Stress to impress - stress analysis

A top Formula 1 driver doesn't give too much thought to any one of the hundreds of components that make up his race car failing while he's teetering on the brink of adhesion at 180mph plus. If he did then he probably wouldn't be able to do it. But there's more to it than that. You see, deep down perhaps, he can always feel safe in the knowledge that someone else has already worried about those components for him - someone like the Stress Analysis and Materials Group headed by Luca Furbatto.

Furbatto's dedicated band of engineers work in the drawing office - an ideal location as they often need to interact with the designers - and comprises experts in both composite materials and metallurgy, most of them recruited from the aerospace industry.

Racing is very much in Furbatto's blood then, which is useful, as the challenge the group faces is one that is central to modern race car design - that is, trying to achieve the balance between driver safety and car performance. It is up to the Stress Analysis and Material Group to ensure that while a component in the car is safe, and meets all the strict FIA regulations, it is also optimised to perform at its very best. It must meet stiffness and weight targets that are crucial to its performance on track. It's an immense challenge, but thankfully they have a quite incredible tool at their disposal to meet it.

"Structural simulation helps tremendously in terms of stiffness and strength visualisation," says Furbatto. "It helps us find out where the structure bends or twists, and then improve it. And all this can be done before a single component is actually made." This 'stress analysis' is carried out by FEM (Finite Element Method - also called FEA, the 'A' for Analysis). Basically, these computer programmes are able to apply a three-dimensional grid of elements to the designer's CAD renditions of the components. As these are then mathematical models, set forces and boundaries - which are derived from track data, limit conditions and FIA regulations - can be applied to them to give a virtual picture of the component under stress, showing it bend, vibrate or deform. Ninety per cent of the car is now modelled with FEM, and tests applied include everything from vibration on a wing, or torque on a gear, to the results of a side impact test on the monocoque.

"The loads we apply to structures are often very large," says Furbatto. "Under a front crash, for example, the load generated by the nosecone can reach 350kN, or 35 tonnes - that's about the size of a large truck..."

These loads can then be represented on the screen using a colour spectrum, which correspond to measurements along the screen edge, for quick reference. The grids, or 'meshes', can be fine or coarse, depending on the level of information required - a finer mesh means more elements, hence more information but longer run time. Yet with between 300 to 400 parts to look at per car, this is by no means a simple job.

As Furbatto points out, with each individual case "the maths behind it is very complicated, you are talking about very large matrices.The job requires a sound understanding of finite element method and numerical convergency." Indeed, with a large component such as a monocoque, analysis can take a full eight weeks from the start of meshing to the finished report.

That said, there's no doubt all this virtual testing has speeded up the build process of the modern Formula 1 car considerably, and has also allowed the designers to come up with components that are at the optimum weight and stiffness. As McLaren Racing's executive director of engineering, Neil Oatley, puts it: "In the past the development process was less precise, and you would always allow yourself much larger safety margins; Inevitably you would be more conservative so components would be heavier."

But in these days of ultra-strict FIA safety regulations it's not enough to simply prove that a part is up to the job on the screen alone, and it's part of the group's responsibility to set the parameters and coordinate the routine lab tests on the finished components too. This all takes place in the test lab, using non-destructive and destructive test frames supplied by McLaren Technology Centre Partner Instron.

"Once a part is designed to spec," says Furbatto, "in order to be approved for track use it has to go through a series of tests, including static tests, fatigue tests and destructive tests." Incidentally, the group also oversees material tests in the lab, working closely with Official Supplier Advanced Composites Group, as well as batch quality checks and failure analysis. All that said, it's the stress analysis that's still the core of the group's work, although in recent times its role has begun to change, as McLaren's chief designer Mike Coughlan explains. "Stress analysis is moving more towards the optimising of components now and higher level structural simulation," he says.

"CAD packages are getting to the point where simple stress analysis can be done by the designer. So the stress group is moving away from analysis of isotropic [that is, a single component with uniform properties] materials... Luca and the group are now looking towards the very high level contact problems, complex assemblies and composite ply-by-ply analysis."

This work sees the group working on the optimisation of everything from the monocoque - where it is looking at optimising the ply direction of the composite material to achieve better stiffness and strength - to the transmission, from aerodynamic part to suspension components.

But driver safety is always the top priority, and a major worry. As Furbatto says, "Every time that something fails on the car when it is a structural failure, not due to contact with other cars, it personally hurts. But even from this kind of event we continue to learn." According to Furbatto, there is always that vital balance to consider: "You can make a tank, and it will pass all the strength/FIA tests, but it is not going to be very quick!"