In order to describe the external and internal deformation states of air springs several FE-related tools (in particular: new material laws) were developed. The calculations were largely based on measurements on different components of the air spring wall.
In particular a new inelastic formulation for the rubber compound was developed by the Universitaet Hannover on the basis of a series of cyclic tensile and shear measurements on rubber samples carried out by Akzo. The constitutive equations reflect most of the observed elastic and inelastic rubber characteristics:
elastic: 'Neo-Hookean' base characteristic
damage: (recoverable) stiffness reduction after 'prestretching'
hysteresis: direction-dependent additional stresses
non-viscous: no strain-rate related effects
Other material models were defined for the description of complete cord layers, individual cords and filament material. In FE calculations stress and strain distributions within the air spring wall were calculated for comparison with a simultaneously established fatigue test data base. Fatigue tests were carried out on different air spring tyres and on adapted pressurized cross-ply tubes.
Although a comprehensive correlation analysis between the calculated stress strain data and the tested failure behaviour of air springs could not be concluded within the project framework a good correspondence could be found between areas of calculated maximum shear stress and the origin of observed fracture phenomena.
For the design and optimization of products based on rubber-cord composites the knowledge of internal micromechanics and related fatigue mechanisms under application loadings is of fundamental importance. Several Finite Element programmes (e.g; ADINA, MARC, ABACUS) offer methods for a realistic simulation of deformations and some internal stresses and strains. Composite structures though are usually modelled using transverse isotropic elemental formulations, which only allow a macroscopic description of the internal deformation states. These methods do not take into account the helical geometry and the nonlinear behaviour of the cords. Also, most interactions between the cords and the surrounding rubber are ignored.
Within the project new experimental techniques and numerical tools are to be developed to enable a better modelling of stress and deformation states of thin walled rubber cord composites (e.g. air bellows)