WO2017047890A1

WO2017047890A1 - Method for predicting wear and lifespan of press tool by using wear model

Info

Publication number: WO2017047890A1
Application number: PCT/KR2016/003279
Authority: WO
Inventors: 김낙수; 천승현; 이준민; 전용근
Original assignee: 서강대학교산학협력단
Priority date: 2015-09-14
Filing date: 2016-03-30
Publication date: 2017-03-23
Also published as: KR20170031906A

Abstract

A method for predicting the wear of a press tool comprises the steps of: calculating an abrasive wear amount of the press tool by applying an abrasive wear model to a finite element analysis simulation program; calculating a fatigue wear amount on the basis of a difference between the abrasive wear amount and an actually measured wear amount obtained by performing an actual press process using the press tool; and calculating a wear amount of the press tool by applying, to the finite element analysis simulation program, an abrasive wear model in which the abrasive wear amount is reflected and a fatigue wear model in which the fatigue wear amount is reflected, and using a wear simulation of the press tool.

Description

Prediction of Wear and Life of Press Tool Using Wear Model

The present invention relates to a method for predicting wear and life of a press tool used in a press process.

The press process for forming a base material such as a sheet using a press tool includes a shearing process, a bending process, a drawing process, and the like.

In the press process, friction occurs due to contact and sliding of the tool and the plate on the tool surface, and powder or dust is generated from the plate during the process. These factors lead to grinding wear on the tool surface, and furthermore fatigue fatigue in the form of non-gradual volume loss due to the constant cyclic loading of the tool due to the nature of the press process. This wear results in dimensional changes in the tool and degrades the precision of sheet forming, which negatively affects formability and product quality. Therefore, it is very important to predict tool wear and replace the tool in a timely manner in order to improve product productivity and reduce cost.

However, until now, a systematic method for estimating tool wear amount and determining tool replacement time is not known. Therefore, in the industrial field, it is determined by tool wearer's subjective experience or know-how.

This requires a lot of trial and error to determine the wear life of the tool, even if the process conditions or environment are slightly different, and it takes much time and money. Furthermore, the systematic determination of tool replacement time saves cost and time in the press process, and thus requires more systematic tool wear prediction techniques.

The problem to be solved by the present invention is to provide a quantitative and reliable tool wear prediction method.

According to an embodiment of the present invention, a method of predicting wear of a press tool may include calculating a grinding wear amount of the press tool by applying a grinding wear model to a finite element analysis simulation program, and using the grinding wear amount and the press tool. Calculating a fatigue wear amount based on the difference of the measured wear amount obtained by performing the press process, and applying the grinding wear model reflecting the grinding wear amount and the fatigue wear model reflecting the fatigue wear amount to the finite element analysis simulation program. Calculating a wear amount of the press tool using a wear simulation of the tool.

The grinding wear model may be a modified Archard model.

The modified Archard model can calculate the wear depth by the following equation.

[Equation]

Where W is the wear depth, P is the vertical pressure acting on the press tool, v is the speed at which the material slides on the surface of the press tool, H is the surface hardness of the press tool, and t is the contact angle between the press tool and the material. The time, K, represents the wear factor of the press tool, and a, b, c represent wear constants that impart freedom to the factors affecting the wear of the press tool.

K, a, b, and c may be obtained by a pin-on-disc test according to ASTM G99 standard.

The fatigue wear model may be a Lemaitre model.

The Lemaitre model calculates the damage value increase by the following equation, and the fatigue wear amount may be calculated based on the damage increase.

[Equation]

here,

Is the increase in damage value due to fatigue accumulation,

Is the increase in strain in the elastic region,

Damage strain energy release rate in the elastic region,

Is the Poisson's ratio,

Is the Young's modulus,

Is the damage value,

Is the effective stress in the elastic region,

Is the hydrostatic stress in the elastic region,

Wow

Represents the damage factor that controls the degree of damage increase that occurs depending on the material of the press tool.

The wear prediction method of the press tool according to another embodiment of the present invention may further include applying a shape update algorithm to the wear simulation to reflect the shape change of the press tool according to the wear.

In the step of reflecting the shape change of the press tool, the surface shape is updated in such a manner as to gradually retract the surface node constituting the shape of the press tool according to the amount of wear for grinding wear, and the accumulated damage value for the fatigue wear is When the threshold is reached, the surface shape can be updated in such a way as to remove the surface elements that make up the shape of the press tool.

The wear prediction method of the press tool according to another embodiment of the present invention may further include the step of simulating the plastic behavior of the press tool to reflect on the wear simulation of the press tool.

The plastic behavior of the press tool can be simulated by the following Johnson-cook flow stress equation.

[Equation]

here,

Is the flow stress,

Is the yield stress at a given reference temperature,

Is the coefficient of strain hardening,

Is the plastic strain,

Is the strain hardening coefficient,

Is the coefficient of strain-rate hardening,

Is the plastic strain rate,

Is a given reference strain rate set to reflect the test results,

Is the temperature,

Is room temperature,

Is the melting temperature,

Is the material constant.

The wear prediction method of the press tool according to another embodiment of the present invention may further include the step of simulating the plastic behavior of the workpiece generated before the necking and reflecting it to the wear simulation of the press tool.

The plastic behavior of the workpiece can be simulated by the following Swift equation.

[Equation]

here,

Represents the flow stress,

Represents the stress coefficient,

Represents strain,

Represents the initial strain,

Represents the strain hardening coefficient.

The wear prediction method of the press tool according to another embodiment of the present invention may further include the step of simulating the fracture model of the workpiece to reflect in the wear simulation of the press tool.

The fracture model of the workpiece can be simulated by the following Enhanced Lemaitre damage model.

[Equation]

here,

Is the damage rate per unit time,

Represents the equivalent plastic strain,

Represents Young's modulus,

Represents a load parameter,

Represents the strain threshold (Threshold logarithmic strain at which Lemaitre damage model),

Represents the triaxiality ratio,

Is

Indicates,

,

Is a material parameter.

According to the present invention, the wear of the press tool can be accurately predicted by applying the grinding wear model and the fatigue wear model to predict the wear of the press tool.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view for explaining an experiment for determining a constant related to material properties of a press tool by a pin-on-disk method in a modified Archard model used in a wear prediction method of a press tool according to an embodiment of the present invention. to be.

FIG. 2 is a view for explaining a process of determining constants related to material properties of a press tool through derivation of a graph corresponding to the wear depth measured by the pin-on-disk method of FIG. 1.

Figure 3 is a graph showing the amount of wear of the press tool (punch) according to the punching number obtained through the experiment using the blanking process as an example and the amount of wear of the press tool obtained through the simulation of the wear prediction method according to an embodiment of the present invention.

Figure 4 is a graph showing the change in the size of the hole formed through the experiment and the size of the hole obtained through the simulation of the wear prediction method according to an embodiment of the present invention by using the blanking process as an example.

Hereinafter, with reference to the accompanying drawings will be described in detail an embodiment of the present invention.

The wear prediction method of a press tool according to an embodiment of the present invention relates to a method of predicting wear by calculating an amount of wear of a press tool using a wear model.

First, the wear prediction method of the press tool according to the embodiment of the present invention calculates the amount of grinding wear of the press tool by applying the grinding wear model to the finite element analysis simulation program.

Here, the finite element analysis simulation program may be any simulation program using the finite element analysis program, for example, it may be a DEFORM simulation program.

In the embodiment of the present invention, the grinding wear model is used to calculate the amount of grinding wear that occurs gradually on the surface of the press tool, and the Archard model modified with the grinding wear model is used. That is, the amount of grinding wear of the press tool is calculated by applying the modified Archard model modified from the previously known grinding wear model Archard model to the finite element analysis simulation program.

The modified Archard model calculates the wear depth (W) by the following equation.

[Equation]

At this time, the constants K, a, b, c of the modified Archard model may be determined by a pin-on-disc test according to ASTM G99 standard. Specifically, these constants can be obtained through experiments using the pin 10 and the disk 20 as shown in FIG. As shown in the figure, the disk 20 may have a disc shape.

For example, the pin 10 may be formed in a cylindrical shape with the same material as the press tool used in the pressing process, and the disk 20 may be formed with the same material as the plate material to be pressed. As shown in FIG. 1, in a state in which the bottom surface of the pin 10 is in contact with the top surface of the disk 20, a constant contact pressure P is applied to the pin 10 in a downward direction, and the disk 20 is in this state. Rotate for a constant time t at a constant speed v. The test is carried out while varying the contact pressure P and the speed v, and the degree of wear of the pin 10 is measured while the test is carried out, and the amount of wear of the pin 10 according to the time or the number of rotations of the disk 20. Create a graph. Experiment by changing the values of unknown K, a, b, c by substituting P, v, H, which is the surface hardness of the press tool, and the amount of wear of the pin W, which are already known in the modified Archard model equation Find the curve that matches the value. For example, as shown in the graph of FIG. 2, the number of cycles of the disk and its corresponding wear depth in the state of applying a predetermined vertical load obtained through the pin-on-disk test is shown on the graph. We can determine the unknown constants K, a, b, and c by deriving a curve that matches

Meanwhile, according to the embodiment of the present invention, the fatigue wear amount is calculated based on the difference between the grinding wear amount calculated using the modified Archard model obtained as described above and the actual wear amount obtained by performing a press process using an actual press tool. . That is, since the wear occurring in the press tool consists of fatigue wear due to fatigue accumulation in addition to the grinding wear, the remaining portion except the grinding wear amount is regarded as the fatigue wear amount from the actually measured wear amount. If damage accumulates inside the press tool due to repetitive loads during the pressing process and the damage value reaches a threshold value, fatigue wear is considered to occur on the surface of the press tool due to fatigue accumulation. To simulate this phenomenon, the Lemaitre model is used as a fatigue wear model that can calculate damage caused by repeated loading.

That is, the fatigue wear amount is calculated by using the difference between the grinding wear amount and the actual wear process obtained by performing the actual press process using the press tool, and the fatigue wear model reflecting the fatigue wear amount is applied to the finite element analysis simulation. The Lemaitre model is used as the fatigue wear model.

In general, damage models such as the Lemaitre model calculate damage only in the plastic zone, but fatigue wear is a phenomenon caused by damage accumulation in the elastic zone, so the fatigue wear is calculated by determining the existing Lemaitre model to the elastic zone. .

According to an embodiment of the present invention, the Lemaitre model uses the damage value increment (

), And the fatigue wear amount is calculated based on the damage value increment.

[Equation]

here,

Is the increase in damage value due to fatigue accumulation,

Is the increase in strain in the elastic region,

Damage strain energy release rate in the elastic region,

Is the Poisson's ratio,

Is the Young's modulus,

Is the damage value,

Is the effective stress in the elastic region,

Is the hydrostatic stress in the elastic region,

Wow

Where the damage factor is

Wow

Can be determined using the difference between the experimental results and the results of the model considering only the grinding wear described above. For example, an additional fatigue wear model can be applied to the simulation model to calculate the amount of wear on the press tool, and the damage factor to minimize the error between the calculated and experimental values.

Wow

Can be derived through an optimization technique.

The wear simulation of the press tool is derived by applying the above-described grinding wear model reflecting the grinding wear amount and the fatigue wear model reflecting the fatigue wear amount to the finite element analysis simulation program.

FIG. 3 shows a comparison between the amount of wear of the press tool according to the punching number obtained through actual experiments and the amount of wear of the press tool obtained through simulation. Referring to FIG. 3, only the grinding wear model is applied (Calculation without Compared to fatigue, the calculation with fatigue model (Calculation with fatigue) is closer to the experimental results.

On the other hand, according to an embodiment of the present invention, by applying a shape update algorithm to the wear simulation can reflect the shape change of the press tool according to the progress of the wear. For example, for grinding wear occurring in a press tool, the surface geometry is updated by progressively retracting the surface node constituting the press tool according to the amount of wear, and for fatigue wear the press is pressed when the accumulated damage reaches a threshold. The surface shape can be updated by removing the surface elements that make up the shape of the tool. By applying the surface shape update algorithm in this way, the shape change of the press tool as the wear progresses can be reflected, thereby enabling more accurate wear prediction.

On the other hand, Figure 4 is a graph showing the change in the size of the hole is obtained through the simulation of the wear prediction method according to the embodiment of the present invention and the shape of the hole formed through the experiment using the blanking process as an example, Determine when to change the tool.

That is, as the press process (for example, the blanking process) proceeds, wear occurs in the press tool, and the size of the hole formed in the plate by the wear of the press tool gradually decreases, and FIG. 4 shows the reduced hole. Shows the area of. Referring to FIG. 4, the area reduction (Experiment) obtained through the experiment and the area reduction (Calculation) obtained through the simulation results are approximated, and the regression line approximating the calculation result by the linear function is shown in FIG. 4. Same as

Using this, the replacement cycle of the press tool can be calculated by the following equation.

[Equation]

here,

Represents the area reduction rate of the hole

Represents the punching number.

For example, assuming that the allowable area reduction of the hole is 0.2 mm 2, the replacement cycle of the press tool is 99,010 hits by the above equation.

On the other hand, according to another embodiment of the present invention, the press tool can be processed with SKD11 material, using the Johnson-cook flow stress model to simulate the plastic behavior of the material of the press tool occurring before the necking (necking) Can be reflected in wear simulation. This can be done by the following Johnson-cook flow stress equation.

[Equation]

here,

Is the flow stress,

Is the yield stress at a given reference temperature,

Is the coefficient of strain hardening,

Is the plastic strain,

Is the strain hardening coefficient,

Is the coefficient of strain-rate hardening,

Is the plastic strain rate,

Is a given reference strain rate set to reflect the test results,

Is the temperature,

Is room temperature,

Is the melting temperature,

Is the material constant.

The Johnson-cook flow stress model is a phenomenological constitutive model for materials that takes into account plastic strain, plastic strain velocity, and temperature effects.

For example, a press tool can be processed from SKD11 material, and general tensile tests can be conducted to determine the material properties of the tool. However, in the case of SKD11 material, the die-only material has a very high initial yield stress, so the load applied to the tool during normal shear deduction does not reach the initial yield stress point of the tool and only causes elastic deformation. Material properties do not contribute a great deal to the evaluation of tool wear. For this reason, in the embodiment of the present invention, the material properties of the SKD11 material used the values in the following table which are already known by the above formula.

[table]

On the other hand, according to another embodiment of the present invention, the plastic behavior of the workpiece (plate) generated before the necking can be simulated and reflected in the wear simulation. For example, the plate may be formed of SUS430, and may simulate the plastic behavior of the material generated before necking using the Swift flow stress model to reflect the wear simulation. This can be done by the following Swift equation.

[Equation]

here

Represents the flow stress,

Represents the stress coefficient,

Represents strain,

Represents the initial strain,

Represents the strain hardening coefficient. At this time, the material constant stress coefficient, strain, initial strain can be determined through the room temperature tensile test and simulation.

The Swift equation can simulate the behavior from the moment the plastic deformation of the material starts until the necking occurs.

Meanwhile, according to another embodiment of the present invention, the fracture model of the workpiece (plate) may be simulated and reflected in the wear simulation using the Enhanced Lemaitre damage model. For example, the plate may be formed of SUS430, and the Enhanced Lemaitre Damage Model may be used to simulate the fracture of the material after necking and to consider the shear properties of the material. This can be done by the following Swift equation.

here,

Is the damage rate per unit time,

Represents the equivalent plastic strain,

Represents Young's modulus,

Represents a load parameter,

Represents the triaxiality ratio,

Is

Indicates,

,

Is a material parameter.

Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements using the basic concept of the present invention as defined in the following claims are also rights of the present invention. It belongs to the range.

The present invention relates to a method for predicting the life of a press tool and has industrial applicability.

Claims

As a method of predicting wear of a press tool,

Calculating a grinding wear amount of the press tool by applying a grinding wear model to a finite element analysis simulation program;

Calculating a fatigue wear amount based on a difference between the grinding wear amount and an actual wear amount obtained by performing an actual press process using the press tool, and

A press tool comprising the step of calculating the wear amount of the press tool using the wear simulation of the press tool by applying the grinding wear model reflecting the grinding wear amount and the fatigue wear model reflecting the fatigue wear amount to the finite element analysis simulation program Wear prediction method.
In claim 1,

The grinding wear model is a modified Archard model wear prediction method of the press tool.
In claim 2,

The modified Archard model is a wear prediction method of the press tool to calculate the wear depth by the following formula.

[Equation]

(W is the wear depth, P is the vertical pressure acting on the press tool, v is the speed at which the material slides on the surface of the press tool, H is the surface hardness of the press tool, t is the contact between the press tool and the material Time, K is the wear factor of the press tool, a, b, c are the wear constants that give the degrees of freedom to the factors affecting the wear of the press tool)
In claim 3,

K, a, b, c is a wear prediction method of the press tool obtained by a pin-on-disc test according to the ASTM G99 standard.
In claim 1,

The fatigue wear model is Lemaitre model wear prediction method of the press tool.
In claim 5,

The Lemaitre model calculates an increase in damage value by the following equation, and the fatigue wear amount is calculated based on the damage increase.

[Equation]

(here,
Is the increase in damage value due to fatigue accumulation,
Is the increase in strain in the elastic region,
Damage strain energy release rate in the elastic region,
Is the Poisson's ratio,
Is the Young's modulus,
Is the damage value,
Is the effective stress in the elastic region,
Is the hydrostatic stress in the elastic region,
Wow
Represents the damage factor that controls the degree of damage increase that occurs depending on the material of the press tool)
In claim 1,

And applying a shape update algorithm to the wear simulation to reflect a change in shape of the press tool according to wear.
In claim 7,

In the step of reflecting the shape change of the press tool, the surface shape is updated in such a manner as to gradually retract the surface node constituting the shape of the press tool according to the amount of wear for grinding wear, and the accumulated damage value for the fatigue wear is And a method of predicting wear of the press tool when the threshold value is reached to update the surface shape in such a way as to remove the surface elements constituting the shape of the press tool.
In claim 1,

And simulating the plastic behavior of the press tool and reflecting it in the wear simulation of the press tool.
In claim 9,

The plastic behavior of the press tool is a wear prediction method of the press tool is simulated by the following Johnson-cook flow stress equation.

[Equation]

(here,
Is the flow stress,
Is the yield stress at a given reference temperature,
Is the coefficient of strain hardening,
Is the plastic strain,
Is the strain hardening coefficient,
Is the coefficient of strain-rate hardening,
Is the plastic strain rate,
Is a given reference strain rate set to reflect the test results,
Is the temperature,
Is room temperature,
Is the melting temperature,
Is the material constant)
In claim 1,

A method of predicting wear of a press tool further comprising simulating the plastic behavior of the workpiece occurring before necking and reflecting it in the wear simulation of the press tool.
In claim 11,

The plastic behavior of the workpiece is a wear prediction method of the press tool is simulated by the following Swift equation.

[Equation]

(here
Represents the flow stress,
Represents the stress coefficient,
Represents strain,
Represents the initial strain,
Represents the strain hardening coefficient)
In claim 1,

A method of predicting wear of a press tool further comprising replicating a fracture model of a workpiece and reflecting it in the wear simulation of the press tool.
In claim 13,

The failure model of the workpiece is simulated by the following Enhanced Lemaitre damage model.

[Equation]

(here,
Is the damage rate per unit time,
Represents the equivalent plastic strain,
Represents Young's modulus,
Represents a load parameter,
Represents the strain threshold (Threshold logarithmic strain at which Lemaitre damage model),
Represents the triaxiality ratio,
Is
Indicates,
,
,
Is a material parameter)