Structures Revision

• Aircraft use accelerometers in components to periodically check for vibration caused be engine RPM, fuel pumps etc.

• These resultant vibrations produce a waveform. Where multiple component vibrations are detected they can be broken down into individual sine-waves,  this process is known as Fourier-analysis.

• Each component under goes testing from 0-Max RPM in order to produce a specific graph known as a component frequency chart. (Frequency in the Y axis, HP speed in the X).

• If a vibration is detected at a specific frequency and speed then the graph can be used to predict the component.

-          reduce number of damaging load cycles per sortie

-          manage the aircraft configuration and mass distribution carefully

-       identify causes of damaging loads through OLM and HUMS

-          increase speeds on roller landings, which prevents lift reduction on wings avoiding bending cycle of wings at the wing root

-          use fuels in wing tips last, so wing bend relief is kept as high as possible

-          reduce time in buffet condition, restrict combinations of speed and load factor

-          High G turns performed at the end of sorties when aircraft weight is less.

• Statement of Operating Intent and Usage review. Ensuring that the aircraft is being flown as it was intended and that the sortie profiles are still accurate

• structural sampling- regular examination of components is conducted regularly. Opportunity sampling for components that are out for maintenance affords the possibility of examining components usually inaccessible. Planned sampling is carried out routinely.

• Teardown- Use of an ex-service airframe is an in depth destructive examination of the aircraft structure, used to calculate the SI and calculate fatigue life of aircraft.

• Maintenance Scheduling- Adjusted as new information is made available.

• Data Recording- recording the fatigue of aircraft via fatigue meters and strain gauges allows for detailed information to be retained. Operational loads measurement (fixed wing) and operational data recording (heli) is used. This is used to validate design assumptions and can be used for trending.

Safe Life

Damage Tolerant

What are they

for a safe life approach a component or the aircraft is given a fixed operational life. 'safe life'

Components are replaced when this is reached.

Aircraft are retired when this is reached unless their life gets extended through additional maintenance.

Components may appear undamaged however internal microstructure has changed. Further usage risk is too high.

Damage tolerant is when an aircraft or a component is not given a fixed operational life. Instead the structure is designed to tolerate certain amounts of damage. Cracks that occur must be easily recognisable so that it takes no more than 3 opportunities to notice.

Safe Life

Damage Tolerant

How are they established

Established through testing. S-N curves are produced which are used to predict the safe life.

Measured in cycles or hours.

tests on the ground during design to show if:  -the cracks take a long time to initiate

-the cracks grow slowly

-are long enough to be seen within 3 attempts.

Safe Life

Damage Tolerant

What aircraft types use what

fast jets + helicopters. - high stress on fast jets

Transport aircraft use this approach. - No high G manoeuvres.

(i)                  Advantages Vs Disadvantages:

Advantages - high strength to weight ratio, tailored directional properties, non-corroding in salt environments, excellent fatigue resistance, manufacture complex shapes.

Disadvantages - susceptible to impact damage, poor electrical conductivity, moisture pick up and + UV damage, badly effected by some chemicals, BVID (hard to see), difficult to repair

(ii)                If they don’t corrode what makes CFRP degrade?

basically above is listed in the disadvantages, UV, damage to panels creating delamination for water to get in etc, osime oils etc causes rapid later splitting

• general surface corrosion                                                                                                                                                    -uniform degree of attack all over an unprotected surface

• dissimilar metal or galvanic corrosion                                                                                                                       when 2 different metals are in direct contact and water becomes trapped between them or a pool of water covers both materials

•  crevice corrosion                                                                                                                                                      when an electrolyte gains access to crevices in or between components                                                                     -corrosion cell produced from the non-uniform concentration of dissolved oxygen in the water i.e. high dissolved oxygen content at the surface and low dissolved oxygen content in the crevice+

• intergranular corrosion                                                                                                                                             - corrosion within the grain boundaries of a metal                                                                                                  - different chemical composition of grain boundaries and the grain itself, thus, able to form an electrochemical cell when exposed to an electrolyte

•  exfoliation corrosion                                                                                                                                                               -extreme form of intergranular corrosion, mainly occurs in aluminium alloys that have been rolled or extruded i.e. grains have been flattened/elongated                                                                                                                                           -localised corrosion at the grain boundaries wedges them open causing the grains to flake away from the surface                            
• pitting corrosion                                                                                                                                                              +appears as a series of small pits on the metal surface (ranging from cavities of small diameter to depressions of large diameter)

•  microbiological corrosion                                                                                                                                  - corrosion of aluminium alloy in integral fuel tanks due to the fungal growth from the micro-organisms in turbine fuel, prevalent in hot damp climates

• stress corrosion cracking (scc)                                                                                                                                 -   combination of a steady tensile stress (below the yield stress) and a corrosive environment
  often caused by residual stresses generated within the metal during manufacture and assembly NOT by the applied service load

-          presence of corrosive fluids leads to the creation of an electrochemical cell

-          creates a potential difference between metals

-          faster process when electrolyte is a better conductor of electricity and when dissolved oxygen is present.

Shot Peening - The component is bombarded with a stream of small hard spheres, which plastically deform a thin layer at the surface. This leaves a residual compressive stress. The mean stress at the surface is therefore reduced, delaying or preventing surface cracks from initiating.

Selection of Surface Finish - A rough surface will allow fatigue cracks to initiate quicker than a polished surface, and fatigue life at a given stress amplitude will be reduced.

Cold Working of Fastener Holes – Holes to be cold worked are drilled slightly undersize, then a tapered pin or mandrel is pulled through the hole to expand it. The edge of the hole plastically deforms, leaving a residual compressive around the circumference, improving fatigue resistance.

Corrosion Prevention – The onset of corrosion will shorten fatigue lives, as pits etc. will act as stress concentrations. Similarly if corrosive fluid can reach the tip of a growing fatigue crack, that crack will propagate faster. Early identification and rectification of corrosion will therefore preserve fatigue life of components.

Fatigue Limit – Some steels and titanium alloys may have a Fatigue Limit, if stress is kept below this limit then no fatigue failure should occur. It is important to note that if stress concentrations are introduced to the surface then fatigue cracks may still initiate.

Low and high cycle fatigue relates to the point at which given materials fail under cyclic loads.

Number of cyclic loadings, stress amplitude of these alternating movements and point of failure can be shown on an S-N curve graph.

Average results for these values are displayed.

Low cycle fatigue for a material/structure is a failure point of less than 10^5 cycles and high cycle fatigue is anything above that.

Uncertainty/unpredictability in fatigue life of individual specimens is greater in structures that reach high cycle fatigue results.

Aluminium alloys:

• Have a steady S-N curve with fatigue failure between 10^6 and 10^7 cycles at low stress amplitudes (25% of their static Design Limit Load-DLL).

• Classical crack initiation, growth and final failure stages are likely.

• At higher stress amplitudes it will fail much earlier.

• Aluminium does have the benefit of allowing lengthy (500mm) critical crack lengths that engineers should spot.

Titanium and some steels

• Seem to have a fatigue limit line.

• Provided the stress amplitude is kept to a fairly low level, cyclic fatigue does not appear to occur provided the surface is smooth and damage/corrosion free.

• Critical crack lengths are as low as a few millimetres.

composites:

• Fail at a similar number of cycles to 'gently-fatigued' Aluminium (~10^6 cycles) .

• However, this is regardless of stress amplitudes experienced- within practical reason. They can take cyclic stress amplitudes of 75% (of their Designed Ultimate Load-DUL) right up to the number of cycles stated above.

• Design limits requested for any given aircraft in service would not usually exceed 67% DUL. Even with small drilled holes, it takes cyclic loads far beyond those commonly experienced in flight to cause any initial minor damage to grow worse.

In summary,

Aluminium alloys experience cyclic fatigue at all stress amplitudes, they could fail at low or high cycle stages but it is predictable through usage.

Titanium and some steels are unaffected at low levels of amplitude. They could theoretically have unlimited cyclic fatigue life provided stress amplitudes are kept low.

Composites will last a predictable number of cycles, i.e. around 10^6 - 10^7 but can do this even under extreme cyclic stress loading amplitudes beyond normal flight parameters.

frames and bulkheads

-

Frames are transverse members in the shape of the fuselage cross-section.

They maintain the cross-section shape of the fuselage.

Frames also divide the skin/stringer construction into shorter lengths to increase the load at which they buckle.

A bulkhead is an upright wall of separation within the fuselage.

Frames that are solid right across the fuselage are usually called bulkheads.

stringers and longerons

-------------------

In aircraft fuselage stringers are attached to frames and run the longitudinal direction of the aircraft.

They are primarily responsible for transferring the aerodynamic loads acting on the skin onto frames and formers.

The terms “Longeron” and “stringer” are used interchangeably. However there is a subtle difference. Longerons travel the whole length of the aircraft and stringers are placed in between the frames.

Kiel’s and intercostals?

-------

A Kiel is a beam located along the bottom of the fuselage. It is used to carry loads around the parts of the lower fuselage where skin has been cut away. For example to provide doors for the landing gear.

An intercostal kielson runs between floors, an intercostal floor runs between kielsons, and an intercostal stringer runs between frames. The intercostal is for dissipating a load from a first frame onto a second frame and/or a skin of the aircraft.

(when other metals have better properties)?

-          Aluminium has the best balance of properties for airframes.

-          Titanium alloys and steels have higher strength to weight ratios, but on airframes buckling resistance is more important than strength.

-          Low density, therefore components can be made thicker, increasing buckling resistance but still remaining light in weight.

-          Corrosion resistant

-          Good fatigue resistance

-          Can be used up to 120 ‘C - not used on engines