CMF moves toward market after passing temperature tests

Composite metal foam – CMF – is closer to commercial applications in packaging or for the transportation of hazardous materials after passing so-called ‘simulated pool fire testing’.

Steel-steel composite metal foam samples before testing (right) and after 100 minutes exposure to 825 deg C (left) (Image: Afsaneh Rabiei, NC State University)

The researchers at North Carolina State University also used experimental data to develop a model for predicting how variations in the CMF would affect its performance. Their findings have been published in ScienceDirect.

CMF armour cuts weight and adds protection

Simulated pool fire testing is an experimental test that materials must pass in order to be considered for use in manufacturing rail tank cars that transport hazardous materials. In simulated pool fire testing, a panel of material is exposed to a temperature of at least 816oC on one side for 100 minutes. A suite of thermal sensors rests on the other side of the panel. If those protected sensors register a temperature of 427oC or higher at any point during the 100 minutes, the material fails the test.

For their tests, the NC State researchers used panels made of steel-steel CMF, which is a foam consisting of hollow, metallic spheres – made of materials including carbon steel, stainless steel or titanium – embedded in a metallic matrix made of steel, aluminium or other metallic alloys. According to NC State, Steel-steel CMF indicates that the spheres and the matrix were both made of steel.

“A solid steel plate with the same thickness hits 427oC in about 12 minutes,” said Afsaneh Rabiei, first author of a paper on the work and a professor of mechanical and aerospace engineering at NC State. “In three rounds of testing, our steel-steel CMF was exposed to the same temperatures of 825oC for the full 100 minutes – and the highest temperatures recorded on the back of the panel using protected sensors were between 351 and 379oC. It is worth noting that the steel-steel CMF panel is only one-third of the weight of the solid steel plate that failed the test in about 12 minutes.

“In other words, the CMF passed the test by a wide margin,” Rabiei said in a statement. “Based on the experimental and modelling results, as well as the uncertainty studies – all of which were reported in this paper – a 15.9mm thick steel-steel CMF met the acceptance criteria for the simulated pool fire test by a large margin. We were testing the CMF for use as novel insulation system for transportation of HAZMAT, but it’s also relevant to applications from military vehicles to architectural structures.”

The new research is said to build on previous work that found CMFs significantly more effective at insulating against high heat than the conventional metals and alloys that they’re made of, such as steel. Taken together, the findings highlight CMF’s potential for use in storing and transporting nuclear material, hazardous materials, explosives and other heat-sensitive materials, as well as space exploration.

The new research also gave researchers data they could use to help fine-tune the desirable characteristics of CMFs, depending on the intended application.

“Because we can control the features of the CMF, such as the size of the hollow spheres in the foam, we wanted to create a model that could be used to predict how different types of CMF would perform in simulated pool fire testing,” Rabiei said. “This would allow us to design future foams in order to find the best balance of physical, mechanical and thermal properties.”

The researchers built the model by drawing on data from their simulated pool fire test experiments. Based on rigorous evaluations of the model, they found that the model’s predictions are accurate to within 10oC.

“Our next steps include expanding the model to allow us to simulate so-called torch-fire testing,” Rabiei said. “Torch-fire testing is also required for materials to be used in tank cars that transport hazardous materials, but it requires larger samples – panels that measure four feet by four feet.”