Chapter 6 Flashcards
What is Poisson’s ratio v?
For elastic deformation, the negative ratio of lateral and axial strains (-εx/εz) that result from an applied axial stress.
What is anelasticity?
Time-dependent elastic (nonpermanent) deformation.
What is design stress?
The product of the calculated stress level (on the basis of estimated maximum load) and a design factor (which has a value greater than unity). Used to protect against unanticipated failure.
What is ductility?
A measure of a material’s ability to undergo appreciable plastic deformation before fracture; it may be expressed as percent elongation (%EL) or percent reduction in area (%RA) from a tensile test.
What is elastic deformation?
Deformation that is nonpermanent—that is, totally recovered upon release of an applied stress. Happens at small deformations and has a linear stress-strain curve (some polymers have non-linear curves).
What is engineering strain?
The change in gauge length of a specimen (in the direction of an applied stress) divided by its original gauge length.
What is engineering stress?
The instantaneous load applied to a specimen divided by its cross-sectional area before any deformation.
What is hardness?
The measure of a material’s resistance to deformation by surface indentation or by abrasion.
What is modulus of elasticity?
The ratio of stress to strain (σ/ε) when deformation is totally elastic; also a measure of the stiffness of a material (materials with larger elastic moduli deform less).
What is plastic deformation?
Deformation that is permanent or nonrecoverable after release of the applied load. It is accompanied by permanent atomic displacements.
What is the proportional limit?
The point on a stress-strain curve at which the straight-line proportionality between stress and strain ceases.
What is resilience?
The capacity of a material to absorb energy when it is elastically deformed. The energy is recovered when the load is released.
What is safe stress?
A stress used for design purposes; for ductile metals, it is the yield strength divided by a factor of safety.
What is a shear?
A force applied so as to cause or tend to cause two adjacent parts of the same body to slide relative to each other in a direction parallel to their plane of contact.
What is tensile strength?
The maximum engineering stress, in tension, that may be sustained without fracture. Often termed Ultimate (Tensile) Strength.
What is toughness?
A mechanical characteristic that may be expressed in three contexts: (1) the measure of a material’s resistance to fracture when a crack (or other stress-concentrating defect) is present; (2) the ability of a material to absorb energy and plastically deform before fracturing; and (3) the total area under the material’s tensile engineering stress-strain curve taken to fracture.
What is true strain εT?
The natural logarithm of the ratio of instantaneous gauge length to original gauge length of a specimen being deformed by a uniaxial force.
What is true stress?
The instantaneous applied load divided by the instantaneous cross-sectional area of a specimen.
What is yield strength σy?
The stress required to produce a very slight yet specified amount of plastic strain; a strain offset of ε = 0.002 is commonly used.
What is yielding?
The onset of plastic deformation.
What does the symbol σ represent?
Tensile stress = force/initial cross-section area
What does the symbol τ represent?
Torsion (shear) = moment/(original cross-section*radius)
What is the formula for compression σ?
Same as tensile stress -> σ = force/initial cross-section area
What is bi-axial tension?
Where tension acts on an object along two axes (ie x and y)
What is hydrostatic compression?
Where an object is compressed from all directions due to a fluid
In what direction are tensile stresses applied?
Perpendicular to the cross-sectional area
In what direction are shear stresses applied?
Parallel to the cross-sectional area
What is the formula for tensile strain εz?
εz = change in length/original length (in direction of force)
What is the formula for lateral strain εx?
εx = - change in diameter/original diameter (perpendicular to force direction)
What is the formula for shear strain γ?
γ = tanθ
What is deflection dependent on?
Material, geometric, and loading parameters
What does α represent?
Angle of twist
What does G represent?
Elastic shear modulus –> G = τ/γ (shear stress/strain)
What does E represent?
Modulus of elasticity or Young’s modulus and is a material property
What is Hooke’s Law?
σ = E * ε (stress = modulus of elasticity * strain)
What are the general values of Possoin’s Ratio?
Metals ~ 0.33
Ceramics ~ 0.25
Polymers ~ 0.40
For most materials falls between 0.15 and 0.50
What does Ur represent?
Resilience –> area under stress-strain curve up until yielding
What are the types of measurements for hardness?
- Brinell (sphere of steel or tungsten carbide)
- Vickers (diamond pyramid)
- Knoop (diamond pyramid)
- Rockwell/Superficial (diamond cone and steel spheres)
What is working stress σw?
The ratio of yield stress (σy) over the factor of safety (N) which ranges between 1.2 and 4
