The benefits of simulation in passive vehicle safety

In recent weeks I have talked about road safety, crash tests and passive safety. Generally speaking, road safety is not limited to crashworthiness, i.e. the ability of a vehicle to protect its occupants, but there are many more facets. Road safety has always been divided into two macro-areas: active safety and passive safety.

Active safety

Active safety refers to all systems and devices whose purpose is to prevent and avoid an accident. Among the devices belonging to this macro area, the best known are the anti-lock braking system (ABS), traction control and the lane keeping system.

Passive safety

Passive safety refers to all systems and devices designed to mitigate injuries resulting from an unavoidable accident . The most important and well-known of these are airbags and seatbelts. In this context, vehicle design is one of the cornerstones of passive safety: here we return to the concepts of the geometric and biomechanical approach to design.

So how can a vehicle layout be more secure?

Basically, the vehicle can be divided into various parts, each of which must meet certain safety requirements. There are many types of intervention that we can take as examples:

The passenger compartment must have the correct integrity to prevent as much as possible deformation from entering the occupants' contact zone and to allow all systems to function properly. Excessive deformation of the steering column can cause the airbag to deploy in the wrong position, which can lead at best to a reduction in its absorption capacity and at worst to increased injury (e.g. it can deploy too close to the driver).

  • the frontal area of the vehicle should be designed with the dual task of preventing parts from penetrating the passenger compartment, but above all to reduce the scale of the accident by absorbing the kinetic energy of the impact, usually by introducing sacrificial structures that deform in a controlled manner.

These two simple examples show how important the study of vehicle layout is for the safety of its occupants and, not least, for pedestrians. Over the last few decades, once the biomechanical approach has taken hold, vehicle safety has improved dramatically thanks to the introduction by manufacturers of various types of experimental tests up to the definition of homologation crash-tests which in themselves already guarantee a definite level of vehicle safety.

However, the experimental tests alone did not allow direct intervention in the design phase, except with the experience of the technicians gained with the previous vehicles, and it was only possible to correct the almost completed layout after testing the first prototypes.

The importance of virtual simulation

Since the beginning of the 2000s, virtual crash-test simulation has become increasingly popular, but initially the limited predictive capacity of software combined with the low computational capacity of computer centres limited its use. The need for an integrated tool during design quickly led to an increase in the fidelity of the codes in use and it took only a few years for the major manufacturers to introduce such tools even in the initial development phase of a new vehicle, saving a lot of time and significantly reducing costs.

Looking at it from our point of view, from the technical point of view of a design engineer, what is the technical advantage of virtual simulation? There are many advantages to simulating a crash test, of course, but one of the advantages I like best is being able to watch the deformation of the vehicle and each of its components without the limitation of the number of cameras and without the encumbrance of other components. A sort of 'pure' observation without limits, which undoubtedly allows a greater understanding of the physics surrounding the problem.

But let's take an example and suppose we want to design and/or improve the behaviour of the "crash-box" of a van, a sacrificial component interposed between the bumper and the chassis that has the task of absorbing kinetic energy in medium-low speed impacts; in the figure below on the left is a Finite Element model for crash analysis of a generic van, the crash box is hidden by the bumper and the front wheel, it is clear the difficulty of evaluating how it deforms in a real crash test. With a simulation, however, it is possible to hide the other parts very quickly and the crash box is visible in its entirety (figure on the left), the advantage is considerable.

In addition to this, it is also possible to highlight the stresses during deformation. In the figure below, VonMises stresses are plotted on the component, but plastic deformations could just as easily be assessed. This small example should make it clear that it is relatively simple to evaluate even small changes to the shape of the crash box with these tools.

This is what we also do at Sinaptica, supporting in the product development phase, providing evaluations of various design solutions in a short timeframe, delving deep into the problem to make it clear how and why solutions must take certain development paths.