عنوان مقاله [English]
Train movement on the railway tracks causes dynamic loads and impact to the track, stress and deformation to the different elements of the tracks and vibration, etc. Since, the defects of the track structure and its components depends directly on the dynamic response, stress and deformation of the track, it naturally cause amplification.
The main important types of ballast defects are ballast settlement, smash of ballast and hardening of ballast layer. The effects of deterioration of ballast on dynamic response, stresses, and deformations of track components, can be simulated by a real train-track modeling. When the ballast is not in a good condition, sleepers provide a small support. This situation can usually be seen on the railway lines. When the ballast undergoes deterioration, the sleepers suspend from the rail. This situation usually speeds up the deterioration of the track and for example, it can create surface corrugation of rail, damage or brakeage of fastenings, loss of elastic characteristic of rail pad, crack in the sleepers and uselessness of the ballast layers.
Grassie , has analyzed a 150cm suspended sleeper on an elastic bed by the method of continuous beams, also complementary studies have been carried out by Dalburg  with regard to its effects on dynamic behavior of track components. He has considered the effect of unfilled distance between the sleeper and the ballast in his dynamic model and has investigated the increase of interaction forces. Also, Gaoliang , has implemented the train-track dynamic modeling in his investigation for replacement of sleepers in high speed lines. Hence, if when replacing a new sleeper with an old one, the track is not tamped, in reality, the replaced sleeper is suspended. It has been tried in this paper to investigate the dynamic behavior of such sleepers in the whole structure.
Due to the fact that in the Iranian railway and in particular in districts such as North, Lorestan and North-East, the problem of rail corrugation, suspended sleepers due to the age of the track or replacement of sleepers, wear of the rails and deterioration of ballast exists, this paper has carried out a case study on the North-East railway district, considering the superstructure with UIC60, good and poor quality of ballast (in some section of the line), Wooden and concrete sleepers with a high speed trains of up to 160 Km/h. For this purpose a suitable model of a track with suspended sleepers has been considered and variation of rail-wheel interaction contact forces, acceleration, vibration and displacement of rail-sleeper and ballast has been investigated. The elaborated model is of a discrete supports with one suspended sleeper. The mathematical model of the track was developed through the former model [4,5], in order to undertake the possibility of suspending the sleepers.In normal situation, the track components dynamic's behavior, takes place by modeling of rail in the form of continuous beam on discrete elastic support. In practice, sometimes due to deficiency of the track bed or the ballast layer, sleepers supports do not perform well, which is termed as "loose sleeper". In this article, mathematical model of the track consisting of several suspended sleepers has been developed. According to this model, equations of motion for the track-train has been formed by considering non-linearity of wheel-rail contacts and has been solved by numerical integration in time domain. Dynamic response of track components with harmonic excitation source has been investigated. From the results it can be seen that, intensive vibrations, would cause an increase in wheel-rail contact forces, e.g. harmonic excitation of 0.4 amplitude and wavelength of 240 mm on the rail (measured in North-East railway district), shows an increase of 25.5% in contact forces (for trains of 160 Km/h) and 74% in vertical displacement of the rail. Also the results showed that; lack of one sleeper on the track would act as an excitation source and would cause irregularity in vibration (vibration acceleration, vibration velocity) for railway track superstructure components.