Development of a design methodology of a composite monocoque chassis.

Abstract

The concept of the composite monocoque chassis has been implemented in many vehicle designs; however, there is little open-access literature defining the primary considerations when simulating one. The purpose of this research is to develop a methodology for determining the structural integrity of a composite monocoque chassis, through finite element analysis, with the intention of developing a lightweight solar powered vehicle. Factors that influence this methodology include; the definition of the vehicle loading conditions, failure criteria, and important design parameters, chief among which is the torsional stiffness. Chassis design specifications were developed from the 2017 Bridgestone World Solar Challenge rules and regulations as these are the most common and complete specifications for this particular type of vehicle. The primary design criteria considered is the torsional stiffness, which was determined from the application requirements and literature, and resulted in a suitable value of 4000 Nm/deg. Siemens NX Nastran was used to develop a torsional stiffness model, which uses the torsional loading condition, to determine the torsional stiffness value. The design methodology then follows an iterative process where various geometry and layup modifications were considered, under the same loading conditions, with the aim of increasing the torsional stiffness to achieve the required value. Aerodynamic properties were adapted from existing UKZN solar vehicle knowledge; however, this research does not consider the optimisation of the aerodynamic properties of a monocoque chassis. Only a structural simulation was conducted. The ultimate strength of the material was also considered throughout the simulation process, however in all cases the model failed to meet the required torsional stiffness parameter before material failure modes. The door recesses had the most significant effect on the torsional stiffness. By compacting the door recesses the torsional stiffness was increased by 29.04 %. A final torsional stiffness was of 4097 Nm/deg was attained with the implementation of an aluminium honeycomb core. Additionally; an analysis of the mounting points was conducted to ensure that the layup can withstand the concentrated loads at the suspension mounts. This analysis is concerned with the principal stresses, where the principal stresses give insight into the most suitable orientation of the layup. The torsional stiffness model resulted in a maximum principal stress of 81.68 MPa, below the 464.4 MPa tensile strength of the reinforcement material orientated in the direction of the fibres. To verify the significance of the torsional stiffness failure criterion, vertical and lateral bending analyses were conducted. A vertical bending model was developed where the chassis is modelled as a simply supported beam, simulating the squatting and diving of a chassis under acceleration and deceleration respectively. The maximum deflection was 5.28 mm, which is below the vi maximum allowable deflection of 12.29 mm, determined from a maximum deflection ratio of 1/360th of chassis length. A lateral bending model modelled the chassis as a simply supported beam with the maximum stress being analysed. The maximum stress experienced by the chassis under this loading condition was 18.73 MPa, which was 75.8 % less when compared to the maximum stress exhibited by the chassis under the torsional loading condition. Flexural bending tests were conducted on various laminate sandwich structures used in the finite element analysis to validate the simulation material properties. The peak load and mid-span deflection of each specimen was recorded to determine the maximum flexural stress and flexural modulus of elasticity. The flexural stress at specific midspan deflections was compared, under the same loading conditions, to that of the bending stress exhibited by a flexural bend test model finite element analysis conducted in Siemen’s NX Nastran. Graphs of the stress versus midspan deflection were plotted for each specimen layup type and the curves of the simulated and experimental results were compared. In each laminate sandwich structure case, the simulation curve exhibited a linear relationship between the midspan deflection and flexural bend stress and the experimental curve exhibited a linear relationship until the elastic limit of the specimens was reached. Thereafter the curve exhibited an exponential relationship as plastic deformation occurs until the specimen failure. An iterative finite element analysis design methodology was used to develop a composite monocoque chassis. The design process of a composite monocoque chassis is simplified by using finite element analysis to iterate through many different configurations, such as core thicknesses, layup orientations, and geometry features, to customise the properties of the structure. With these properties, it is possible to determine chassis performance. The finite element analysis results illustrated that geometry modifications, such as compacting door recesses, and applying strategic layup orientations, such as implementing a honeycomb core, significantly affected the torsional stiffness of a chassis. In addition, a chassis with sufficient torsional stiffness exhibits sufficient bending stiffness. The methodology presented in this research stands to be supportive in designing a fully composite monocoque chassis for lightweight race vehicle applications.