Chapter 1

Characteristics of Stamping and

Properties of Sheet Metal Forming

Stamping is a kind of plastic forming process in which a part is produced by means of the plastic forming of the material under the action of a die.

Stamping is usually carried out under cold state, so it is also called cold stamping.

Hot stamping is used only when the blank thickness is greater than 8~100mm. The blank material for stamping is usually in the form of sheet or strip, and therefore it is also called sheet metal forming. Some non-metal sheets (such as plywood, mica sheet, asbestos, leather) can also be formed by stamping.

Stamping is widely used in various fields of the metalworking industry, and it plays a crucial role in the industries for manufacturing automobiles, instruments, military parts and household electrical appliances, etc.

The process, equipment and die are the three foundational problems that needed to be studied in stamping.

The characteristics of the sheet metal forming are as follows:

1. High material utilization. 2. Capacity to produce thin-walled parts of complex shape.

3. Good interchangeability between stamping parts due to precision in shape and dimension.

4. Parts with lightweight, high-strength and fine rigidity can be obtained.

5. High productivity, easy to operation and to realize mechanization and automatization.

The manufacture of the stamping die is costly, and therefore it only fits to mass production. For the manufacture of products in small batches and rich varieties, the simple stamping die and the new equipment such as a stamping machining center, are usually adopted to meet the market demands.

The materials used for sheet metal stamping include mild steel, copper, aluminum, magnesium alloy and high-plasticity alloy-steel etc.

Stamping equipments includes plate shear and punching press. The former shears plate into strips with a definite width, which would be pressed latter. The later can be used both in shearing and forming.

1.1 Characteristics of stamping forming

There are various processes of stamping forming with different working patterns and names. But all of them undergo plastic deformation. The common conspicuous characteristics in stampingforming processes are as follows

（ 1 ） The force per unit area perpendicular to the blank surface is not large but is enough to cause so plastic deformation. It is much less than the inner stresses along plane directions of the plate. In most cases stamping forming can be treated approximately as that of the plane stress state to simplify vastly the theoretical analysis and the calculation of the process parameters.

（ 2 ） Due to the small relative thickness, the anti-instability capability of the blank is weak under compressive stress. As a result, sometimes the stamping process is difficult to proceed successfully without using the anti-instability device (such as blank holder). Therefore the varieties of the stamping processes dominated by tensile stress are more than those dominated by compressive stress.

（ 3 ） During stamping forming, the inner stress of the blank is equal to or sometimes less than the yield stress of the material. In this point, the stamping is different from the bulk forming. During stamping forming, the influence of the hydrostatic pressure of the stress state in the deformation zone in forming limit and the deformation resistance is not so important as to the bulk forming. In some circumstances, such influence may be neglected. Even in the case when this influence should be considered, the treating method is also different from that of bulk forming.

(4) In stamping forming, the restrain action of the die to the blank is not severe as in the case of the bulk forming (such as die forging). In bulk forming, the constraint forming is proceeded by the die with exactly the same shape of the part. Whereas in stamping, in most cases, the blank has a certain degree of freedom, only one surface of the blank contacts with the die. In some extra cases, such as the forming of the suspended region of sphere or cone, and curling at the end of tube, neither sides of the blank on the deforming zone contact with the die. The deformation in these regions are caused and controlled by the die applying an external force to its adjacent area.

（ 1 ） The importance of the strength and rigidity of the die in stamping forming is less than that in bulk forming because the blank can be formed without applying large pressure per unit area on its surface. Instead, the techniques of the simple die and the pneumatic and hydraulic forming are developed.

Due to the characteristics of stamping deformation and mechanics mentioned above, the stamping technique is different from the bulk metal forming as follows:

（ 2 ） Due to the plane stress or simple strain state in comparison with bulk forming, more research on deformation, force and power parameters have been done. So stamping forming can be performed by more resonable scientific methods. Based on the real time measurement and analysis on the sheet metal properties and stamping parameters, by means of computer and some modern testing apparatus, research on the intellectualized control of stamping process is also in proceeding.

（ 3 ） It is shown that there is a close relationship between stamping forming and raw material. The research on the properties of the metals for stamping forming, mainly includes the, forming ability and shape stability, has become a key point in stamping technology. The research on the properties of the sheet metal stamping not only meets the need of the stamping technology development, but also enhances the manufacturing technique of iron and steel industry, and provides a reliable foundation for increasing sheet metal quality.

1.2 Categories of stamping forming

Many deformation processes can be done by stamping, the basic processes of the stamping can be divided into two kinds: cutting and forming.

Cutting is a shearing process that one part of the blank is cut from the other. It mainly includes blanking, punching, trimming, parting and shaving, where punching and blanking are the most widely used.

Forming is a process that one part of the blank has some displacement from the other. It mainly includes deep drawing, bending, local forming, bulging, flanging, necking, sizing and spinning.

In substance, stamping forming is such that the plastic deformation occurs in the deformation zone of the stamping blank caused by the external force. The stress state and deformation characteristic of the deformation zone are the basic factors to decide the properties of the stamping forming.

Based on the stress state and deformation characteristics of the deformation zone, the forming methods can be divided into several categories with the same forming properties and to be studied systematically.

The deformation zone in almost all types of stamping forming is in the plane stress state.

Usually there is no force or only small force applied on the blank surface. When it is assumed that the stress perpendicular to the blank surface equals to zero, two principal stresses perpendicular to each other and act on the blank surface produce the plastic deformation of the material. Due to the small thickness of the blank, it is assumed approximately that the two principal stresses distribute uniformly along the thickness direction.

Based on this analysis, the stress state and the deformation characteristics of the deformation zone in all kinds of stamping forming can be denoted by the points in the coordinates of the plane principal stresses (diagram of the stamping stress) and the coordinates of the corresponding plane principal strains (diagram of the stamping strain). The different points in the diagrams of the stamping stress and strain possess different stress state and deformation characteristics.

• （ 1 ） When the deformation zone of the stamping blank is subjected to the plane tensile stresses, it can be divided into two cases, that is σr>σθ>0, σt=0 and σθ>σr>0, σt=0. In both cases, the stress with the maximum absolute value is always a tensile stress. These two cases are analyzed respectively as follows.

• 1) In the case that σr>σθ>0 and σt=0,

• according to above analysis, it is known that this kind of deformation condition is in the region AON of the diagram of the stamping strain (see Fig. 1.1), and in the region GOH of the diagram of the stamping stress (see Fig. 1.2).

• 2) When σθ>σr>0 and σt=0,

• this kind of deformation is in the region AOC of the diagram of the stamping strain (see Fig. 1.1), and in the region AOH of the diagram of the stamping stress (see Fig. 1.2).

• (2) When the deformation zone of the stamping blank is subjected to two compressive stresses σr and σθ (σt=0), it can also be divided into two cases, which are σr<σθ<0, σt=0 and σθ<σr<0, σt=0.

• 1) When σr<σθ<0 and σt=0,

• this kind of deformation condition is in the region GOE of the diagram of the stamping strain (see Fig. 1.1), and in the region COD of the diagram of the stamping stress (see Fig. 1.2).

• 2) When σθ<σr<0 and σt=0,

• this kind of deformation is in the region GOL of the diagram of the stamping strain (see Fig.1.1), and in the region DOE of the diagram of the stamping stress (see Fig.1.2).

• (3) The deformation zone of the stamping blank is subjected to two stresses with opposite signs, and the absolute value of the tensile stress is larger than that of the compressive stress. There exist two cases to be analyzed as follows:

• 1) When σr>0, σθ<0 and |σr|>|σθ|,

• this kind of deformation condition is in the region MON of the diagram of the stamping strain (see Fig. 1.1), and in the region GOF of the diagram of the stamping stress (see Fig. 1.2).

• 2) When σθ>0, σr<0, σt=0 and |σθ|>|σr|,

• this kind of deformation condition is in the region COD of the diagram of the stamping strain (see Fig. 1.1), and in the region AOB of the diagram of the stamping stress (see Fig. 1.2).

• (4) The deformation zone of the stamping blank is subjected to two stresses with opposite signs, and the absolute value of the compressive stress is larger than that of the tensile stress. There exist two cases to be analyzed as follows:

• 1) When σr>0, σθ<0 and |σθ|>|σr|,

• this kind of deformation is in the region MOL of the diagram of the stamping strain (see Fig. 1.1), and in the region EOF of the diagram of the stamping stress (see Fig. 1.2).

• 2) When σθ>0, σr<0 and |σr|>|σθ|,

• such deformation is in the region DOF of the diagram of the stamping strain (see Fig. 1.1), and in the region BOC of the diagram of the stamping stress (see Fig. 1.2).

The four deformation conditions are related to the corresponding stamping forming methods. and are shown in Figures 1.1 and 1.2.

Fig. 1.1 Diagram of stamping strain

Fig. 1.2 Diagram of stamping stress

• The four deformation conditions analyzed above are applicable to all kinds of plane stress states, that is, the four deformation conditions can sum up all kinds of stamping forming into two types, tensile and compressive. When the stress with the maximum absolute value in the deformation zone of the stamping blank is tensile, the deformation along this stress direction must be tensile. Such stamping deformation is called tensile forming.

• Based on above analysis, the tensile forming occupies five regions MON, AON, AOB, BOC and COD in the diagram of the stamping strain; and four regions FOG, GOH, AOH and AOB in the diagram of the stamping stress.

When the stress with the maximum absolute value in the deformation zone of the stamping blank is compressive, the deformation along this stress direction must be compressive. Such stamping deformation is called compressive forming. Based on above analysis, the compressive forming occupies five regions LOM, HOL, GOH, FOG and DOF in the diagram of the stamping strain; and four regions EOF, DOE, COD and BOC in the diagram of the stamping stress.

The tensile forming is located in the top right of the boundary, and the compressive forming is located in the bottom left of the boundary.

MD and FB are the boundaries of the two types of forming in the diagrams of the stamping strain and stress, respectively.

Because the stress produced by the plastic deformation of the material is related to the strain caused by the stress, there also exist certain relationships between the diagrams of the stamping stress and strain. There are corresponding locations in the diagrams of the stamping stress and strain for every stamping deformation.

According to the state of stress or strain in the deformation zone of the forming blank, and using the boundary line in the diagram of the stamping stress MD or the boundary line in the diagram of the stamping strain FB, it’s easy to know the properties and characteristics of the stamping forming.

• The locations in the diagrams of the stamping stress and strain for various stress states and the corresponding relationships of the two diagrams are listed in Table 1.1. It shows that the geometrical locations for every region are different in the diagrams of the stamping stress and strain, but their sequences in the two diagrams are the same.

• One key point is that the boundary line between the tensile and the compressive forming is an inclined line at 450 to the coordinate axis. The characteristics of the stamping technique for tensile and compressive forming are listed in Table 1.2.

• Table 1.2 clearly shows that the characteristics of the force and deformation in the deformation zone of the blank, and the relevant to the deformation for each type of stamping methods are the same. Therefore, in addition to the research on the detail stamping method, it’s feasible to study stamping systematically and comprehensively.

• The characteristic of the systematic research is to study the common principle of all different types of stamping method. The results of the systematic research are applicable to all stamping methods. The research on the properties and limit of the sheet metal stamping has been carried out in certain extents. The contents of the research on the stamping forming limit by using systematic method are shown in Fig.1.3.

1.3 Methods to study stamping forming

The objective of the stamping forming research is to realize and apply the principles to solve various engineering problems through study on the stamping forming procedure.

Recently, the research on stamping forming has been carried out in wide range with various methods. The research can be summed up substantially into following types.

The double line arrows in Fig.1.4 denote the

procedure of formal theoretical research.

In such research, the actions of the equipment and die are decomposed into micro-ingredients of the force and applied on the microstructure of the metal according to the multi-crystal microstructure of the deformation metal, the deformation of the metal microstructure is then investigated and the deformation of the microstructure to macro- deformation of the sheet metal is summed up.

Fig.1.4 Diagram of stamping research

methods

This scientific method is perfect, but

current development in mechanics and

metallurgy cannot meet such demand.

Therefore this kind of formal theoretical

research is still in burgeoning, and is not

applicable in practice.

This research assumes metal as the ideal homogeneous solid and simplifies the property parameters, the boundary conditions and the blank geometrical parameters of the metal, the stamping process and principle are analyzed and described by mathematical method.

Since the formal theoretical research method encounters formidable difficulty, a kind of simplified theoretical method appears in the research field of the stamping forming. The characteristic of this kind of research method is denoted by continuous line in Fig.1.4, which is the main trend of the recent research.

Naturally, the assumptions adopted in the analysis would induce some deviation between the real stamping process and its simulation. The result is certainly approximate and does not reflect the real stamping process completely. Especially in analyzing complex stamping forming, this theory is not so valid. In recent years, due to the development of the finite element method and computer technology, this kind of theoretical method has greatly progressed, it shows applicable prospect even in analyzing complex stamping forming.

The application of this kind of theoretical method mainly focuses on some special stamping deformation investigation, and it is expected that more achievements can be obtained in the foundamental principles of stamping forming. On the other hand, due to the simplifications and assumptions, experiments are indispensable to test the validity of this theory.

The third method to study stamping forming is

shown by dashed dotted line in Fig.1.4.

The characteristic of this method is that ignoring the deformation process of the blank under the action of the load during stamping forming, trying to build the relationships and rules directly among the original forming conditions (including the structure of the die, the geometrical parameters of the die working portion and the properties of the stamping equipment) with the final results of the stamping forming.

Fig.1.4 Diagram of stamping research

methods

It is a rational empirical method, and widely used in stamping technology field in recent years. This method is intuitional, simple and easy to be adopted by engineers, but it cannot be used to reveal the real procedure of the stamping deformation, so it is not the radical method to investigate the principles of the stamping forming more thoroughly and its usage is strictly limited.

The fourth method to study stamping forming is

denoted by dashed line in Fig.1.4. Based on the fundamental knowledge of the mechanics and metallurgy, the essential characteristics and principles of the stamping forming are investigated to solve the practical problems in stamping process. In comparison with other plastic working processes, the characteristics of the stamping forming and the principles of the sheet metal deformation are unique. So this method has definitely aim and the analysis results can be used directly to solve different problems in stamping forming .

• The characteristics and effects of this research method are illustrated by following examples.

• (1) The sequence of the stamping process can be decided by the trend rule of the stamping deformation.

• (2) According to the rule of uniform velocity distribution in the outer flange of the deformation zone during multi-pass deep drawing for the box parts, the shape and dimension of the blank during process sequence can be calculated. Therefore the calculation for deep box parts of the multi-pass deep drawing technology is based on scientific foundation to improve the technique level of stamping forming.

• (3) According to the research on the wrinkle during the sheet metal forming caused by non-uniform tensile and shearing force, a new research field is developed beyond the traditional compressive instability theory. The results obtained offer an effective measure to cope directly with wrinkle during stamping forming.

• (4) The theory of sorting stamping forming based on the characteristics of the stamping deformation and mechanics is an easy approach to systematize the researches of the stamping forming, and can be used to deepen the investigation on stamping forming limit, to point out a clear and definite direction for improving the stamping forming limit.

• Above examples indicate that this method is closely relevant to the real stamping forming process. It can be used to analyze and solve the stamping forming problems effectively. Although this method is in the junior stage, mainly focusing on the qualitative analysis for stamping forming now. With the constant progress of this method and combined with modern mechanics, the breakthrough of this method is expected to further enhance stamping forming technology.

1.4 Raw materials for stamping forming

There are a lot of raw materials used in stamping forming, and the properties of these materials may have large difference.

The stamping forming can be succeeded only by determining the stamping method, the forming parameters and the die structures according to the properties and characteristics of the raw materials. The deformation of the blank during stamping forming has been investigated quite thoroughly. The relationships between the material properties decided by the chemistry component and structure of the material and the stamping forming has been established clearly.

Not only the proper material can be selected based on the working condition and usage demand, but also the new material can be developed according to the demands of the blank properties during processing the stamping part. This is one important domain in stamping forming research.

(2) Method to judge the stamping property of the material, find parameters to reflect exactly the property of the material to be stamping formed, establish the relationship between the property parameters and the practical stamping forming, and investigate the testing methods of the property parameters.

The research on the material properties for stamping forming is as follows:

(1) Definition of the stamping property of the material.

(3) Establish the relationship among the chemical component, structure, manufacturing process and stamping property.

The raw materials for stamping forming mainly include various metals and nonmetal plate.

The sheet metal includes both ferrous and nonferrous sheet metals. Although a lot of sheet metals are used in stamping forming, the most widely used materials are steel, stainless steel, aluminum alloy and various composite metal plates.

1.4.1 Ordinary steel plate

In stamping forming, steel plate is the most widely used material. It is used for products in vehicles including automobile, tractor and train, and for electrical, petrochemical, mechanical and architectural products.

Due to the different functions and demands for different products, the types and shapes of the steel plate are also different.

Fig. 1.5 shows the procedures for manufacturing various steel plates and the variation of the program during manufacturing and logistics.

Fig.1.5 Procedure for manufacturing various steel plate and logistics variation

1 Hot rolled plate

There are two kinds of hot rolled plate available. One is the plate with black oxide coating 10μm deep formed directly after hot rolling. The oxide coating is hard and brittle. During stamping forming, especially when peeling exists, the die may be damaged. In order to overcome this problem, the hot rolled plate with its oxide coating being eliminated by superficial treatment as acid-washing etc is also supplied by the steel inustry. The surface of this kind of steel plate is rough, but it also has advantage in easy lubrication and fits for forming process.

The hot rolled plate does not have the texture organization of the cold rolling plate, so its stamping forming property is not as good as the cold one.

On the other hand, the thickness and properties of the hot rolling steel fluctuate greatly, which is not beneficial to stamping process. Besides the chemical component, the crystallite size also influences its strength and n value.

Due to low price of the hot rolling steel,

the steel plate with favorite stamping

properties and fitting to deep drawing

forming is being developed.

The properties of the hot rolled plate are

usually adjusted by controlling the crystallite

size.

2 Cold rolled plate The superficial quality and the stamping properties of the cold rolled plate are excellent. The properties and thickness of the plate are also stable. Therefore it is widely used in stamping forming.

By adjusting the chemical components, controlling the processes of smelting, hot and cold rolling as well as annealing, the cold rolled plates with various properties can be obtained.

Through the deformation during rolling and the recrystallization treatment during annealing, the texture organization with increasing r can be obtained, hence improving the deep drawing property of the cold rolled plate and the die fitting ability during curved part forming.

Table 1.3 shows an approximate comparison of the quality and properties between cold and hot rolled plate.

Table 1.3 Comparison of the quality and properties between cold and hot rolled plate

Item Hot rolled plate Cold rolled plate

Roughness Rγ / μm 20~50 0.25~25

Thickness tolerance (mm) ± (0.18~0.25) ± (0.08~0.13)

Percentage elongation δ (%) 27~35 37 ~ 42

Thickness directivity

coefficient r 0.8~0.95 1.1~1.8

There are two kinds of cold rolled plates, aged

and non-aged.

Usually, in the tensile diagram of the ordinary annealed cold rolling mild carbon steel plate, there is a yield point elongation. The reason is due to discontinuous yield phenomenon formed by C and N atoms. Slip lines which would damag surface smoothness would appear during stamping forming.

In order to overcome this unfavorable phenomenon, rolling with certain reduction is applied to steel plate after annealing, called sizing rolling.

Although this method is very effective, but its effect cannot sustain for a long time. After a certain perio, the actions of C and N atoms would restore around dislocation. This kind of steel plate is called aged cold rolling plate.

Adding Al or Ti can restrain completely the influence of C and N on dislocation, even eliminate aging phenomenon. In this way the effect of the sizing rolling can be maintained permanent. This kind of steel plate is called non-aged cold rolled sheet. The recently widely used 08A1 for stamping forming is one kind of non-aged mild carbon cold rolled plate.

1.4.2 High-strength steel plate

With increasing of the strength, the thickness of the steel plate can be decreased without decreasing the strength and rigidity of the part, hence both the weight and cost of the part can be decreased. Therefore, both domestic and international investigations focus on producing high-strength steel plate and developing corresponding stamping forming technique.

High-strength steel plate is used more often in automobile industry to meet the demands of the strength, toughness and forming properties.

The tensile strength of the ordinary high-strength steel plate is in the range of 350~500MPa.

Now many kinds of high-strength steel plate are used successfully in automobile industry to substitute ordinary steel plate for reducing weight and cost.

Some ultra-high-strength steel plates, with the strength as high as 1000MPa, have been developed.

1 High-strength steel plate modified with phosphorus

Recently, high-strength steel plates used in production are listed as follows:

This kind of high-strength steel plate belongs to solution hardening type. It was used rather early in automobile industry, and is well developed.

Modifying with phosphorus, the strength increased to 350~440MPa; r and n are decreased a little, with r=1.4~1.8, and n=0.2~0.24 respectively.

2 BH hardening high-strength steel plate

The stamping forming property of this kind of steel plate is very similar to that of mild carbon steel.

After stamping forming, spraying painting and low temperature baking, its strength is increased due to BH hardening and it turns to high-strength steel plate.

3 Biphase high-strength steel plate

There are mild ferritic and hard martensite organizations in this kind of steel plate, and hence both the strength and plasticity of the plate are rather good.

This kind of steel plate is mainly used to manufacture automobile parts, such as pillars and chassis.

1.4.3 Superficial treatment steel plate

In order to prevent the corrosion of the part manufactured by steel plate during utilizing, after cold and hot rolling, the steel plate is electroplated or processed by hot dipping in 450~500˚C corrosion-resistance metal solution, and then the superficial treatment steel plate is made.

Tin-plating, zinc-plating and aluminum-plating steel plates are the commonly used superficial treatment steel plate

Due to the thin coating of the superficial treatment steel plate, the influence of the coating on stamping forming property is small. The stamping forming property of the treated plate is roughly the plate without superficial treatment.

Whereas, the influence to the friction properties of the plating surface on stamping process cannot be neglected. Particularly when using drawing rib to contract friction resistance in stamping forming, whether the stamping forming is successful or not may be dependant on its surface friction condition.

Research results show that with the increasing of the friction coefficient of the superficial treatment steel plate, the formability range of the experimental used vehicle cover panel decreases. In other words, the demands to the forming conditions and parameters tend to be more strict.

1.4.4 Stainless steel plate

The stainless steels used in stamping forming include ferritic (Cr-system) and austenitic (Cr-Ni-system). Since their component and organization are different, their stamping properties are also different. Although both are called stainless steel, due to vast difference in stamping properties, the problems occurred during stamping forming of these two kinds of steels during forming must be disposed differently.

The stamping property of the ferritic stainless steel plate is near to that of the cold rolled plate. Its texture organization can be obtained by hot rolling, cold rolling and annealing during manufacturing procedure, with r increasing to 1.2~1.8.

The ferritic stainless steel plate is considered owning perfect deep drawing property.

The hardening index of the ferritic stainless steel plate is about 0.2 and its percentage elongation is about 0.25~0.3.

Both of then less than those of the austenitic stainless steel plate. Its tensile stamping forming property is a little poor and its bulging property (Erichsen value) is lower than that of the austenitic stainless steel plate.

The deep drawing property of the austenitic stainless steel plate is not so good, but its hardening index n is much larger than that of the ferritic stainless steel plate. Its Erichsen value is also large, so it owns perfect tensile stamping forming property, such as bulging.

The austenite phase in austenitic stainless steel plate would transform into martensite organization with quite high-strength during plastic deformation playing the role in the phase changing intensification. Therefore the stability extent of the austenite phase changing has great influence on the hardening index n of the austenitic stainless steel.

A lot of brands and types of stainless steel have been used in stamping forming to meet various demands of products and to obtain different stamping properties. Even for the same type of stainless steel, readjusting its chemical components or adding some chemical elements (such as copper, nickel and titanium) would change its property greatly.

Table 1.4 shows an approximately comparison of stamping properties between the commonly used ferritic and austenitic stainless steel plates.

Table 1.4 Comparison of stamping properties between commonly used ferritic and austenitic stainless

steel plates  

ItemMd30/

˚C

σ0.2

/MPa

σb

/MPa

δ/ %

n rErichsen

value/mm

LDR

Ferritic stainless steel pla

te-

300~

390

430~

590

25~33

0.18~

0.25

1.25~

1.75

8.5~10

2.25~

2.4

Austenitic stainless

steel plate

-46~45

240~

300

540~

700

48~65

0.4~

0.6

0.95~

1.04

12~13

2.2~

2.42

Some problems not existing in the ordinary steel plate may occur in the processing of the stainless steel plate, for instance, the aging crack and surface scratch due to die surface bonding during deep drawing. Although mechanisms of these problems have not been clearly discovered, some effective measures have been taken to solve them successfully.

1.5 Stamping forming property of the sheet metal and its assessing method

The stamping forming property of the sheet metal is the adaptation capability of the sheet metal to stamping forming. It has crucial meaning to the investigation of the stamping forming property of the sheet metal.

In order to produce stamping forming parts with most scientific, economic and rational stamping forming process and forming parameters, it is necessary to understand clearly the properties of the sheet metal, so as to utilize the potential of the sheet metal fully in the production.

On the other hand, to select plate material accurately and rationally in accordance with the characteristics of the shape and dimension of the stamping forming part and its forming technique is also necessary so that a scientific understanding and accurate judgment to the stamping forming properties of the sheet metal may be achieved.

There are direct and indirect testing methods to assess the stamping property of the sheet metal, both including many experiment methods (see Fig. 1.6).

Fig. 1.6 Outlines of the assessing methods for the stamping property of the sheet metal

Practicality stamping test is the most direct

method to assess stamping forming property

of the sheet metal. This test is done exactly

in the same condition as actual production by

using the practical equipment and dies.

Surely, this test result is most reliable. But

this kind of assessing method is not

comprehensively applicable, and cannot be

shared as a commonly used standard between

factories.

The simulation test is a kind of assessing method that after summing up and simplifying actual stamping forming methods, as well as eliminating many trivial factors, the stamping properties of the sheet metal are assessed based on simplified axial-symmetric forming method under the same deformation and stress states between the testing plate and the actual one.

In order to guarantee the reliability and generality of simulation results, a lot of factors are regulated in detail, such as the shape and dimension of tools for test, blank dimension and testing conditions (stamping velocity, lubrication method and blank holding force, etc.).

The widely used and ‘generally accepted simulation tests and its characteristics are listed in Table 1.5.

Table 1.5 Characteristics of generally used simulation tests

Testing method

Experimental objective and testing content

Expressive method of the testing results

Erichsen test

Bulging propertyErichsen value (m

m)Deep

drawing test

Deep drawing property

Limit deep drawing ratio LDR

Expanding test

Expanding property Expanding coefficient

Bending test

Anti-necking capability for large plasticity and

strain gradient

Minimum bending radius (mm)

Indirect testing method is also called basic testing method. Its characteristic is to connect analysis and research on fundamental property and principle of the sheet metal during plastic deformation, and with the plastic deformation parameters of the sheet metal in actual stamping forming, and then to establish the relationship between the indirect testing results (indirect testing value) and the actual stamping forming property (forming parameters).

Because the shape and dimension of the specimen and the loading pattern of the indirect testing are different from the actual stamping forming, the deformation characteristics and stress states of the indirect test are different from those of the actual one. So, the results obtained from the indirect test are not the stamping forming parameters, but are the fundamental parameters that can be used to represent the stamping forming property of the sheet metal.

The uniaxial tensile test of the sheet metal is studied extensively now, so the indirect testing method is usually used.

This method has been applied in industry for many years and is a quite ma’ture way to determine the mechanical property of the material.

In order to fit the demand for testing the stamping property of the sheet metal, necessary modification and renovation of the traditional tensile testing method is made to form the presently widely used uniaxial tensile testing method for assessing the stamping property of the sheet metal, and to establish the corresponding guidance standard about the testing methods and conditions.

Although the uniaxial tensile testing method for assessing the stamping forming property of the sheet metal is basically the same as the traditional one, the contents and meanings of the tensile testing value are different from those of the traditional one.

According to different characteristics of the stamping properties related to corresponding test values, the test results are divided into three types, namely, mechanical, deformation properties and stress-strain relationship.

(1) Mechanical properties

σs - yield strength

σb - ultimate strength

σs /σb - ratio of yield strength to ultimate strength

(2) Deformation properties

δ - total percentage elongationδu –homogeneous percentage elongation

δs - yield percentage elongation

r - plastic strain ratio, ‘r value’ for shortψ - area reduction ψu - homogeneous area reduction

n - Hardening coefficient, ‘n value’ for short

E - Elastic modulus

(3) Properties of the stress-strain relationship

Above various uniaxial tensile testing values related to the stamping properties of the sheet metal express the material properties in different plastic deformation stages. Their relationships are shown in Table 1.6.

Table 1.6 Characteristics of uniaxial tensile test

 

Stages of tensile test

Mechanical properties

Deformation properties

Stress-strain relationship

Start tension      

↓     Elastic modulus E

Elastic deformation

Yield strength σs

Yield percentage elongation δs

 

↓   Plastic strain ratio r  

Plastic deformation

  Homogeneous percentage elongation δu

Hardening coefficient n

↓   Homogeneous area reduction ψu

Hardening curve

NeckingUltimate

strength σb

   

↓   Total percentage elongation δ

 

Damage  Area reduction ψ  

Figure 1.7 shows the curve of the tension P versus elongation ΔL for uniaxial tension, it is usually called tensile curve. The nominal stress is σ=P/F0, the ‘relative

elongation is δ=ΔL/L0, where F0 is the original area

and L0 is the original length of the specimen. Another types of tensile curve expressed by σ versus δ can also be obtained. These two kinds of curves are practically the same, the only difference is that the scales of the axis are different.

The actual deformation situations of the specimen at various stages in Fig. 1.7 are shown in Fig. 1.8.

Fig. 1.7 Tensile curve

Homogeneous tensile Plastic tensile instability Instability in thickness deformation stage (necking) direction (damage)

Fig. 1.8 Uniaxial tensile testing process of sheet metal

Original state of specimen(before tension)

The above various tensile testing values related to the stamping property of the sheet metal can be obtained from Fig. 1.7. But some values such as n can only be obtained based on analyzing and calculating the original tensile testing data. Now, with the development of the computer technology, the values of the tension and elongation in tensile experiment can be measured by the force and displacement sensors. After analyzed and dealt with by computer, all the tensile testing values related to the stamping property of the sheet metal can be printed out directly.

Presently, the companies producing and utilizing the sheet metal represent the stamping forming property by values of the uniaxial tensile test. These values are the basis for ordering or checking and accepting the sheet metal. A brief analysis of the main values of the uniaxial tensile test and the relationship between the values and the stamping forming property is given as follows.

The yield strength σS, the ultimate strength σb and

the ratio of the yield strength to ultimate strength σS /σb influence the stamping forming property of the

steel plate in view of the force.

When σS and σb increase, the stamping force

increases, and the contact compression between the sheet metal and the die surface also increases. This results in an increase in the stamping force as well as the forming difficulty, and a decrease in die life.

In addition, when stripping the forming workpiece from die and unloading, high yield strength σS causes large springback.

This would influence the dimension precision of the forming workpiece. σS and σb can be calculated

conveniently by the measuring results of the tensile curve in Fig. 1.7.

Furthermore the surface of the forming workpiece may be scratched (caused by the die surface bonding failure) and resulting in producing waste.

The percentage elongation δ and the homogeneous percentage elongation δu are the

parameter showing the plastic deformation capacity of the sheet metal. They influence the forming property in view of the plastic deformation capacity.

The homogeneous percentage elongation δu is the percentage elongation when local necking just appears, or instability of plastic deformation in sheet metal just occurs.

After necking, the plastic deformation concentrate

s on the necking section. This causes not only the err

or of the shape and dimension of the stamping formi

ng workpiece would surpass the acceptable range, b

ut also the damage would take place quickly. Theref

ore, the homogeneous percentage elongation δu is us

ually the main parameter to determine the stamping

property of the sheet metal.

High homogeneous percentage elongation δu of the sheet metal is conducive to all kinds of tensile forming.

Experimental results show that the larger the homogeneous percentage elongation δu, the larger would be the limits in tensile forming, such as bulging, flanging and expanding. Therefore most of the high quality sheet metal for stamping forming own high homogeneous percentage elongation δu.

r is also called the ratio of the plastic strain. It is

a parameter to express the aniso’tropic property of

the sheet metal.

Because the sheet metal undergoes rolling and annealing processes during manufacturing, hence the crystal orientation of the texture organization tends to approach uniformity. The sheet metal shows ani’sotropy in the macro-view, that is, the property of the sheet metal is different in different orientations. The anisotropy of the sheet metal greatly influences its stamping forming property.

For anisotropic sheet metal, in tensile test, the strain in the width direction is not equal to that in the thickness direction, and these two strains do not equal to half of the strain in the length direction. The larger difference between strains in width and thickness directions, the more severe anisotropic would be the sheet metal.

In practice, the anisotropy of the sheet metal is expressed by r, which is the ratio of the strains in width and thickness directions expressed by the logarithmic strain, and can be calculated by Equation 1.3.

0 0/ ln / / ln /b tr b b t t (1-3)

For isotropic sheet metal, its property is the same

in any direction.

In the uniaxial tensile test, the strain in width direction εb is equal to the strain in thickness direction εt.

Both strains are half of the longitudinal strain εl, so r

is equal to 1 for any homogeneous material. When r is greater than 1, εb is larger than εt in the uniaxial ten

sile test, showing that the deformation in thickness direction is more difficult than in width direction.

According to the constant volume condition of the plastic deformation, Equation 1.3 can be rewritten as follows:

/b l br (1-4)

In order to improve the measuring accuracy when using the tensile test to measure the r value of the sheet metal, instead of measuring the strain in the thickness direction directly, the strains in the width and length directions are measured by an extensometer or a displacement sensor, and then the r value can be obtained by Equation 1.4.

The anisotropy of the sheet metal (r value) mainly influences the property of the deep drawing.

Both theoretical analysis and practice result confirm that the larger the r, the better would be the property of the deep drawing.

Figure 1.9 shows the influence of r on the limiting drawing ratio (LDR) of the cylindrical shell parts.

Fig. 1.9 Effect of r on LDR of the cylindrical shell parts

In addition, large r is beneficial to the stamping forming of the curved parts with inclined sidewall (hemispherical, cone shaped and parabolic parts, and the panel part of the automobile). When forming these parts, the tensile force would cause the tensile strain and the transverse compressive strain perpendicular to the tensile strain in the sidewall of these parts.

The larger the r, the greater would be the compressive strain, and it is more beneficial to the displacement of the material moving towards the surface of the punch, that is, more easy to perform the forming process under the tensile force.

Experiment show that r almost remains constant during uniaxial tensile test, so the value can be measured at any tensile strain.

Generally, the value is measured when tensile strain is between 15%~20% in order to eliminate the measuring error.

For anisotropic sheet metal, if the specimen is cut in different directions, the values of the uniaxial tensile test are also different.

The phenomenon of the property of the sheet metal being related to the direction on the plate plane is called in-plane anisotropy.

Generally, the in-plane anisotropy is harmful to stamping forming. The in-plane anisotropies for different materials are different.

Usually the rolling direction is regarded as the datum when estimate the anisotropy of the sheet metal, and this direction is called 0˚ direction. The property parameters of 45˚and 90˚ directions are compared with the datum.

Among various parameters of the tensile test, the influence of anisotropy expressed by r on stamping forming property is the most important.

When the difference Δr between r in different directions being used to express the in-plane anisotropy, it can be calculated as follows:

0 90 45/ 2 r r r r (1-5)

The larger the absolute value of Δr, the greater would be the in-plane anisotropy of the sheet metal.

Δr is ‘relevant directly to the height and orientation of the earring in the cylindrical deep drawn workpiece.

The larger the absolute value of Δr, the higher would be the earring height. When Δr >0, earring occurs in the directions of 0° and 90°; when Δr<0, earring occurs in the direction of 45° (see Fig. 1.10 and Fig. 1.11).

Fig. 1.10 Effect of the in-plane anisotropy of sheet metal on the earring height

－ average height of cylindrical deep drawn workpiece h

－ average value of r,

r 0 45 902 / 4r r r r

Fig. 1.11 Relationship between earring orientation and Δr

n value is also called work hardening coefficient,

or hardening ‘index.

In stamping technique, n represents the property that deformation resistance (strength) of the material increases with increasing deformation during cold deformation procedure. Its influences on stamping forming property of sheet metal are diverse, and the mechanism of the influence is rather ‘complicated. So the research work on n value is also quite active.

If changing the y-coordinate of the tensile curve

in Fig. 1.7 to the real stress ( ) and the x-

coordinate to the real strain expressed by the

logarithm ( ), the hardening curve as shown

in Fig. 1.12 can be obtained.

PF

0ln

LL

Although the hardening curve can express the hardening property of sheet metal clearly and directly, it’s not convenient in practice.

Fig. 1.12 Hardening curve

To meet the demands for practical analysis and theoretical calculation, presently the modeling method is widely used, in which the hardening curve is expressed approximately by the mathematic equation.

The linear expression is very convenient in calculation, but the error between the real curve and the expression is large. As a result, the calculation error is also large and the calculation results are not quite applicable.

Usually, the following power function is adopted to simulate the hardening curve in stamping forming.

where, C is the strength coefficient, n is the hardening coefficient, also known as n value.

nC (1-6)

C represents the strength property of the sheet metal, its value depends on the types of the material.

n is the work hardening coefficient of the sheet metal during plastic deformation, and is closely related to the stamping property of the sheet metal.

The influence of n on the shape of the hardening curve is shown in Fig. 1.13.

When n is small, percentage of plastic deformation, the material is in the saturated state of work hardening. This is harmful to the spreading of the local deformation, resulting in excessive concentrated deformation and the damage of the workpiece . Therefore large n is necessary for tensile forming.

Fig. 1.14 shows the effect of n on Erichsen value, which represents the bulging property.

Fig. 1.13 Effect of n on hardening curve shape

Fig. 1.14 Effect of n on Erichsen value

Both the work hardening coefficient n and the str

ength coefficient C can be obtained simply by calc

ulating the measuring data of the real stress

and the real strain in the uniaxial tensile test.

PF

0ln

LL

By selecting the real stresses and strains of the two points in the uniaxial tensile test curve, that is, two groups of (σ1, ε1) and (σ2, ε2), the work hardenin

g coefficient n can be calculated according to Equation 1.6.

Lo’garith’mizing Equation 1.6, we obtain:

Substituting σ1, σ2, ε1 and ε2 into Equation 1.5, we

obtain:

ln ln lnC n (1-7)

2

1

2

1

ln

lnn

(1-8)

By substituting n into Equation 1.6 or Equation

1.7, the strength coefficient C can then be calculated.

Inasmuch as the rule of the work hardening described by the expression of σ=Cεn after modeling is not exactly the same as the practical hardening curve, the former itself is an approximate curve, and the positions of the two points selected on the curve influence the n value.

In order to reduce the calculation error, these two points should be located at the homogenous tensile stage (stable plastic deformation stage) of the curve with the distance between them as large as possible.

Thank You Thank You ！！