chapter 8: mechanical properties of solids Flashcards
Define Deforming force.
A force which produces a change in shape or size of the object on applying it.
Define elasticity AND perfectly elastic body
ELASTICITY - The property of the body to regin its original form (length, volume & shape)
- it is a deforming force.
PERFECT ELASTIC BODY - the body which regains its original configuration immediately.
What is plasticity AND perfect plastic body?
PLASTICITY- The inability of a body to return to its original form/shape immediately.
PERFECT PLASTIC BODY - the inability of a body to return to original form at ALL.
Define stress WITH formula.
STRESS - defined as the ratio of internal force (F), when object is deformed, to the area (A) over which this force acts.
stress = applied force/area
unit is N/m2 or Pascal (Pa)
Dimensional formula [ML-1T -2]
What are the types of stress.
- LONGITUDINAL/NORMAL STRESS - Restoring force per unit area perpendicular to the surface.
(a) Tensile stress: if theres an increase with the length of the object under applied force,
(b) Compressional stress: if theres an decrease in length of the object under applied force. - SHEAR STRESS - When elastic restoring force acts parallel to surface area.
Define strain WITH formula.
The ratio of change in shape/size to original form
STRAIN = change in dimension/original dimension
no UNIT therefore NO DIMENSION
What are the three types of strain?
- LONGITUDINAL STRAIN - change in length/original length
- VOLUMETRIC STRAIN - change in volume/original volume
- SHEAR STRAIN - tangential applied force/area of face
What’s the Hooke’s Law given with derivation.
Hooke’s law states that, within elastic limits, the ratio of stress to the corresponding strain produced is a constant. This constant is called MODULUS OF ELASTICITY (k).
stress DIRECTLY PROPOTIONAL strain
F (spring) = kx
There are two types of Modulus of rigidity. What is the Young’s modulus of rigidity?
YOUNG’S MODULUS OF RIGIDITY (Y) : The ratio of longitudinal stress to longitudinal strain.
Y = stress / strain
unit is N/m^2 or Pascal (Pa)
Metals have large value of young’s modulus. “ Higher the young’s modulus of the material the more elastic it is. “
What is the second type which is Bulk’s modulus of rigidity?
BULK’S MODULUS OF RIGIDITY (B) - within elastic limits, the bulk modulus is defined as the ratio of longitudinal stress and volumetric strain.
B = normal stress/volumetric strain
= -F/A / △V/V
= -pV / △V
unit is N/m^2 where p=pressure & v=volume
Define compressibility in Bulk’s modulus
Compressibility of a material is the reciprocal of bulk’s modulus of elasticity
C = 1/k
unit = N^-1 M^2
What is the Modulus of Rigidity OR Shear Modulus?
Defined as the ratio of the tangential stress to the shear strain. (𝜼)
(𝜼) = shear stress/shear strain
= F/A / Y
= F/ AY
unit is N/m^2
The shear modulus of a material is always considerably smaller than the young’s modulus of it.
What is Limit of elasticity?
The maximum value of deforming force for which elasticity is present in the body.
Name all the parts of a Shear-strain curve graph giving all points. THIS IS FOR A METAL WIRE WHICH IS GRADUALLY BEING LOADED
(a) The beginning straight line indicates Hooke’s law is obeyed called PROPOTIONAL LIMIT.
(b) A curve is formed and the last point of curve is called ELASTIC LIMIT.
(c) Beyond the B point the strain increases rapidly more than stress. The material has scquired PERMEANANT SET from point 0.
(d) Point beyond C called PLASTIC BEHAVIOUR and eventually the metal wire breaks at the last point which is called as FRACTURE POINT.
Define Elastic after effect.
The delay in the elastic body regaining back to its original form from the deforming force.
What is Elasticity Fatigue?
The property of the elastic body by virtue of which its behaviour becomes less elastic under the action of repeated alternating deforming forces.
What are Ductile, Elastomers and Brittle materials?
DUCTILE - The type of materials that have a large plastic range of extension.
example: silver, copper, aluminium
BRITTLE - The type of materials that have a small plastic range of extension and can break easily.
example: glass, ceramics etc.
ELASTOMERS - The type of materials for which strain produced is much larger than stress applied. (no plastic range)
example: rubber, large vessel carrying blood from heart
Explain Elastic potential energy of a stretched wire.
When a wire is stretched, interatomic forces come into play which opposes the
change. Work has to done against these
restoring forces.
Talk about Poissons ratio.
The ratio of change in diameter (∆D) to original diameter (D) is called LATERAL STRAIN. The ratio of change in length (∆l) to original length (l) is called LONGITUDINAL STRAIN.
∴ Poisson’s Ratio (σ) = 𝐿𝑎𝑡𝑒𝑟𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛/𝑙𝑜𝑛𝑔𝑖𝑡𝑢𝑑𝑖𝑛𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛
= -∆D/D / ∆l/l
= l∆D / D∆l
no unit and no dimension
The negative sign (-) shows that longitudinal
and lateral strains are in opposite sense.
