experimental evaluation of coating delamination in vinyl-coated metal forming

Journal of Mechanical Science and Technology 26 (10) (2012) 3223~3230

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-012-0824-6

Experimental evaluation of coating delamination in vinyl-coated metal forming†

Young-ki Son1, Chan-joo Lee1, Jung-min Lee2, Sang-doek Byoen3, Soen-bong Lee4 and Byung-min Kim5,*

1Precision Manufacturing Systems Division, Pusan National University, Busan, 609-735, Korea 2Dongnam Regional Division, Korea Institute of Industrial Technology, 1274 Jisa-Dong, Gangseo-Gu, Busan, 618-230, Korea

3HA Digital Engineering Gr., LG Electronics, Gaeumjeong-Dong, Seongsan-Gu, Changwon-Si, Gyeongsangnam-Do, 642-711, Korea 4Faculty of Mechanical and Automotive Engineering, Keimyung University, 1095 Dalgubeoldaero, Dalseo-Gu, Daegu, 704-701, Korea

5School of Mechanical Engineering, Pusan National University, 30 Jangjeon-Dong, Kumjeong-Gu, Busan, 609-735, Korea

(Manuscript Received October 15, 2011; Revised March 12, 2012; Accepted May 2, 2012)

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Abstract In this paper, a new evaluation and prediction method for coating delamination during sheet metal forming is presented. On the basis

of the forming limit diagram (FLD), the current study evaluates the delamination of PET coating by using a cross-cut specimen, dome test, and rectangular-cup drawing test. Dome test specimens were subjected to biaxial, plane strain, and uniaxial deformation modes. Rectangular cup- drawing test specimens were subjected to the deep-drawing deformation mode, and compression deformation mode. A vinyl-coated metal (VCM) sheet consists of three layers of polymer on the sheet metals: a protective film, a PET layer and a PVC layer. The areas with coating delamination were identified, and the results of the evaluation were plotted according to major and minor strain values, depicting coating delamination. The constructed delamination limit diagram (DLD) can be used to determine the forming limit of VCM during the complex press-forming process. ARGUS (GOM) was employed to identify the strain value and deformation mode of the delaminated surface after the press forming. After identifying the areas of delamination, the DLD of the PET coating can be con-structed in a format similar to that of the FLD. The forming limit of the VCM sheet can be evaluated using the superimposition of the delamination limit strain of the coating onto the FLD of VCM sheet. The experimental results showed that the proposed test method will support the sheet metal forming process design for VCM sheets. The assessment method presented in this study can be used to determine the delamination limit strain under plastic deformation of other polymer coated metals. The experimental results suggested that the pro-posed testing method is effective in evaluating delamination for specific applications.

Keywords: Vinyl coated metal (VCM); PET coating; Delamination limit diagram (DLD); Deformation mode; Coating and forming

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1. Introduction

In the manufacturing process of outer panel of home appli-ance, most parts are formed from sheet metal into the required final shape, and then coated with paint. The curing process in a conventional coating cycle produces harmful substances known as volatile organic compounds (VOCs) or hydrocar-bons. VOCs can cause health disorders in humans and destroy natural vegetation. In addition, after each cycle of the conven-tional coating process, large amounts of waste water, used solvent, and paint sludge are generated. Various laws and acts have been passed by governments around the world in order to prevent pollution from being produced by the paint and coat-ing industry [1]. Vinyl (polymer) coating is an alternative method that may

reduce VOC emissions and prevent the formation of hazard-ous wastes during the coating process. It is a continuous and automated process for coated metal sheets prior to forming parts. The vinyl coating process comprises a number of basis steps, or operations, such as unwinding the metal coil, clean-ing and treating the coil with chemicals, applying the primer, curing the coat, cooling, and finally ageing the coil for ship-ment. The process can control the VOC emissions by elimina-tion of incinerators and a treatment unit for waste water and toxic gases in the production lines. Compared to most of the other painting processes, vinyl coating is an energy-efficient process which has several other advantages, such as low run-ning cost, low transportation and storage costs [2]. The vinyl-coated metal (VCM) sheet has several advantages,

such as low friction, corrosion prevention and excellent glossy appearance. Since many polymers have superior friction prop-erties, the VCM sheet can reduce or eliminate potentially harmful lubricants. The vinyl coating thickness can be accu-

*Corresponding author. Tel.: +82 51 510 3074, Fax.: +82 51 581 3075 E-mail address: [email protected]

† Recommended by Associate Editor Vikas Tomar © KSME & Springer 2012

3224 Y. Son et al. / Journal of Mechanical Science and Technology 26 (10) (2012) 3223~3230

rately controlled during the process. This control ensures con-sistent chemical and mechanical properties in all the sheets. Since the VCM sheet is free from dirt, residual oil, uneven surfaces and other defects, it provides improved resistance against harsh weather and corrosive environments [2]. In spite of all the above advantages, the vinyl coating on a

VCM sheet has to meet industrial requirements. The VCM sheet constitutes of four different layers; the protective film, polyethylene terephthalate (PET) layer, polyvinyl chloride (PVC) layer, and substrate. The vinyl coating has to be free from contact damage, and the adhesive bond has to be main-tained during the sheet metal forming process. The vinyl coat-ing should not be damaged under various deformation modes, such as uniaxial tension, plane strain, biaxial tension, com-pression, and deep-drawing modes. Further, it has to be free from defects, such as delamination, tearing, and development of scratches. However, the widespread use of the VCM sheet is limited by a shortage of analytical and experimental meth-ods to evaluate the delamination limit of the PET layer in the sheet metal forming process. The typical defects in the VCM product during sheet metal

forming are shown in Fig. 1. Moreover, the PET layer be-tween the protective film, PVC, and substrate should remain intact during the manufacturing process. Therefore, it is im-portant to evaluate the delamination limit of PET during plas-tic deformation of VCM sheets. Owing to the increasing interest in deformable coated metal,

investigations have been conducted on the deformation and rupture of polymer-coated metal. Gay investigated adhesion between a solid and a stretched elastic material. For such a material, it was confirmed that the adhesion at the molecular scale is not affected by stretching [3]. However, adhesion at the microscopic scale is affected because of the elastic re-sponse of the material. Chang et al. devised a test method called the notched coating adhesion (NCA) test to estimate the

adhesive performance and durability by using special speci-mens that accelerate humidity conditioning [4]. The speci-mens were subjected to uniaxial tensile strain and the strain at which the coating de-bonded was used to calculate the critical strain energy release rate. The NCA test method was used to measure the interfacial fracture toughness of a certain coating. Subsequently, Dillard et al. conducted finite element analysis to determine the critical strain energy release rates for the NCA specimen [5]. Even though plastic deformation is occurs when the NCA specimens are placed under tension, the NCA test cannot predict the adhesion failure of coatings on de-formed substrates in a practical complex metal forming proc-ess. Jaworski et al. studied the application of polymer-coated metal to strip ironing [6]. The experiments demonstrated that the coating integrity was affected by tooling design and proc-essing conditions. Huang and Huang et al. further explored the effects of temperature on the survivability of coatings in strip ironing [7, 8]. Hatanaka et al. investigated coating adhesion after deep drawing in relation to the pre-treatment of alumi-num [9]. It was concluded that the deterioration in adhesion by drawing was due to the cohesive failure of films, which re-sulted from pre-treatment and the change in the underlying surface topography. Deflorian et al. evaluated the performance of polyurethane-and polyester-coated stainless steel after Erichsen cup drawing [10]. The adhesion was evaluated through electrochemical impedance spectroscopy. Braunlich et al. evaluated the formability of polymer-coated steels in production conditions [11]. Bending, folding, embossing, deep drawing, and other tests were conducted. Coating parts were inspected for gloss loss, coating tear-off,

etc. The study provided a method for evaluating coating per-formance. However, the use of this approach for coating and application development is costly and time consuming. Ravi et al. evaluated the durability of polymer-coated metal in plas-tic deformation [12]. A tensile test, rectangular stretch bend test, tape pull test, and other tests were conducted. The parts were inspected for coating durability under environmental conditions, such as moisture and salt. However, this method for coating considers only the deep drawing deformation mode. However, the above tests are costly and time consuming. As

the specimens in the dome test are subjected to different de-formation modes by varying the sample width, this study fo-cuses on using the tests conducted by subjecting the specimen to each deformation modes. This study proposes a new testing method based on the key

features of the dome test and rectangular cup drawing test. Dome test specimens were subjected to biaxial, plane strain, and uniaxial deformation modes. Rectangular cup drawing test specimens were subjected to deep-drawing deformation mode and compression deformation modes. Experiments were con-ducted to assess the delamination of sheet metal coating with PVC and PET. On the basis of delamination occurring by under each deformation mode, the data collected in the ex-perimental were used to construct the delamination limit dia-gram (DLD) for a given coating structure. The feasibility of

Fig. 1. Typical defects of VCM products.

Y. Son et al. / Journal of Mechanical Science and Technology 26 (10) (2012) 3223~3230 3225

using a DLD to predict vinyl coating delamination during sheet metal forming is discussed. In the sheet metal process, the blank undergoes large plastic

deformation. Plastic deformation is characterized by different deformation modes. Hence, in-depth investigation of VCM under biaxial, plane strain, uniaxial, deep drawing, and com-pression deformation modes is necessary. Therefore, this study focuses on using the tests that will subject the specimen to the each deformation mode. The principal strains at the end of the sheet metal forming

are calculated as follows:

11

0

lnd

dε = ; 2

20

lnd

dε = ; 3

0

lnt

tε = (1)

where d0 and t0 are the initial dimensions, and d1, d2 and t are the final dimensions of the process. It is usually assumed that the strain path is linear, i.e., that

the strain ratio remains constant, which is given using Eq. (2).

2 02

1 1 0

ln( / )

( / )

d d

ln d d

εβ

ε= = (2)

From Eq. (1), the thickness strain is determined by meas-

urement of thickness, or alternatively from the major and mi-nor strains assuming constant volume deformation. The thickness strain (ε3) and current thickness (t) are deter-

mined using Eqs. (3) and (4), respectively.

13 1

0 0

ln (1 ) (1 ) lnt d

t dε β ε β= = − + = − + (3)

0 3 0 1exp( ) exp[ (1 ) ]t t tε β ε= = − + (4)

By using the above equations, the principal strains and the

strain ratio can be determined, and the straining process is conveniently described using the principal strains, i.e.

11

0

lnd

dε

=

; 2

2 10

lnd

dε βε

= =

;

3 10

ln (1 )t

tε β ε

= = − +

(5)

where β is constant. The diagram shown in, Fig. 2 does not represent any par-

ticular process, but will be used to discuss the different defor-mation processes. The ellipse shown is a contour of equal effective strain, ε ; each point on the ellipse will represent strain in a material element. The path 0A indicates equal biax-ial stretching. A sheet stretched over a hemispherical punch will deform in this way at the center of the process shown in Fig. 2. The strains are equal in all directions and the grid circle expands, but remains circular. As 1,β = the thickness strain is 3 12ε ε= − , so the thickness decreases more rapidly with

respect to ε1 than those in any other deformation modes. In the process illustrated by path, 0B, the sheet metal ex-

tends only in one direction and the circle becomes an ellipse in which the minor axis is unchanged. The plane strain is ob-served in the side or about 125 × 200 (mm) specimen by dome test. It will be shown later that in plane strain, the sheet is par-ticularly prone to failure by splitting. The point C is the process in a tensile test and occurs in

sheet when the minor stress is zero. The sheet stretches in one direction and contracts in the other. This deformation mode will occur whenever a free edge is stretched, as in the case of hole extrusion or in about 25 × 200 (mm) sample by dome test. In the process, indicated by point D, stresses and strains are

equal and opposite, and the sheet deforms without a change in thickness. This process is called drawing as this observation is made when the sheet is drawn into a converging region. It is also called pure shear and occurs in the flange of a deep-drawn cup. Splitting is unlikely and in practical forming op-erations, large strains are often encountered in this deforma-tion mode. The uniaxial compression, indicated by the point E, is ex-

treme case and occurs when the major stress is zero, in the edge of a deep-drawn cup. The minor stress is compressive and the effective strain and stress are 2ε ε= − and 2σ σ= − respectively. In this mode, the sheet thickens and wrinkling is likely [13].

2. Experimental procedure

Based on the concept of the FLD, the present investigation evaluates the coating behavior in VCM sheets under plastic deformation by using a cross-cut specimen, optical strain measuring system ARGUS (GOM), and the Universal Sheet Metal Forming Test Machine USM 500D (Tokyo testing ma-chine). Most coatings are sensitive to the deformation mode, and

for coating delamination, a certain amount of time is required and specific conditions are involved. Hence, the cross-cutting

Fig. 2. Strain diagram and deformation modes : the different deforma-tion modes corresponding to different strain ratios are shown.


specimen, conditioned specimens becomes an excellent means for delamination testing of VCM sheets. The cross cutting serves as the initial debonding site that could lead to the occur-rence of delamination and further damage as the material is strained. Instead of the single notch used by Chang et al. and Dillard et al., cross-cutting grids similar to cross-hatch grids (ASTM D3359-97) are created in the present test specimens. The cross-cutting grids serve an important purpose. They ac-celerate the occurrence of the delamination under different deformation modes by reducing the time required to delami-nate the adhesive bond and the interface region. Moreover, similar to the etched or printed circular grids in the dome test, the cross-cutting grids can be used for strain measurement. In this study, however, dots on the specimen surface were used for strain measurement by ARGUS. The cross-cutting specimens in the dome test and rectangu-

lar drawing test are subjected to different deformation modes by varying the sample width. Strain measurement is con-ducted after testing each specimen to inspect strain of delami-nation initiation. To study the effects of the deformation mode on the coating, the specimens are subjected to deformation using the dome test and rectangular drawing test. In this study, the major strain and minor strain are measured

from the deformed specimens and punch stroke can be con-trolled by the sheet metal testing machine. Also, various de-formation modes are applied by the sample with different width. Once the coating delamination is identified, the results of the evaluation are plotted according to the major and minor strains to depict the critical delamination strain of the coating. The constructed DLD can be used to determine the delamina-tion limit during the complex forming process. Definitive results are presented in terms of the strain state of the coating area that is delaminated for different applied deformation modes.

2.1 Experimental materials

Experiments were conducted on the VCM sheet supplied by a roll coating company (BN STEELA, Republic of Korea). The coating and substrate thickness of the VCM are listed in Table 1. The VCM sheet consists of four different layers; the substrate, polyvinyl chloride (PVC) layer, polyethylene terephthalate (PET) layer, and protective film. The SEM im-ages of the VCM structures are shown in Fig. 3. A VCM sheet with a thickness, t = 0.94 mm is used in this study. Its me-chanical properties are listed in Table 2.

2.2 Preparation of specimen

For the dome test, the VCM specimen was prepared from coil-coated sheet metal according to ASTM standard E2218-02. The cross cutting grids were fabricated using the industrial field method in the whole section of the specimen. A blade with cross-cutting edges spaced 10 mm apart was used to cre-ate the grids. The first cut was made along the length of the whole section. The second cut was made at 90° to the first cut along the width of the whole section creating a cross cutting grid pattern with 10 × 10 (mm). Extreme care should be taken to ensure that the cut is made by steady motion using just suf-ficient pressure on the blade to have the cutting edge reach the substrate. Dot printing is applied to the surface of a VCM sheet to provide an array of precisely spaced gauge points prior to forming the metal into a final shape by the application of a force. An array of circles is printed on the surface of the specimen. The dot patterns remain adhered to the VCM so that they do not move on the coating surface or get rubbed off during the forming operation. Dot printing of the VCM sheet and measurement of strain are shown in Fig. 4. After the VCM has been formed, delaminated grids are measured by ARGUS.

Table 1. Coating and substrate thickness.

Coating thickness

Substrate thickness

VCM Sheet (mm)

Protective Film (mm)

PET (mm)

Vapor- deposition (mm)

PVC (mm)

GI – DQ Grade (mm)

0.94 0.046 0.03 0.017 0.046 0.8

Table 2. Mechanical properties of the coating film and metal substrate.

Elastic modulus (GPa)

Yield strength (MPa)

Ultimate tensile strength (MPa)

Elongation (%)

Substrate 201 163 388 51.43

PET 1.54~2.2 81.6 195 82.5

PVC 2.48~2.82 25.23 45.55 293.56

Fig. 3. Schematic representation and SEM image of the VCM sheet.


2.3 Dome test

The dome test was performed by using a Universal Sheet Metal Forming Test Machine according to ASTM standard E2218-02 [14-19]. In this study, the experimental procedure mainly involves three stages: preparation of VCM sheet specimen, punch–stretching the dot printed samples to onset of delamination, and measuring the strains. Dot printing was performed on the VCM using a contacting grid circles with a diameter of 1 mm. The dot pattern was printed on the speci-mens using conventional printing equipment. Punch-stretching experiments were carried out on a double-action hydraulic press with a capacity of 500 kN (50 tons). Different punch-die assemblies were designed and fabricated according to the thickness of the sheets. A typical punch-die assembly used in the experiment is shown in Fig. 5. A draw bead of 7 mm was provided on the dies to restrict

the material flow from outside. Sufficient blank holding force was applied using the lower die to clamp the material in the draw bead. The punch velocity was maintained almost con-stant at 5 mm/min. The VCM sheet samples were subjected to different modes of deformation, i.e. tension to tension, plane strain, and tension to compression modes by varying the width of the specimen. Test specimens shall be wiped clean and dry to remove grit and soil. A lubricant of any type can be used between the hemispherical punch and the specimen’s surface.

In this study, lubricant is used. Lubricants increase the amount of stretch before localized necking and move the location to-ward the nose of the punch. Lubrication changes the ε1 and ε2 strains, but does not affect the FLC and DLC. Date points move along the curve due to the change of the ε2 strain be-coming more positive in the stretch region and more negative in the draw region of the FLD when effective lubricants are used in press forming. Therefore, DLC proposed in this study will not change according to selection of other lubricants. The length of the blank was 200 mm and the width was varied between 200 and 25 mm. In this study, samples of size 200 × 200 mm, 200 × 150 mm, 200 × 125 mm, 200 × 100 mm, 200 × 75 mm, 200 × 50 mm, and 200 × 25 mm were cut by shear-ing. The dimensions of the specimen are shown in Fig. 6. For each blank width, at least four of five specimens were tested to obtain the maximum number of delaminated points. In this study, the blanks were stretched, and the experiments were discontinued at the onset of delamination in this study. The major and minor diameters of the ellipses were meas-

ured using ARGUS with a strain accuracy of 0.1% and the major strain (e1) and minor strain (e2) were calculated. The DLD was drawn by plotting the minor strain on the abscissa and corresponding major strain on the ordinate and by draw-ing a curve that separates the safe region from the unsafe re-gion (occurrence of delamination).

2.4 Rectangular cup drawing test

In this test, rectangular specimens were tested using the Universal Sheet Metal Forming Test Machine. The VCM sheet samples were subjected to deep drawing deformation mode in the rectangular cup drawing test. In this study, sam-ples of size 105 × 105 mm were cut by shearing. The punch dimension of the rectangular cup drawing is as follows: length × width = 100 × 52 mm, as shown in Fig. 7(a). The clearance between die and punch was 0.875 mm. The shoulder radius of

Fig. 4. Dot printing and strain measurement (all dimension in mm).

Fig. 5. Geometry of the punch-die assembly used in the experiment. (all dimensions in mm).

Fig. 6. Dome test specimen (all dimensions in mm) .


punch (rp) and die (rd) are 10 mm and 7 mm, respectively. Fig. 7(b) shows the specimen of rectangular cup drawing test. The blank holding force was 50 kN, and the punch stroke was 35 mm. The punch velocity was maintained almost constant at 5 mm/min. For each blank, at least four or five specimens were tested to obtain the maximum number of delaminated points. In this study, the blanks were stretched and the experiments were discontinued at the onset of delamination.

3. Result and discussion

On the basis of the proposed method, the qualitative obser-vation of the delamination test was conducted for specimens subjected to different modes of deformation and different levels of strain. According to the specimen width, the strain level and deformation modes of delaminated grids were ob-tained to construct the DLD.

3.1 Deformation mode: tension to compression

Fig. 8 shows the delamination limit strain of the tension to compression deformation mode.

In the deep-drawing mode, no delamination was observed on the coating for a strain levels of up to e1 (major strain) = 0.307 and e2 (minor strain) = -0.444. The major and minor strain values of the grids observed in the specimens are given in the DLD shown in Fig. 8. The triangular marks on the graph represent the major and minor strain values of the de-laminated grids. The maximum major and minor strains in the specimens for

the rectangular drawing test were around 0.6, and -0.4 at sub-strate failure, respectively. It is noted that most of the delami-nation occurred in grids where the values of the major strain were (between 0.29 and 0.31) and those of the minor (strain were between -0.43 and -0.45). Similar results were obtained when the specimens were sub-

jected to the uniaxial tension mode. In the uniaxial tension mode, no delamination was observed

on the coating for a strain levels of up to e1 = 0.387 and e2 = - 0.189. The degree of delamination of the grid was high for each specimen when compared with those subjected to the deep-drawing deformation mode. Moreover, the values of the major and minor strains at de-

lamination were attained before substrate failure occurred in the uniaxial tension mode. The maximum major and minor strain value for the specimens for the dome test were around 0.5, and -0.15 at substrate failure, respectively. In the compression mode, the minimum major and minor

strain values in the specimens for the rectangular drawing test were around 0.14 and -0.28, respectively, when wrinkling occurred in the substrate. It is observed that most of the de-lamination occurred in grids with low values of major (be-tween 0.18 and 0.20) and minor (between -0.35 and -0.47) strains. All grids subjected to high strain states were found to be de-

laminated. This observation confirmed that the PET coating and adhesive bond in these grids were affected by the strain level in the tension-compression deformation mode.

3.2 Deformation mode: tension to tension

In the balanced biaxial tension deformation mode, delami-nation was observed on the coating for a strain levels higher than e1 = 0.250, and e2 = 0.250. The major and minor strain values of the grids obtained from the specimens are given in indicate the delamination diagram shown in Fig. 9. The trian-gular marks on the graph indicate the major and minor strain values of the delaminated grids. The maximum major and minor strain in the specimens for the dome test were around 0.5, and 0.3 at substrate failure, respectively. Most of the de-lamination occurred in grids where the values of the major strain were between 0.24 and 0.26. Similar results were obtained when the specimens were sub-

jected to the biaxial tension mode. In the biaxial mode, de-lamination was observed on the coating for a strain level higher than e1 = 0.25 and e2 = 0.125. The delaminated state of the grids was similar for each specimen when compared with

(a) (b) Fig. 7. Rectangular cup drawing test machine and specimens (all di-mensions in mm).

Fig. 8. Delamination limit diagram for tension to compression defor-mation mode.


those subjected to the balanced biaxial tension deformation mode. The maximum major and minor strain value in the spe-cimens for the dome test were around 0.4 and 0.125 at sub-strate failure. Most of the grids subjected to high strain states were found

to be delaminated. This suggested that the PET coating and adhesive bond in these grids were affected by the strain level in the tension-tension mode.

3.3 Deformation mode: plane strain

In the plane strain mode, no delamination was observed on the coating for strain levels of up to e1 = 0.203 and e2 = 0.005. The major and minor strain values of the grids collected from the specimens are given in the delamination diagram shown in Fig. 9. The triangular marks on the graph represent the major and minor strain values of the delaminated grids. The maxi-mum major and minor strain value in the specimens for the dome test were around 0.34 and 0.01, respectively, at substrate failure. Most of the delamination occurred in grids with low values of major (between 0.20 and 0.22) and minor (between 0 and 0.01) strains. Therefore, the PET coating and adhesive bond in these grids were affected by the strain level in the plane strain mode. When the deformation modes approached plane strain, the

delamination limit strain was lower, and the delaminated grids were deformed severely. In the plane strain mode, value of major strain at delamination is lower than those of the samples subjected to other deformation modes. The observation about the surface roughness dependence on stress provide a possible explanation for the minimum in the delamination limit curve at plane strain. The VonMises equivalent stress in the plane strain is bigger than other that of deformation mode. And sur-face roughness is influenced by the VonMises equivalent stress. Thus, VCM specimen under plane strain occurred

foremost as compared with other deformation modes. There-fore, plane strain mode was an adverse condition for delami-nation [20].

4. Conclusions

This study presents a testing method for evaluating PET coating delamination in sheet metal forming. Most of the coat-ing adhesion tests do not involve a deformed substrate in vari-ous deformation modes. Therefore, the commonly used tests for delamination of coatings are not effective in evaluating the coating behavior during plastic deformation of the substrate. On the other hand, the current sheet metal formability test does not evaluate PET coating delamination. The proposed method combines the primary factors of the cross-cutting specimen, dome test, and rectangular cup drawing test. Using the proposed test method, the experimental investigation showed that the PET coating can be affected by the deforma-tion modes and strain level. While the delamination cannot be apparently observed in a low strain state, the delamination of PET coating is evident under a high strain state. After identifying the area of delamination, the delamination

limit diagram (DLD) of the coating system can be constructed in a format similar to that of the forming limit diagram. The feasibility of using VCM sheets in complex press forming can be evaluated through the superimposition of the delamination limit strain of the coating system onto the forming limit dia-gram of the VCM sheet. The experimental investigation sug-gested that the proposed test method will support the sheet metal forming process design for VCM sheets. The assessment method presented in this study can be used

to determine the delamination limit strain under plastic defor-mation of other polymer coated metals. The experimental results suggested that the proposed testing method is effective in evaluating delamination for suitable applications. It is also believed that additional work is required to develop other ex-perimental models to investigate the deformation mode at the coating-metal interface under deformation conditions.

Acknowledgment

This paper is supported by the PNU-IFAM JRC (NRF-2009-K20601000004-09E0100-00410), MKE and KTF (KOTEF) through HRT Project for Strategic Technology.

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[20] C. M. Wichern, B. C. De Cooman and C. J. Van Tyne,

Surface roughness of a hot-dipped galvanized sheet steel as a

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Young-Ki Son is currently a Ph.D candidate at the Precision Manu- facturing Systems Division at Pusan National University in Busan, Korea. His current research interests include polymer coated metal forming.

Byung-Min Kim received his B.S., M.S. and Ph.D degrees from Pusan National University, Korea, in 1979, 1984 and 1987, respectively. He is currently a professor at the School of Mechanical Engineering at Pusan National Univer- sity in Busan, Korea.

experimental evaluation of coating delamination in vinyl-coated metal forming

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