Lecture 6 - Design, Certification & Fatigue

AE1110-II Introduction to Aerospace Engineering - Materials

Jointly responsible parties (structural safety)

Manufacturer

Operator

Authority

Familiarization & Certification basis → establishment of certification program → compliance demonstration

Structural requirements related to

Strength (resistance to failure and standards with regard to risks)

Load cases (exact ‘estimation’ of external forces on aircraft during its life)

Life time

The unit of time

Flights is more relevant than flying hours

Flying hours is often easier for aircraft operator

Each part has its own unit of lifetime depending on type of lifetime reducing factor (fuselage pressurization vs engine part rotation)

Structural design philosophies

Safe life

Fail Safe

Damage tolerance

Safe life

No. of flights, landings, flight hours during which there is a low probability that the strength will degrade below its design strength (safety by reitrement)

Design with safety factor

Fatigue becomes and issue when:

• Increase of design lives (economic reasons)

• Increase of loads (e.g. cabin pressure at higher altitudes)

• Improvement of (accuracy of) methods

• Application of stronger materials, but with poor fatigue properties, crack growth and residual strength

Damage during service is not taken into account

Fail safe

Attribute of structure that permits it to retain required residual strength for a period of un-repaired use after failure or partial failure of a principal structural element

Emphasis on multiple structural member concept

Static strength analysis of various damage scenarious

Each structural item adequate safe life

Economically more viable than safe-life concept

In time damage detection possible (increase safety)

But:

• Not all failure modes anticipated

• No partial failure anticipated

• Multiple site damage (MSD): Fail safe ineffective (aging aircraft)

Damage tolerance

The ability of the structure to sustain anticipated loads in the presence of fatigue, corrosion or accidental damage until such damage is detected through inspections or malfunctions and is repaired

DT concept is not a replacement: Combination of Safe Life, Fail Safe and damage tolerance needed

Damage and imperfections assumed to be present (from day 1)

Sufficient durability (economics)

Fundamental asssumption of Damage tolerant philosophy: Repair restores structural strength to original level

Durability

The ability of the structure to sustain degradation from sources as fatigue, corrosion, accidental damage and environmental deterioration to the extent that they can be controlled by economically acceptable maintenance and inspection programs

Modern structural design must satisfy

Damage tolerance

Durability

Damage tolerance: design approach

Critical location (primary structure): Damage tolerance

Safe life is only allowed for landing gear attachments

Flat panel without notch

The normal stresses are the same everywhere

Flat panel with notch

A notch creates a disturbance in the stress flow

Notch not only decrease in cross section (increase average σ) also Concentration of stress (K_t)

Stress-strain concentrations

Nominal stress = average stress in the net-section higher than σ_applied

σ_nom = W/(W-D) σ_applied

Stress concentration factor K_t

K_t = σ_peak/ σ_nom

Stress-strain concentrations (flat panel with notch)

Consider an isotropic infinite sheet with an elliptical hole

K_t = σ_peak/ σ_nom = 1 + 2a/b = 1 + 2sqrt(a/r)

For a circular hole: K_t = 3 independent of hole dimensions

Linear elastic & anistropic materials (composites) can be very sensitive for notches

Saint Venant

Disturbance is limited to the direct neighbourhood of the notch causing the disturbance

Stress concentraction in finited dimensions (Heywood equation)

Stress concentration reduces from K_t = 3 (infinite width) towards K_t = 2

K_t = 2 + (1-d/W)³

Plane stress - superposition

1. Determine stresses for σ_1

2. Determine stresses for σ_2

3. Superimpose both stress systems

4. Use superposition stress principle

Stress concentration

Repair/reinforcement may attract stress because of increased stiffness

K_t effect on ultimate strength

Plasticity at notch reduces peak stress

Concentration of stress & strain

• A stronger, but more brittle material can not be loaded as high as the weaker, more ductile material

Ductile materials are less notch sensitive under tensile loading

• Composites do not yield → peak stresses are not leveled off (not sensitive)

The strength of the ductile material can be used much better

Fatigue (Definition)

Damage phenomenon induced by large number of load cycles below ultimate strength resulting in permanent deterioration of material or structure causing a reduction in load bearing capability

Fatigue of notched materials

Stress concentration reduces fatigue limit

Fatigue

Initioation and growth of small cracks = 80-90% of total life (Remainder of life fast growth to failure)

Thickness of fatigue critical parts is greater than required for static strength

Means to increase life:

• Damage tolerance approach

• Avoid stress concentrations and damage initiators in design

• Less fatigue sensitive materials

Characteristic features of fatigue

Fatigue appears different in metals and composites

• Metals sensitive to tension-tension fatigue

• Composites sensitive to compression - tension fatigue

Static strength requirement for composites often covers fatigue related aspects

Damage tolerance: Residual strength

Stress intensity factor: parameter describing singularity in elastic field at the crack tip

σ = K f(r,θ)

K = S sqrt(πa)

Three modes to open a crack tip

Mode I: Tension

Mode II: Shear

Mode III: Transverse Shear

Critical K in mode I is K_1c and is called fracture toughness (material property indicating the sensitivity for cracks under static loading)

Fracture toughness

Dependent on material property (ductility)

Related critical stress dependent on geometry:

σ_crit = K_1c / sqrt(π a_crit)