The rules of the Formula Student competition pay close attention to safety and are therefore very strict in this respect as far as the battery box. The minimum wall thicknesses, material, number of screws that the box holds and especially the load of 40g (for our box 15kN) in the longitudinal and transverse direction X, Y and 20g in the vertical direction Z are defined. These forces are introduced into the center of gravity of individual stacks. However, the defined material can be replaced by an equivalent one. The defined material can be replaced by an equivalent one if you prove its equivalence by a real bending and shear test. . We took this path when designing the battery and replaced the defined steel with an equivalent carbon fiber design.
The sandwich structure consists of two coatings and a core between them. The core is usually a low density material with a significantly lower modulus of elasticity than the coating. The task of the core is to connect the two coatings at a constant thickness. The advantage of this structure is the provision of greater bending stiffness, which increases with the thickness of the plate. Due to the low core density (typically 50-80kg / m3), a very light construction can be achieved. The total weight of the box itself is 2,4 kg.
A large part of the car's weight is made up of fasteners. It is therefore an effort to replace it with the top adhesives of our partner LOCTITE. With glued joints, it was possible to lighten the car by a few grams, and after experience with adhesives, we decided to use them in the assembly of individual parts of the battery box. Equivalence against the rules defined by the M6 screws awaited us
Will it endure the stress?
Thanks to detailed data on LOCTITE adhesives, it was possible to predict damage to the glued joints using FEM (finite element method). Creating a simulation in the ABAQUS program was one of the most challenging parts to prove that our design is able to withstand a defined load in front of the commissioners from Austria. Cohesive elements were used to implement the glued joint into the simulation, which is a tool for mathematical expression of the results of real glue tests. The result of the simulation will then show the damage of the glued joint in a dimensionless amount of damage from 0 to 1. After 13 hours of calculation, the simulation showed that the surfaces of the glued joints and the composite structure are able to transfer the defined load.
For the FSE.05 model, we will retain the composite structure with glued joints. The construction worked well and a survey of other teams showed that it is also one of the lightest in the world.
Thank you HENKEL for the provision of material equipment.