The growth rate of composite materials is 6% to 7% a year. They are used in a wide range of sectors, from aeronautics to health, often as a replacement for metals. However, the design of composite parts is far more complex than that of their metal counterparts, and there are just as many different types of failure.
Sophie Toillon, engineer at Cetim and referent in the failure analysis of polymer and composite parts, takes a closer look.
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Why is the failure analysis of composite parts a complex process?
Sophie Toillon: As a relatively recent development, extensive R&D is still being carried out on composites to gain a better understanding of the damage mechanisms involved. Given their organic nature, composites are particularly sensitive to their operating environment (humidity, chemicals, temperature, pressure, etc.).
Furthermore, each composite is inherently distinct, which adds to the complexity of the analysis. This stems from the choice of resins and fibres making up the composites, combined with factors such as the reinforcement stacking sequences. For example, two composite specimens with different orientations will not have the same mechanical behaviour. Another reason is the large number of different manufacturing processes and the types of potential defects associated with them. There is currently no general rule for defining the level of harmfulness of a defect. Specific skills in materials-products-processes are therefore essential to properly understand the failure mechanisms of these materials.
So how can we prevent failures in composite materials and extend the life of a product?
There are many causes of failure in composite parts. These include design errors, defects or singularities generated during manufacture which are not taken into account during dimensioning. Failure can also result from inappropriate or unanticipated use of the product. One example would be a composite pipe that breaks as a result of water hammering or a reduction in its mechanical properties when in contact with the fluid being transported. These failures could have been avoided by carrying out a hydraulic study of the network beforehand, or through mechanical tests on pre-conditioned specimens in the operating environment.
Feedback from the expert assessment of parts returned from the field, as well as from qualification tests, help to improve our understanding of the damage mechanisms and its influential factors. It also provides the basis for optimizing the design of a composite part, enhancing the robustness of a manufacturing process and thus improving the final quality and durability of the product.
Moreover, failure analysis can be used to decide on the relevant controls or inspections to be carried out on the structure during service (SHM). In this area, optical fibres are used, for example, to monitor strain in the hulls of racing boats. Acoustic emission is used for in situ measurements in pressure vessels. These continuous testing make it easier to detect damage upstream, so that repairs can be made before failure, and also give access to reliable data for establishing the remaining useful life of parts.
How can we use this analysis to help manufacturers meet the new environmental challenges?
The use of composite materials meets some of the challenges of the environmental and energy transformation, i.e. the development of new sources of energy, sustainable mobility through weight reduction, durability and repairability. But it also brings new challenges, requiring us to anticipate the behavior of composite materials under potentially more severe conditions, in new environments. It also requires us to take an interest in new materials such as bio-based products, and to understand the impact of recycling on a structure’s lifespan.
Cetim leverages its R&D and operational feedback to identify and remove certain technological hurdles.
In particular, Cetim is involved in European projects such as the Thor project, which aims to develop a recyclable thermoplastic storage tank demonstrator for hydrogen. Cetim’s laboratories have also acquired new equipment to characterise the behaviour of materials in specific operating conditions that include cryogenic environments.
Lastly, Cetim develops the skills of manufacturers to anticipate these changes through support initiatives and new Cetim Academy® training courses.
For more information on the topic Attend our free webinar:
“Failure Analysis: a key factor in product improvement and durability”
special focus on Composite Tank
April 2nd from 9.15am to 10.15am (UTC+2)
- Introduction: approach and contribution of expertise to improving part quality and durability – example of application with composite tanks.
- Design concept for composite tanks with thermoplastic liner
- Overview of materials and processes used to manufacture tanks with composite TD and TP shells and plastic liners
- Expected manufacturing defects – Notions of harmfulness
- Understanding failure modes – Feedback from qualification of parts – Methodology and main tools
- Conclusion
- Q&A
