INNOTRACK Deliverable 4.2.3 -- Improved model for loading and subsequent deterioration of insulated joints
Numerical simulations have been carried out to assess the influence of key parameters on the deterioration of insulated joints. The measures of fatigue damage were adopted; one low-cycle fatigue based and one ratcheting based. Both of these models are based on the stress–strain evolution, which consequently needs to be accurately modelled. This calls for a constitutive model that can deal with plasticity under general multi- axial loading. In the current study the material parameters in the constitutive model have been calibrated to reflect the characteristics of the rail steel grade 900A.
The simulations verify previous findings that the insulating layer does not carry any significant load. Consequently severe strains occur at the rail head edge. The ratcheting based fatigue criterion was found to be the most suitable for the current study and the accumulated plastic strain after four load passages, εeff, was adopted to quantify the ratcheting. The numerical simulations showed that increasing the insulating gap from 4 mm to 6 mm gives roughly the same effect as an increase of the vertical load from 150 kN to 200 kN, which is an increase in εeff of about 10%. In addition, a very detrimental effect of traction and braking was found: Increasing the longitudinal load from 0 kN up to 45 kN caused an increase in εeff magnitudes with up to 95%.
The predictions of plastic deformation and rolling contact fatigue have been performed. A fixed traction coefficient was presumed giving a wear pattern that is a linear function of pressure, which is in turn proportional to the cube-root of the applied normal force. The highest wear is predicted immediately after the insulated joint, with additional high-wear spots appearing a few metres later as the wheel bounces down and settles.
In addition to numerical simulations four operational joints have been monitored in field to follow the degradation. It was found that material damage is induced very fast. In accordance to numerical simulations the rail ends closest to a nearby station showed the largest degree of damage. This damage had a wear-like appearance. After 8 months it was found that also “cavity-like” damages had formed in the vicinity of the insulating joint.