had become brittle as a result of aging.

A special eyebolt was used to lift prefabricated concrete panels weighing approximately 16 cwt. Two eyebolts were used with a spreader bar to give a vertical lift on each eyebolt. Following failure of one eyebolt, which resulted in dropping of the load and subsequent failure of the other one, a complete eyebolt was submitted for assessment. Microscopic examination indicated a medium carbon-manganese steel had been used for the lower screwed portion of the eyebolt. Failure may have been due to brittle fracture or to fatigue, both of which could have been initiated at cracks in the hardened material in the region of the weld securing the screwed portion to the intermediate collar and which may have formed at the time of manufacture. Out-of-squareness of the thread with the collar, as was seen in the example submitted, gave rise to bending stresses when the bolt was tightened down, and this could have been a further factor which promoted failure. It was suggested that the design and construction could be improved by either making the component in one piece or, if it was desired, to adapt a standard eyebolt.

A steel eyebolt which attached a rear lift strut to the right wing of a helicopter failed by fatigue. As a contributing factor, thread cutting produced sharp notches at thread roots, reducing fatigue life. Also, design fatigue life may have been exceeded as the part was in use about 10,000 h. Cumulative damage resulting from a previous accident could have occurred too. Because of this accident, inspectors were instructed to examine threaded zones of eyebolts by magnetic particle inspection after every 100 h in service. A maraging steel drive shaft of a helicopter also failed because of corrosion (pits), and continuous abnormal misalignment as well. Corrosion probably developed from moisture and water droplets on shaft diaphragm profiles. Improved diaphragm pack seals and coatings made by an electron-coat process (such as a Sermetal finish) are now used in new shafts.