Elastic properties and tensile testing Flashcards
Name the 9 mechanical properties of materials discussed in class
- Elastic (Young’s) Modulus
- Poisson’s Ratio
- Shear Modulus
- Yield Strength
- Ultimate Tensile Strength
- Strain to Failure (Ductility)
- Fracture Toughness
- Hardness
- Fatigue Life
What are the properties of elastic deformation?
Non-permanent and reversible
Elastic deformation is recoverable; a material
returns to its original shape when an applied load is removed
What type of relationship can be present between the force (N) and elongation (m)?
The relationship between
force and elongation can be
linear (as shown) or nonlinear.
What is Young’s Modulus?
A measure of material stiffness
What is resilience?
The recoverable energy absorbed by a material
during elastic deformation
How is modulus of resilience (MOR) determined?
Determined by the area under the stress-strain curve up to
yield point
What is the relationship between stored energy, yield stress, and elastic modulus?
Stored energy increases with
increased yield stress and/or
decreased elastic modulus
What is the uniaxial tensile test?
This is the most common test used to determine the
relationship between load and deformation in a material
What type of properties does the uniaxial tensile test measure?
- Elastic Properties (Elastic Modulus)
- Plastic Properties (Yield Stress)
- Failure Properties (Ultimate Tensile Stress)
What are common standardized testing procedures for stress and strain?
ASTM (ASTM E-8), CSA, BS, DIN, JIS
What do standards specify and require when it comes to mechanical properties?
- Standards specify size and shape of sample
- Standards require calibration of testing machines
What is Poisson’s Ratio and what does it characterize?
- characterizes the
contraction perpendicular
to the extension caused by
a tensile stress - defined as the negative of
the ratio between lateral
and axial strains
What is the elastic modulus a function of?:
i) Bond stiffness
ii) Density of bonds (i.e. atomic packing)
In Poisson’s ratio, which axis is along the positive strain (elongation)?
The z-axis.
In Poisson’s ratio, which axes are along the negative strains (lateral contractions)?
X and y-axis
What is the relationship between the lateral strains if the applied stress is uniaxial (only in z-direction), and the material is isotropic?
The lateral strain across the x-axis = lateral strain across the y-axis
What does poisson’s ratio depend on?
It depends on atomic
arrangements and bonding
When do atoms experience negligible atomic interactions?
At large distances since the atoms are too far apart to have an influence on each other. So there is no attraction or repulsion.
When do atoms exert forces on each other?
At small separation distances.
How is the equilibrium interatomic distance determined graphically?
By the distance at which the interatomic force is 0.
Why do the atoms experience repulsive energy when separated by a
distance less than the equilibrium distance ro?
Due to electrostatic repulsion between positively charged nuclei of atoms and electron clouds of atoms.
What is the interatomic force between the atoms at the equilibrium
distance r0 and why?
0 N because the attractive and repulsive forces cancel each other out.
What does the minimum potential energy correspond to in the potential interatomic energy vs. interatomic distance graph?
equilibrium spacing, r0
What is the relationship between Young’s Modulus and the slope of the interatomic force over interatomic distance? When is this relationship valid?
Young’s Modulus is proportional to the slope of the interatomic force over interatomic distance. This relationship is valid only at the equilibrium distance.
What type of material would require a greater stress to elastically deform the material?
A material with high stiffness and high Young’s Modulus requires greater stress to elastically deform the material.
What is stiffness?
A material’s resistance to elastic deformation.
What is the relationship between Young’s Modulus, stiffness, and elastic strain?
This modulus may be thought of as stiffness, or a
material’s resistance to elastic deformation. The greater the modulus, the stiffer
the material, or the smaller the elastic strain that results from the application of
a given stress.
What is a brittle metal?
A metal that experiences
very little or no plastic deformation upon fracture (oppose of ductile)
Do the expressions of ductility yield the same numerical value?
No, ductility being expressed in percentage elongation or percentage reduction in area doesn’t yield the same numerical value.
Order these types of materials (metals, polymers, ceramics) from the highest Young’s Modulus to the lowest Young’s Modulus.
Ceramics, Metals, and Polymers
Why do tensile specimens have a dog-bone geometry?
To localize the deformation of the material in a particular region of the sample during tensile testing.
