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INDUSTRY FOCUS MILITARY, AEROSPACE & DEFENCE


Meeting aerospace cai standards


As the use of advanced composites continues to grow, reliable methods of assessing impact


resistance have become increasingly important. Stephanie Williams, senior product specialist at Instron, explains how drop towers are key to Compression After Impact (CAI) standards in the aerospace industry


C


ompression After Impact (CAI) testing is being used to investigate


the condition of composites, enabling engineers to learn how and where damage is situated and helping avoid potential accidents. The process is used in many industries, including aerospace and defence, as well as automotive. Drop towers, also known


as drop weight impact testers, aid the CAI testing process because they measure and evaluate where the damage starts, how it is initiated and the severity of the impact, enabling engineers to assess potential composite damage in panels. The first step involves


using an impact drop tower to cause Barely Visible Impact Damage (BVID) on the panel, followed by a compression test. This uses an electromechanical testing unit to measure the composite’s strength after being damaged by the impact event. CAI testing is about


how the damage develops internally, so considerations include where the first crack originates, the load or energy level of the crack, and how the internal damage spreads over time. A panel may look fine, but have significant


damage internally. Take an aircraft wing, for example. On the outside, the skin may look undamaged but internal damage could be weakening the structure of the composite. This internal damage can then impact the compressive strength of the composite’s structure.


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InspeCtIon The aerospace industry is bound by standards that were created as the result of numerous failures that have happened in real world scenarios. These are very specific about the fixture, mass, geometry and how the impact is delivered for the composite. In December 2025, Airbus had to


inspect ‘as many as 600 planes’ to review a ‘supplier quality issue with metal panels’ on their aircraft, as reported on BBC News. The metal plates were located at ‘the front of the aircraft’ and found to be ‘overly too thick or too thin’. This highlights the commercial and reputational risk of insufficient testing during component design. Historically, aerospace


companies had their own internal standards, but a lot of those have now been rolled into American Society for Testing Materials (ASTM) standards, like ASTM D7136. The ASTM D7136 standard outlines the impact test, the evaluation and documentation of impact damage post-test.


It specifies the test format for this


method, using ‘a flat, rectangular composite plate, previously subjected to a damaging event’ that is ‘tested under compressive loading using a stabilisation fixture’. The International Organisation


for Standardisation (ISO) has a standardised test method for CAI testing called ISO 18352:2009 which is suitable for continuous-fibre-


reinforced polymer matrix composites. The standard also outlines that it’s limited to ‘fibre-reinforced plastic laminates with multidirectional reinforcements manufactured from unidirectional prepreg tapes/fabrics or woven fabrics’.


2 DESIGN SOLUTIONS SEPTEMBER 2026 www.designsolutionsmag.co.uk A customer of Instron, for example, was


working with a global aerospace company to design a carbon fibre component to help the OEM reduce weight. Such companies, like Boeing and Airbus, are very strict about component specifications, with every fixture being created from steel. But an inspector told the company its self-designed setup didn’t meet the required CAI testing standard. Moving to a professionally manufactured and installed drop tower with the correct fixtures made it easier to demonstrate compliance. In the aerospace industry, you must repeat


the damage state with the panels reliably, shift after shift. Drop towers make it easier to achieve CAI standards because having a system that controls the impact energy, fixture, mass and height removes the variability that the standards are trying to eliminate.


Improper CAI testIng And loss of Customer trust Choosing the right materials testing equipment is key for performing gold standard CAI tests. The end user wants to know how the composite damage was created in the drop tower and not just the outcome value of the test. When the end user questions a set of CAI test results, it makes it difficult to move a programme forward, particularly for electronics, construction, sports equipment, aerospace and automotive companies. In the real world, composites get damaged.


CAI testing should be designed for this reality. Failing to assume that impact damage will happen leads to a lack of understanding about how and why composites get damaged. Drop tower testing offers a controlled way


to understand what happens to a composite structure after it has been impacted, and knowing how CAI testing works when designing composites is key for recognising how they behave when damaged. Instron’s range of drop tower machines


are built to test the damage of composites, including plastics and light alloys. Its 9400 series of drop tower testing units are designed for handling highly demanding analysis for materials and composites.


ITW Test And Measurement www.instron.com


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