The effect of the head easement is examined by using FEA
to calculate contact forces and pressures between the bar and
the rail. Although FEA was conducted for both supported and
suspended joints, only results for suspended joints are presented
in this paper. In the results that follow, the number of bolts is
varied (4 versus 6 bolts) with and without the easement in 6-
hole standard joint bars connecting 136 lb rail. Table 5 shows
elevation views for contact forces acting on standard joint bars.
The contact forces acting on the top of the bar have a triangular
shape with a sharp peak or maximum value. The maximum
contact forces are higher in bars without the easement than in
bars with the easement. Contours of contact pressures greater
than 1 psi acting on the joint bar and 136 lb rail are presented in
Table 6. Contact pressures are shown to be greatest adjacent to
the easement. Furthermore, contact pressure on the bottom of
the joint bar is lower than on the top for either case. High
contact stresses are likely to cause plastic flow, which may
ultimately lead to initiation of cracking.
Bolt tension was also calculated using FEA. A nominal
value of initial tension was assigned to each bolt in the
assembly, prior to any externally applied loads. Tension in the
bolts was then calculated when the wheel is located directly
over the center of the joint. Table 7 shows bar graphs for bolt
tension before and after the wheel load is applied. In all
suspended joint cases, bolt load increases in every bolt when the
wheel load is introduced. Independent of whether 4 or 6 bolts
are used in the assembly, the two inner-most bolts are shown to
have the greatest increase in tension. In all supported joint
cases, bolt load decreases in every bolt when the wheel load is
introduced.
Measurements of residual stresses in joint bars were
conducted by the Transportation Technology Center, Inc. [16].
Residual stresses are generally compressive at the top of the
rail. However, the effect of residual stresses was not examined
in the present FEA study.