Study Of Coach Body Structure Based On Nonlinear Method

Integral bus is becoming the most popular body structure because of its lighte weight,good rigidity and so on. It is a spatial truss structure which is welded by rectangular steelpipes. Due to the cancellation of the frame, the coach body bears all the working load and avariety of impact load. Higher requirements of the body structure strength and stiffness areneeded. Therefore, how to describe the stress level of the structure more accurately hasimportant significance on the integral bus body structure design.Traditional research on coach body structure is based on the linear analysis method. Butunder extreme cases, plastic deformation will occur inevitably and the stress is so high thatsome is even more than the fracture limit. The linear analysis method is not able to calculatethe stress accurately. At the same time, during mechanical design process, the safetycoefficient is based on the yield criterion. Therefore, the structure is not recommended to beused when the stress is beyond yield limit.However, the fracture doesn’t happen at drivingconditions except some structure bearing alternating load. Thus plastic deformation likelytakes place when local structure stress is beyond yield limit, which leads to stressredistribution and intensification. Therefore, linear analysis method cannot describe yieldcharacteristics.At first，the nonlinear analysis method about rectangular pipe fracture is explored.According to the tensile experiment, an effective stress-strain curve is obtained. Ductilityfailure criteria is chosen and the strain value at fracture is determined as well as the damagefactor. At the same time, the element stiffness softening evolution is also confirmed to realizethe nonlinear analysis method on considering material nonlinearity, nonlinear contact andgeometry deformation.Finally, the pipe failure and crack propagation mode of therectangular steel are obtained.In order to validate the effect of the nonlinear analysis method, a fracture experiment isdesigned. To improve the success rate of the experiment and experimental efficiency, a semicircle hole is opened to guide the fracture to happen at the middle part of the pipe. Thereaction force-displacement curve, the crack position, orientation and size of the hole provethe effect of the nonlinear analysis method well.Basing on the effect of the nonlinear analysis method, the structure characteristics arerecounted by nonlinear analysis method at three typical working conditions. According to theanalysis results, the stress is lower than the results of the linear analysis method, and thehigher the stress is, the bigger the gap is. Not only that, the calculation results also prove thestress is redistributed, the plastic area is lager than it which is calculated by linear analysismethod, and the stress concentration coefficient is smaller. All show that linear analysismethod is not a good way to calculate structure stress whose level is higher than yield limitand is not conducive to light the weight.However, there are so many iterations during the nonlinear analysis process. Therequirements of the computer resource are more and the computation time is longer. Basingon the existing model, a curve is produced which describes the relationship of linear analysisstress value and the stress ratio. And the linear analysis stress value is corrected.