PSI - Issue 14

Sudheer Patri et al. / Procedia Structural Integrity 14 (2019) 688–695 Sudheer patri et. al.,/ Structural Integrity Procedia 00 (2018) 000–000

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4.1 Metallurgical investigations Metallurgical investigations on the failed screws were carried out to find out the root cause of failure, so that necessary steps can be adopted to prevent such failure in future. Initially, fractured surfaces of failed screws were examined by scanning electron microscopy (SEM), and the fractographs are shown in Fig. 4 (a) to Fig. (c). These fractographs revealed fully ductile mode of fracture. Energy dispersive x-ray spectroscopy (EDS) was also carried out on the fracture surfaces, which revealed traces of oxide inclusions as shown in Fig. 5. Chemical analysis from EDS indicated deviation in chemical composition of screw material from the standard as shown in Table 1. Chemical composition clearly indicates that the material used for construction of these screws is not comply with the depicted standard i.e IS 1367. Further, macro-hardness measurements were also carried out on polished surfaces of these failed screws using Vicker’s macro-hardness tester with a load of 10 kgf. Results revealed an average hardness of 229 HV 10 against specified value of 356 – 414 HV 10 as per same standard. The hardness result clearly indicates that the material for the failed screws is not as per standard. However, due to material constraint, it was difficult to measure other mechanical properties of this screw material; reference of hardness value was only considered for comparison. 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.

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Fig. 4. Fracturograph of failed screws showing (a) complete cross section with deformed region, (b) dimples indication ductile mode of failure and (c) presence of inclusions