Study of car roof crushing due to falling tree trunk under quasi-static & dynamic loading for varying heights & materials

Abstract

During vehicular accidents, there are several scenarios where the roof of the vehicle comes under direct load. The vehicular components like airbags and restraints (seatbelts) are responsible for providing safety, but the structural integrity of the roof determines the amount of cabin headspace the driver and the passengers get. Vehicular Rollover is one such accident where the roof can prevent fatal neck injury if it is structurally robust. When a heavy object falls from above, the roof becomes the only component that separates the passenger’s head from the object. One such example is a heavy tree branch, or a trunk that has fallen on moving cars causing damage to the roof and compromising cabin head space. A limited amount of literature which analyses vehicular crashes is publicly available. For studying this phenomenon, we conducted roof crushing simulations in ANSYS software. A NASCAR Car Frame CAD model obtained from the internet, was then modified slightly. The tree trunk was assumed to be 0.4 meters in diameter, 0.57 kg/m3 dense, and weighs 1074 kg. We have broadly simulated four types of test cases. First, a quasi-static loading of the trunk on the centre of the roof. Second, a transient analysis of the trunk having a free fall vertically downwards on the roof from 0 m height above the roof. Third, a series of transient simulations of the tree trunk having a free fall from a height of at least 1m from the car roof. The height of the tree trunk from the roof was increased by 1m after each successful convergence till it reached 5m. Simulations were also conducted for 7.5m and 10m for a better understanding. Fourth, a set of two transient simulations where the height of free fall was fixed at 5 m above the roof. Each of these two simulations were performed with a material different from what was used to perform the previous simulations. A total of eleven simulations were performed out of which one was static, and the rest were transient structural simulations. Eight out of the ten transient simulations were performed using the same material for the car frame while the height of free fall was varied from 0 m to 10 m. Three out of these transient simulations had the free fall height fixed at 5 m, but the material was varied for each and every simulation. Seven parameters namely, the Equivalent Plastic Strain (𝜀􀭣􀭯 ), Equivalent Stress (𝜎􀯘􀯤 ), Roof Force (𝐹􀭖), Suspension Spring Force (𝐹􀭗), Suspension Damping Force (𝐹􀭈), Total Suspension Force (𝐹􀭘), and the Roof Distortion (𝑢􀭖) were extracted and analysed. Close attention was also given to the roof and suspension forces while observing their change in behaviour as the height of free fall of the trunk and the car frame material was varied. The Car frame did not undergo any plastic deformation when loaded under quasi-static conditions. The roof of the car kept crumbling to a dangerous amount as the height of free fall of the trunk was gradually increased beyond 1 m. Maximum plastic strain was observed locally at the location where the trunk and the roof made contact. For some cases, the maximum was also observed in the location where the rim of the roof was connected to the supporting pillars. The intrusion of the roof into the passenger cabin i.e., the roof distortion exceeded 8 cm when the height of fall was increased to 2 m which can cause serious injuries. The cushioning effect offered by the roof and the frame structure along with the suspension was prominent when the height of free fall of the trunk was under 2 m. This effect gradually became negligible when the impact velocity exceeded 7 m/s. An approximate relation was established between the quasi-static and dynamic loading of the car frame. This can be utilized to estimate the forces acting on the roof when compared to static behaviour which is still the trademark to formulate the Roof Strength of a passenger car. The first chapter of the thesis is the introductory chapter. In this chapter we discussed about some basic terminologies like vehicular accidents, a vehicle’s crashworthiness, different types of material behaviour, Isotropic and Kinematic hardening models, etc. related to this study to provide a better understanding. This was followed by the literature review comprising of the available literatures on several topics like the impact of vehicular crashes, different types of vehicular crashes that are analysed, different plasticity models, and the parameters utilised for conducting this study. The chapter ends with the scope of work and objectives of this study. In the second chapter we discussed about the different methods which were implemented in order to conduct the simulations and extract the necessary results. A detailed description on the model preparation and simulation parameters is present in this chapter. The chapter also contains the necessary APDL commands implemented in pre-processing and post-processing of the simulations. The third chapter encloses the meaningful and key observations deciphered after analysing the results generated from the simulations. The chapter also discusses in detail about the correlation between static and transient analyses results. In the fourth and final chapter we discussed about a better variation of the scenario that has been considered for this research. Instead of the trunk falling perfectly aligned to the roof, it falls obliquely at an angle causing an imbalance of impact forces on the car frame. The chapter also provides necessary parameters and boundary conditions which can be implemented to conduct analyses on the aforementioned scenario.