| Off-road vehicles are prone to rolling over during working, which may cause personal injury even death once the accident happens. Operator protection system mainly includes Roll-over Protective Structure (ROPS), operator restraint system and interior design of the cab. After ROPS equipped in off-road vehicles, the injury rate of drivers is greatly reduced in the roll-over accidents. ROPS which is proved qualified by laboratory testing may become deformation and impended DLV (Deflecting Limited Volume), and thus threatens the safety of driver. Different restraint systems can cause different protective benefits, the dynamic response of ROPS can affect the human injury and the injury mechanism should be study deeply etc. So the forth study of operator protection system performance during rolling over accidents is required.This paper based on national natural science foundation program"Roll-over safety technology for off-road vehicle"(No.50775095) and national"863"program"Digital design platform of safety for Roll-over and Falling-object of off-road vehicles (No.2007AA04Z126). The relationships between human injury and some parameters such as body parameters, seat belts, ground parameters and roll-over protection structures energy absorption and loading method were studied in theory and experiment during rollover accident of off-road vehicles.The basic theory of occupant injury simulation and relevant biomechanics knowledge during off-road vehicles rolling over accidents were studied in this paper. According to the main injury parts of the operator during roll-over accidents, the related injury criteria were discussed. Motion equations and solving methods in multi-body dynamics were studied. The control equations, and the integration method, time-step control as well as the hourglass control having influence on the simulation accuracy and computational efficiency in the explicit finite element method were taken into consideration. And the calculation of contact forces during rolling over accidents was given.The multi-body model of a certain wheel loader was established. The modeling processes of ROPS, bucket, tires and seat belts, as well as connections between components were mainly discussed. The tire stiffness tests were performed, including radial stiffness and lateral stiffness. The influence of changing inflation pressure and vibration frequency on the tire stiffness and damping were analyzed. The stiffness properties of different ground were calculated by using the load-sinkage theory, in order to simulate the contact characteristics of the ground.The restraint effectivenesses of lap belt, three-point and four-point seat belts were analyzed applying this multi-body dynamic model. The dummy's motion and the injury values under different restraint systems were obtained. The results show four-point seat belts can protect the driver's safety better during the roll-over accident.The dynamic clearance space for the driver is put forward, and the DLV should be extended to it in ROPS dynamic test, then the injury criterions are used to evaluate the performance of ROPS.Taking the human body, occupant restraint system, seat belts, driver protection structure, vehicles and ground as a system, the rigid-flexible hybrid virtual prototype model was established using nonlinear large deformation finite element method and multi-body dynamics methods. The dynamic responses of vehicle parts were simulated using this model during the vehicle collision with the ground. The simulation results are verified correctly through the wheel loader model roll-over tests. Based on this, the injury values of human body essential parts were predicted in rollover accident. Through studying the human injury values with different stiffness of ROPS and different slope angles, we can get the following conclusions: the energy absorbed in ROPS dynamic roll-over test is larger than the static test, impending and non- impending DLV can not verify the quality of ROPS in the dynamic rollover test, the injury criterion should be the evaluation standard; under ensuring a certain survival space, the ROPS stiffness is smaller, the injury value is smaller, this is mainly due to the increase of stiffness of ROPS can lead to a shorter contact time and the development of larger reaction forces and consequently the transfer of increased peak decelerations to the vehicle's occupants. It can make the human body range of motion increased and the risk contact with the parts of the cab also increased; the increase of slope angle can make a shorter contact time and make the contact angle and slip distance different, so the slope angle is greater, the deceleration suffered by operator is larger, the injury value is greater. And the injury parts are closely related to the angle of slope; overturning in large slope angle, the four-point seat belts can effectively reduce the probability of head injury, but the neck injury is similar to the result under wearing two-point lap belt, having little effect on the neck injury.As the three-point seat belt restraint effectiveness great influenced by the roll-over direction, it was optimized by orthogonal experimental design in this paper. 25 times simulation were carried out choosing three design parameters including seat belt stiffness, seat belt hanging point height and lap belt installing angle. Then the experimental data were processed based on Range Analysis and the relationship between primary and secondary of the three parameters were obtained. Finally, the value of head injury is reduced 40.242%.The innovative points in this paper include: (1) The virtual test was performed to simulate the rollover accident of off-road vehicles by taking the human body, occupant restraint system, seat belts, driver protection structure, vehivles and ground as a system, using the rigid-flexible hybrid virtual prototype model; (2) The human injury criterions are regarded as the performance requirement of the protection system,and the dynamic clearance space for operator is determined according to the trajectory of human head in rollover accident which is regarded as guiding the interior design of the cab in order to enhance the rollover safety for off-road vehicles; (3) The restraint effectivenesses of lap belt, three-point and four-point seat belts were analyzed applying the wheel loader multi-body dynamic model. As the three-point seat belt restraint effectiveness greatly influenced by the roll-over direction, it was optimized by experimental design in this paper, and then found the relationship between primary and secondary among the optimization parameters.The results of this paper develop the rollover security technology for off-road vehicles, which is now represented by the static performance analysis of independent components such as the protective structure and the restraint system, to dynamic rollover accident simulation about the system, which is composed by the operator, vehicle, restraint system and the protective structure. And the human injury criterions are regarded as the performance requirement of the protection system. These results can provide basis for enhancing the safety design lever for off-road vehicles.
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