Yield Criteria Flashcards
What does a uniaxial tensile test measure?
Mechanical properties like stress and strain.
What is the formula for engineering stress?
σ = P / A
What is the formula for engineering strain?
ε = δL / L
What does the gradient of the stress-strain curve in the elastic region represent?
Young’s modulus (E)
What separates the elastic and plastic regions on a stress-strain curve?
The yield point (σy₀)
What happens in the elastic region?
All deformation is recovered.
What happens in the plastic region?
Not all deformation is recovered.
What happens when material is loaded past the yield stress?
It undergoes permanent deformation.
How does unloading occur after plastic deformation?
Along the same gradient as the elastic portion but leaves permanent strain.
What kind of stress-strain curve does grey cast iron show?
A steep curve with little strain – brittle behavior.
What kind of stress-strain curve does mild steel show?
A curved path after yielding – ductile behavior.
What kind of stress-strain curve does aluminium alloy show?
Smooth transition from elastic to plastic – no clear yield point.
What is a ductile material?
A material that undergoes large deformation before failure.
What is a brittle material?
A material that fractures with little deformation.
When does failure occur in ductile materials?
At the onset of plastic deformation.
When does failure occur in brittle materials?
At fracture.
What is the yield criteria used for?
Predicting the initiation of yielding in ductile materials.
How is ductility expressed?
As percentage elongation or percentage reduction in area at failure.
What is used when a clear yield point isn’t observed?
Proof stress (e.g., 0.2% offset method).
How is the 0.2% proof stress determined?
Draw a line parallel to the elastic gradient starting from 0.2% strain.
What type of failure occurs in ductile materials under tensile test?
Cup and cone failure.
At what angle does ductile fracture typically occur?
45°
On what plane does ductile fracture occur?
Plane of maximum shear stress.
What does the torque-angle graph for ductile materials like mild steel show?
Linear elastic region followed by yielding.
What is the formula for torque in the elastic region?
T/J = Gθ/L
Where does failure occur if torque is increased until failure?
On the plane of maximum shear stress.
What is the orientation of the fracture surface under torsional failure?
Transverse to the axis.
What kind of fracture surface does a brittle material like cast iron show under tensile loading?
Flat and perpendicular to the loading direction.
On what plane does brittle fracture occur?
Plane of maximum principal stress.
What kind of failure is observed in torsion tests on brittle materials?
45° helical failure.
What stress causes failure in brittle materials under torsion?
Maximum principal stress.
What fracture surface is observed in mild steel?
Cup and cone (ductile).
What fracture surface is observed in cast iron?
Flat, perpendicular or helical (brittle).
