Infrared alerts for composite materials

Materials World magazine
5 Feb 2012
infrared imaging of house

A novel infrared imaging method could improve understanding of the thermal properties and fire resistance of composite materials. The research, conducted at the Universidad Carlos III de Madrid (UC3M), Spain, might benefit many industries, including aerospace, rail and construction.

According to UC3M team member Professor Fernando Lopez, existing contact procedures, such as thermocoupling, fall short in several areas. They require direct contact with the sampling material, which can damage the detector, making measurements more expensive and awkward to perform. Lopez explains that when temperatures exceed 1,000°C, the exact temperature of the sampling object can be obscured.

‘The thermal energy emitted by a flame at high temperatures saturates the detectors in the hotter zones. If you prevent measuring in this zone, you lose the hotter points. In addition, in the cooler zones the smoke acts as a wall, which hides everything.’

In order to address these shortcomings, the team has devised a contactless technique based on an infrared (IR) camera and an excitation system that is adapted to a sample’s properties. Lopez notes, ‘The measurement is made by an IR camera tuned at a proper wavelength. After measurement, the images obtained are processed with algorithms to obtain the parameters of interest.’

The method, which provides 2D images rather than point measurements, is said to measure in milliseconds, and is less expensive. Lopez claims, ‘One thermocouple is cheaper than our system, but you can only measure one point. Several thermocouples would be necessary in each process if several points were to be measured. After the experiment, thermocouples have to be replaced due to the damage caused in fire experiments. The same happens with glass fibre sensors. Because our system is contactless, its does not suffer any damage in the test and can be used a lot of times, which reduces cost.’

The technique is currently industry-ready, though studies that adjust the method for specific material types still need to be conducted. The team is looking to measure the thermal parameters of new materials. These include thermal diffusivity, specific heat, thermal conductivity, emissivity, convection factor and Nusselt numbers.

They are also developing a system to examine hidden subsurface stains. ‘The most commonly used technique in the examination of structural defects is ultrasound,’ he says. ‘Ultrasound needs contact liquid as an interface of measure, which is invasive. This technique is complex and slow, which results in a bottleneck in the manufacturing process.

On the other hand,’ he adds, ‘non-invasive thermography is fast. It is a real alternative in identifying structural defects in materials such as hidden sub-surface defects, broken zones, humidity and excessively stressed zones.’