Investigating and interpreting failure analysis of high strength nuts made from nickel-base superalloy

The United States Air Force experienced failures of three separate zero-time, 220 ksi Inconel 718 nuts that were installed on A-10 aircraft. During the investigation, another nut fracture occurred on an operational jet. This bolt/nut combination had very few service hours on it and was well below the expected service life. A thorough review of the procurement data revealed that all the failed nuts had passed lot tests successfully. This led to concerns that the current procurement test requirements were insufficient to identify nuts that could unexpectedly fail in service. Another factor convoluting the situation was evidence of shanking on one of the four failed nuts. In order to determine the risk for potential future nut fractures, a novel experimental approach was developed and executed to evaluate the strength and integrity for all 220 ksi Inconel 718 nut manufacturers and sizes utilized on the A-10 aircraft. These experiments were designed to have a combination of shanking and pre-load and push the nuts to extreme limits with the intent of uncovering sub-par capability. The amount of torque at failure and the distance traveled by the nut were the two measures of performance. In addition to these experiments, a rigorous metallurgical evaluation was performed to determine if there were any metallurgical anomalies within the fractured and tested nuts. The measured torque values were then used to correlate to the metallurgical evaluations and the results are presented. This novel testing method provided quantitative data to determine the impact of the manufacturing process on the performance of the bolt/nut combination and demonstrated that a characterization of the microstructure alone may not provide evidence of corresponding nut performance.