Intro to Aero II - Structure and Materials Flashcards
(30 cards)
<p>What equation gives us a geometrical understanding of how something deforms under a force ?</p>
<p>strain</p>
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<p>Which is equal tothe extension normalized by its initial length</p>
<p>What do stress-strain graphs show us ?</p>
<p>they show us how an object deforms according to a forces (stresses)</p>
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<p>*where the cross sectional area is taken into account*</p>
<p>How do you determine on a stress-strain graph the transition between linear elastic slope and the yield point ?</p>
<p>a common (but arbitrary) approach is to take the intersection between the stress-strain curve and the 0.2% offset of the linear elastic slope,</p>
<p>What do you call amaterial that becomes fully plastic after yielding, i.e. the slope continues horizontally ?</p>
<p>perfect plastic</p>
<p>What does a steep stress-strain curve say mean ? (curve after yield point )</p>
<p>A steep curve means a 'large strain hardening'</p>
<p>and vice versa a gentle slope is a 'small strain hardening '</p>
<p>What does thehighest point in the stress strain curve tell us about the material ?</p>
<p>It tells us theultimate strength of a material.</p>
<p>If this strength value is very high, it indicates a strong material. If the strength is low, it indicates a weak material.</p>
<p>What does the yield strength tell us about a material ?</p>
<p>The yield point (transition from elastic to plastic) may either be located at small values of the stress (low yield strength) or at high values of stress (high yield strength).</p>
<p>The first transition point indicates a <strong>soft</strong> material, whereas the second indicates a <strong>rigid</strong> material.</p>
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<p>What happens at the end of a stress-strain curve ?</p>
<p><strong>Fracture</strong> occurs at the end of the curve. The<u> elastic deformation still present causes spring back</u>. This is illustrated by the dotted lines parallel to the initial elastic slope of the curve. The remaining deformation is plastic deformation.</p>
<p>What does the amount of plastic deformation say about the material ?</p>
<p>A small degree of plastic deformation indicates a <strong>brittle</strong> material.</p>
<p>A high degree of plastic deformation indicates a <strong>ductile</strong> material.</p>
<p>What is the equation for the normal stress in the linear elastic part of the stress strain curve?</p>
<p>The constant E is called the modulus of elasticity, or the Young’s modulus. The value of this Young’s modulus is a characteristic value for a material; a high value indicates a stiff material, a low value a flexible material.</p>
<p>The epsilon is the strain.</p>
<p>What is the the relation between the lateral and transverse strain?</p>
<p>And why is it useful ?</p>
<p>transverse strain = to poisson's ratio x strain in lateral direction = poisson's ratio x stress/E-modulus</p>
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<p>Allows you to relate the stress in one direction to the strain in the other...</p>
<p>The strain of one direction is equal to the poisson's ratio of the object times the strain in the perpendicular direction divided by the</p>
<p>What is the relationship between the shear strain and shear stress ?</p>
<p>where G is the shear modulus of elasticity and γ the shear strain (equal to tanθ)</p>
<p>Youn'gs modulus and uni-axially loaded sheets andbiaxially loaded sheets</p>
<p>For a uni-axially loaded sheet, the stiffness relates directly to the Young’s modulus.</p>
<p>In a bi-axially loaded situation however, the <u>apparent stiffness </u>may be different from the material stiffness.</p>
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<p>The apparent young's modulus is given by:</p>
<p>What do the subscripts of the normal stress and normal strain indicate?</p>
<p>The direction of the of the stress and strain.</p>
<p>What is the difference between the tensile deformation and shear deformation for isotropic ans anisotropic sheets?</p>
<p>For anisotropic sheets constants such as the poisson's ratio and the young's modulus now have to be defined in particular directions.</p>
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<p>The image attached shows the shear and tensile deformations written in terms of a matrix, with constants in one matrix and stress in the x and y as well as the shear stress are written in another matrix.</p>
<p>Relationship between stress-strain and toughness.</p>
<p>Unit of toughness</p>
<p>The toughness of a material is defined as resistance against fracture, and it is in general considered to be represented by the area underneath the stress-strain curve. This area represents the mechanical deformation energy per unit volume prior to failure.</p>
<p>Definition of Brittleness</p>
<p>Property of material that allows little bending or deformation without shattering</p>
<p>Definition of Contraction/Expansion</p>
<p>Reaction produced in material as the result of heating or cooling</p>
<p>What is Ductility ?</p>
<p>Property of metal that allows it to be permanently drawn, bent, or twisted into various shapes without breaking</p>
<p>What is engineering load defined as ?</p>
<p>Load divided by the original cross-section of the material/component</p>
<p>What is the hardness of a material?</p>
<p>Ability to resist abrasion, penetration, cutting or permanent distortion</p>
<p>what does isotropic mean ?</p>
<p>Material having identical mechanical properties in all directions</p>
<p>What is the Toughness of a material ?</p>
<p>Property of a material that withstands tearing or shearing and may be stretched without being deformed or breaking</p>
<p>What is the True Stress ?</p>
<p>Load divided by the actual cross-section of the material/component</p>
What are the functions of a rib?
1. They maintain the aerodynamic profile of the wing
2. to transfer the aerodynamic and fuel loads acting on the skin to the rest of the wing structure
3. The ribs provide stability against panel crushing and buckling. Without any ribs the upward ending of the wing would crush the upper and lower wing panels. With ribs at certain length the upward bending would cause buckolng to the upper wing skin but keep the lower wing skin at distance of the upper wing skin. With the optimal rib spacing both crushing ad buckling are prevented and the shape of the wing is preseved,
4. It can help withstand local loads. Aside from aerodynamic and fuel loads the wing structure is locally loaded by landing gears, taxiing and take-ff and uring flight the engines, flaps and ailerons will locally apply laods to the structure.
5. The sealing fucntions in case the integral fuel tanks run through the wing strucutre. The ribs prevent surge and splashing.
What is the function of Spars?
What form do they usually take ?
And how do they cahgne across a wing
The main function of the spars is to the wing bending moment.
The basic form of spars is often the I-beam, in which the girders pick up the normal forces and the web plate the shear forces.
Although the deflection near the wing root is significant smaller than near the wing tip, the loads near the wing root are significant larger. As a result the spar, the girders and web plate, must be thicker near the wing root than near the wing tip.
What is the issue with manufacturing spars with varing cross section ?
Extrusion of an I-beam (see chapter 4 for the extrusion process) can only be performed on constant cross-sections
Either the girders are separately manufactured and connected to a web plate by riveting for example, or a spar with constant thickness is extruded and subsequently reinforced by bonding additional sheet material to web plate and girders.
Advantages of a torsion box over the two spar concept ?
A completely unsupported and load bearing structure can be achieved that does not require support or struts. At given wing span, the wing can be thinner in case a torsion box concept is applied, or similarly, at given wing thickness the wing can be longer. Additionally, by designing the torsion box carefully, the torsion stiffness and the bending stiffness can be engineered separately.
In general, the torsion box concept results in lower structural weight compared to the two spar concept.
List the structural concepts
Give an example for each
Truss structure (wire bracing, sheet, I-beam)
Shell structure
sandwich structure (honeycomb)
Integrally stiffened structures
Give two methods to geometrically stiffen a sheet for compression?
corrugation, sitffners
