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StM3 > Short Questions > Flashcards

Short Questions Flashcards

1
Q

Hot working temp for metals?

A

1/3 to 1/2 absolute melting point

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2
Q

For bolted lap joint in tension, why is pre-load important?

A

It can reduce stress concentration near the bolt hole and promote bearing failure which prevents shearing failure.

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3
Q

What causes thermal residual stress within the adhesive layer of adhesive joints?

A

High curing temperature and mismatch between the coefficient of thermal expansion of the adhered and adhesive materials.

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4
Q

What is the effect of increasing the carbon content of steel on the ductile to brittle transition temperature?

A

Increases the transition temperature.

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5
Q

What are several conditions that affect the ductile to brittle transition temperatures?

A
  • Smaller grain size lowers the temp.
  • More stress concentrations due to flaws/cracks/defects increases the temp.
  • Increasing the loading rate lowers the temp.
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6
Q

How many elastic constants define a state of transverse isotropy?

A

4

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7
Q

Describe a symmetric laminate.

A

A laminate with a ply +(theta) angle x distance from mid-plane and a ply +(theta) -x distance from mid-plane. angle

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8
Q

Describe load spectrum truncation.

A

The limit of upper load levels.

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9
Q

List the 3 parts of classical fatigue analysis.

A
  • Load idealisation.
  • Data manipulation.
  • Damage accumulation.
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10
Q

List the 3 fatigue design guidelines for metal alloys.

A
  • Ensure selection of suitable alloy.
  • Ensure correct heat treatments.
  • Account for grain directions.
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11
Q

List the mechanisms of primary stage creep in metals.

A
  • Increasing dislocations.
  • Work hardening.
  • Reducing creep rate.
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12
Q

What is the equation for the deflection of a beam?

A

(kPL^3)/EI

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13
Q

Describe shear flow for pure free torsion.

A

Shear flow is the force per unit of circumferential length. Shear flow q = force per unit length.

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14
Q

What is the Semi-Monocoque shear flow equation?

A

q = -(Sy/Ixx){Sum} Aiyi + qo

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15
Q

What is the torsional axis of a wing?

A

It is the locus of shear centres along the span and is the axis about which the wing will rotate during pure torsion. It is also called the ‘flexural axis’ and the ‘elastic axis’.

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16
Q

What is the Centre of Pressure along a wing?

A

The point at which the resultant force of aerodynamic pressure around the aerofoil acts.

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17
Q

What is the Aerodynamic Centre of a wing?

A

The point at which moment due to lift and drag forces remains constant. This allows for us to replace the lift/drag forces at the CP with forces at the AC plus a constant pitching moment.

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18
Q

What is the equation that relates the rate of twist to the resultant shear flow?

A

(theta)’ = (1/2AiG){Sum} q (ds/t)

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19
Q

What is the equation relating the torque and resultant shear flow of a closed cell?

A

T = 2Aq

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20
Q

For the landing condition, define what is meant by the reaction factor (lambda).

A

It is defined as the ratio of peak load (Zmax) during the landing to the static load at Max Landing Weight (MLW) per main gear leg.

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21
Q

What is the equation relating the equation relating the manoeuvring speed to the stalling speed?

A

Va = Vs x sqrt(n)

where n = 2.5g for large civil aircraft.

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22
Q

What are the macro features observed on the fracture surfaces of brittle and ductile materials under tension?

A

Brittle - Grainy or faceted surfaces.

Ductile - Cup and cone pattern.

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23
Q

List 3 classic fracture modes.

A
  • Opening mode.
  • Shearing mode.
  • Tearing mode.
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24
Q

What is the ratio between the plastic zone radii of metals under plane stress and plane strain?

A

rp (plane stress) = 3 x rp (plane strain)

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25
Q

Why does Griffith’s theory not apply to ductile materials?

A

It is based solely on the atomic bond energy preventing the crack to grow, neglecting small-scale plasticity ahead of the crack.

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26
Q

What is the equation used to calculate the radius of the plastic zone?

A

rp = (1/6pi)(Kc/sigmay)^2

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27
Q

For plane stress, what is the relationship between Kic and Gic?

A

Gic = (Kic^2)/E

28
Q

What is the equation for Inglis’ theory?

A

Sigma(max) = Sigma(1+2sqrt(a/pc))

29
Q

What dictates the strength of a unidirectional lamina?

A
  • Longitudinal tensile and compressive strengths : Xt and Xc
  • Transverse tensile and compressive strengths: Yt and Yc
  • Shear strength: S
30
Q

What aluminium alloys are commonly used in aircraft structures?

A

2000 and 7000 series.

31
Q

List the 3 main sources of heat generation in metal machining.

A
  • Work done on the shearing chip.
  • Energy dissipated by friction at the tool-chip interface.
  • Friction between the tool and the machined surface.
32
Q

Describe ‘spring-in ‘ in composites manufacturing.

A

The through-thickness chemical and thermal shrinkage of composites is much greater than the in-plane shrinkage during curing. Therefore, the angle of an angled composite laminate becomes smaller than that of the mould after curing.

33
Q

List the mechanical fastening failure modes.

A
  • Bearing failure (strongest).
  • Shear tear-out.
  • Tensile tear-out.
  • Tension failure of member.
34
Q

How can shear and tensile tear-out be avoided?

A

By spacing the fasteners far away from the edge.

35
Q

Give two applications where creep will be a significant issue.

A
  • Aero turbine blades.

- Stressed airframe components subjected to kinetic heating.

36
Q

Describe a spring and dash-pot model for a Maxwell series model.

A

Spring and dash-pot in series.

37
Q

What happens to the Poisson’s ratio when adding extra 90(deg) unidirectional plies?

A

A reduction in Poisson’s ratio.

38
Q

Describe the coupling represented by D16 and D26 terms.

A

Bend-Twist Coupling.

39
Q

Describe the mechanism of creep in metals.

A

Grain boundary slip.

40
Q

List the assumptions of boom & skin idealisation of a light aircraft structure.

A
  • Booms carry only direct load and represent stringers, spar caps and portions of skin, and act at the median skin line.
  • Skins carry only shear load.
  • Warping restraint and shear lag are ignored.
41
Q

Write the differential relationship between the internal shear and bending moment in a beam.

A

Sy = dMx/dz

42
Q

For a thin closed-wall section, what do we need to find the location of the shear centre?

A
  • Equilibrium of torsional moments.
  • The equation giving the rate of twist and the compatibility statement of zero twist for transverse loading through the shear centre.
43
Q

Why does different loading of wings require different idealisations of the same structure?

A

Different loadings result in a different sense of direct stress which means that the modelled boom areas may need to account for different portions of skin.

44
Q

Briefly describe the phenomenon of fatigue in metals.

A
  • Catastrophic failure can occur under moderate levels of cyclic loading.
  • Due to accumulation of microscopic damage at a detail/flaw which leads to cracking.
  • This causes progressive loss of strength.
  • Micro fatigue is traced to local plastic deformations that can occur even when global stress is well below elastic limit.
  • Stress concentrations at details/flaws produce dislocations that can move under higher stresses.
  • The accumulation of dislocations can cause local work hardening.
  • The only way to continue energy absorption is to create new surfaces by cracking.
45
Q

What are plastic and elastic strain energy responsible for?

A

Plastic strain energy: crack initiation.

Elastic strain energy: crack propagation.

46
Q

Describe Safe-Life fatigue design.

A
  • Aircraft design usually estimates life using fatigue analysis assuming an initial finite crack length.
  • Safe-Life is used for components that are difficult to inspect for cracks.
  • Usually for high-strength alloys that exhibit short initial crack lengths.
  • After the Safe-Life time is reached, the component is removed from service despite no apparent damage.
  • This method is also used as an overall estimate of fatigue life for general aircraft structures.
47
Q

Describe Fail-Safe fatigue design.

A
  • Component is designed so that a certain amount of fatigue damage can be sustained and confidently detected before reaching a critical level.
  • Usually fatigue and fracture analysis are both carried out to characterise an aircraft structural life.
48
Q

List assumptions for fatigue life analysis.

A
  • Block loading is representative of ‘real’ case and the loading sequence has no effect on fatigue life.
  • Linear Goodman relationship is representative of the whole fatigue life performance envelope.
  • Linear relationship is an accurate representation of fatigue performance at R = -1.
  • Constant mean = Sigma1g
  • Damage sum according to P-M Rule is a representative failure criterion.
49
Q

What is meant by rake angle in metal cutting?

A

The angle between the front face of the cutting tool and the normal to the surface of the part being cut.

50
Q

What is meant by the term ‘traceability’ with regard to manufacturing?

A

Each element of the manufacture is fully documented:

  • Design
  • Stress analysis
  • Drawings
  • Specification of bought in parts
  • Certificates of conformity against those specs
51
Q

What are the major features of Resin Transfer Moulding?

A
  • Dry reinforcement is clamped into a fixed mould cavity.

- Resin is then introduced under pressure to wet out the reinforcement.

52
Q

When would brittle failure be preferred to ductile failure?

A
  • When failure must best localised

- When distortion would be more damaging than separation (some mould applications)

53
Q

What is the equation for the stress intensity factor for a small crack in an infinite body?

A

K = 2Sigma(a/pi)^(0.5)

54
Q

What does the term ‘quasi-isotropy’ mean wrt composite materials?

A

Elastic properties are the same in all directions.

55
Q

What is meant by the term ‘balanced’ wrt composite materials?

A

A laminate with ply +(theta) must also have a -(theta) ply with such that the number of pluses and minuses are equal (assumes equal thickness of ply).

56
Q

Which fibre angles contribute most to shear stiffness, 0, 45 or 90 deg?

A

45 deg

57
Q

What is the equation for the R-ratio in fatigue?

A

R = Sigma(min) / Sigma(max)

Remember, alternating stress is the +/- from the mean, not the difference between the max and min.

58
Q

List 3 design guidelines for the manufacture/assembly of metal aircraft structures.

A
  • Minimise/avoid fretting
  • Shot peen for compressive residual stresses
  • Ensure proper shimming for fit-up
59
Q

List 3 design guidelines for stress levels.

A
  • Allow for detail stresses
  • Limit maximum design guidelines for stress levels
  • Limit fuselage hoop tensile stress
60
Q

Describe the mechanism of creep in plastics.

A

Polymer chain sliding/alignment.

61
Q

Describe the mechanisms of the secondary stage of creep in metals.

A

Balanced work hardening and thermal softening.

62
Q

List 3 effective failures due to creep.

A
  • Excessive deformation exceeding allowable value
  • Excessive deformation leading to instability
  • Creep rupture/fracture
63
Q

Describe a spring and dash-pot model for a standard linear solid model.

A

Spring connected to a spring and dash-pot that are parallel.

64
Q

What is the engineers beam-bending relationship equation?

A

-Sigma/y = M/I = E/R

65
Q

What are the 3 compressive failure modes?

A
  • Transverse tensile
  • Microbuckling / Extensional/Shear
  • Shear
66
Q

Explain the term ‘Crack Tip Singularity’.

A
  • Comes from the fact that continuum mechanics predicts infinite stresses at a perfectly sharp crack, i.e. the stress field is singular in the presence of a sharp crack.
  • Inglis’ theory states that for an elliptical crack in an infinite solid the peak stress is… where sigma is the far-field stress and a is the crack half length. A perfectly sharp crack will have a zero radius of curvature at the tip so as Pc tends to 0, Sigma max tends to infinity.
  • Crack tip singularities dont happen in nature as no material can withstand infinite stress. In reality, either local plastic deformation ahead of the crack or other damage mechanisms (void nucleation, microcracking) will relieve stresses. Stresses will therefore be high but not infinite at the crack tip.

StM3 (4 decks)

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