Putting on the heat

Changing the temperature of materials can reveal important data about their engineering merits. Ed Hill spoke to Dr Phil Williams at Mettler Toledo about its approach to testing composites. A large amount composite material testing is carried out by destructive processes such as pulling shearing and splitting test coupons, but another important area of research, particularly when new manufacturing processes and ingredients are introduced is thermal analysis.

Dr Phil Williams is product specialist for thermal analysis at Mettler Toledo in the UK. The Swiss headquartered company offers a contract testing service and also sells thermal analysis equipment to the composites market.
He explains that this kind of testing not only reveals the constituent chemical, structural and performance features of new materials, but it is also useful when non-destructive testing is required.

Doubling up

“Thermal analysis of composites often uses two elements, thermogravimetry (TGA) and differential scanning calorimetry (DSC). TGA measures the changes in weight of a sample as it is heated, cooled or held at constant temperature. It looks at the material characterisation and identifies what sort of plastic it is, such as the resin content and the physical properties that these produce.

“You can identify the main material a composite is made from along with any additives including the concentration of chemicals such as plasticisers and oxidising agents. These can be measured and quantified using different techniques.”

Differential scanning calorimetry measures enthalpy (or energy) changes in a sample due to alterations in its physical and chemical properties when temperature changes occur over a period of time.

“From DSC and TGA you can test not only the composition of the material, but also its melting and glass transition temperatures. This means you can test a small sample for things like impurities or contamination.
“It can also be used as a quality control test without having to destroy a whole structure. It allows you to take a small sample from an edge of a large airframe or wind turbine blade, for example, without damaging the whole product.”

Another method of thermal analysis, known as mechanical testing, is also carried out by Mettler Toledo’s products and services.

“We have mechanical measuring instruments designed for dynamic mechanical analysis (DMA) and thermomechanical analysis (TMA).”

DMA measures the viscous and elastic properties of a sample in relation to temperature, time and frequency. TMA is used to measure the dimensional changes of a material as a result of temperature.

Adds Williams: “These two types measure mechanical analysis. They are similar to dynamic/stress testing, but use much smaller sample sizes and perform more subtle and technical measurements like modular stiffness and expansion efficiency.”

The need for speed

Thermal analysis can also be a useful tool when testing processes have to be accelerated.

“As you heat things reactions are faster. Higher temperatures accelerate the failure of a material so you can use temperature information to force it to fail more quickly. This means that tests that could potentially take months or years to carry out can be completed in half a day.

“For example, with prepreg semi-cured material used for aircraft wings and similar structures, thermal techniques can measure their shelf life. Prepreg materials are expensive, especially if they have to be kept in cold storage. Manufacturers don’t want to waste them so they need to know how stable these materials may be over six months, for example. We can do a rapid test in 20 minutes on one of our instruments to see how viable the material is and whether it will deteriorate.”

With ever more demanding standards and specifications required by numerous industry bodies testing can only increase, but the biggest reason according to Williams for its current growth is the natural demand for stronger and lighter materials but with lower production costs for manufacturers.

“Our support for our customers in testing work and consultancy is growing all the time. We help them develop new materials and improve established ones. The main increase we are seeing is in carbon fibre composites with high carbon loads. To improve the physical properties of these composites further, the ratio of carbon fibre to resin is being increased. However, there is a trade-off and by using higher fibre loading it can cause the composite to fail in use. Both the properties of the resin and the mechanical effects of high carbon loadings can be investigated by using thermal analysis.

“A big driving factor for manufacturers is trying to make things cheaper. Traditional composites have tended to be thermoset products which need to be oven baked, are labour and energy intensive and difficult to recycle. For large-scale structures, such as aircraft the costs make sense, but for smaller parts that are not safety critical they are not really cost effective.

“Thermoplastics, which melt and then go solid, are a much more economical, so a major driver in testing is of thermoplastic materials that can be used for smaller parts in higher volume manufacturing such as car and motorcycle production.”

http://uk.mt.com