PSI - Issue 37

Theodoros Marinopoulos et al. / Procedia Structural Integrity 37 (2022) 139–144 T Marinopoulos et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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called offsets , are given for each load level, at the ground, ankle, knee and top transverse planes of the limb. Since the socket is the only component tested in this occasion, only knee and top offsets where identified, with the top point representing the top of the femoral bone (Fig. 3).

Figure 3. Discretised total height between new calculated planes

Although a socket tested strictly following the standards’ instructions would result in a safe product, for the paediatric application of the current study the loads would be unreasonably high. For this reason, new load requirements and corresponding load-application points were calculated. A second-order curve was fitted to the load requirements of the three described classes. The fitted equation was solved for the pa tient’s weight of 73.4 kg corresponding to the 98 th percentile of its age group (NatCen Social Research and University College London 2017) to obtain the new minimum load levels that the socket should withstand, resulting in 3850 N. The same method was used to calculate the offsets. A set of square beams in the identified dimensions were produced able to transfer the load in the described by the standards direction. The load was applied to the sockets using a residuum limb manikin, which was beforehand casted using silicone rubber (Dragonskin 10 SLOW) with material properties similar to those of the human tissue (Mak, Liu, and Lee 1994). All sockets were initially loaded up to 1020 N (this level acted as a settling force) and then unloaded to what would be the starting point of the static test. After that the sockets were loaded with a constant rate until their failure. To avoid effects of the time-dependent properties of the manikin, the compression was conducted at 0.5 mm/s. A universal Instron Series 3000 with a 10 kN loadcell was used for the tests. 3. Results Initial results suggest that 3D printed sockets can sustain the required loads when material and design properties are met. More specifically, the initial design of the round-ended socket failed to meet the requirements after failing below 3400 N. Still, both the CFN and the redesigned PLA socket (Fig. 2B) managed to exceed – although marginally – the identified load requirements, reaching the loads of 4050 N and 4280 N, respectively (Fig. 4).