ICAR Technical Series no. 7 - Nitra Proc.

Modelling of liner behaviourMaterial andmethodsGeometry andmaterialcharacteristicsThe liner material properties were measured for three similar liners bycarrying out tensile tests on the materials in accordance with BS903:A2(BSI, 1995).The liner geometry was obtained from both part drawings and internalmould impressions. The internal mould impressions were taken using asurface impression compound, Technovit 3040, which was subsequentlypoured into the liner barrel. After a few hours when the compound hadsolidified the liner was split and the internal liner impression was removed.Dimensions were measured from the liner impression.Finite elementanalysisA nonlinear finite element model was developed with 8 noded solidelements using Ansys 5.6 to model the collapse of the liner. The modelgeometry was defined parametrically in terms of upper barrel bore, lowerbarrel bore, barrel length, wall thickness, axial stretch and radial stretch.The elastic modulus for the material was also entered as a parameter. ThePoisson’s ratio was fixed at 0.49. A two dimensional section of half theliner was constructed by defining various keypoints and areas.These areas were meshed with a special element (MESH200), which isintended for multi-step meshing operations that require a lowerdimensionality mesh to be used for the creation of a higher dimensionalitymesh, i.e. sweeping a 2-D mesh into a 3-D mesh. Model meshing wasadditionally parameterised which included the number of elementsthrough the wall thickness, the element size and the number of elementsin a 180° rotation.This mesh was rotated about 180° and the mesh elements converted tosolid 8 noded elements (SOLID45) used for 3-D models with materialsobeying linear material constitutive laws. The materials were linear andisotropic in the strain area of interest. Despite the axisymmetric linergeometry it was necessary to model half the liner as the deformed modelshould only have one plane of symmetry. Specific groups of nodes weredefined as components C1 to C5 within the geometry to simplify theapplication of loads and constraints.Since the geometry and loading pattern are symmetrical the finite elementmodel has no material imperfections and will not deform correctly whenloaded. This is because the liner barrel will contract in the radial andcircumferential directions and the material will compress therefore it wasnecessary to introduce an imperfection. Eigenvalue buckling analysisdetermined the initial buckling mode shape upon which miniature nodaloffsets were applied to introduce the imperfection. The nodal offset wascomputed as 10% of the normalised buckling mode shape, i.e. a maximumoffset of 0.1mm.264Conference on "Physiological and technicalaspects of machine milking"