Failure analysis of structural screw joint in a start-up neutron detector handling mechanism

Abstract This paper deals with the failure analysis of a structural screw joint located in the load transfer path of a complex mechanism called Start-up Neutron Detector Handling Mechanism (SNDHM). It is a carrier mechanism for high temperature neutron detectors, which measure the neutron flux during initial criticality. In installation testing of SNDHM, failure was observed in all the 6 no’s of M5 screws of this structural joint. These screws were as per standard IS 1367-3 with property class 12.9. Subsequently, these failed screws were analysed to find out the root cause of failure, so that necessary measures can be adopted to prevent such failure in future. Chemical analysis from EDS indicated deviation in chemical composition of screw material from the standard. Hardness tests results revealed an average hardness of 229 HV10 against specified value of 356 – 414 HV10. Based on the hardness values, which is nearly 40% lower than the expected value, it is assumed that the estimated strength of the screws would be much lower than the specified values as per standard. This implies that the joint has failed at load, which is much lower than the designed value. However, the possibility of over load either due to errors in packing or due to abnormal operation cannot be ignored. It is postulated that the un-noticed motor over-travel during final stages of testing or packing errors would have resulted in over load. It is proposed to incorporate a torque limiter in the drive line of the mechanism to prevent over loading due to erroneous motor operation in future, apart from use of screws which comply with the standard.