An integral part of the engineering design process is the selection of appropriate materials. Whether the materials involved are homogeneous or composite, their internal consistency will be taken for granted and the overall design will have to take into account their relevant properties.
But suppose that for composite materials this constraint on designers was removed and that instead they could design the materials at a “micro-structural” level. Then it might be possible to fabricate structures with a “designed in” capability for damage-induced crack propagation in particularly crucial areas to occur in a controlled manner that would minimise the extent to which fundamental structural integrity was compromised. In turn, that might enable designers to reduce the need to ensure necessary safety levels through over-use of materials with a consequent weight penalty. For aerospace applications in which weight reduction is a constant preoccupation, the advantages of such a capability are obvious.
Futuristic, yes; fantasy, no. The development of such capabilities is the objective of research being carried out with the department of aeronautics at one of the UK’s foremost technical universities, Imperial College London. The research is led by Professor Silvestre Pinho, who says its roots stretch well back into the preceding decade, and in particular to his own fascination with the use of finite element analysis (FEA) software to explore material properties and behaviour. When the technique is used to investigate the behaviour of large carbon fibre-reinforced composite structures of the type used in aerospace applications, it has to be able to cope with the way such materials behave in actual use, he says.
“One of the challenges involved in simulating the failure of very large components of that sort is that the failure mechanisms occur at a very small scale,” he says. Fortunately, he adds, FEA software works well at the scale involved which he describes as “microscopic” rather than “molecular”.
Designing materials
Nevertheless, simulating failure modes is not enough. Instead, the focus of the work carried out by Pinho and his team at Imperial – about half a dozen strong – has shifted to a new goal, that of “designing new types of material”, he says.
His preferred tool for carrying out FEA work is the Abaqus software system that now forms part of the Simulia family of products from Dassault Systèmes. One of the features of the software that lends itself to the work carried out at Imperial College is the ease with which the researchers can input modelling information in their own bespoke format and then use the commercial product purely as a “solver,” he says. One consequence of this is that the team has also been able to use the Abaqus product for work at “molecular level”, particularly for investigations into the relatively new material graphene, as well as for more conventional work involving carbon composites, he says.
But there are also other areas in which the team at Imperial College has made advances in the use of FEA software. One of them, says Pinho, involves “the numerical representation of the propagation of cracks”. Although the physics involved is well understood, accurately modelling the process has proven more problematic, he says. But the team has developed what he describes as a “floating node” methodology that has made it possible to simulate “at a continuing mechanics level”, over distances measured in centimetres, the propagation of up to several hundred cracks – something that was previously challenging.
Going up the size scale even further, the team has developed a means of “superposing” – effectively, seamlessly integrating – both very large models of whole structures, such as might be used to represent, say, an aircraft wing, with more narrowly focused models of the sort necessary to represent stresses at very small scales, says Pinho. Models at different scales are blended rather than simply interfaced, so the whole process is in consequence much more efficient, he says.
The primary objective of the research is an enhanced capability to model the “structural responses of carbon composite materials” – particularly in aerospace applications, says Pinho. At the moment, the deficiencies of existing methods mean that such materials have to be employed with extreme levels of redundancy, compromising the benefits they can bring in weight saving, he says.
Fracture toughness
So far, the work at Imperial College has not advanced to implementation in practice. Nevertheless, it has moved from the world of modelling to the development of experimental carbon composite materials that are considerably more damage-tolerant than has previously been the case, says Pinho. The key metric is fracture toughness, a measure of how much energy the material dissipates when it breaks. The more energy that is required, the less damage will be caused to the structure as a whole by any given input of destructive energy. The team has designed materials “with micro-structures that enable them to dissipate five times more energy than before,” he says.
To indicate how this has been achieved, he displays on his computer screen some magnified images of carbon composite materials that have been forced to fracture in laboratory tests. The fracture surface of older, unmodified material is “uneventful” – quite smooth. But in the case of modified materials, the opposite is the case – both sides of the fracture surface consist, instead, of a series of spikes that look like bristles on a brush.
Enabling the material to behave in this way brings several benefits for its fracture toughness, he says. One is that the undulations create fractures with a greater immediate surface area, which by definition requires more energy and so tends to confine the fracture to a more limited region of the part as a whole. A second, related, factor is that, as the material splits apart, the spikes on each opposing surface rub against each other, generating friction that again uses up further energy.
The key to achieving this behaviour is something that initially seems counter-intuitive – the deliberate severing of reinforcement fibres in a controlled pattern across the surface of the material so that the internal structure is not continuous, says Pinho.
But totally homogeneous materials such as glass tend to be brittle, because once a crack starts to propagate there is nothing to stop it spreading. In addition, controlled discontinuities as a means of making materials tougher also occur in nature.
The discontinuities in the material are produced by a surface treatment technique involving a laser, with the maximum depth of cut into
the material being about 0.03mm, says Pinho. Importantly, the chemical composition of the material is not altered – a fact that has caused excitement when he has outlined the work at academic conferences, he says.
Damage limitation
At the level of actual structures, all this means that crack propagation caused by, for instance, some form of impact damage could be made to happen in a manner that would be mitigated in three ways – deflection, deviation and diffusion, he says. This means, respectively, that cracks occurring at a particular point are made to propagate in a predetermined direction, that they do so in an energy-intensive zig-zag pattern rather than a straight line, and that they spawn numerous sideways micro-cracks that again consume energy as they form. In all cases, the macroscopic consequence would be limiting damage that might compromise overall structural integrity.
No weight penalty
If this research were to be turned into real aerospace engineering, then Pinho envisages it being used initially as a means of creating extra resilience without a weight penalty in areas of “geometrical discontinuity” in the structure, such as where an engine is attached to a wing. Altering the microstructure of materials in this way might help to achieve other goals, such as increasing compressive strength in a particular area, he says.
Ultimately, the research could lead to a fundamental change in the way engineering designers approach the task of matching materials to requirements. They now do so by identifying existing materials that most closely satisfy the demands made by the overall design objective – an objective that may need to be modified according to the capabilities of the materials involved.
But in the future that order of priorities could change, making it possible to design materials to enable product performance objectives to be satisfied without compromise. It might be possible, says Pinho, to define and produce “materials for purposes” rather than to find “purposes for materials”.
IN FOCUS – DESIGNING STRONGER AND LIGHTER AIRCRAFT
The demands of aerospace applications mean that some of the most advanced materials development happens in this sector. Engineers are constantly striving to make the structures of aircraft stronger and lighter.
The latest generation of aircraft from Boeing and Airbus embodies this pursuit.
More than half of the Airbus A350 XWB is made of composites. It is the first Airbus aircraft to use more composites than metals. The fuselage panels, frames, window frames, clips and doors are made from carbon fibre-reinforced plastic, with a hybrid door frame structure consisting of this material and titanium being used for the first time.
The use of composites has enabled Airbus to extend the service intervals for the A350 XWB from six to 12 years, which reduces maintenance costs for customers. The higher percentage of composites also reduces the need for fatigue-related inspections, and the requirement for corrosion-related maintenance checks.
Similarly, Boeing’s 787 makes greater use of composite materials in its airframe and primary structure than any previous aircraft from the company.
Nearly half the airframe is made of carbon fibre-reinforced plastic and other composites, delivering a weight saving of 20% compared to conventional aluminium designs.
Engineers have to analyse every area of the airframe when determining which material to use. So, where aluminium is sensitive to tension loads but handles compression well, composites are not as efficient in dealing with compression loads but are good at handling tension. The use of more composites, especially in the tension-loaded environment of the fuselage, therefore reduces maintenance issues arising from fatigue when compared with an aluminium structure.
This type of analysis has also led to an increased use of titanium. Titanium can prove the best low-maintenance design solution. Titanium can withstand comparable loads better than aluminium, has better fatigue strength, and is highly resistant to corrosion. Around 14% of the 787’s airframe is titanium.