How an F1 composite monocoque could be defective

It became known that Lewis Hamilton and Valtteri Bottas’ chassis were swapped for the French Grand Prix this weekend.

This is not uncommon in Formula 1 and there can be numerous reasons for it. The reasons for the change were discussed by the team and will not be explored in this discussion.

Occasionally, a driver will be told that their chassis does not feel right or is defective. This discussion briefly looks at what constitutes a contemporary F1 chassis and how it can be defective.

To clear up any confusion, first consider the semantics of the term “chassis”. In the context of this discussion, the correct term is monocoque or informal tub. Technically, a chassis is an assembly of components, including the tub.

The monocoque or tub is an important structural element of the F1 chassis that houses the driver, attaches the front suspension to it, houses the fuel cell, and allows for the attachment of the fully loaded engine block and the loaded drying pan. The longitudinal, axial and torsional rigidity of the tub is crucial, as the structural member transfers all dynamic-mechanical loads between the front and rear of the vehicle.

A modern f1 tub is made of a composite matrix, generally made of carbon, kevlar and cylon fabrics and epoxy resin. The composite matrix is ​​laid in a mold and over structural and collapsible cores such as aluminum and Nomex honeycombs, divinycel foams for reinforcements and partitions, and structural pickups such as engine and suspension brackets.

The tub manufacturing method of choice in F1 is the closed-form autoclave process. In this method, layers and structures are placed in a mold with a cloth pre-impregnated with resin. The lay-up is done in a clean room to minimize the risk of dust and other debris accidentally getting into and trapped in the lay-up. For the same reason, composite layering technicians wear hairnets and gloves to ensure that oils from their skin do not get into the layering and contaminate it.

Prepreg materials are expensive and are stored in freezers to prevent premature activation or curing of the resin. After the part has been placed in the mold and trimmed, the assembly is coated with a layer of plastic and sealed. The “bags” have connections that are connected to vacuum lines. The placed and packaged form is then transferred to the autoclave and the sealed clave door is closed. Inside the autoclave, the vacuum lines are connected to a pump, further pumps act on the packaged form from the outside and computer-controlled heating elements subject the lay-ups inside to heating cycles of up to 250 ° C for a period of up to eight to 12 hours, depending on the type of materials used. The initially applied heat activates the pre-impregnated resin into a viscous state, which then hardens during the clave cycle. The vacuum lines evacuate air from the sealed mold package with the intent of drawing air pockets out of the build-up and ensuring that the resin flows into the evacuated inclusions and, with the external pressure applied, presses the build-up onto the mold surfaces to create a homogeneous, inclusion-free composite matrix .

It is often believed that this process is chosen because it gives the highest strength results, but while autoclaved parts are actually high in strength, other processes such as cold resin infusion actually produce higher strength composite dies. The composite tub offers an F1 design team an excellent balance, especially between torsional rigidity and weight. While an F1 tub can very well be even stiffer and possibly lighter by using chrome moly tubing and epoxy resin bonded to sheet aluminum, the choice of composites and manufacturing processes is far more accommodating to create complex curves and geometries.

Another advantage of using the production method with a closed form autoclave is that non-deformed surfaces are produced with high-quality, defect-free surfaces.

In the past, an f1 tub was made in two different mold halves and joined together due to the geometric complexity of its shape. The connection line connecting the two halves has been an area of ​​risk in the past as defects in that connection would compromise torsional stiffness and become prone to brittle fatigue and defect propagation. However, as manufacturing, and particularly molding methods and technologies, have evolved over the years, a modern f1 tub is generally made in one piece using a monolithic mold. This reduces the risk that this lead bond line will fail.

As discussed, a tub with integral deformable structures is manufactured in accordance with the FIA ​​F1 technical regulations and is a homologated part. Once the tub design has passed destructive FIA ​​tests and has been accepted, which is generally achieved by abandoning tub serial numbers 001 and 002, the design cannot change for the remainder of the season.

In terms of safety, weight, center of gravity and aerodynamic goals, rigidity is one of the most important performance characteristics of a tub. As the transmitter of all loads between the front and rear end, every deflection must be kept to a minimum. In general, if a driver is critical of the performance of a monocoque, it is more than likely that the tub will bend out of normal parameters under some sort of load combination.

In the lay-up process, it is crucial that the connections between laminate layers, structural cores and mechanical pickups are faultless. Sometimes even micro-inclusions in a laminate can be a potential source of error. Inclusions in composite matrices can lead to larger and smaller delaminations, in which layers separate from one another, which can spread and tear. Other causes of delamination of layers of fabric and structural points are excessive loads, for example from jumping curbs and accidents.

Most F1 teams have non-destructive testing (NDT) departments with x-ray and ultrasound equipment used as part of their preventive maintenance and life cycle management systems to monitor the integrity of a tub construction. However, there are cases where micro-defects were not detected internally, but were noticed by the driver at the racetrack. It is not unheard of that a defect affecting the performance of an F1 monocoque can be identified but not located and repaired, and that it will be condemned as either a show car, or worse, a trash can.