Fire protection for composites: Fire behaviour not sufficiently studied yet
Composites can withstand temperatures of up to 650°C. And they need to be highly resistant to heat, too, since fire protection requirements are strict for fibre-reinforced composites in the offshore sector, the marine market, the construction industry and for rail vehicles. Taking place in Stuttgart from 6 to 8 November, COMPOSITES EUROPE 2018 will present an overview of the standards the materials are required to meet, along with information on fire retardants and extinguishing agents.
CFRP components burn for days
On 23 February 2008, when the “Spirit of Kansas”, a United States Air Force B-2 bomber, crashed on the runway shortly after take-off from Andersen Air Force Base in Guam and caught fire, it wasn’t just that $1.4 billion went up in smoke. The most expensive plane crash in US Air Force history also illustrated the challenges presented by burning CFRP components. The time it took to put out the fire and the amount of extinguishing agents needed far exceeded expectations. What’s more, embers were found burning even several days after the accident and after several attempts to extinguish the fire.
Since then, the fire behaviour of fibre-reinforced composites repeatedly has been the subject of expert debates and scientific research. Fire resistance and smoke development standards are particularly strict in aviation, the offshore wind turbine sector, the marine market, the construction sector and for rail vehicles. Since organic resins are typically used in composites, this was long considered an obstacle to development. In the meantime, however, new applications are being launched year after year which meet even the requirements of the higher-level classifications EN 13501-1 (fire behaviour) and DIN 4102-1 (building material class).
Carbon fibres can withstand temperatures of about 650°C
In addition, numerous research institutions are engaged in studying and analysing the fire behaviour of composites. In his much noticed 2015 investigation entitled “Prüfungen von Bauteilen aus GFK/CFK hinsichtlich Brandschutz und Tragfähigkeit” (Testing of CFRP/GFRP Components Regarding Fire Protection and Load-bearing Capability), Prof. Dr.-Ing. Jochen Zehfuß from the Institute of Building Materials, Concrete Construction and Fire Safety (iBMB) at Technische Universität Braunschweig tackled the issue with a series of tests and experiments.
He found the thermal stability of the carbon fibres to be at about 650°C, whereas the degradation of the plastic matrix already takes place at temperatures of 300°C to 400°C. Besides the composition of the material, fibre content and composition also impact fire behaviour, according to the expert’s insights. For example, a higher fibre content reduces the amount of smoke released, and the amount of organic materials is reduced. It was also noted that the materials exhibit different, temperature-dependent degradation processes.
Conventional reactive fire protection systems respond too late
When the material is subjected to thermal stress, the matrix degrades first. The uppermost layer of a burning CFRP component facing the heat source mainly consists of carbon fibres and charred residues of the matrix. This layer is extremely porous and acts like thermal insulation, which can lead to a brief weakening of the fire due to the reduction of outgassing of combustible degradation products. Over the course of the fire, the degradation layer gradually advances to the opposite side, away from the heat, until the entire component consists entirely of charred residues.
These findings aside, researchers note that when it comes to the fire resistance properties of fibre-reinforced composites, there are knowledge gaps regarding the material behaviour at high temperatures. The effect of protection systems is still largely unexplored, and conventional reactive fire protection systems respond “too late”. They’re calling for better differentiation of fire protection requirements and for the use of performance-based methods.
Fire protection of composites systems subject to several unknowns
Dipl.-Ing. Dominique Max from Karlsruhe Institute of Technology (KIT) similarly explored the issue for the Standing Conference of Interior Ministers and Senators of the German Federal States in the 2015 Research Report 177 titled “Eigenschaften und Abbrandverhalten von Faserverbundwerkstoffen, speziell Kohlefaserverbundwerkstoffen (CFK)” (Properties and Burning Behaviour of Fibre-reinforced Composites, Specifically Carbon-reinforced Composites [CFRP]). The report took a closer look at key characteristics, the material behaviour in certain scenarios and the material-specific hazards that could arise in connection with CFRP moulded parts. The bottom line: during the fire, excess pressure can develop inside, which leads to swelling. This internal pressure can lead to the detachment of fibre layers. Carbon fibres released in this process can get into the lungs, where they can potentially cause harm.
Regarding the toxicological potential of carbon fibres, it can be assumed that they are less hazardous than crystalline quartz or asbestos. When CFRP materials burn, increased release of critical, fibrous debris should be expected. Overall, however, there is a lack of conclusive expert assessments, which is why additional research is needed, particularly a critical review of the effectiveness of proposed measures. In other words: fire protection of composite systems remains a formula with several unknown variables.
 Forschungsstelle für Brandschutztechnik (Fire Protection Research Centre at KIT)
 Arbeitskreis V, Ausschuss für Feuerwehrangelegenheiten, Katastrophenschutz und Zivile Verteidigung (Working Group V, Committee for Fire Brigade Affairs, Disaster Management and Civil Defence of the Standing Conference of Interior Ministers and Senators of the German Federal States)