Experimental and numerical investigation of filled thermoset polymers as mechanical interface in fine tolerance applications
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As a part of reducing costly processes in production of marine thrusters, casting and machining of large steel parts is proposed to be replaced by welding of smaller elements. This does however require a procedure for compensating for the welding deformations, as these would most likely be to large compared to the tolerance limits for machine parts. The deformations are proposed to be compensated for by accurate aligning of parts followed by fixing them together using a casting compound. An interface compensating for welding deformations between the thruster stays holding the propeller shaft, and the shaft is examined, and a downscaled, simplified model for mechanical testing of the interface is constructed and produced. A casting method using injection of thermoset resin with 3 bar pressure results in a bushing with few and small bubbles, and a smooth surface finish. The mineral filled thermoset resin HUNTSMAN CW2215 and slow curing agent HY5160 is found to be applicable for the application in terms of strength, fracture parameters, pot life, curing shrinkage and water absorption. Through mechanical testing the material is found to have a viscoelastic (creep) behavior, where the behavior is estimated to fit into a exponentially decreasing material model (two-part prony series) through comparison with a finite element analysis (FEA). The strength of the interface is validated through mechanical testing of maximal transient and steady state axial loads from DNV ice loads regulations, conservatively scaled to match the downsized model. Fatigue strength of the interface is not tested due to time limitations. The results from mechanical testing is compared with an axisymmetric FEA and Mohr-Coulomb failure criterion and is showing no failure for maximal transient loads. Stress intensity factors for linear elastic fracture mechanics (LEFM) of high stress concentrations is found, and are low compared to the estimated fracture toughness of the material, resulting in a factor of safety of 8,8 for the full scale geometry. The FEA also shows that the interface design works as supposed, transferring loads as compressive stress rather than shear stress, utilizing the material characteristics. A 3D FEA analysis is performed to investigate the influence of non-axisymmetric loads and geometry, and shows that these effects are significant and will reduce the factor of safety.