NMC123 Chap 6 - mechanical properites

What is the difference between plastic and elastic deformation

When a material is only elastically deformed when the applied stress is released the material returns to its original shape

If it is deformed plastically then it would be permanently deformed even when the stress is relieved

What is the most general formula for stress and strain

Stress = F/A0

Strain = Delta L / L0

How does Shear stress and strains formula differ

Shear stress (tau) = Fs/A

Shear strain (y) = W/L0 where W is the amount the corner of the material moved

What is an elastic moduli and how is it defined on a stress strain curve?

The elastic moduli is a measure of a materials stiffness and is expressed as the gradient of the elastic region of a stress strain curve

What is poissons ratio and how is it expressed?

When a material has a stress applied to it the material will elongate but simultaneously shrink is diameter this ration between stretch and shrink is poissons ratio

V = -Elat/Elong

V = -(Delta d/d) /(Delta L/L)

What is the poissons ratio for most metals

0.3

Why do we need a shear Modulus (Gm) and how is it expressed mathematically

In a crystal lattice metals deform via a shear stress over a slip plane not a tensile force. This means we need to express a shear modulus.

Gm = E/2(1+v)

What is the elastic modulus of resilience and how is it expressed mathematically

It is the amount of elastic energy absorbed when a spring is compressed. It is the area under the elastic region of a stress strain curve

Ur = (Yield Strength)²/2E

What are the standards for a tensile test

A gauge length of 50 or 200mm and a gauge diameter of pi/4(d0)²

What property of ceramics causes them to require a compression test instead of tensile

They are very brittle

Why is a 0.2% offset yield strength sometimes used?

Because of the gradual transformation from elastic to plastic deformation in ductile materials it makes it hard to read off the sharp yield point

What is UTS/Rm

It is the point where necking begins

Give 2 measurements of a metals ductility and their units

%Elongation - A5

%Reduction - Z

What is the difference between engineering stress/strain and true stress/strain

In the standard engineering stress and strain formulas it is assumed that

Is it true by the engineering stress/strain equations that a materials strength descreases as a greater load is applied?

No infact if you plot true stress/strain curves the materials strength actually increases.

What are the formulas for true stress and strain

True stress = F/Ai (Ai instantaneous cross sectional area

True strain = ln(Li/Lo) or ln(Ao/Ai) Where Ai and Li are instantaneous lengths and cross sectional areas

What are the equations to relate true stress/strain and engineering stress/strain

True stress = stress(1+E)

True strain = ln(1+E)

Where E is the engineering strain at that point