MATERIAL SELECTION FOR AIRCRAFT COMPRESSOR BLADEAhmad Bamasq

Ahmad Al Dakhil

Outline• Introduction• Constrains• Selection Procedure• Decision• Future approach• References

Introduction• The majority of the thrust for propulsion in a modern

commercial jet engine comes from a large diameter fan at the front of the engine, which is driven by the low-pressure turbine at the rear of the engine. The fan, similar to a room fan, consists of multiple blades that rotate about the fan axis at high speed, and push the air backward past the engine.

Constrains: Mass & Shape• It is desired to minimize the weight of the fan blade to

decrease engine weight and then the takeoff weight. The mass of the blades rotating at high speed creates high stresses in the blades. It also requires the fan disk should be strong enough to hold the blades.

• The blade has a specified size and shape.

Constrains: Strength• The blade is to withstand rotational stresses equivalent to

70,000 psi (483 Mpa) in a material with density of titanium (4500 kg/m3).

• We can find relation of σ/ρ > 107333 Pa / (kg/m3)

Constrains: Fracture Toughness • Tolerance to damage (dents, cracks) from impact of

foreign objects (rocks, birds) is also important. A .02” (0.51 mm) deep impact-induced crack should not propagate under the cyclic loads imposed by centrifugal force.

• Fast fracture will occur if the fracture toughness • (Kc) > σ (πa)1/2

• (Kc) > ρ107333 (π 0.00051)1/2

• (Kc) / ρ > 4300 Pa (m)1/2 / (kg/m3)

Constrains: Cost & Temperature • Cost is always a constraint in jet engines, particularly

commercial ones, and it is desired (though not essential) to keep blade cost below $2000.

• The maximum service temperature is 200 C

Design Requirements

Aircraft compressor blades. Function

Size and shape are specified .

Strength: must not fail under design stresses.

High fracture toughness.

Maximum service temperature is 200 C

Constraints

Minimize mass Objective

Choice of material Free variables

Indexes

• σ/ρ > 107333 Pa / (kg/m3)

• (Kc) / ρ > 4300 Pa (m)1/2 / (kg/m3)

Tensile strength / Density1000 10000 100000

Frac

ture

toughnes

s /

Den

sity

10

100

1000

10000

Nickel-based superalloys

Titanium alloys

Wrought magnesium alloys

Low alloy steel Stainless steel

CFRP, epoxy matrix (isotropic)

Tensile strength / Density1000 10000 100000

Frac

ture

toughnes

s /

Den

sity

10

100

1000

10000

Titanium alloys

Nickel-based superalloys

Wrought magnesium alloys

Low alloy steel Stainless steel

CFRP, epoxy matrix (isotropic)

Tensile strength / Density200000 300000 400000 500000 600000 700000

Frac

ture

toughnes

s /

Den

sity

5000

10000

15000

20000

25000

Titanium alloys

Age-hardening wrought Al-alloys

Low alloy steel

Stainless steel

Wrought magnesium alloys

Nickel-based superalloys

CFRP, epoxy matrix (isotropic)

Materials for bladesIt is much lighter than normal (metallic) blade and very strong but it’s expensive. CFRB

Titanium has very good balance between weight, drag and durability against vibrations, damage - such as bird strikes - and erosion through sand, and rain. However, it’s expensive.

Titanium alloys

Both have much bigger density Steel & Nickel alloys

Lighter and cheaper than Ti but the safety is low. Al & Mg alloys

Ti blade vs. composite blade• Today, the largest engine producers are Roll Royce and General

Eclectic. RR use hollow Ti blades while GE uses a composite blade.• • The choice of blade construction depends on a number of

considerations, thus there is no clear ‘right or wrong’ answer. Each blade has advantages and disadvantages.

‘Preferred’ material Factor

Composites Fatigue strength

Ti alloys Impact strength

Both about equally (high) Cost

Depends on fan diameter Weight

Both appear adequate Durability

• All RR aircraft engines use hollow titanium fan blades including the Trent 1000 engine which is used in Boeing 787 Dreamliner.

• RR claims that CFRP blade is not aerodynamically efficient as Ti blade. It has to be thicker to have the strength to deal with actual requirements. In addition, Ti blades are more economical.

• However, Rolls-Royce is planning to replace the Ti blades by CFRB blades.

• Rolls-Royce and GKN have developed a CFRP blade that is as thin as the titanium blades with manufacturing costs.

• This fan blade has already undergone ground tests, including blade-off and bird strike tests.

• It is to begin flight tests on a Trent 1000 in the 2013 in Boeing 787.

• It could become available on a new engine in the end of the decade (beyond the Trent XWB).

• Since 1995, GE uses a CFRB fan blades for their engines.

• Starting from GE90 for Boeing 777 and now: GEnx for Boeing 787 Dreamliner which has both a front fan case and fan blades made of carbon fiber composites.

• The CFRP with has titanium leading edge for extra protection were a lightweight and durable solution.

• Each fan blade weighs between 15 and 22 Kg. Every engine contains 22 of these fan blades, which add approximately 900 kg to the engine's thrust capability, providing better fuel burn.

References• http://www.me.jhu.edu/hemker/MatSel/Labs/Lab%202%20

Fan%20Blade.doc• http://www.sciencedirect.com/science/article/pii/S0921509

398011794• http://www.flightglobal.com/news/articles/rolls-royce-come

s-full-circle-362251/• http://www.geaviation.com/aboutgeae/presscenter/ge90/g

e90_20041116.html• http://www.compositesworld.com/articles/aviation-outlook-

composites-in-commercial-aircraft-jet-engines

Thanks ..!