Do current composite materials handle bird strikes better than their predecessors? Or they are not part of forward-facing components/areas?
For example, the Boeing Dreamliner or the Airbus A350-XWB.
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$\begingroup$ Composite rotor blades use a nickel (extremely wear-resistant) strip on the leading edge to reduce erosion from blown dust/sand. $\endgroup$– Nick TCommented Oct 19, 2015 at 22:56
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3 Answers
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Out of the box, no. They are worse at handling birdstrikes. But with some thought they can be made much better.
All materials have an elastic range of deformation, and if that is not exceeded, they will assume their original shape without damage. Composites fail if this range is exceeded. First the fibers with the highest elongation fail, which enables the other fibers to take more load, but at some point too few fibers remain, and the part fails catastrophically. This is audible: The first small failures cause a faint crackling, and the eventual failure a loud bang.
Metals add a plastic deformation before ultimate failure, and this enables them to absorb a lot of energy in the process. Since energy is force times distance, the small elastic deformation will not absorb much energy. However, the plastic deformation is magnitudes bigger and makes metals the much better energy absorbers. Even a deformed structure can take a sizable fraction of the load it took before deformation. A birdstrike will cause a dent, but the rest of the structure will not see much of the load since the dent-forming could absorb a lot of energy. However, the changed contour can provoke changed aerodynamic loads, so new failure modes might result.
Impact loads on composites have a second disadvantage: The impact creates shockwaves which travel through the structure and cause delamination (separation between fibers and resin matrix) when they hit a boundary. Therefore, the biggest damage occurs on the opposite side of the impact. A birdstrike might not leave a visible dent on the outside, but can shatter the material on the opposite side such that the structure will break at a fraction of the load it would have carried without the bird strike.
Therefore, some composite wings use metallic leading edges to protect the composite structure from birdstrikes. But also the composite structure itself can be improved: If very tough fibers like Kevlar or Spectra are added, the impact energy will not break those fibers, and the load will be distributed over much more of the resin matrix. This will delay delamination, and when it happens, it will be spread over a bigger area and cause the fibers to break at multiple locations. The energy needed for the widespread delamination and the multiple fiber failures will increase the energy absorption dramatically. This structure must be designed as a protective armor, so the load-carrying structure below is not affected. Proper crashworthy design with composites allows to make the structure even tougher than a comparable metallic structure.
answered Oct 19, 2015 at 12:21
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3$\begingroup$ The important difference between the energy absorbed in elastic and plastic deformation is that plastic deformation is non-reversible, and the energy is retained within the material permanently as locked-in internal stresses. The energy stored in elastic deformation is only temporary, and is released when the structure returns to its original shape. This often causes vibrations to be transmitted through the structure which can take a relatively long time to die away, and may cause collateral damage at places a long way from the original impact site as they do so. $\endgroup$ Commented Oct 19, 2015 at 18:27
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In general, the composite materials are poor compared to metals in impact resistance (in general, composites are not good in resisting compressive loads). This is because the metals and composites respond to impact in different ways.
In case of metals , most of the impact energy is absorbed by the metals through plastic deformation. As the plastic region is pretty large compared to the elastic region, this means that they have a good impact resistance.
Source: www.spaceflight.esa.int
However, the (fiber reinforced) composites undergo little plastic deformation. The response of the composites in case of impact depends on a number of characters like the speed of impact, fiber orientation etc. In case of low intensity impact, elastic deformations may appear near the impact area. A high speed impact, however, causes deteriorations of the material, which includes separation of the fibers from the matrix, matrix cracking and fiber breaking.
Comparison of stress- strain curves of composites and metals. Image from Circular and Square Concrete Columns Externally Confined by CFRP Composite: Experimental Investigation and Effective Strength Models by Riad Benzaid et. al.
The absorbed energy consequent to the impact depends, among others parameters, on the fiber – resins link resistance. If this link is strong, a continuous crack may spread along the material. In the case of a weak link, the generated crack may have an irregular form, leading to a rapid separation of the fibers from the matrix and to a considerable absorption energy. However, the adhesion between the fibers and the matrix shouldn't be too weak as a low shear resistance also influences the impact behavior in a negative manner.
The primary mechanism is that the fibers are subjected to bending, which breaks them, consequent to which the load is transferred to the resin base, which usually cracks. The the load is transferred to the other fibers, which get damaged.
There are some ways to make the composites better impact resistant, like using materials like Kevlar, which has higher impact resistance. These tough materials have higher failure loads,which helps in absorbing the impact load and transferring to to the resin matrix (which carries the compressive load).
Impact strength of various composites, from The Impact Behaviour of Composite Materials by Chircor Mihael et. al.
The composites can also be tailored to have a better impact resistance. Also, the places where the impacts are expected, like the leading edges can be made from metals. Interestingly, both A350XWB and Boeing 787 use metals in the wing leading edges, though I'm not sure how much of this is attributable to impact resistance.
Source: www.1001crash.com
answered Oct 19, 2015 at 15:09
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$\begingroup$ On the other hand, Kevlar and kin are considerably harder than glass or carbon fiber to incorporate successfully into composite aircraft structure. $\endgroup$– VikkiCommented Jul 19, 2019 at 22:36
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May be it's better to answer the question the other way around. There are regulations which need to be followed. This also includes bird-strikes. In order to get certified all the requirements have to be accomplished.
The reason new materials are used is because they feature desirable attributes (they are for example lighter or maybe cheaper) while achieving the requested properties.
It is not only a specific property of the material which makes it sustaining a bird strike but also the way the whole part was designed by the engineers. Depending on the material this design process might be different and might contain certain challenges. However, the final part will only be as strong as the regulations require. This will be true for aluminium, titanium or composite materials.
