In our previous work  we described our simulation method and investigated the influence of important parameters (for instance the particle shape or the wall-normal restitution coefficient).
As these four equations show the coefficient of kinetic friction [[micro].sub.k] and the wall-normal restitution coefficient [e.sub.n] have to be known to calculate the particle velocity after the wall collision.
(After all, the impacted area of the struck vehicle straddles its center of mass.) Separation velocities are obtained by integrating the appropriate accelerometer channels, from which the restitution coefficient (not accounted for in Crash3) is easily calculated.
where [epsilon].sub.0] is the restitution coefficient at the no-damage threshold.
where [e.sub.ij] is the restitution coefficient. The effective mass [m.sup.*] = ([m.sub.i] + [m.sub.j])/[m.sub.i][m.sub.j], where [m.sub.i] and [m.sub.i] are the mass of surface sphere.
So, coefficient of friction [mu] = 0 and the restitution coefficient e =1.
and where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]--contact stiffness; [e.sub.k]--speed restitution coefficient; [E.sub.R].
sub.e,i]) = 0.3 Railway vehicle 1/8 car body mass car body damping [C.sub.bg4] = coefficient 10 kNs/m 1/4 bogie mass frame damping [C.sub.bg3] = coefficient 100 kNs/m 1/2 wheelset mass wheelset damping [C.sub.bg2] = coefficient 50 kNs/m Mass in contact damping coefficient of [C.sub.bg1] = mass in contact 44.2 kNs/m Car body stiffness wheel Radius [R.sub.W] = 0,495 m Frame stiffness elastic modulus of wheel [E.sub.W] = 210 Gpa Wheelset stiffness restitution coefficient [[??].sub.max] = 10 m/s Maximal Pisson's coefficient e = 0.65 penetration of wheel velocity Exponent Fig.
The restitution coefficient is equal to the ratio between the relative speed of the separation of the mass centers just after the impact, [([V.sub.B]).sub.2] - [([V.sub.A]).sub.2], (Relative speed just after the impact) and the relative speed of the approach just before the impact [([V.sub.A]).sub.1] - [([V.sub.B]).sub.1] (Relative speed just before the impact).
Table 2 shows the value of the final speed of the test vehicle and the restitution coefficient with respect to the mass of the previously selected vehicles.
This value was also employed for characterisation of the particle-particle interaction, thus the normal restitution coefficient
magnitude is [c.sup.n] = 0.5.
To calculate the parameters of collision, the theorems of change in linear and angular momentum and two restitution coefficients
of normal impulse [R.sub.1] and [R.sub.2] by sides 1 and 2 (1), (2) can be used (Lavendelis et al.