Despite decades of innovation in composite materials, understanding and measuring crashworthiness still relies heavily on trial and error. Yet composites are being increasingly selected due to their high strength-to-weight ratios and greater energy absorption. The demand for more consistent and meaningful crashworthiness data is therefore growing. Here, Lorenzo Vigna, mechanical engineer at Instron, discusses the challenges of testing composites and the need for a more standardised approach.
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Composite crashworthiness is defined by three issues: difficulty in predicting behaviour during impact, the high cost of destructive testing and a continued reliance on iterative development. A serious challenge lies at the root of all three. Currently, there is no widely accepted benchmark for evaluating material crashworthiness.
The hidden complexity behind composite performance
Unlike metals, composites do not behave in a uniform or predictable way under load. Crash-scenario performance is influenced by a wide range of interacting variables such as fibre type, matrix, layup sequence, thickness, geometry and even the selected manufacturing technique. Furthermore, the same material can produce completely different results, depending on the load conditions including impact velocity, load direction and testing temperature. This variability makes crashworthiness difficult to predict and creates huge inconsistencies in testing.
With no universal standard for composite crashworthiness testing, research teams and manufacturers often develop their own internal methods, tailored to specific materials or applications. However, this creates an issue where data cannot be easily compared.
Inconsistency is holding the industry back
Without comparable data, engineers face significant challenges when evaluating new materials. Variations in fibre, matrix and even manufacturing process can mean a new full cycle of redesign, testing and validation. This is both time-consuming and expensive, particularly when it involves full-scale destructive testing.
Results obtained at the material level do not always translate directly to real structures. Some carbon fibre coupon tests have shown specific energy absorption (SEA) values of around 81 kJ/kg, while full-scale components dropped to 24 kJ/kg, due to low energy absorption failure modes triggered by the different geometry.
This gap between controlled testing and real-world performance could also explain why many industries are conservative in their material choices, even when better-performing alternatives may exist. In some cases, standardisation becomes a necessity simply to manage the cost and complexity of crashworthiness testing.
Take for example, Formula 1. The Fédération Internationale de l'Automobile (FIA), which governs motorsport, decided to standardise the material, geometries and manufacturing processes of specific crash structures because of the high design costs with developing and validating a new component, especially safety-critical ones.
In addition, the industry increasingly relies on the building block approach to design. This involves progressing from small material samples to sub-components and finally full-scale structures. To successfully achieve this, the approach depends on reliable, scalable data at each stage.
Gaining control
If the problem lies in testing, does the solution lie there too? Given the complexity of composites, creating a single universal test is unrealistic. However, establishing a framework that allows easier comparison should be considered.
One approach is to focus on controlling test conditions, rather than observing outcomes. For example, using flat coupon specimens provides several advantages such as being simple to manufacture, more cost-effective and less prone to defects. Additionally, when combined with a well-designed fixture, engineers can actively control how a material fails. This is critical, because failure mode has a direct impact on energy absorption.
Two key modes are often observed during testing. Splaying, or delamination, is where layers separate and energy is absorbed primarily by the matrix, while tearing of fibres results in significantly higher energy absorption. In fact, depending on how a composite material fails, energy absorption values can increase from around 40 kJ/kg to over 60 kJ/kg, for the same sample made of the same material. This highlights just how sensitive crash performance is to test conditions.
Modern testing systems, such as the 9400 series of drop towers developed by Instron, minimise these effects through precise alignment, robust fixtures and advanced data acquisition. Drop towers allow testing of more representative loading conditions, providing detailed insight into how energy is absorbed and dissipated during an impact event.
Traditional universal testing machines provide a simple and cost-effective way to evaluate composites under quasi-static conditions, but their low testing speeds restrict their ability to accurately represent real crash events or generate reliable SEA values. Other methods such as sled testing systems can closely replicate full crash scenarios, but their cost and complexity means they are not well suited for routine material evaluation required by research and development teams. Drop tower testing provides a balance between these approaches, offering a more representative impact velocity than quasi-static tests while being far more accessible than full-scale crash setups.
With fixtures that can reliably trigger one failure mode or another, engineers can remove a major contributor of variability. Combined with consistent specimen geometry, controlled constraints and metrics such as SEA, this approach moves the industry closer to securing crucial, comparable data.
Composites are increasingly being selected for their high impact performance, but inconsistency in crashworthiness results makes it difficult to standardise results. Crashworthiness is not limited by the material itself, but by the industry’s inability to consistently measure and compare. It’s not about improving the materials, but improving the way they are tested.