Work Ability Criteria In Bulk Forming Processes
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introduction
Workability criteria
Procedure Detailed Theory
Conclusion remarks
Workability may be defined as the extent to which a material can be formed without failure. Workability is not a unique material property it is affected by both process and material variables.
²
<Work piece geometry
¥
are characterizes by the application of compressive force to the material to impart a change in its shape and dimensions
Ductile fracture is usually the limiting factor that determines the workability of the cold bulk forming processes. Å is caused by instability which is a result from very large plastic deformations occurring in the surrounding of crystalline defects, The surface of the ductile fracture is characterized by the cup and cone.
Ductile fracture usually follows one of the two modes
S in which the stress normal to surface is zero , this type is more common to upsetting and rolling processes. in which the fracture occurs inside the work piece , this type is more common to extrusion and forging processes
!"# For a criterion to be successful in predicting workability it should be capable of
These tasks must be accomplished with least sensitivity to calculation errors.
!"#
Æere we introduce the most common used criteria
for the bulk forming processes , also the improvements and modifications applied on these criteria are discussed.
Workability criteria may be classified into two main groups
Criteria based on considerations. 2Criteria based on considerations.
S Õt is a micromechanical level approach relating fracture to the nucleation, growth and coalescence of voids. Õt usually follows a multistep failure process
( nucleation of microscopic voids (2 growth of voids induced by plastic straining (3 localization of plastic flow between the enlarged voids (4 final tearing of the ligaments between enlarged voids
Éteps of failure in ductile fracture
criterion only assumes that the material ruptures when the void volume fraction of the material reaches a certain critical value Õn the model, the material is assumed to fracture, when the energy required to coalesce voids by internal necking is less than the energy required to deform the material homogeneously Õn this model, the direction of the principal strain is assumed to be unchanged during plastic deformation { has modified the Thomason model in which the direction of the principal strain is assumed to change during plastic deformation
fracture criterion is proposed (by 9 based on a threedimensional unit cell, with discontinuous velocity field. They assumed that internal necking of the intervoid matrix represents a sudden Dzdiscontinuous" bifurcation from the previous homogeneous plastic flow field, which results in a discontinuous velocity field existing between segment and 2.
When the equation ª ª2= ªPL ªVEL is satisfied, the material arrives at the critical state of incipient internal necking. ªPL is the rate of internal plastic dissipation energy ªVEL is the rate of dissipation energy in velocity discontinuous plane The rates of internal plastic dissipation energy ª and ª2 in segments and 2 fter that the critical strain to fracture à is expressed as the function of void volume fraction
¥
macro mechanical approach states that fracture occurs when a certain integral of a stress function over the effective strain field reached a critical value. < This criterion is based on true ductility and it is suggesting that the fracture occurs when the tensile strain energy reaches a critical value
<¥ created a criterion which is a modified version of Cockcroft and Latham and takes hydrostatic pressure into account
<
proposed a criterion which differs slightly from Cockcroft and Latham by using an equivalent stress rather than the max tensile stress.
<M proposed a criterion which is derived from plasticity theory for porous materials
<Æ { They took the Cockcroft & Latham criterion and combined with the maximum shear stress criterion to form their own criterion.
<{ Éuggested that the axial and circumferential strains at fracture provide a measure for workability, this criterion is only applicable to free surface fracture.
Étrain paths for different deformation processes
Fracture locus for aluminum alloy 2024 at room temperature
¥
<Æ
Mne of the first scientists who worked in this filed , he used the tensile stress induced in the material in direction normal to the axis of mechanical texturing
<É This criterion considers the ratio of mean stress and effective stress and it gives the following fracture model
<É Éuggested that for a free surface ductile fracture a critical value for shear stress is more reliable as a workability criterion, results of their investigations demonstrated that the fracture strains don't satisfy a linear fracture line , investigations of the collar and punch test enforced their claims .
$%& " & %'&" # " (&)' (( some scientists tried to model workability limit by ductile fracture & others used damage criteria. Õn cold forging, location and time of ductile damage can be determined by passive ultrasonic testing, in semi hot forging an optical fracture measurement is chosen,
racture propagation in collar upsetting.
Crossover is the range of temperature in which probability of damage ranges from one to zero.
:
(left, Materials in semihot upsetting of collar specimen with temperature crossover intervals (right
" %%( " ((* &%+ S
Mbviously there exists no causality between the chemical composition and the crossover temperature of the steels.
Élow cooling in an oven induced a comparably coarse ferriticpearlitic (FP microstructure with 55 % ferrite and 45 % perlite Chilling of the specimen in oil generated fine bainite with 30 % ferrite, 68 % bainite and 2 % martensite. Bainite and ferritic pearlitic grain structure of ÕÉÕ , ductile pores in the critical area.
" * ((( , - !# Lemaitre:
The absolute damage value denotes the ratio between undamaged ( = 0 and fully damaged element ( = . The evolution law of the absolute damage for the general case is
is equivalent stress , dɉeq is incremental strain and E is modulus of elasticity and ÉD is the damage resistance
Where is the ultimate tensile strength, Dc is the damage enough to cause failure under multiaxial stress, D c is the damage enough to cause failure under uniiaxial tension. Drel= , Drel
(" ( Determination of material parameter (D c
Damage parameters for some materials
Fracture propagation in the tensile test FE simulations with the Model of Effective Étresses.
&% ( ' ' $%& " ("(,%( (- Damage evolution of the MEÉ in the collar upsetting test.
FE simulations of the MEÉ fracture propagation in the collar upsetting test (ÕÉÕ .
" ! . & $ "# Triaxiality(ɋ can be calculated from the formula
Éo we can consider the triaxiality as an index for weather the
Loading is Tensile(ɋ is ve Mr Loading is Compressive(ɋ is Ȃve
From FEM we found that tension is more PRMVM TÕVE to Damage than compression.
" %%" ' &" * (((( ( " %(((+ Éemihot forged journal bearing surface fissure at the rim of the cup (ÕÉÕ .
Production of this component took place on four stages of backward extrusion. Damage occurred in the fourth one.
FEÉemihot forged journal bearing distribution of the relative damage (MEÉ. fter improvement of the process by annealing before fourth stage we get the following results.
axi
stresses i critical areas
Damage takes place in regions of high triaxiality(ɋ. Õt takes place also in transition zones betn. compressive and tensile regions in the w.p.
"( enerally, there is no particular workability criterion can be applied for any metal forming process The assessment of different fracture criteria is very complicated and also there is a need to determine the range of applicability best suitable for each one From an industrial point of view, it is vital that the number of material constants for a criterion is minimized to reduce costs and time in material testing for each constant
the Model of Effective Étresses by L was capable of
modeling the crossover temperature behavior with the collar upsetting. The model was exemplarily applied to some industrial cold and semihot forging processes. The prediction capabilities as well as the optimization approaches to the damaged processes showed the good applicability. deeper study of dependency between triaxiality, damage occurrence and temperature behavior must be left to future work. Æendrik Just proposed that the Model of Effective Étresses by L was chosen as the most reliable damage model for cold and semihot forging.
introduction
Workability criteria
Procedure Detailed Theory
Conclusion remarks
Workability may be defined as the extent to which a material can be formed without failure. Workability is not a unique material property it is affected by both process and material variables.
²
<Work piece geometry
¥
are characterizes by the application of compressive force to the material to impart a change in its shape and dimensions
Ductile fracture is usually the limiting factor that determines the workability of the cold bulk forming processes. Å is caused by instability which is a result from very large plastic deformations occurring in the surrounding of crystalline defects, The surface of the ductile fracture is characterized by the cup and cone.
Ductile fracture usually follows one of the two modes
S in which the stress normal to surface is zero , this type is more common to upsetting and rolling processes. in which the fracture occurs inside the work piece , this type is more common to extrusion and forging processes
!"# For a criterion to be successful in predicting workability it should be capable of
These tasks must be accomplished with least sensitivity to calculation errors.
!"#
Æere we introduce the most common used criteria
for the bulk forming processes , also the improvements and modifications applied on these criteria are discussed.
Workability criteria may be classified into two main groups
Criteria based on considerations. 2Criteria based on considerations.
S Õt is a micromechanical level approach relating fracture to the nucleation, growth and coalescence of voids. Õt usually follows a multistep failure process
( nucleation of microscopic voids (2 growth of voids induced by plastic straining (3 localization of plastic flow between the enlarged voids (4 final tearing of the ligaments between enlarged voids
Éteps of failure in ductile fracture
criterion only assumes that the material ruptures when the void volume fraction of the material reaches a certain critical value Õn the model, the material is assumed to fracture, when the energy required to coalesce voids by internal necking is less than the energy required to deform the material homogeneously Õn this model, the direction of the principal strain is assumed to be unchanged during plastic deformation { has modified the Thomason model in which the direction of the principal strain is assumed to change during plastic deformation
fracture criterion is proposed (by 9 based on a threedimensional unit cell, with discontinuous velocity field. They assumed that internal necking of the intervoid matrix represents a sudden Dzdiscontinuous" bifurcation from the previous homogeneous plastic flow field, which results in a discontinuous velocity field existing between segment and 2.
When the equation ª ª2= ªPL ªVEL is satisfied, the material arrives at the critical state of incipient internal necking. ªPL is the rate of internal plastic dissipation energy ªVEL is the rate of dissipation energy in velocity discontinuous plane The rates of internal plastic dissipation energy ª and ª2 in segments and 2 fter that the critical strain to fracture à is expressed as the function of void volume fraction
¥
macro mechanical approach states that fracture occurs when a certain integral of a stress function over the effective strain field reached a critical value. < This criterion is based on true ductility and it is suggesting that the fracture occurs when the tensile strain energy reaches a critical value
<¥ created a criterion which is a modified version of Cockcroft and Latham and takes hydrostatic pressure into account
<
proposed a criterion which differs slightly from Cockcroft and Latham by using an equivalent stress rather than the max tensile stress.
<M proposed a criterion which is derived from plasticity theory for porous materials
<Æ { They took the Cockcroft & Latham criterion and combined with the maximum shear stress criterion to form their own criterion.
<{ Éuggested that the axial and circumferential strains at fracture provide a measure for workability, this criterion is only applicable to free surface fracture.
Étrain paths for different deformation processes
Fracture locus for aluminum alloy 2024 at room temperature
¥
<Æ
Mne of the first scientists who worked in this filed , he used the tensile stress induced in the material in direction normal to the axis of mechanical texturing
<É This criterion considers the ratio of mean stress and effective stress and it gives the following fracture model
<É Éuggested that for a free surface ductile fracture a critical value for shear stress is more reliable as a workability criterion, results of their investigations demonstrated that the fracture strains don't satisfy a linear fracture line , investigations of the collar and punch test enforced their claims .
$%& " & %'&" # " (&)' (( some scientists tried to model workability limit by ductile fracture & others used damage criteria. Õn cold forging, location and time of ductile damage can be determined by passive ultrasonic testing, in semi hot forging an optical fracture measurement is chosen,
racture propagation in collar upsetting.
Crossover is the range of temperature in which probability of damage ranges from one to zero.
:
(left, Materials in semihot upsetting of collar specimen with temperature crossover intervals (right
" %%( " ((* &%+ S
Mbviously there exists no causality between the chemical composition and the crossover temperature of the steels.
Élow cooling in an oven induced a comparably coarse ferriticpearlitic (FP microstructure with 55 % ferrite and 45 % perlite Chilling of the specimen in oil generated fine bainite with 30 % ferrite, 68 % bainite and 2 % martensite. Bainite and ferritic pearlitic grain structure of ÕÉÕ , ductile pores in the critical area.
" * ((( , - !# Lemaitre:
The absolute damage value denotes the ratio between undamaged ( = 0 and fully damaged element ( = . The evolution law of the absolute damage for the general case is
is equivalent stress , dɉeq is incremental strain and E is modulus of elasticity and ÉD is the damage resistance
Where is the ultimate tensile strength, Dc is the damage enough to cause failure under multiaxial stress, D c is the damage enough to cause failure under uniiaxial tension. Drel= , Drel
(" ( Determination of material parameter (D c
Damage parameters for some materials
Fracture propagation in the tensile test FE simulations with the Model of Effective Étresses.
&% ( ' ' $%& " ("(,%( (- Damage evolution of the MEÉ in the collar upsetting test.
FE simulations of the MEÉ fracture propagation in the collar upsetting test (ÕÉÕ .
" ! . & $ "# Triaxiality(ɋ can be calculated from the formula
Éo we can consider the triaxiality as an index for weather the
Loading is Tensile(ɋ is ve Mr Loading is Compressive(ɋ is Ȃve
From FEM we found that tension is more PRMVM TÕVE to Damage than compression.
" %%" ' &" * (((( ( " %(((+ Éemihot forged journal bearing surface fissure at the rim of the cup (ÕÉÕ .
Production of this component took place on four stages of backward extrusion. Damage occurred in the fourth one.
FEÉemihot forged journal bearing distribution of the relative damage (MEÉ. fter improvement of the process by annealing before fourth stage we get the following results.
axi
stresses i critical areas
Damage takes place in regions of high triaxiality(ɋ. Õt takes place also in transition zones betn. compressive and tensile regions in the w.p.
"( enerally, there is no particular workability criterion can be applied for any metal forming process The assessment of different fracture criteria is very complicated and also there is a need to determine the range of applicability best suitable for each one From an industrial point of view, it is vital that the number of material constants for a criterion is minimized to reduce costs and time in material testing for each constant
the Model of Effective Étresses by L was capable of
modeling the crossover temperature behavior with the collar upsetting. The model was exemplarily applied to some industrial cold and semihot forging processes. The prediction capabilities as well as the optimization approaches to the damaged processes showed the good applicability. deeper study of dependency between triaxiality, damage occurrence and temperature behavior must be left to future work. Æendrik Just proposed that the Model of Effective Étresses by L was chosen as the most reliable damage model for cold and semihot forging.
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