Failure Analysis and Simulation of Sheet Metal Analysis... · 2018. 5. 12. · minimal material scrap [3]. In this process, a flat sheet metal was kept under a blank holder force

International Journal of Engineering Research and Reviews ISSN 2348-697X (Online) Vol. 6, Issue 2, pp: (30-38), Month: April - June 2018, Available at: www.researchpublish.com

Page | 30 Research Publish Journals

Failure Analysis and Simulation of Sheet Metal

Products during Deep Drawing Process - A Review

Siddharaj V. Kumbhar

Solapur University, Solapur, M.S., INDIA

Email: [email protected]

Abstract: Sheet metal forming is an important process in most industries where large sized products are deep

drawn for obtaining the complex shape of products at higher productivity rates. During deep drawing process

industries often experience failures viz. wrinkling, fracture, tearing and earing of the sheet metal. Different aspects

of these failures are discussed in this paper so as to minimize these failures. The methodology followed is by

adopting theoretical design, computational analysis using software package ANSYS and corresponding

experimental investigations. A particular set of industrial data is used for this analysis which helps to evaluate the

causes of failures. It is determined that low pressure leads to wrinkling on/nearby the flange area; while excess

pressure causes sudden fracture or tearing through the sides/wall surface. Also optimum pressure is determined

experimentally for standard commercial products so as to avoid the failures during the process. The simulation

results of ANSYS are in conformance with experimental data showing the maximum thickness reduction ratio and

the maximum depth of draw. Failures can be predicted for different designs of products from simulated values.

Keywords: Sheet metal; Deep drawing process; Optimum pressure; Wrinkling/Fracture Failure; Simulation in

ANSYS.

1. INTRODUCTION

In sheet metal industries, where products are manufactured by adopting processes right from melting of scrap to cold

working and up to press operation, annealing is one of the important processes. Annealing has to be done properly to

avoid failures at various intermediate stages of cold working processes. The use of trial and error methods is very

expensive with regard to both money and time along with material loss.

The importance of this project lies in its effectiveness of reduction of trial and error methods, material wastage, and time

by determining the optimum pressure to deform the sheet metal to form the desired shape without any failures. This is

possible because of analysis of the problem using ANSYS which eliminates all the disadvantages associated with

conventional methods.

The problem occurring during the press operations of aluminium sheet metal to get the desired shape is that the worker

has to conduct the trial and error method to get the correct amount of pressure at which sheet metal deforms to desired

shape without failures such as necking, wrinkling, earing or shell fracture. This increases wastage of sheet metal blanks,

time and labor costs.

The objective of the project is to determine the optimized pressure for deformation of sheet metal and compare with

theoretical and practical results. This would result in formation of utensils without any failures or fractures like necking,

wrinkling or earing with saving of time and money. Also the failures can be analyzed by using ANSYS.

The Gajanan Industries, Solapur currently performs the operations by trial and error methods for each batch of production

until there are no failures associated with the formed product. These failures may be like wrinkling, cracking or earing.

There is no standard pressure set for any of the product and hence trial and errors are performed for obtaining successful

draws.

mailto:[email protected]



For the theoretical approach of the study as in [1-2], the design process of the punch and die and the pressure required, is

adopted. Sheet metal forming is basically conversion of flat sheet metal into a product of desired shape without defect like

fracture or excessive localized thinning. Sheet metal is characterized by high surface to thickness ratio. The drawing of

sheet metal or commonly known as deep drawing is a process in which a punch is used to force a sheet metal to flow

between the surfaces of a punch and die. As a result, a cylindrical, conical or box-shaped part is formed in the die with

minimal material scrap [3]. In this process, a flat sheet metal was kept under a blank holder force (BHF). The blank holder

should allow the material to slide into the die surface but at the same time, that force must be a sufficient enough to

prevent wrinkling of the sheet.

Excessive punch force would result in shell fracture suddenly as it increases the load on the blank, causing the shell to tear

or fracture once it exceeds the material plastic limit. The fracture toughness and allowable flaw size of material decreases

with the increase of the materials yield strength. Wrinkling is also one of the major defects.

Wrinkling is a kind of buckling phenomenon that prevents from forming of the sheet [1-3]. If the buckling take place in

flange area, it is called puckering and it take place on the wall of the cup. Earing is one of the defects which is commonly

observed in deep drawing process, defined as the formation of waviness on the top of the drawn cup.

Several studies are available in open literature on modeling and analysis of bending simulations. This includes analytical,

empirical and Finite Element Analysis (FEA) of various bending processes.

From [4], the FEM results are in good qualitative agreement with the experimental findings. To improve the robustness of

the deep drawing process it is suggested to make the die with a circular profiled die land in place of the cylindrical die

land, because by making the die land circular profiled, a slight tilt of the die will only give rise to minor changes in the

contact conditions in the die land and wall interface.

Mathematical models for plain-strain sheet bending to predict springback, bendability, strain and stress distributions and

the maximum loads on a punch and a die are used [5].The accuracy of sheet metal formation depends on many factors like

yield stress, temperature and die rigidity etc. Also nature of load affects the accuracy of the deep drawn objects [6].

Areas of high strain in the press formed component can be located and by studying the development of the strains during

the press forming the cause of the strains can be found [7]. Finite elements method is more accurate than the experimental

method in predicting the best die design, and a good match between the two methods can be found [8]. Experimental and

numerical results show that blank holder and die shapes influence punching and blank holder forces, drawing ratio and

material flow [9]. Optimization of process parameters [10] – punching tonnage, the sheet thickness, the sheet length and

the sheet width is necessary for sheet metal forming.

In hydroforming deep drawing process [11], the uniform deformation of blank is obtained to get a required shape and also

blank failure is prevented during deformation due to fluid pressure and blank holding pressure being equal. The wrinkling

is reduced in the blank due to the support of high pressurized viscous fluid.

Wrinkling in a drawn part consists of a series of ridges that form radially in the undrawn flange of the work-piece due to

compressive buckling. Wrinkling in the wall is when the wrinkled flange is drawn into the cup; these ridges appear in the

vertical wall. Tearing is an open crack in the vertical wall, usually near the base of the drawn cup, due to high tensile

stresses that cause thinning and failure of the metal at this location. This type of failure can also occur as the metal is

pulled over a sharp die corner. Earing is the formation of irregularities (called ears) in the upper edge of a deep drawn

cup, caused by anisotropy in the sheet metal. If the material is perfectly isotropic, ears do not form [12].

Finite element method is used to minimize the impact earring piece. The optimum blank shape design for deep drawing of

cups with a uniform allowance at the flange is determined. The non-uniform flange is caused by non-uniform material

flow in the sheet [13].

2. FAILURE ANALYSIS – DIFFERENT CASES

2.1 Effect of Blank Holder Force (BHF)

Sheet metal forming is basically conversion of flat sheet metal into a product of desired shape without defects like fracture

or excessive localized thinning. Sheet metal is characterized by high surface to thickness ratio. The drawing of sheet metal

or commonly known as deep drawing is a process in which a punch is used to force a sheet metal to flow between the

surfaces of a punch and die. As a result, a cylindrical, conical or box-shaped part is formed in the die with minimal



material scrap [1]. In this process, a flat sheet metal was kept under a blank holder force (BHF). The blank holder should

allow the material to slide into the die surface but at the same time, that force must be a sufficient enough to prevent

wrinkling of the sheet.

Excessive punch force would result in shell fracture suddenly as it increases the load on the blank, causing the shell to tear

or fracture once it exceeds the material plastic limit. The fracture toughness and allowable flaw size of material decreases

with the increase of the materials yield strength. Wrinkling is also one of the major defects.

Wrinkling is a kind of buckling phenomenon that prevents from forming of the sheet. If the buckling takes place in flange

area, it is called puckering and it takes place on the wall of the cup. Earing is one of the defects which are commonly

observed in deep drawing processes, defined as the formation of waviness on the top of the drawn cup. Several studies are

available in open literature on modeling and analysis of bending simulations. This includes analytical, empirical and finite

element analysis (FEA) of various bending processes.

Ismail M. S. B. [1] investigated the effect of friction between blank and punch in deep drawing process. It is observed that

higher blank diameter and BHF raises the value of limiting draw ratio (LDR) and thus increases the drawability of

aluminium AA1100 sheet and copper sheet. For forming limit diagram (FLD) test, the level of FLD is increasing with

increase of blank thickness of aluminium AA1100. The use of lubricant also raises the level of FLD and thus lessens the

tendency of aluminium AA1100 sheet to rupture. It is found that the formability of sheet metal is increasing due to

increase of blank diameter, blank thickness, blank holder force (BHF) and use of lubricant. Also the formability of

aluminium AA1100 sheet is better as compared to copper sheet.

2.2 Effect of die shape

From the study by Faris S. T. [2], the FEM results are in good qualitative agreement with the experimental findings. To

improve the robustness of the deep drawing process, it is suggested to make the die with a circular profiled die land in

place of the cylindrical die land, because by making the die land circular profiled, a slight tilt of the die will only give rise

to minor changes in the contact conditions in the die land and wall interface.

In order to investigate if a slight tilt of the die land cup give rise to a cup height variation, the deep drawing and ironing

process was simulated using the FEM through program code (ANSYS 5.4) . In Figure (3) is shown the FEM model.

In Fig. 2 the equivalent strain distribution in the deep drawn and ironed cup with the cylindrical die land is shown. It can

be seen that a slight tilt of the punch does give rise to an uneven cup height and an uneven strain distribution. Fig. 3 shows

the equivalent plastic strain distribution in the deep drawn and ironed cup with the circular die land. It is clear that initial

blank thickness at the region of a flat bottom face of the punch does not change. This is because the flat face of the punch

is in contact with blank and with drawing force; friction comes in to play which prevents any deformation of the metal

under the punch.

Fig. 1: FEM model of the punch and die in mesh [2]



Fig. 2: Cup with the cylindrical die land showing the plastic strain distribution with tilt angle between die and

punch of 0.4 degree [2]

Fig. 3: Cup with the circular die land showing the plastic strain distribution with tilt angle between die and punch

of 0.4 degree [2]

The FEM simulations show that when the deep drawing and ironing is carried out with a conventional cylindrical die

land, a slight tilt of the die land in relation to the punch can give rise to variations in the wall thickness and in the cup

height. Also the cup quality (cup wall thickness and height) is nearly unaffected by a slight tilt of the die in relation to the

punch when the die is made with a circular profiled die land in place of the cylindrical die land.

2.3 Stress-Strain development for blanking and deep drawing

Mathematical models for plain-strain sheet bending to predict spring back, bendability, strain and stress distributions and

the maximum loads on a punch and a die are used, Kaonga M. [3]. The location of the critical areas on the workpiece

were obtained by using contour plots of the stress and strain for the blanking, deep drawing and bending simulations were

in good agreement with the theory and experimental results. The results obtained from coupling deep drawing and

bending processes showed that the processes are the major strain contributors in a production line. Blanking has less or no

impact on subsequent processes.

a. b.

Fig. 4: Contour Plots for a Blanking Process showing

(a) von Mises Stresses and (b) Strains, [3]



The contour plots shown in Fig. 4 (a & b) illustrate the evolution of the damage field and the propagation cracks in this

field. It is evident in both plots that the stress and strain in the damage zone increases towards the shear line. The stresses

in the material close to the cutting edges reach a value corresponding to the material shear strength (350MPa). The

maximum strain occurred at the middle of the blank along the shear line.

a. b.

Fig. 5: Contour Plots for a Deep Drawing Process showing

(a) von Mises Stresses and (b) Strains, [3]

Fig. 5 shows the contour plot for the deep drawing process. Both the strain and stress plots show that the blank became

thicker at its outer portions as it was forced into the cavity. This is normally observed in deep drawing. As the punch

forces the blank into the die cavity, the blank diameter decreases and cause the blank to become thicker at its outer

portions due to circumferential compressive stresses to which the material elements in the outer portion is subjected. The

von Mises contour plot shows that these circumferential compressive stresses reached 360 MPa. The contour plots shown

in Fig. 5 (a) show a maximum Von Mises stress of 360 MPa which is below the ultimate tensile strength of 420 MPa for

the blank and above the initial yield stress of the material of 282 MPa. This confirms that forming process (deep drawing)

occurred within the plastic region. It can also be seen that the portion of the blank between the die wall and the punch

surface underwent some considerable tensile force and tended to stretch and become thinner. Similarly, the portion of the

formed cup which wrapped around the punch radius was strained under tension in the presence of bending as shown in

Fig. 5. This portion becomes the thinnest portion of the cup and usually is the first place to fracture.

2.4 Contact pressure distribution between punch-sheet-blank holder-die

The accuracy of sheet metal formation depends on many factors like yield stress, temperature and die rigidity etc. Also

nature of load affects the accuracy of the deep drawn objects (Nimbalkar et al.) [4]. Simulation of elastic-plastic behavior

of mild steel sheet is carried out with non-linear condition to investigate sheet metal forming process to compare

experimentally available data. The contact elements are generated between sheet metal and the dies for forming process.

Displacement convergence is considered in the problem to get correct shape. The critical stresses of radial, hoop and von

Mises stresses are represented for the problem along with contact pressure distribution.

a. b.

Fig. 6: Punch, sheet, blank holder and die assembly showing (a) Contact Pressure

(b) Contact pressure distribution, [4]



Results in Fig. 6 (a) & (b) shows increased punch loads with inclination of blank holder. An inclination of 20

gives the

best stress distribution in the sheet metal. Higher blank holder forces which induces interference between sheet metal and

blank holder also reduces the load on punch, but increases the stress generation in the sheet metal which is not desirable

for error free sheet metal formation. By inclusion of blank holder angle, the contact pressure is reducing at the blank

holder showing reduced load on the blank holder. But increase in punch load can be observed. As the blank holder angle

increases, the load on blank holder is reducing but after certain angle, punch loads changes are less as the blank holder is

not playing any role on load sharing.

2.5 Effect of die and blank holder shapes

The effect of die and blank holder shapes in deep drawing process are investigated by Malekani et al. [7], which influence

punch and blank holder forces, drawing ratio and material flow. Fig. 7 shows punch load vs. displacement, where increase

in slopes of die and blank holder shows better condition of drawing and improved process quality and also maximum

depth of drawings.

Fig. 7: Punch load vs. punch displacement [7]

Fig. 8: Blank holder load vs. punch displacement [7]



Fig. 9: Stress distribution in blank for different die and blank holder [7]

Fig. 8 shows the effect of die and blank holder shape on the optimum blank holder load. Wrinkling is caused due to less

BHF and tearing due to excessive BHF. Also increase in slope of tool lowers the BHF for similar stress distribution as

seen in Fig. 9.

2.6 Failures in sheet metal during deep drawing process

The study by Kumbhar S. V. et al. [12] describes different failure criteria for sheet metal during deep drawing process.

The excess pressure (punch & BHF) leads to cracking failure while low pressure (BHF) form wrinkles on the flange area.

The maximum depth of deep draw is estimated for the product desired for standard sizes. The pressure ranges for different

failures – wrinkling and cracking, are determined as shown in Table 1.

Table 1- Pressure ranges for standard products for failures and successful draw [12]

The blank used for deep drawing process is simulated in ANSYS for determination of failure regions from stress

distribution. The mesh plot and stress distribution are shown in Fig. 10 & 11, giving the maximum displacement and

regions of failure of blank while deep drawing.

Fig. 10: Mesh plot showing blank in meshing, with maximum displacement in vertical downward direction as

408.319 mm [12]

Sr. No. Type of Product Pressure Range (kg/cm

2)

WRINKLING SUCCESSFUL DRAW CRACKING

1. 30” Cylindrical POT 600 – 660 700 800 – 850

2. 28” Cylindrical POT 400 – 450 500 550 – 600

3. 24” VESSEL 300 400 420 – 500

4. 36” Cylindrical POT 550 – 600 610 – 750 750 – 800



Fig. 11: Stress distribution in blank showing region of failure in red colour [12]

3. CONCLUSIONS

From various studies it can be concluded that deep drawing process is influenced largely by – blank holder force, die

shape, fracture due to thinning, strain development around cutting edges, contact pressure between punch-sheet-blank

holder-die, blank holder shape, etc. There are many more factors which do influence the drawing process and not only

limited to these major ones. Also these parameters contribute to the failure of sheet metal during deep drawing process

leading to wrinkling, cracking/tearing, earing, types of fractures. The pressure need to be maintained and optimized

depending on the material type, thickness, size, shape, friction, lubricant used, hardness of material and temperature

conditions. Many failures like cracking/tearing may be predicted by simulation in advance of prototyping. This will

minimize trial & error methods and lead to maximize the savings of time, money and labour. The optimization of

pressure/force must be done to get error free deep draw process and good quality product without any fractures induced.

REFERENCES

[1] Muhammad Safwan Bin Ismail, 2010, “Experimental Study of Formability of Sheet Metal in Deep Drawing

Process”; Malaysia University Pahang.

[2] Saad Theyyab Faris, June 2009, “Study of the Stress And Strain Distribution during Deep Drawing Process”; Diyala

Journal of Engineering Sciences, Vol. 02; pg. no. 80-95.

[3] Mathews Kaonga, Nov. 2009, “Simulation and Optimization of a Full Deep Drawing Process”; the University of

Zambia.

[4] Laxmiputra M Nimbalkar, Sunil Mangshetty, Oct. 2012, “Analyzing the Effect of Blank Holder Shape in Deep

Drawing Process Using Fem”; International Journal of Engineering Research and Development, Volume 4, Issue 1;

pg. no. 23 – 28.

[5] R.Uday Kumar, Feb. 2013, “Analysis of Major Strains and Minor Strains in Sheet Metal Forming”; International

Journal of Application or Innovation in Engineering & Management (IJAIEM), Volume 2, Issue 2; pg. no. 194 –

198.

[6] Kadhim M. Abed, “A Design Calculating System for Deep Drawing Die by using Simulation Model”;

2011,Thi_Qar University Journal for Engineering Sciences, Vol. 2, No. 4.

[7] J. Malekani, A. M. Pour, M. Eskandazade, and A. Totonchi “Numerical and Experimental Investigation of Die and

Blank Holder Shape in Deep Drawing Process”; 2008, Journal of Applied Sciences, ISBN 1812-5654.

[8] Mohd. Amiruzamin B Mohd Shaufi, “Optimization of Process Parameters in Sheet Metal Forming By Using

Taguchi Method”; Nov. 2008, Universiti Malaysia Pahang.



[9] R.Uday Kumar and P. R. Reddy, “Influence of Viscosity on Fluid Pressure in Hydroforming Deep Drawing

Process”; 2012, International Journal of Mechanical Engineering and Technology (IJMET), pp. 604-609.

[10] R. V. Reddy, T.A. Janardhan Reddy and G.C.M. Reddy, “Effect of Various Parameters on the Wrinkling In Deep

Drawing Cylindrical Cups”, International Journal of Engineering Trends and Technology- Volume3 Issue1- 2012.

[11] J. Wang, A. Goel, F. Yang and J-T Gau, “Blank optimization for sheet metal forming using multi-step finite element

simulations”, International Journal Advance Manufacturing Technology; volume-40 ( 2009); pp-709-720.

[12] Kumbhar Siddharaj V. and Sonage B. K., “Failure Analysis of Sheet Metal Utensils during Deep Drawing Process”,

Journal of Mechatronics and Automation; Vol.3 Issue 3, 2016, pp. 27-33.

Documents

Failure Analysis and Simulation of Sheet Metal Analysis... · 2018. 5. 12. · minimal material scrap [3]. In this process, a flat sheet metal was kept under a blank holder force