Freight rail cars may experience high input forces during a coupling event, which could potentially cause damage to the car body and/or lading. The AAR recommended practice states that cars should not be coupled at speeds greater than 4 mph. However, this recommendation is not always followed and cars are often coupled at much higher speeds. As a result, accelerometers on the car body are sometimes used to monitor impact events. Threshold levels may be set to determine if an over-speed or high-force impact event has occurred. However, a single acceleration value can be difficult to interpret because its relationship to impact force is dependent on many factors, including car type, end-of-car device type, lading type, and loading condition. Dynamic modeling and parametric studies may be used to determine these relationships which can be applied in practice.
This paper presents a study on the relationship between struck coupler force and car body acceleration for a series of impacts on a tank car in both loaded and unloaded states. For the loaded condition, the tank was filled with water. The simplest change from an unloaded tank to a loaded tank is the decrease in acceleration for a given force due to the added mass. However, there is additional complexity added to the system due to the sloshing liquid inside the tank. When attempting to model this dynamic system there is added uncertainty in struck coupler force estimation because of the non-linearity in low frequency car body oscillations. Several example data sets are presented in the time and frequency domains to illustrate this point. The data is then used to generate an empirical model using system identification techniques. The results show that the proposed model offers improved characterization of the system as compared to conventional techniques by accounting for the uncertainties introduced by the sloshing liquid in the tank. The proposed technique is computationally efficient and can potentially be implemented in real time. The model is used to estimate struck coupler force and is validated with real data.