Science of Materilas - Module 2

What type of loading is this?

Compression

What type of loading is this?

Tension

What type of loading is this?

Torsion

What type of loading is this?

Bending

What type of loading is this?

Shear

Tension Test

Specimen is deformed by tensile force usually to fracture by at a constant rate in a tensile testing machine. Output: Load/Force vs Elongation

Compression Tests

Specimen is deformed by compressive force usually to fracture by at a constant rate. Output: Load/Force vs Elongation by convention compressive forces a negative.

Torsion

A variation of pure shear in which an object is subjected to a twisting force (torque) causing shear stress to develop across its cross section.

Elastic deformation

Elastic deformation is the temporary, reversible change in an object's shape or size when a force is applied allowing it to return to its original shape once the force is removed.

Plastic Deformation

Plastic deformation is the irreversible change in a material's shape or size that occurs when forces exceed its yield strength, causing permanent distortion.

Strain (Atomic Scale)

Small changes in interatomic spacing and stretching of interatomic bonds.

Modulus of Elasticity

E is a measure of the resistance to separation of adjacent atoms,

Poisson’s Ratio

Ratio of lateral (x & y) and axial (z) strains

Yield Strength

Stress at which noticeable plastic deformation occurs.

Yield Point

x,y coordinates at which the strain deviates from being proportional to the stress. (Proportional limit)

Ductility

Ductility is a measure of the amount of plastic deformation sustained at fracture.

Brittle

A metal that experiences very little or no plastic deformation with low energy absorption before fracture.

Ductile

Exhibit substantial plastic deformation with high energy absorption before fracture

Necking

Necking is a phenomenon in ductile materials where deformation localizes to a small region, causing a significant reduction in cross-sectional area.

Toughness

Toughness is defined as the ability of a material to absorb energy and plastically deform before fracturing.

Resilience

Resilience is the capacity of a material to absorb energy when it is deformed elastically, and then upon unloading, have this energy recovered.

Hardness

Hardness is a measure of a material’s resistance to localized plastic deformation (small dent or scratch)

Tensile strength

Tensile strength is the maximum stress a material can endure while being stretched or pulled before it permanently deforms or breaks.

Hardness Measurement

Rockwell, Brinell, Vickers

Plastic Deformation (atomic)

Plastic deformation corresponds to the movement of dislocations, or slip, in response to an applied stress.

Dislocation Motion

A dislocation moves along a slip plane in a slip direction perpendicular to the dislocation line.

Mechanisms of Strengthening in Metals

The ability of a metal to plastically deform depends on the ability of dislocations to move. Restricting or hindering dislocation motion renders a material harder and stronger. (less ductile)
Grain size reduction
Solid-solution alloying
Strain hardening

Impact of Cold Working

Plastic deformation at low temperatures produces:
Change in grain shape
Strain hardening
Increase in dislocation density

Annealing

se of appropriate heat treatment to decrease Tensile Strength and increase %Elongation

Annealing Stage (Reversal)

Recovery
Recrystallization
Grain growth

Driving force

Difference in internal energy between the strained and unstrained material.

Steps in fracture

Crack Formation
Crack propagation
Catastrophic Failure

Transgranular

Cracks pass through grains; have a grainy or faceted texture

Intergranular

Crack propagation is along grain boundaries

Fatigue

A form of failure that occurs in structures subjected to dynamic and fluctuating stresses.

Extending Fatigue Life

Polishing – removes machining flaws
Imposing compressive stresses (compensate for applied tensile stresses) into thin surface layer
Case hardening – create C- or N- rich outer layer in steels by atomic diffusion from the surface.
Optimize geometry – avoid internal corners, notches etc.
Reducing mean stress level

Creep

Time-dependent & permanent deformation of materials when subjected to a constant load or stress above 0.4 times the melting temperature.

Creep is minimized in materials with:

High melting temperature
High elastic modulus
Large grain sizes (inhibits grain boundary sliding)