PSI - Issue 14

7

Rajesh Sharma et al. / Procedia Structural Integrity 14 (2019) 738–745 Author name / Structural Integrity Procedia 00 (2018) 000–000

744

Fig. 9. (a) SEM SE microstructures near fracture edge and (b) EDS spectrum.

3.4. Chemical analysis and hardness The chemical composition of the cartridge case has been determined by energy dispersive spectroscopy attached with SEM. The composition is near to 70/30 brass. Hardness was measured at different locations from 1 to 6 using Vicker’s hardness tester employing 500 gm load for 15 sec. The average hardness values at each location are shown in Table 1.The hardness values of locations 1 and 2 are nearly same and are equivalent to typical annealed microstructure.The high hardness values at locations 4 to 6 are not surprising since corresponding microstructures display deformed microstructures. On the other hand, the high hardness value of location 3 in comparison to those of 1 and 2 are due to very fine microstructure.

Table 1: Vickers hardness values of cartridge case at different locations. Vicker’s Hardness (Hv), 500 gm load

Locations Hardness

1

2

3

4

5

6

101

100

128

183

178

190

As mentioned above, the microstructures of the cartridge case are different at different locations. The corresponding hardness values also support the same. In addition, the cracks are observed at different locations along with typical branching. The microstructure near fracture is highly deformed and reveals the presence of curved flow lines. This reflects localized deformation due to high pressure. The corrosive pits present at inner surface of the cartridge case provide crack initiation point for cracks. Subsequently these cracks propagate from inner to out surface and finally results in fracture [Fontana, 1986; Jones, 1992; Warke, 2002]. The cartridge case failure in present case is therefore attributed to stress corrosion cracking. 3.5. Concluding remark The cartridge case has failed due to stress corrosion cracking (SCC). The SCC is seen extending deep into the section from inner to outer surface causing loss of strength of the material. Acknowledgements Authors are grateful to The Director DMRL for his kind support and encouragement.