Determination of hyperelastic and viscoelastic properties of washing machine door sealing and finite element analysis of its dynamic behavior

Elastomer materials have relatively low stiffness compared to other materials. Elastomers are used on washing machine door due to the low gas permeability, high deformation without damage and sealing properties. As a result of high deformation without damage washing machine tub can move easily relative to front board. In this study static and dynamic properties of door sealing of washing machine were studied. Static and Dynamic test of washing machine door sealing were performed with specimens. For the hyperelastic material properties, specimens were tested with different deformation modes. Uniaxial tension, planar tension and equibiaxial tests were conducted to represent three different deformation modes. Test results were fitted to hyperelastic material models by Msc. Marc software. After determining the best suited hyperelastic model, uniaxial, planar and equibiaxial tests were conducted on Msc. Marc software. Experimental and finite element results are compared. Uniaxial test was made by tensile test machine. Specimen was cut accordance to ISO standard from 2 mm rubber sheet. Force required to deformate the rubber specimen was calculated by tensile test machine. Engineering stress was calculated by force and cross section area of specimen. Strain was measured by extensometer that was attached to upper and lower part of the specimen. Force and displacement were measured simultaneously. Engineering stress – engineering strain curves were obtained. Planar tension test is used for understanding the shear properties of material. Test specimen was cut from 2 mm rubber sheet. Dimensions of the test speciment were determined in accordance to Msc. Marc user guide. To conduct planar tension test fixture was designed. Fixture was attached to tensile test machine. Engineering stress was calculated by force and cross section of the specimen. Force was measured by tensile test machine. Engineering strain was measured by optical system. Markers were placed on the center of the specimen to minimize the boundary condition effects. To conduct equibiaxial tension test fixture was designed to make test on tensile test machine. Equibiaxial test specimes were cut from rubber sheet which has 2 mm thickness. The fixture converts compression force to equibiaxial tension force. Force required to deformate spicemen was calculated by tensile machine. Engineering stress was measured by converting compressive force to equibiaxial tension force by kinematic analysis of the fixture. CAD model of fixture was modelled on NX software to make kinematic analysis. Force calculated by tensile test machine converted to equibiaxial tension force by kinematic analaysis of fixture. Markers were placed on the center of the specimen to capture pure equibiaxial strain. Strain was measured by optical system. Engineering stress – strain curve was obtained. After making three different experimental tests and generating engineering stress – engineering strain of the tests, test results were entered to Msc. Marc software. Elastomer data fit tool was used to fit experimental data to existing hyperelastic material model on Msc. Marc software. Uniaxial, planar and equibiaxial stress – strain curves were used in the same time to calculate constants of the hyperelastic models. Ogden, Neo-Hooken, Mooney Rivlin and Yeoh model parameters were obtained by elastomer data fit tool. With the positive constant option, only positive material constants were obtained. Thus obtained material model did not have stability problem. Ogden model with 3 parameters was closest to experimental test results. For this reason Ogden hyperelastic material model was chosen. After determining the material model and its constants finite element model of there different experimental tests were created in Msc. Marc software. Hex elements with Hermann formulation were used to mesh the specimens. Same boundary conditions and displacement was applied to finite element model of three different test. Stress and strain of the specimens were obtained by static analyses. Stresses and strains were converted to engineering stresses and strains to compare experimantel results. Satisfying closeness between experimental results and finite element analyses results were obtained. Due to the operating condition of washing machine, door sealing is exposed to dynamic cyclic load. For this reason dynamic test was conducted with DMA testing equipment. Double shear specimen was used on DMA test. Specimens were cut from rubber sheets with 2 mm thickness. Dimensions of the specimens were determined by the DMA testing machine. Test was conducted with constant strain rate in 30 ° C degree temperature. Rubber specimen was exposed to cyclic shear load from 1 Hz to 100 Hz frequency. Storage modulus, loss modulus and phase angle were measured in frequency range by DMA testing machine. DMA experimental test results showed that storage and loss modulus increased by frequency. Because of working condition of the washing machine only modules in 1 Hz to 30 Hz entered to Msc. Marc software. Storage modulus and loss modulus were imported in Msc. Marc software. Experimental data fit tool was used. Storage modules in 1 Hz to 30 Hz were entered as a Hyperelastic Shear Storage Modules. Loss modules in 1 Hz to 30 Hz were entered as a Hyperelastic Shear Loss Modules. Long Term Stiffness value was calculated from Ogden hyperelastic material model. Storage modulus and loss modulus were fitted in to prony series by Msc. Marc software to represent viscoelasticity. Five coeffients of prony series were obtained. After determining viscoelastic and hyperelastic material model, finite element model of door sealing was created on Msc. Marc software. Shell mesh was used to model door sealing because of small thickness. Material of the door sealing finite element model was defined as hyperelastic material model that had been obtained before. Door sealing was analyzed statically on Msc. Marc software and Radioss software. Region of the door sealing that is attached to washing machine tub was fixed. Imposed displacement was applied to region of the door sealing that is attached to front board of the washing machine. Reaction force of the region that is connected to washine machine tub was calculated. By displacement and reaction force stiffness of the door sealing was calculated. Same analysis was conducted on Radioss software and same result was obtained. After static analyses, door sealing was analyzed dynamically with 1 Hz to 30 Hz imposed displacement. Ogden hyperelastic and viscoelastic material was used. Displacement and reaction force were obtained. It was seen that with hyperelastic material model, door sealing did not have damping properties. With hyperelastic and viscoelastic material model, door sealing showed damping properties.Phase angle was calculated from result of the analysis. Damping and rigitidy of door sealing on 1 Hz to 30 Hz frequency was obtained. As a result of the dynamic analysis of door sealing, rigidity of door sealing increases with frequency. Damping of door sealing increases with frequency as well.