experimental investigation and analysis … the shape of sheet metal through plastic deformation...

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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 8, Issue 11, November 2017, pp. 252–264, Article ID: IJMET_08_11_028

Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=11

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

EXPERIMENTAL INVESTIGATION AND

ANALYSIS OF TENSILE AND FORMING

BEHAVIOUR OF COLD ROLLED SHEET

METAL

K. Logesh

Research Scholar, Department of Mechanical Engineering,

Sathyabama University, Chennai, TamilNadu, India

V. K. Bupesh Raja

Professor & Head, Department of Automobile Engineering,

Sathyabama University, Chennai, TamilNadu, India

D. Gokula Krishnan, Md Azeemudeen M, A. Nikhil Andrew and K.S. Hariprasath

UG Scholar, Department of Mechanical Engineering,

Veltech Dr.RR & Dr.SR University, Chennai, Tamil Nadu, India

ABSTRACT

Automotive and aircraft industries extensively use sheet metals for creating body

sheel and instruments panels. Improper handling during the production process and

misapplied load would result in defects such as wrinkling, localized thinning and

tearing of the finished part. Literature articles published from past research activities

throws little light on the formation and control of defects in sheet metal during its

working. This paper makes an attempt to understand the formability parameters which

influences the quality of finished part during sheet metal forming process.

DYNAFORM and LS-DYNA 10 were used to simulate and optimize the deep drawing

processes of the sheet metal through Finite Element Analysis. Theoretical results were

validated with experimental work. It was found that the theoretical results were in

similar to the experimental values

Key words: Formability parameters, Simulate, optimize, Deep drawing behaviour,

Finite Element Analysis

Cite this Article: K. Logesh, V.K. Bupesh Raja, D. Gokula Krishnan, Md

Azeemudeen M, A. Nikhil Andrew and K.S. Hariprasath, Experimental Investigation

and Analysis of Tensile and Forming Behaviour of Cold Rolled Sheet Metal,

International Journal of Mechanical Engineering and Technology 8(11), 2017, pp.

252–264.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=11

Experimental Investigation and Analysis of Tensile and Forming Behaviour of

Cold Rolled Sheet Metal


1. INTRODUCTION

1.1. Sheet Metal Forming

Sheet metal Forming is one of the prominent and conventional methods in the manufacturing

industry for converting raw material into finished product. Metal forming is a process of

changing the shape of sheet metal through plastic deformation [1]. After converting solid

metal piece into product form by metal forming processes, the mass as well as volume

remains unchanged. The bulk metal work piece deformed plastically has high volume to

surface area ratio.For example forging, extrusion etc. [2].

In case of sheet metal forming there is a high ratio of surface area to thickness. Some

typical examples are some of the home appliances like fridge, washer, DVD players etc. The

designing of dies, selection of sheet materials and lubricants is still more of an art than a

science [1].

1.2. Deep Drawing Process

Deep drawing is a method of forming under compressive and tensile load conditions, whereby

a sheet metal blank is transformed into a hollow cup. Hollow cups are formed by placing a

blank of the optimized size in the vicinity of the die and the punch wherein the cup is formed

by pressing the blank in the die with the help of the punch. The outer periphery of the die

which supports the blank holder is termed as a flange [3]. It is desirable to avoid the forming

defects during deep drawing and there is a need to know the mechanism of forming [4].

In a deep drawing process, only the punch force is the dependent variable. The prominent

independent variables are given below:

The material properties of the sheet metal

The ratio between blank diameter and punch diameter.

Thickness of sheet material.

The clearance between the punch and the die.

Punch and die corner radii.

Friction & lubrication at the punch, die and workplace interfaces.

During the course of deep drawing, the following five processes take place:

Pure radial drawing between die and blank holder with one principal strain tensile in nature

and the others in compressive nature [5].

Bending and sliding over the die profile.

Stretching between the die and the punch (Biaxial tensile strain).

Bending and sliding over the punch profile radius.

Stretching along the punch bottom (Biaxial tensile strain).

1.3. Formability Problems

The ultimate defect in a formed sheet metal chip or part is the development of crack which

destroys its structural integrity and finishing of the material. The usefulness of the part may

also be destroyed by local thinning , necking or by buckling in regions of compressive stress

[6]. Some of the problems are: Fracturing, Buckling, Shape distortion, loose metal.

K. Logesh, V.K. Bupesh Raja, D. Gokula Krishnan, Md Azeemudeen M, A. Nikhil Andrew and

K.S. Hariprasath


1.4. Formability Tests

There are different types of tests to examine the formability for a deep drawing process some

of them are: tensile test, cup test, hemispherical dome test and Fukui conical cup test.

2. OBJECTIVE OF STUDY

The combination of DYNAFORM-PC and LS-DYNA is used for finite element analysis of

the sheet metal [1].The objectives of this study include:

Determination of weak areas,

Percentage reduction in the blank thickness,

Stress and strain distribution,

Forming Limit Diagram for the blank material,

Failures in sheet metal formability, etc.

2.1. Need for Study

The need of the study about optimization of sheet metal forming includes:

Reduction in lead times of forming,

Reduction in number of die-punch sets used,

Reduction in experimentation work, etc.

3. MATERIAL

The cold rolled steel sheet JED 657 of thickness 2.5 mm is considered for the study. The

actual chemical composition of the sheet is presented in the Table 1. [3].

Table 1 Chemical composition of the cold rolled sheet JED 657

Composition C Mn S P

Wt. % 0.066 0.347 0.006 0.012

4. EXPERIMENTAL PROCEDURE

4.1. Finite Element Modeling and simulation

Determination of material properties is done through tensile test and the pre-processing which

includes meshing, defining of tools, material definition and providing boundary conditions for

deep drawing of the drum brake cylinder is elaborated. DYNAFORM is used as a design tool

for the punch, die, binder and blank and is than exported in IGES format which in turn is used

as an import file for LS-DYNA pre-processor [6-7].

4.1.1. Finite Element Model Building

DYNAFORM software can be used to develop punch, die, binder and sheet blank for a sheet

metal forming operation. During the design of punch and die, an appropriate clearance is to be

provided between them. This clearance should be greater than sheet thickness and can be

obtained from equation [8].

c = t + K√ (1)




Where,

c =clearance

t=material thickness

K = coefficient

4.1.2. Die

A die is a specialized tool used in manufacturing industries to fissure or tailor material using a

press. Like molds, dies are generally customized and specialized to the item they are used to

create.

Create a center line by selecting Pre-process -> Line/Points -> Line. Type the x, y, z

coordinates to draw the line by mentioning the end points.

Create horizontal and vertical lines as per the dimensions of the outer profile of the die with

the same options as mentioned above.

Select Pre process -> Line/Points -> Circle, draw an arc tangent to the two lines with Tangent

to two lines option and select the two lines and type the radius of the arc as per the profile of

die and click Ok.

Select Pre process -> Line/Points -> Combine and combine the lines forming die profile.

Select Pre process -> Surface -> Revolve, choose points and select the centre line as the axis

of die and then select combined line. Click Ok.

Select Part from the main menu and type PUNCH, and then click Ok.

4.1.3. Punch

Select Part -> current; in the option bar choose PUNCH to make current working part as

punch. Select Part -> Turn on/off; in the option bar choose PUNCH to make other parts

invisible. Repeat the steps in the modeling of die as per the dimensions of the punch to create

a punch. Select Part from the main menu and type BLANK. Then click Ok.

4.1.4. Blank

Select Part -> Current in the option bar and choose BLANK to make current working part as

blank. Select Part -> Turn on/off; in the option bar choose BLANK to make other parts

invisible.

Create a centerline by selecting Preprocess -> Line/Points -> Line. Type the x, y, z coordinates

to draw the line by mentioning the end points.

Select Preprocess -> Line/Points -> Circle, draw a circle by typing radius with Center radius

option and then click Ok.

Select Tools -> Define Blank and select the part blank, then select Tools -> Blank generator

and the pick the draw circle for the blank surface to be generated.

Determine the Blank material. Select Part from the main menu and type BINDER, and then

click Ok.

4.2. Tensile Test

Material properties like young‟s modulus (E), yield stress, strain hardening factors,

anisotropic factor and the true stress versus true strain curve are some of the basic input

factors and parameters required. The basic input parameter required for deep drawing process

need to be procured and get the result from Tensile Test and this test is the mostly used as per


K.S. Hariprasath


the ASTM E8/E8M standards shown in Figure 1. A Test specimen of 50 mm long and

12.5mm wide is used [8].

Figure 1 A Tensile test specimen strip as per ASTM E8/E8M [8]

Tensile test specimens were prepared along three directions, with the specimen taken such

that they are aligned 0◦, 45

◦ and 90

◦ to direction of rolling of the sheet. These samples were

tested in a universal testing machine and the result was noted down. The prepared specimens

are shown in Figure 2.

Figure 2 Specimen prepared for tensile test

4.2.1. Tensile Properties

The standard tensile properties like yield stress (σy) and ultimate tensile stress (σu) can be

found out from the stress versus strain data. The engineering stress-strain curve does not give

the true indication of the deformation characteristics of the material since it is based entirely

on the original dimensions of the specimen. Whereas in real case, the work piece undergoes

appreciable change in cross sectional area [10-11].

σ = s (e + 1)

True strain (ε) was determined from the engineering strain by using the relation:

ε = ln (e + 1)

Where, σ - True stress

- True strain

4.2.2. Strain Hardening Coefficient (n) and Strength Coefficient (k)

The strain hardening coefficient (n) and strength coefficient (k) of the work piece is

determined from the slope of the true stress v/s true strain curve when plotted on the




logarithmic coordinates assuming the uniform plastic deformation. A plot of logarithmic

value of true stress and logarithmic value of true strain up to the maximum load results in a

straight line. The slope of the line „n‟ is strain hardening and strength coefficient „k‟ is the

true stress at ε =1.0. The values of (n) and (k) are calculated from log – log plot and are

recorded [12].

4.2.3. Plastic Strain Ratio

Plastic strain ratio is the resistance of steel sheet to thinning during forming operation. This is

the ratio of the true strain in width to the true strain in thickness of the plastically strained

sheet metal [7].

r = εw/ εt

εw = ln (w/w0)

εt = ln (t/t0) = ln (L0w0/Lw)

Where,

w - change in width

w0 - original width

t - Change in thickness

t - Change in thickness

t0 - original thickness

To determine the (r) value, change in the width and final gauge length of the specimen

should be measured in addition to the initial width and gauge length [8-9]. The value of (r)

varies with respect to the test direction in the anisotropic materials. An average value (rm)

represents the normal plastic anisotropy of the sheet. The (rm) value can be calculated using

the equation and is recorded in Table 2.

rm=[r0 + 2r45 + r90]/4

Where, r0, r45 and r90 are the plastic strain ratio in 00, 45

0 and 90

0 orientation to the rolling

direction.

Table 2 Values of „r‟ and „n'

r0 r45 r90 rm n

1.8653 1.6322 1.8829 1.7934 0.2005

5. RESULT AND DISCUSSION

The post-processing of the two stages of the drum brake cylinder whose simulation was done

using LS-DYNA solver [10,12].

5.1. Post Processing

Post-processing provides the details like stress distribution in all the directions, nodal

displacement, shell thickness at various elements, percentage thickness reduction in the

component, FLD plot for the component, energy levels, elemental view, vector representation


K.S. Hariprasath


of displacement and strain at various elements. Moreover the percentage thickness reduction

of various analysis performed during the study is presented [10] [12].

5.1.1. Stress Distribution

The analysis was performed initially for the first stage and then the component obtained after

the first stage was used as the blank for the second stage for subsequent forming. The stress

distribution in the component after first and second stage Figure 3 is represented [13-14].

Figure 3 Stress distribution analysis

5.1.2. Nodal Displacement

It is essential for the elements to be properly meshed throughout the material at all nodes [13-

14]. The nodal vectors like the displacement viz, rotation, translation are available at the

nodes. When the nodes displace, they drag and take the elements along in a certain manner

dictated by the element formulation. In other words, displacements of any points in the

elements are interpolated from the nodal displacements shown in Figure 4, and this is the

main reason for the approximation and ambiguos nature of the solution [13-14].

Figure 4 Nodal Displacement of Material Analysis.

5.1.3. Shell Thickness

The metal at the center of the blank surface gets attached to the punch surface and in doing so

it is thinned down. The metal in this region is subjected to biaxial tensile stress due to the

action of the punch. Due to the biaxial tensile stress caused by the punch,metal blank gets

drawn radially towards the die cavity. As the blank gets drawn in, the exterior circumference

is continuously decreased and reduced from that of the original blank. This means that it is




subjected to a compressive strain near blank in circumferential direction and tensile strain in

radial direction [14]. As a result of these two principal strains there is continuous increase in

the thickness as the metal moves inward literally. However as the metal passes over and top of

the die radius, it is first bent and then straightened while at the same time subjected to tensile

stress. The representation of shell thickness is shown in Figure 5. This plastic bending due to

the tension leads to the thinning caused by circumferential shrinkage[15-16].

Figure 5 Shell thickness representation

5.1.4. Forming Limit Representation

Sheet metal can be blemished only to a certain stage before local thinning and fracture occur.

This level depends principally on the combination of strains imposed, that is, the ratio of

major and minor strains. The lowest level occurs at or near plane strain, that is, when minor

strain is zero. This information can be plotted graphically and is known as forming limit curve

[15].

For plotting an appropriate FLD curve in LS-DYNA we need to provide some basic

parameters like allowable thinning and anisotropic value to the system. The software provides

the output showing the areas with good quality of forming, cracks and risk of cracks, severe

thinning, wrinkling and wrinkling tendencies [15-18]. FLD representation for the first and

second stage is represented in Figure 6&7.

Figure 6 FLD representation for first stage


K.S. Hariprasath


Figure 7 FLD representation for second stage

5.1.5. Forming Limiting Diagram

FLD representation can be plotted on a graph which is shown Figure 8.

Figure 8 Representation of Forming Limit Diagram

5.1.6. Elemental Representation

The deformation of nodes leads to distortion of the elements. The h-adaptive method is used

to subdivide an element into smaller element whenever an error indicator will show that

subdivision of element will provide improved accuracy. As there is an element restriction in

our case are shown in Figure 9, the number of elements is seized at 9999. [19-25].

Figure 9 Elemental representation of second stage




6. OUTPUT

Since the quarter portion of the component was considered for analysis, as it reduces the

analysis time, the output from the first stage and second stage can be mirrored and displayed

as a whole cup. The output of first and second stage is represented in Figure 10&11. [26-27].

Figure 10 Output from first stage

Figure 11 Output from second stage

Blank material forms the raw material for sheet metal forming. Appropriate blank size

will reduce the raw material wastage and helps in reduction of inventory required [24-25].

The blank diameter can be optimized with the help of simulation technique. The optimized

blank diameter for our component is found to be 316 mm. The structure of blank is shown in

Figure 12. The current blank diameter used is 320mm. So reducing the diameter to 316 mm

will lead to 2.4% saving of material [26-30].

Figure 12 Blank punch diameter


K.S. Hariprasath


Table 3 Simulation results

Simulation was performed changing various parameters like velocity given to the die, die

force and binder force. The maximum thinning level in each case is reflected in the following

Table 3. Considerable reduction in thinning was observed with increase of die velocity to a

certain limit i.e. 500 mm/sec. It was also observed that the component had undergone tearing

with increase in velocity to great extent. The waving of the blank material at the center of the

blank was arrested with the help of appropriate pad which did not affect the thinning in the

component as shown in Figure 13. The stress and the strain for each of the orientation

through which the specimen were cut, i.e., 0o, 45

o and 90

o respectively. The results obtained

are discussed below.

7. CONCLUSIONS

The modeling software DYNAFORM and the finite element analysis software LS-DYNA

were used to model and simulate the sheet metal forming process for the drum brake cylinder

of JED 657 material and 2.5 mm thick and the tensile tests was performed using Universal

Testing machine for the material provided to obtain its material properties and stress v/s strain

curve.

Virtual try outs were performed duplicating the actual process and the blank diameter was

optimized. The optimized blank diameter was found out to be 316 mm and hence 2.4% saving

in material is observed.

It can be observed that the flange of the cup and the small portion of the wall adjacent to the

flange is identified to have wrinkling tendency and hence can be considered as a weak area.

This can be seen from its FLD representation.

Simulations were performed by varying various parameters like velocity for die, die force and

binder cushioning force and their dependency on thickness reduction was observed. It was

observed that there is considerable less thinning in the case where velocity of the die was

specified to be 500mm/sec. With the higher velocity of the die, cracks were observed in the

component and hence leading to failure at the first stage itself.

Since the quarter section of the cup was considered for the simulation purpose, it was

observed that there was waving in the center portion on the blank. It was found that the

waving can be arrested with the use of appropriate pad during the first phase of forming

operation i.e. closing without affecting the shell thickness of the component.

Serial

Number Velocity Die Force

Binder

Force

Percentage

Thinning

1 150 90 30 32.63

2 150 80 20 32.75

3 150 - - 32.89

4 200 90 30 29.93

5 200 80 20 30.12

6 200 - - 30.27

7 300 90 30 25.14

8 300 80 20 25.45

9 300 - - 25.56

10 500 - - 24.39




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