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Joined: Feb 2011
18-02-2011, 09:53 AM
Dr. Arindam Kumar Chanda
Forming Full.pptx (Size: 377.18 KB / Downloads: 174)
Elastic and plastic deformation
• Elastic deformation is a change in shape of a material at low stress that is recoverable after the stress is removed.
• This type of deformation involves stretching of the bonds, but the atoms do not slip past each other
• When the stress is sufficient to permanently deform the metal, it is called plastic deformation
• Plastic deformation involves the breaking of a limited number of atomic bonds in the crystal by the movement of dislocations
• Done at temperature above recrystallization temperature but below melting point.
• Work hardening is eliminated.
• Improvement of mechanical properties like elongation, reduction of area and impact value
• Refinement of crystal occurs
• Cracks and blow holes are welded up.
• Promotes uniformity of material
• Internal and residual stresses are not developed in metal
• Done at temperature below re-crystallization temperature
• Work hardening is not eliminated.
• Decrease of mechanical properties like elongation, reduction of area and impact value
• Crystallization does not occur. Grains are elongated and distorted.
• Possibility of crack formation and propagation
• Uniformity of material is lost
• Internal and residual stresses are developed in the metal.
• Bars, plates, sheets, rails and other structural sections are made through plain and grooved rolls.
Deep drawing is defined as the process of making of cup shaped parts from flat metal sheet. This process is also called cupping.
Piercing or Seamless tubing
Here a hot rolled billet is passed between two conical shaped rolls and over a mandrel which assist in the piercing and control the size of the hole as the billet is forced over it.
Tresca’s Yield Criteria
• Since the plastic flow depends on slip which essentially is a shearing process, Tresca suggested that “The plastic flow initiates when the maximum shear stress reaches a limiting value”. This limiting value is defined as shear yield stress K.
• If the principal stresses at a point in the material are σ1 , σ2 , and σ3 , then the maximum shear stress τmax , is given by
τmax = (σ1 - σ3 ) / 2
Hence Tresca’s criteria becomes
(σ1 - σ3 ) / 2 = K
Von Mises’ Yield Criteria
• Von Mises’ proposed that the plastic flow occurs when the shear strain energy per unit volume reaches a critical value(say A). The shear strain energy per unit volume is given by
1/6G[(σ1 - σ2 )2 + (σ2 – σ3 )2 + (σ3 – σ1 )2 ] , where G is the shear modulus of the material. Hence according to this criteria
1/6G[(σ1 - σ2 )2 + (σ2 – σ3 )2 + (σ3 – σ1 )2 ] = A
or, (σ1 - σ2 )2 + (σ2 – σ3 )2 + (σ3 – σ1 )2 = 6GA
i.e. (σ1 - σ2 )2 + (σ2 – σ3 )2 + (σ3 – σ1 )2 = C(Constant)
Joined: Apr 2012
03-09-2012, 03:33 PM
METAL FORMING.ppt (Size: 2.22 MB / Downloads: 69)
Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces
The tool, usually called a die, applies stresses that exceed yield strength of metal
The metal takes a shape determined by the geometry of the die
Stresses in Metal Forming
Stresses to plastically deform the metal are usually compressive
Examples: rolling, forging, extrusion
However, some forming processes
Stretch the metal (tensile stresses)
Others bend the metal (tensile and compressive)
Still others apply shear stresses
Material Properties in Metal Forming
Desirable material properties:
Low yield strength and high ductility
These properties are affected by temperature:
Ductility increases and yield strength decreases when work temperature is raised
Strain rate and friction
Bulk Deformation Processes
Characterized by significant deformations and massive shape changes
"Bulk" refers to workparts with relatively low surface area‑to‑volume ratios
Starting work shapes include cylindrical billets and rectangular bars
Forming and related operations performed on metal sheets, strips, and coils
High surface area‑to‑volume ratio of starting metal, which distinguishes these from bulk deformation
Often called pressworking because presses perform these operations
Parts are called stampings
Usual tooling: punch and die
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