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Trends in new materials for aerospace

Stefania Cantoni Italian Aerospace Research Centre Head of Structures and Materials Dept.

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Evolution of aircraft : the role of materials

Dedicated design approaches

how to “think composite”

how to “think metal”

Toward Intelligent structures

Morphing

Outline

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The beginning 17 December 1903 - The Wright Flyer

Evolution of aircraft : Focus on Materials

"...the first powered, heavier-than-air machine to achieve controlled, sustained flight with a pilot aboard”

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Evolution of aircraft : Focus on Materials

• The wood was listed as a candidate material in early aircraft designs. They did consist mainly of wooden structural members (spruce or bamboo), glued or screwed together to form a frame, which was then covered in canvas. The wings were supported by wooden compression struts and steel tension wires.

• Most of the aircraft built until the early 1920s used this 'stick and stringer' type of construction.

Wooden aircraft were extremely successful, but as aircraft became bigger, serious problems due to fungal rot forced designers to consider metallic aircraft.

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• In 1911 the discovery by the German of the aluminum alloyed with copper opened new frontiers. This material stronger than mild steel paved the way for aluminum framed and skinned aircraft.

• The use of Duralumin alloys as they became known, enabled some of the aerodynamic forces to be carried by the stressed skin of the wings and fuselage. This resulted in very efficient airframes. Most of the aircraft in World War II were aluminum alloy stressed skin designs.

• Remarkably, the early aluminum alloys developed in the 1930s (2024) and 1940s (7075) are still used extensively today

Evolution of aircraft : Focus on Materials

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Operating cost sharing for a civil aviation long range airplane (Airbus source)

NEEDS To reduce acquisition cost, through

improved material utilisation, design and manufacturing processes

To reduce airframe weight, leading to

lower fuel consumption and reduced environmental impact

long

72 m

height

24 m

wing span

80 m

Weight 580Tons

Cost 350M$

Evolution of aircraft : Focus on Materials

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A350 XWB

Never in the history of civil aviation so many composites have been used on an airplane

New engineering solutions for aerospace structures are promoting new industrial applications at the gates

Time gap between the creation of a 'laboratory curiosity' material and its application in an engineering context.

Evolution of aircraft : Focus on Materials

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Increasing demands on materials development, inspection and repair technologies as well as affordability and environmental compatibility. Developments in new materials technologies include: Removing the idea of composite as black

aluminum

Dedicated design approaches

Completely automated process

New metal alloy and additive processes

Additive Layer manufacturing

Implementing more functions into the same structure

smart structures

Evolution of aircraft : Focus on Materials

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Structures are designed using a semi-monocoque concept- a basic load-carrying shell reinforced by frames and longerons in the bodies, and a skin-stringer construction supported by spars and ribs in the surfaces

Dedicated design approaches

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An isogrid is a type of partially hollowed-out structure formed usually from a single metal plate (or face sheet) with triangular integral stiffening ribs (often called stringers)

Isogrid structures are related to Sandwich-structured composite panels; both can be modeled using sandwich theory, which describes structures with separated, stiff face sheets and a lighter interconnecting layer. The triangular pattern is very efficient because it retains rigidity while saving material and therefore weight.

Dedicated design approaches

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USA the composite get into the traditional metal isogrid stiffened structures Minotaur : grid fairing

Dedicated design approaches

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Russia the Composite Anisogrid concept Proton Launcher : interstages and cone adapters

Dedicated design approaches

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- The theoretical structural efficiency of lattice shells under heavy axially compressive/bending loads is higher than equivalent layered shells

- Orthotropic elastic properties are demonstrated to increase the weight efficiency with respect to the Isotropic ones under specific stiffness, strength, and buckling constraints

- The helical ribs sustain compression, inducing a circumferential tension in the hoop ribs (skin), producing an effect similar to the action of internal pressure

- This “pressure” stabilizes the circular form, reduces the shell sensitivity to shape imperfections and increases the critical load

Composite Anisogrid concept made of unidirectional ribs

is a perfect combination of such theoretical pros

Dedicated design approaches

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- Filament Winding (FW) and Fiber Placement (FP) are the most efficient and low cost automated method to lay up unidirectional interlaced tows on axisymmetric shapes - Expansion tools are mostly useful for Out Of Autoclave process, still having an high compaction on composite material

Composite Anisogrid Manufacturing

based on unidirectional winding/placement in grooved

expansion tool is a perfect combination of both

theoretical pros

Completely automated process

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15

Despite the positive results of theoretical studies (expected weight 60% less than metal) proposed in aerospace applications both in Russia, with PROTON launcher, and USA, with Minotaur, no real case nowadays exists of a composite grid airplane

Completely automated process

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The basic grid wet FW process:

fiber tows are wound into the grooves formed in an expansion rubber coating, alternating and interlacing helical and hoop ribs

once the skin has been wrapped, a tensioned layer is over-wrapped to compact the grid during vacuum bagged oven curing

The basic grid Fiber Placement (FP) process:

the ribs are built up by loosely stacking the tows on top of each other, creating a free standing grid on a smooth-surfaced carbon fiber composite male tool.

Thick rubber inserts are placed among fiber placed ribs, before over-place the skin.

Autoclave is usually used to cure

Completely automated process

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The use of prepreg does

not seem really effective in the interlacing of tows, in order to keep straight fibers in the knot region. This can lead to poor compressive strength.

Completely automated process

The use of wet seems more effective, due to the resin exceeding content in the ribs and the easy compaction of nodes.

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18

dry fibers parallel robotic winding + resin infusion

a new approach for a real low cost process:

- fully automated and continuous - Out of Autoclave - no dummy helix - no exposure to wet resin - no shelf life problems - rapid, complete infusion due to high permeability - low porosity due to resin degassing during infusion

Manufacturing achievements

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The dummy helical problem: an extra rib to pass through separate hoops

Manufacturing achievements

A parallel feeding for continuous separate hoops without dummy helix

An infinite rotation of the deposition eye

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Mass Demonstrator

(Kg)

Mass Full-scale composite

(Kg)

Mass Full-scale

benchmark (Kg)

Helical ribs 11 37 48 (stringers)

Hoop ribs 2.0 6.8 n.a

Reinforcing Frames 4.2 14.2 5.4

Flange (interface rings) 3.8 12.8 44

Skin 4.0 9 ÷13.5 32

Overall weight (without bolts) 25 80÷85 ≅135

The VEGA Launcher I/S

Vega is a single-body launcher with 3 solid-propellant stages and 1 liquid-propellant upper module for attitude and orbit control, and satellite release (300–2000kg). The first Vega lifted off on 13 February 2012

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Increasing demands on materials development, inspection and repair technologies as well as affordability and environmental compatibility. Developments in new materials technologies include: Removing the idea of composite as black

aluminum

Dedicated design approaches

Completely automated process

Designing in 3D

Additive Layer manufacturing

Implementing more functions into the same structure

smart structures

Evolution of aircraft : Focus on Materials

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Additive Layer Manufacturing

Additive layer manufacturing is a modern fabrication process in which three dimensional parts are built up in two-dimensional layers as little as 0.05 mm thick Unlike all the traditional techniques ALM produces successive layers of material made time to time by the fusion of powders. Layer after layer such systems rebuild the object starting from its mathematical model. Pollution of residuals does not exhists or is neglegible w.r.t. to other «dirty technologies», cost of disposal of dangerous waste is hence greatly reduced. Two complementary process categories based on the melting source: Laser beam melting (suitable for low melting alloys) Electron beam melting (suitable for high melting alloy)

“…new workers are mastering the 3-D printing that has the potential to revolutionize the way we make almost everything….. ask this Congress to help create a network of 15 of these hubs and guarantee that the next revolution in manufacturing is made in America" Obama’s speech Feb. 2013

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Additive Layer Manufacturing

Free-form manufacturing, complex geometries Fit for materials “difficult to manufacture” Less/no material waste 1-piece concept, instead of several assembled parts Fast time-to-market, no tools needed Design iterations affordable Higher density than the one obtained with older

sintering processes Good material properties (comparable to forging) High repeatability, low scrap High technology content / low labour content Technology improvement as fast as software

technology growth

Property (Ti6Al4V) EBM Standard (ASTM) Yield strength 875 MPa >860 (ASTM F1472)

Ultimate Tensile Strength 960 MPa >930 (ASTM F1472) Elongation 13% >10 (ASTM F1472) Rockwell hardness 30-35 HRC High-cycle fatigue test* 550 MPa Fracture Toughness 80 MPa·m½ >65 (AMS4999) Roughness 1-20μm * 10 million cycles

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The Freedom of Design Allows to Integrate Two Separate Functional Parts in One Design

Oil Separator and Cooler

Conceptual Heat Exchanger

Additive Manufacturing Allows Completely New Design Structures

Source: Within, 3T, EOS

Lattice Structure Brake Disk

Integrated Cooling Channels Help to Reduce Weight and Increase Performance

Source: ka.race.ing Karlsruhe , Turbine Blades

Source: Avio Aero

AM is best suited to produce complex integral parts

Additive Layer Manufacturing

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Positive effect of Scandium on the mechanical properties of Aluminum Alloys is obtained by dispersive hardening, generated by the disperse Al3Sc precipitates. Al3Sc phase is characterized by high structural and dimensional match with the aluminum matrix. This intermetallic structure has the same FCC crystalline lattice as Al, and parameters of their lattices differ only by 1.3%. The addition of Scandium leads to a smaller grain size and a lower sensitivity to recrystallization, with consequent

• higher yield stress • better mechanical strength • improved metal weldability • and higher corrosion resistance.

New Metal Alloy

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New Metal Alloy

26

Fuselage stringers and panels of large cargo aircrafts, and some parts of the MiG 21, 29 and 31 military aircrafts are made of Al-Sc based alloys belonging to the 7XXX series

Nose cones of some Soviet submarine-launched ballistic missiles (SLBM)

NASA has developed an Al-Sc alloy belonging to the AA7XXX series for aerospace application. This alloy is deigned to build cryogenic tank for H2O2 propellant because of its high mechanical properties at cryogenic temperatures, high corrosion resistance and good weldability [9].

EADS has proved the applicability of a laser additive manufacturing process with an Al-Sc powder patented as SCalMAlloy

CIRA in cooperation with CSM has developed a Al series 2xxx added with scandium

* Courtesy by ARCAM

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Increasing demands on materials development, inspection and repair technologies as well as affordability and environmental compatibility. Developments in new materials technologies include: Removing the idea of composite as black

aluminum

Dedicated design approaches

Completely automated process

Designing in 3D

Additive Layer manufacturing

Implementing more functions into the same structure

Toward intelligent structures

Evolution of aircraft : Focus on Materials

Use or disclosure of the information contained herein is subject to specific written approval from CIRA

• Smart structures promise to revolutionize the way engineers think about the structural safety

• Due to their specific features, Smart

Structures are capable of monitoring their integrity (SHM), with consequent enhancement in terms safety and performance.

Smart structures - Structural Health Monitoring

“Structural health monitoring (SHM) systems can be arrayed in similar fashion to the human nervous system, with sensors concentrated in key areas where loads are highest” Source: Holger Speckmann, Airbus

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Planned Maintenance Operational Readiness

Fixed inspection rates The costs for the operational readiness are function of maintenance periods.

Pres

ent

Condition Based Maintenance

Inspections only if needed.

Continuos Health Monitoring Operational Readiness Reduction of costs for operational readiness

Weight reduction Damage Management SHM to reduce structural behaviour uncertainties

Futu

re

Damage Management Use of Knock-down factors

Structural Efficiency

Smart structures - Structural Health Monitoring

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Fiber Bragg diffraction grating sensors — Fiber-optic cables embedded in the structure are laser marked with optical interference patterns. Any local strain causes a slight change in the sensor's light transmission wavelength.

Acoustic emission — Transducers listen for acoustic signals generated by cracks, delaminations or fiber breakage.

Acousto-ultrasonics — Low-frequency acoustic pulses are sent through a part and received by transducers. Damage causes a change in the reflected acoustic energy.

Smart or sensitive coatings — Coatings or paints with integrated piezo- and ferro-electric elements or carbon nanotubes can function to detect strain.

Microwave sensors — Small microwave sensors embedded in the structure send and receive signals that indicate moisture ingress. The method is good for monitoring composite sandwich structures.

Imaging ultrasonics — A small ultrasonic wave transducer generates a signal that passes through the material. Changes in wave reflection indicate flaws or damage.

Smart structures - Structural Health Monitoring

A number of SHM sensing technologies are currently under development or in use. The most common are the following:

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Aerospace morphing. The idea is to have an aerodynamic surface (e.g. a wing, a tail or parts of them), able to change its geometry according to the specific flight regime. Expected advantages • Enlargement of the flight envelope, due to

adaptivity to different fly regimes • Better distribution of the loads with respect to

the conventional architecture (hinged flaps and slats)

• Weight reduction due to an optimized multi-disciplinary design approach

A key role is played by the flexibility of the structure that has to fit the target shape and at the same time preserve it under the external aerodynamic loads. The skin, in particular, is demanded to follow the deformation imposed by the inner structure and to cooperate in absorbing and transmitting the external loads.

Leading edge morphing concepts

Toward intelligent structures

Lockheed Martin and NextGen Aeronautics morphable wing concept

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The Segmented/Multilayered Skins is another promising approach whose idea behind is to concentrate the stiffness within small components segmented each other’s, thus enabling the global deformation of the structure.

Function model of fish scales

Morphing Structures : Segmented/Multilayered Skins

The important features pertaining to scaled skins are: • A strongly elastic tensile behavior that is independent from

the presence of rigid scales, • A compressive response that prevents buckling and

wrinkling instabilities, which are usually predominant for thin membranes,

• A bending response that displays non linear stiffening mechanisms arising from geometric constraints between neighboring scales

• A robust structure that preserves the above characteristics upon the loss or damage of structural elements.

These important properties make fish skin an attractive model for the development of very thin and flexible armors and protective layers, especially when combined with the high penetration resistance of individual scales. Scaled structures inspired by fish skin could find applications in ultra-light and flexible armor systems, flexible electronics or the design of smart and adaptive morphing structures for aerospace vehicles. Fish scales body armour

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Shape Memory Materials (SMA) are other good candidates: the metallic alloys, suitable embedded within the skin, can cooperate in withstanding large forces and deformations;

Morphing Structures: Shape Memory Materials

Morphing trailing edge actuated by SMA ribbons, changing the local curvature. Smart Flap national project.

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Shape Memory Polymers, analogously, offer the possibility of recovering a pre-imposed shape and withstanding deformations up to 1100%. These features, jointly with the wide range of mechanical properties obtainable in the design and manufacture phase, are exploited to produce honeycomb or foam components, morphable over the transition temperature and rigid at lower temperatures.

Morphing Structures : Shape Memory Polymers

The process begins at the top and proceeds clockwise around to the center. Under thermal stimulus, the wing quickly unfolds and expands to its appropriate chord and shape.

The coupon begins with a rectangular memory shape. When heated above its transition temperature, it becomes elastic and can be manipulated into a different shape and then cooled to maintain the new shape in a rigid state. When reheated above its transition temperature, it will return to its memory shape if left unrestrained.

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……and more

Tailored resin to improve impact resistance and environment requirements (included thermoplastic materials)

Embedded damping veil for acoustic improvements

Embedded EME and Lightning features (magnetostrictive particles)

Boeing 777 In-Service Experience Sydney Hailstorm