At first it was fire. There is About a million years ago, man domesticated the person who will change the course of mankind permanently. It must be remembered that for the progress of civilization it
Prometheus stole fire from the gods. It is this fire that evokes many contradictory emotions: safety, danger, fear, comfort, pain and hope. Sometimes that everyday friend gets out of our control. From wildfires to house fires, everyone fears these catastrophic scenarios. Even more so when on a plane or a fire breaks out during the flight. Who among us has never shuddered in front of a disastrous movie in which a burning plane crashes to the background of dramatic music?

But don’t panic! Engineers and researchers select materials and design aircraft with respect Safety Standards Among the most demanding. Thus, materials selected for aviation applications must strictly meet several criteria: toxicity, fire resistance, smoke production, combustion, etc.

In aviation, composite materials are the most widely used (50% of the mass of the aircraft) because it offers a good middle ground between mechanical properties (hardness and resistance) and lightness. These so-called “composite” materials generally combine fiber reinforcements (mainly carbon fibres, as in tennis rackets or Tour de France racer bikes) and a polymer matrix (also known as an adhesive or glue between the fibers). of plastic). The reinforcement gives good mechanical properties to the material while the matrix allows the fibers to be bonded together.

The plane is made up of different parts assembled together. The choice of material for these different parts mainly depends on the area in which they are located. The engine area and adjacent rooms are among the most important areas of an aircraft. Especially when the engine catches fire. In this case, the aircraft are designed to be owned by the pilot 15 minutes small to put his device. During these 15 minutes, the flame (from the combustion of fuel – kerosene – and plastic) must not pass through the composite parts and the part must maintain sufficient mechanical strength.

Illustration of thermomechanical coupling © B. Vieille (via The Conversation)

To prevent these critical conditions and avoid potentially dramatic consequences, it is necessary to study the effect of heat and mechanical loading (the forces exerted on different parts of the aircraft) to fully understand their effects. interactions. Imagine what happens when you place a cup on a chocolate bar (figure above) and it begins to melt… Concretely, when you expose a composite part to a flame, the plastic will soften, melt (turn to liquid) and then decompose by heat (turning to gas). These gases then fuel the flames and facilitate the spread of the fire. It is clear that the ability of the part to support the force (the weight of the engine for example) will be significantly reduced. It is these interactions that must be understood.

The characteristic dimensions of aviation composite parts vary from a few tens of centimeters to structures on the order of a meter. The difficulty then lies in reproducing the real conditions of thermal attack accompanied by mechanical convection at best on a laboratory scale (and thus on a small scale). The Certificate Rules (License) Aeronautics defines a flame temperature of 1150°C and heat flux (heat emitted per unit area) of approximately 120 kW/m2.

It is therefore necessary to develop specific technical means that make it possible to measure all the physical quantities (temperature, force, deformation) involved in the physical phenomena involved in engine fire. Under these harsh conditions, there is still good news. For thousands of years, mankind has acquired knowledge and tools that enlighten engineers and researchers on how to design composite parts in aeronautics.

The test bench reproduces the simultaneous effect of flame and mechanical loading © B. Vieille (via The Conversation)

In this spirit, our research laboratories are GPM (Material Physics Group -) and
Korea (Professional Research Complex in Thermal Air Chemistry -) pooled their skills and combined their efforts to develop an original test platform (a prototype machine dedicated to studying specific phenomena – see above) as part of a project called Aeroflamme, funded by the Normandy region and Europe.

This bench combines a kerosene stove (which imposes a flame) and a hydraulic cylinder (which forces a force). It also integrates various measuring instruments: infrared camera (temperature measurement), displacement sensor (distortion measurement) and force. Using this test bench, it is then possible to better understand the fire behavior of composite materials and more specifically the coupling(s) between the effect of flame/heat on the evolution of mechanical properties/behaviour.

Under these conditions, the installed part stays well for fifteen minutes … but it can resist more. Flag on board, have a nice flight!

The use of innovative composite materials in today’s aviation applications faces more pressing safety standards for which it is necessary to provide reliable and relevant answers. Also, it is essential to enable aviation manufacturers to understand/predict the fire resistance of their materials and, ultimately, of their parts and assemblies. Our research work on aviation materials is therefore part of the logic for adopting new materials for applications in a high temperature environment or even during a dangerous fire event. These materials are provided by the manufacturers of aeronautics.

This analysis was written by Benoît Vieille, Professor of Aeronautical Materials Mechanics at the National Institute of Applied Sciences (INSA) Roanne Normandy.
The original article was published on the website of Conversation.