# Shared Flashcard Set

## Details

Structures
55
Aerospace Engineering
12/08/2010

Term
  We derived that the stress tensor is symmetric. Is this due to force or moment equilibrium of the material element?
Definition
 Due to the moment equilibrium
Term
 What is the difference between a body force and a boundary force?
Definition
 Body forces act across the whole object, not just across the boundary layer
Term
 Give an example of a body force.
Definition
 Gravity
Term
 What does moment equilibrium of the stresses on an infinitesimal material element tell us about the stress tensor?
Definition
 That it is symmetricU�kB
Term
 There are never shear stresses on the principal planes.
Definition
 True
Term
 What is the name of the stress state for which the stress field does not change with rotation?
Definition
 Hydrostatic Stress
Term
 What are the principal stresses?
Definition
 Stresses where the shear stress is zero and normal to principle planes
Term
 What are the assumptions for plane stress?
Definition
 τxy, τyz, and σz are all zero
Term
 §  Give one example of a structure for which plane stress is a good assumption. Why?
Definition
 Sheet metal
Term
 The equations of equilibrium apply to a material that is elastic or inelastic.
Definition
 True
Term
 The condition l2+m2+n2 = 1 always holds.
Definition
 False
Term
 §  Label the stresses on an infinitesimal material element.
Definition
Term
 What is the definition of τxy?
Definition
 o   The Shear stress on the x plane in the y direction
Term
 The maximum normal stress vector is always oriented 90° from the minimum normal stress vector.
Definition
 True
Term
 The maximum shear stress is always one-half the difference between the maximum and minimum principal stresses.
Definition
 True
Term
 §  What are rigid body motions?
Definition
 Movement of the body with no deformation
Term
 Any rigid body motion can be written in terms of a ____ and a ____
Definition
 Translation & a Rotation
Term
 Do we use engineering strain or tensor strain in this course?
Definition
 Engineering Strain
Term
 What are the principal strains?
Definition
 o   There are two mutually perpendicular planes on which the shear strain γ is zero and normal to which the direct strain is at a maximum or minimum
Term
 What are the assumptions for plane strain?
Definition
 o   Principle strain in the longest direction can be assumed to be zero
Term
 §  Why do strain fields have to satisfy the compatibility equations?
Definition
 Because the displacement fields have to be continuous
Term
 A structure in a state of plane stress is also in a state of plane strain.
Definition
 False
Term
 Draw the definition of engineering strain and tensor strain
Definition
 [image]
Term
Definition
Term
 What is the difference between an isotropic and orthotropic material?
Definition
 o   Orthotropic materials’ elastic properties change depending on the direction of loading       o   Isotropic materials’ elastic properties are the same no matter the direction of the loading
Term
 A viscoelastic material is one for which creep deformations are significant.
Definition
 True
Term
 §  A material constitutive law relates which 2 parameters?
Definition
 o   Stress and Strain
Term
 Stress is a single-valued function for an elastic material.
Definition
 True
Term
 Stress is a single-valued function for an inelastic material.
Definition
 False
Term
 An orthotropic material has how many Poisson’s ratios?
Definition
 o   Up to 6
Term
 An orthotropic material has how many elastic modulii?
Definition
 o   Up to 3
Term
 §  Describe a creep test.
Definition
 o   Weight loaded (stress held constant) strain measured
Term
 §  Describe a stress relaxation test.
Definition
 specimen is deformed a given amount (Strain held constant) and decrease in stress is recorded over period of time
Term
 §  Draw the spring-damper equivalent for the Maxwell model of a viscoelastic material.
Definition
Term
 §  Draw the spring-damper equivalent for the Voigt model of a viscoelastic material.
Definition
 [image]
Term
 What is the physical meaning of the principal axes of a beam section? Do they depend on the applied loading?
Definition
Term
 What is the physical meaning of the neutral axis of beam section? Does it depend on the applied loading?
Definition
 Axis about which the stress σz equals zero when a bending moment is applied.  It is dependent on applied loading
Term
 §  For a beam in bending, the Neutral Axis always passes through the centroid of the §  beam section.
Definition
 True
Term
 §  For a beam in bending, the Principal Axes always pass through the centroid of the beam section.
Definition
 False
Term
 All moments of inertia of a cross-section must be greater than zero.
Definition
 False
Term
 For a symmetric section, the principal axes are the axes of symmetry
Definition
 True
Term
 §  What is the “rule of thumb” criteria for a section to be considered thin-walled?
Definition
 o   Thickness is 1/10th the length
Term
 §  What is the physical meaning of a shear center?
Definition
 Point about which moment due to shear flow is zero
Term
 §  For what types of thin-walled sections are the shear-center and centroid located at the same point?
Definition
 o   Sections with two axis of symmetry
Term
 §  For a typical aircraft wing section the axial load is resisted by the (a) stringers, (b) skin or (c) both. (Choose a b or c ).
Definition
 (a) stringers
Term
 §  For a typical aircraft wing section the axial load is resisted by the (a) stringers, (b) skin or (c) both. (Choose a b or c ).
Definition
 (a) stringers
Term
 §  For a typical aircraft wing section the shear load is resisted by the (a) stringers, (b) skin or (c) both. (Choose a b or c ).
Definition
 §  (c) both.
Term
 §  For a typical aircraft wing section torsion is resisted by the (a) stringers, (b) skin or (c) both. (Choose a b or c ).
Definition
 (c) both.
Term
 §  We neglected the hoop stress in our calculation of thin-walled beams. For what aircraft structure would this stress component be important?
Definition
 Fuselage
Term
 We assume that open sections support shear loads.
Definition
 True
Term
 §  For a thin-walled closed section, the shear flow due to a torque is constant
Definition
 True
Term
 §  For a thin-walled closed section, the shear stress due to a torque is constant throughout the section.
Definition
 False
Term
 §  For multi-cell sections, what condition do we impose at junctions to determine the amount of torque supported by each cell?
Definition
 o   Shear Flow in = Shear Flow out
Term
 We assume that open sections support torques.
Definition
 False
Term
 Draw the spring-damper equivalent for the Voigt model of a viscoelastic material.
Definition
 [image]
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