Fatigue testing: the challenge of composites

In the world of materials testings, things very much ain’t what they used to be, with new composite materials emerging and replacing more traditional metals or plastics.

This is particularly true of the automotive industry, where the quest for better fuel consumption has led to the replacement of traditional steel parts with hydroformed aluminium and increasing use of composite materials such as polymer matrix composites (PMC) and carbon fibre reinforced polymers (CFRP).

Fatigue testing for automotive components

The automotive industry also need to test its components for fatigue during extensive use, especially as components in cars can experience stresses and forces that can crack or damage the materials even at levels well below the yield strength. A fascinating article in Quality magazine gave an insight into the unique challenges of fatigue testing for the automotive industry.

All structural elements are tested at material and component level for their ability to survive a crash, and the ability to stand up to years of vibration. However, new composite materials prove more of a challenge, as Element Engaged Expert Dr. John Tartaglia explained:

“Compared with metals, these materials provide interesting challenges for fatigue testing. From the standpoint of setting up the tests themselves, composite testing requires advanced machining techniques to avoid fiber pullout, sensitive load cells to measure low loads and advanced gripping techniques to avoid grip fractures that do not occur with metals.”

 

Unlike most industries where testing focuses on assessing the life expectancy during normal use, automotive testing has to focus on what happens in a crash, and the life expectancy without a crash. As Alex Johnson, an applications engineer for Instron, explains, there are financial as well as safety benefits to this approach:

“Fatigue behavior allows manufacturers to better understand the lifetime of their product. In turn, this then allows for the marketing of extended warranty periods and how the product is supported in the future which can be a very powerful tool to differentiate from the competition… The new components and materials now have to undergo durability testing to ensure they can be used to optimize performance whilst maintaining safety.”

 

The problem is, long-term fatigue testing takes time, and in a fast-moving industry, time is a luxury. So, manufacturers have been looking for new ways to predict long-term fatigue life.

• Sound waves can be passed through samples of composite materials rather than applying strain directly and measuring it.
• Fatigue testing can be modelled, creating virtual casting and testing processes for metals. As Dr Tartaglia explains:

“These virtual processes will inevitably deliver significant savings on current physical metal processing and test methods, as certifying materials requires significant expense.”

• New testing methods/models will be required to keep pace with advances in 3D printing, assembly and adhesive technologies.

Computer modelling testing

While computer modelling is in its relative infancy, the technology of materials testing using load cells is proven technology that has, no doubt, saved countless lives over the years. Traditional style testing is comprehensive and thorough for the vast majority of materials and components. It’s also efficient and remarkably cost-effective given the durability and reliability of well-made, high quality load cells such as ours.

 

Call us to discuss how our load cells can help you test your products, components or materials, or for your own branded testing machines, etc.