br Funding This study was undertaken as part of

Funding This study was undertaken as part of a University of Birmingham Year 4 (MEng) Final Year Project, with funding made available from internal resources at the University of Birmingham\'s School of Electronic, Electrical & Systems Engineering.
Ethical Statement
Acknowledgement The University of Birmingham co-authors (Stone and Cooke) would like to express their sincere thanks for the early briefings and material assets provided by the Defence Diving School at HMS Excellence, Portsmouth.
Introduction Maraging steels are high strength steels widely used for aerospace applications where a combination of high specific strength and fracture toughness are the basis for selection. These steels are based on the discovery about the fact that controlled additions of cobalt and molybdenum to the iron-nickel martensitic matrix result in a combined age hardening effect significantly higher than the additive effects of each of these elements [1]. Further, minute additions of titanium and aluminum make the iron-nickel-cobalt-molybdenum matrix amenable to supplemental age hardening [2]. Maraging steels of different grades have been designed to obtain high proof strength and optimum toughness catering to varied applications [3]. Among the family of Ni-Co-Mo maraging steels, 18Ni-8Co-5Mo (with a 0.2% yield strength of “1700 MPa” or “250 kilo pounds per square inch”) steel is widely used. Some of the applications in aerospace systems include solid rocket motor casings, merman bands for stage separation, fasteners requiring a good strength and toughness combination [4]. In recent years, there is an increased interest in these materials in view of their uses for a range of applications [5–8]. In one of the applications, M250 grade maraging steel was used to fabricate shear screws used in the Mermen band ionomycin system of a satellite launch vehicle. These Mermen band separation mechanisms are generally used in the upper stages of launch vehicles, where higher shock levels are not allowed due to proximity to satellite and other sensitive avionics system packages. These types of systems are also used to separate the payloads from the launch vehicles as well as for circumferential separation of the payload fairing. This system consists of a fore end ring attached to the upper stage and an aft end ring connected to the lower stage [9]. The fore and aft end rings are clamped together by floating wedge blocks and are held together at the flanges by two preloaded semi-circular bands which are known Mermen bands. The separation system consists of 40 Nos. of wedge block integrated with two merman bands and band end block by wedge block holder and shear screws. The band assembly is connected by a bolt assembly and bolt cutter. The bolt cutter cuts the bolt and two band assemblies separate the stages. Failure of any of the components of separation system will lead to the mission failure. Therefore the fabrication and quality control are important aspects in the development of these critical systems. Separation mechanisms are designed to withstand ultimate structural load without failure and should separate only on command without causing damage to any other parts. Fig. 1(a) shows the photograph of the band with shear screws in place except the failed one left with an empty slot. The schematic of the fastener is presented in Fig. 1(b) and c shows the photograph of the failed shear screw head (left) and shank (right). In Fig. 1(c), the fracture surfaces are on the top. A closer look at Fig. 1(b) reveals that the shank diameter is 6 mm with k6 tolerance. Hence it can vary from 6.001 to 6.009 mm. This will be assembled to a hole with a diameter of 6 mm with H7 tolerance, which can vary from 6.000 to 6.012 mm. Hence the clearance ensured is −0.009 to +0.011 mm. This is a transition fit shear screw and if a normal screw is used on a clearance hole, obtaining a 180 degree bearing surface is not possible. Further, the screw may bend resembling a three point bending test.