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Hierarchical composites for improving toughness: modelling and experimental validation

Periodic Reporting for period 1 - HierTough (Hierarchical composites for improving toughness: modelling and experimental validation)

Reporting period: 2015-08-17 to 2016-08-16

• What is the problem/issue being addressed?
Carbon fibre-reinforced polymers (CFRPs) are a rapidly growing class of materials as they possess excellent stiffness and strength in combination with a low density. They are vital in reaching the European objectives to reduce emissions of greenhouse gases and to achieve more efficient material usage. They are becoming highly popular in aerospace and automotive industries, but their introduction is hampered by their low damage tolerance. This fellowship proposes a novel approach to increase the toughness and hence the damage tolerance of CFRPs by intelligently designing the microstructure of the material.
• Why is it important for society?
Carbon fibre-reinforced polymers (CFRP) are increasingly being used in structural applications. This implies that failure of CFRP structures can have dramatic consequences for the structure as a whole. To maximise safety (for example, for passengers in a car or airplane), it is important that structures have a high damage tolerance. The translaminar fracture toughness is a key parameter that controls the damage tolerance of composite materials, and hence has an important contribution to increasing the safety of CFRP structures. At the moment, microstructures are the uncontrolled result of the manufacturing process. By changing the paradigm to deliberately controlling the microstructure via the manufacturing process, large improvements in CFRP damage tolerance and hence safety are possible.
• What are the overall objectives?
The objective is to predict the microstructure that maximises toughness through modelling, to manufacture this microstructure and to validate the predicted toughness experimentally. The translaminar fracture toughness of CFRPs is expected to be increased by 50-100%. This is realistic given the successful examples from nature as well as recent, but preliminary modelling predictions.
A finite element (FE) model of the compact tension test was developed that allows the incorporation of complex microstructures as well as size-dependent toughness of the constituents. This model was then used to predict microstructures that can lead to increased translaminar fracture toughness values. The initial results indicated that toughness values could be achieved that are higher than the toughness values of the constituent materials. The model was then used to predict how such synergies can be maximised.

A methodology to manufacture such complex microstructures was therefore developed and optimised. To maximise the design freedom, the chosen approach was to 3D print continuous carbon fibre composites with a MarkForged MarkOne 3D printer. The methodology for 3D printing was optimised so that model microstructures of carbon/glass hybrid composites could be printed. The sample design for the compact tension test also required optimisation. This was needed to prevent unwanted failure mechanisms such as buckling and compressive failure at the back. Once optimised, compact tension tests were successfully performed.

The experimental results showed good correlation with the modelling predictions. This implies that the model can be used to predict even more complex and better microstructures, even though they may not be printable yet. The final step in the model was therefore to move away from layered or striped microstructures (see Figure 1a and b respectively), and into more complex microstructures.

In the final step, more complex microstructures were modelled and optimised. This led to even larger and more sustained synergetic effects. Such complex microstructures would be challenging but not impossible to manufacture. At the moment however, the state of the art in 3D printing is not sufficiently advanced to print them.
The project has modified the compact tension test and accompanying data reduction scheme for ductile samples that are prone to buckling and compressive failure. This methodology can now be used by other researchers on a much wider variety of materials.

We have identified pathways to exploit the microstructure of fibre-reinforced composites to increase translaminar fracture toughness and hence damage tolerance. This can potentially lead to a shift in thinking of the microstructure as the uncontrolled result of the manufacture to adapting the manufacture to create the optimal microstructure. In the future, this could lead to safer and more reliable use of fibre-reinforced composites in applications such as airplanes, cars and trains.

Through our presence at Imperial Festival, we have made the general public more aware of the importance of damage tolerance and toughness for structural applications.
Figure 1: Potential microstructures that can be 3D printed and their printing direction.