Deformation characteristics and formability of fibre-reinforced thermoplastic sheets

Deformation characteristics and formability of fibre-reinforced thermoplastic sheets T.A. Martin,

D. Bhattacharyya

and R.B. Pipes

(Received 9 July 1992; accepted in revised form 26 August 1992) In order to quantify the deformation in sheet materials when formed into complex shapes, an experimental measurement technique is necessary. This paper presents the findings of large strain analysis when applied to thermoplastic composite sheets. Three materials have been investigated by forming flat laminates into a circular cavity with a hemispherical die. With increasing blank size and forming speed, and a decreasing forming temperature, the loads of deformation increased. These data have been presented and related to the problem of flange wrinkling in conjunction with the strain analysis results. Arrow diagrams, and strain contour maps clearly identify the regions of severe deformation, and highlight large compressive strain gradients where buckling occurs. Keywords:

forming

parameter

effect;

deformation;

quality

buckling;

INTRODUCTION For many years products have been manufactured

from sheet materials by bending and/or stretching them into particular shapes. In order to improve the quality of these products methods for quantifying the deformation have been developed. One such method commonly used in today’s sheet metal industry is known as grid strain analysis’. By printing square and/or circular grids onto an undeformed sheet surface, and then measuring the coordinates of each point after deformation, surface strains can be calculated with each small element. By fitting splines to the deformed surface to account for the curvature in each element, and then performing large strain analysis, better results have been obtained2. An advantage with this measurement technique is that it does not require a knowledge of material properties; however, process failures must be defined in terms of critical strains. A few assumptions regarding the forming process are also inherent. Within each element the deformation is assumed to be homogeneous and monotonic, and the principal directions of strain are assumed to remain fixed during forming. One of the most difficult tasks associated with studying the forming characteristics of fibre-reinforced thermoplastic sheets (FRT) is in quantifying the actual deformation in the material as a result of the forming process. A macroscopic picture of the overall deformation is essential in order to illustrate a material’s response to process parameters, such as, forming temperature, forming speed and blank geometry. It also permits experimental verification of theoretical models. This paper aims to illustrate the usefulness of utilizing such a grid strain analysis technique to gain some fundamental understanding of factors contributing to forming 0956-7143/92/030165-08

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Manufacturing

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@ 1992

strain

analysis

problems. In order to do this three distinct thermoplastic composite laminates were formed into a circular cavity with a hemispherical punch under isothermal conditions. The punch loads required to form the specimens were recorded during deformation, and these have also been presented in association with the strain analysis results.

EXPERIMENTAL

DETAILS

Experimental results from a number of tests carried out on three distinct thermoplastic composite materiali have been presented in this paper. The relevant details characterizing these consolidated laminates are summarized in Table 1. For each sheet of material a silk screening process was used to apply an array of square grids to the surface prior to deformation, After forming a component the 3-D co-ordinates of each point were measured and large strain analysis was performed. Strips of various width, as shown in Figure 1, were cut from the pre-consolidated laminates with a diamond tip cutting wheel. Tables 2(a) and 2(b) detail the combinations of forming speed, forming temperature, and blank size used to analyse their effects on product quality for each material. As an additional test, two 112.5 mm square APC-2 blanks were cut from laminates. This was coo, 90”1,, and [ +45”, -45”],, done in order to examine the relationship between the onset of gross buckling and the initial fibre orientation for two blanks with the same geometry. The equipment used to deform the specimens allowed the use of double diaphragms which supported the composite during forming. Each strip was placed between two diaphragms, which were clamped onto a vacuum ring so that the air between them could be evacuated. Butterworth-Heinemann

Ltd 165

Table

I

Material

Experimental

materials

details

Matrix Fibre type V, Fibre length Manufacturer Melt temperature Laminate Thickness * LDF

: Long

Discontinuous

Table 2 a Experimental Sheet width

variables

for PLYTRON

Polypropylene Glass 35% Continuous ICI 145-170°C [+45”, -45”],, 2.0 mm

PEEK Carbon 65% Continuous ICI 340-420°C [+45”, -45O],, 1.05 mm

PEK Carbon 60% 200-250 mm Du Pont 340-420°C [+45”, -45”],, 1.1 mm

(J)

variables

temperature

185

190

J

J

J

for LDF

(J)

and APC-2

Forming 195

200

J

J J J J J J J

XJ

370

0.25

12.5 J J J J J J J

J

J

temperature 380

(“C)

Forming

400

420

0.25

Test specimen

mini)

25

50

125

J

J

J

12.5

XJ

XJ XJ

XJ

(mm

speed

XJ XJ XJ

XJ

dimensions

The APC-2 and long discontinuous fibre (LDF) materials were formed with Upilex-R diaphragms, while the PLYTRON strips were formed with Vat-Pak HS6262 Co-Ex (blue) diaphragms. The clamping plate which held these diaphragms, was located in an Instron tensile testing machine on a lower die with a circular cavity. A hemispherical punch, attached to the loadcell above the die, was used to form the sheet into the cavity as the crosshead was driven upwards. Figure 2 shows a close-up view of this apparatus just after retraction of the punch

(mm

min-‘) 25

50

XJ XJ

X

XJ

XJ XJ XJ

1

speed

( x ) Forming

360

166

(“C)

J

(mm)

12.5 25 37.5 50 75 112.5 175

Forming 180

J J J J J J J

Experimental

Figure

LDF*

(mm)

12.5 25 37.5 50 75 112.5 17.5

Sheet width

APC-2

Fibres

175

b

PLYTRON

XJ XJ

Figure

2

Close-up

view

of experimental

apparatus

from a formed part (punch diameter = 50 mm, cavity diameter = 57 mm). An Applied Test Systems oven (type 3710), mounted in the Instron, was used in conjunction with an LFE 2010/2011 microprocessor controller to heat both the specimen and the test equipment up to the desired forming temperature. After a 15 min soak period at the appropriate temperature, which ensured a fairly even temperature distribution, forming commenced. Having reached a punch depth of approximately 25 mm the

Composites Manufacturing No. 3 1992

crosshead was stopped, the oven door was opened, and the oven temperature was allowed to drop to 50°C before the punch was retracted. In order to analyse the strains in each specimen, a digitizing machine capable of measuring the Cartesian co-ordinates of each grip point in space was used to acquire the 3-D data. This data was stored in an ASCII text file and then transferred to an IBM 4341 Mainframe computer so that large strain analysis could be performed. FORTRAN graphics subroutines were utilized by the CAD software to provide standard facilities for scaling, rotating and translating 3-D pictures, editing, and spline fitting*.

I

RESULTS AND DISCUSSION The results obtained from the large strain analysis carried out on some of the formed parts are illustrated in this section with (i) arrow diagrams, which indicate the magnitudes and directions of principal surface strains, and (ii) strain contour maps on the undeformed surface. Arrow diagrams for three 25 mm wide specimens formed at 12.5 mm min-’ from PLYTRON, APC-2, and LDF strips respectively are shown in Figure 3(a)-(c). In all these tests the two fibre directions were initially orthogonal to one another. The strong influence of the fibres on the deformation can clearly be observed for

Surface Plytron

[+45O/-45O12s

fibre

sheet

I

I

direction

\

Elmax

= 27.8%

c2max

LDF

[+45°/-45014s

= -24.1%

I a

Surface I

I

fibre

direction

sheet /

Elmax =

10.5%

E2max

= -10.9%

I

b Surface APC2

[+45°1-45014s

direction

sheet

I

I

fibre

/

.5,max

E2max =

= 11.9%

-13.8%

I

C Figure

3 Arrow diagrams for (a) PLYTRON,

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(b) LDF, and (c) APC-2 parts. Width = 25 mm, forming speed = 12.5 mm min-’

167

each part in spite of the fact that the matrix polymer in each case was quite different. Because every laminate deformed as a unified material, the tensile strains along each strip were accompanied by compressive strains in the transverse direction. In many instances the compressive strains were greater than the tensile strains, and this led to an increase in product thickness, as would be expected for an incompressible material. This result appears to be consistent with the observation that thermoplastic composites with two directions of fibre reinforcement behave like woven fabrics when deformed3. These arrow diagrams also indicate the magnitudes of deformation possible in a particular thermoplastic composite sheet without incurring material damage. The ability of a sheet of this type to continue in this mode of deformation depends on its resistance to out-of-surface buckling, which might occur as a result of an unfavourable stress distribution. The following photographs in Figure 4 show the significant effect of initial fibre orientation on the final quality of the product. Both parts shown were formed from 112.5 mm square blanks to a similar depth; however, it is clear that while the [ +45”, - 45”] laminate in Figure 4(a) has remained virtually flat in the flange region, the CO”, 9W] sheet in Figure 4( b) has undergone considerable flange wrinkling. Gross buckling in both sheets started off-axis to the fibre directions and then the undulations were propagated in to the rest of the sheet as forming continued. The considerable difference between the shape at the boundary of each part is also worth noting. The basic difference between these two blanks is geometric with respect to the fibres. In both cases the lines of geometric symmetry coincide with lines of material symmetry ; however, for the [ +45”, -45”] laminate the longest fibre extends radially into the flange region much further than it does for the [O’, 90”] laminate. Since the majority of in-surface deformation takes place off-axis to the fibres, the volume of material being deformed in the flange region of the COO,90”] laminate is far greater than that of the [ +45”, -45”] laminate. Therefore, under isothermal conditions, the energy required to draw the COO,90”] sheet into the die should exceed that required to draw in the [ + 45”, -45”] sheet. This deduction is further supported by Figure 5, which illustrates the punch load versus cross-head displacement recorded for each specimen as it was formed in the oven. The punch load necessary to form the [Oo, 90”] sheet remained consistently higher than that required to form the [ +45”, -45”] sheet, and the difference between them grew as deformation proceeded. The diaphragm load has also been included in this figure to indicate the net force required to form the specimens themselves. For these two laminates with the same matrix material and formed at the same temperature, it can be concluded that the amount of energy required to buckle the flange was reached in the [O’, 90”] sheet before it was reached in the [ +45”, -45”] sheet. Although the constitutive properties of this material are not exactly defined, and the critical buckling energy is not known, an insight into how to alter blank geometry to form a given shape has been obtained. By removing regions in the blank where large deformations can take place, the energy required to draw the flange into the cavity can be reduced ; consequently, the inception of buckling can be delayed so that complete forming may be accomplished successfully. This concept is in line with

168

a

4 Photographs (b) [W, 90’1 4$ APC-2

Figure

of 112.5 mm square (a) [+45”, -45”],,, parts formed at 12.5 mm min- ’

and

that utilized in the sheet metal forming industry for designing blank shapes. Redundant regions in a blank are removed to avoid problems such as wrinkling, thinning, and fracturing. The relationship between formability and forming temperature is another important factor which cannot be overlooked. The significance of this parameter depends on the state of the polymer matrix within or above the material’s melting range only, since this is when shaping is performed. The fact that gross buckling does occur indicates that thermoplastic polymers retain some form of elasticity in their melt temperature range which causes them to deform in an out-of-surface

Composites Manufacturing No. 3 1992

200

v

+450/-450

0

Diaphragm

only

t

0

5

10

Punch

15

20

displacement

Figure 5 Punch load vs displacement parts formed at 12.5 mm min ’

for

25

30

(mm) 112.5 mm

square

APC-2

specimens could be formed to a depth of 25 mm and undergo very large strains without any wrinkling in the flange region, the specimens that were wider than 75 mm could not. The depth at which buckling started may be called the critical punch depth in the context of these experiments. It was observed that with increasing strip width the critical punch depth decreased, as might be expected. Since Figure 7 indicates the work done to form each specimen under similar conditions, it is clear that more energy is needed to form wider specimens. Consequently, it would be unadvisable to make a product with a larger blank than is absolutely necessary as it could lead to forming problems in practice. A digitized mesh plot of the surface of a 112.5 mm wide LDF sheet is shown in Figure 8. The wrinkles in the directions off-axis to the fibres are evident from this diagram. Corresponding to Figure 8 is the arrow diagram shown in Figure 9. This figure demonstrates the usefulness of large strain analysis to highlight regions where large amounts of deformation have occurred during forming. In this case large tensile strains directed towards the centre of the punch are accompanied by compressive

250

V.175 200

75 mm

n

v 37.5

Figure Width

6 Photograph = 50 mm, forming

of

an APC-2 sheet speed = 12.5 mm min-

formed ’

at

mm

mm

0

50mm

0

25 mm

360°C.

manner when a critical load is reached. Figure 6 shows an APC-2 strip formed at 360°C. At this temperature the sheet deformed by buckling at the punch nose rather than conforming to it. It can be concluded that by increasing the forming temperature, i.e., by lowering the matrix viscosity, the formability of these materials can be improved ; however, for some polymers the rate of change of viscosity with respect to temperature may be small, and at higher temperatures matrix degradation may become a problem. Polypropylene exhibits a substantial change in viscosity over its melting range and beyond. As a result PLYTRON specimens formed at 180-200°C did not exhibit any gross buckling while those formed at 175°C did. As the matrix viscosity decreased with increasing temperature, problems with squeeze flow arose and localized fibre buckling became the predominant problem. For this particular material the forming temperature was identified as a very important variable, since it significantly affected the fibre/matrix interaction in the sheet. The size of the blank was found to be another factor affecting the formability of the materials. As stated earlier, a series of parts were formed from each material with different strip widths. Figure 7 shows the increase in load required to form the LDF parts as the sheet width was increased. This trend was repeated with the other two materials as well (not reported). While the narrow

Composites Manufacturing No. 3 1992

0 112.5

mm

50

0

5

15

10

Punch

20

displacement

25

(mm)

Figure 7 Punch load vs displacement for LDFstrips ofdifferent I-ormmg speed = 12.5 mm mu-‘, temperature = 370°C

LDF

[+45°/-45014s

width.

sheet

Figure 8 Digitized mesh diagram of an LDF specimen. Forming speed = 12.5 mm min-‘, width = 112.5 mm, temperature = 370°C

169

LDF

Elmax

[+45°/-45014s

= 10.2%

Figure 9 Arrow diagram for a [ +45”, -45”],,

= -10.1%

LDF sheet. Forming speed = 12.5 mm min-‘,

strains close to the hemispherical indentation. In the flange region away from the indentation, the strain magnitudes are much smaller, but they indicate that the sheet has been deformed. It is interesting to note that because the +45” fibre direction does not represent a plane of geometric symmetry in this blank, the degree of deformation in the short sides exceeds that in the longer side. However, the most prominent buckle developed in the long side of the blank. Earlier work has shown that this gross buckling is associated with a large compressive strain gradient across the sheet4, and is not directly related to the amount of strain in the material. A high compressive strain gradient indicates a high strain rate during forming, and hence a higher stress distribution, leading to buckling. Markers have been placed on the

170

EZmax

sheet

width = 112.5 mm, temperature = 370°C

arrow diagram to indicate where large strain gradients are present for this particular specimen. They coincide with the buckles in the sheet. Although the strain magnitudes are smaller near the indentation on the longest side, the strain gradients are higher, and buckling began in this region first during forming. The aforementioned effect becomes more conspicuous on a strain contour map for the compressive strain, s2, in Figure 10. Here again markers have been placed on the diagram to highlight regions of high strain gradient where buckling has occurred. Finally, the effect of forming speed on the material formability has also been investigated. Figure 11 illustrates the forming load recorded for a couple of PLYTRON sheets formed at different speeds to a depth

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No. 3 1992

LDF

[+45°/-45014s

sheet

Surface

Strain

Figure

10

Straincontourmap

increment

= 0.5%.

(~~)fora[f45”,

EZmax=

-45”],,

LDFsheet.

Forming

speed = 12.5 mm min-‘,

A few results obtained from a large number of tests have been presented in this paper to demonstrate the effects of various forming parameters on the quality of the parts

width

= 112.5 mm, temperature

= 370°C

formed, and the following remarks can be made: l

l

REMARKS

Composites Manufacturing No. 3 1992

direction

-10.0%

of 18 mm. Also shown are the loads required to form the diaphragms alone at the same speeds. Both composite parts showed flange wrinkling as a result of the deformation ; however, the wrinkling was less severe in the part formed at 12.5 mm min-‘. Because these materials behave as viscous fluids in their molten state, the internal stress generated during forming is related to the forming speed, and this affects product formability for reasons previously mentioned.

CONCLUDING

fibre

l

The large strain analysis technique provides a snap-shot of the deformation process, which illustrates the characteristic surface strains generated during forming. It has been successfully used to highlight the inextensible nature of the fibres in thermoplastic composite sheets with respect to the large overall deformation of the matrix. For all three materials investigated, the fibres are shown to impose such severe constraint on the deformation that the matrix polymer has little effect on the final strain distribution. A laminate with two directions of fibre reinforcement seems to behave like a bi-directional woven fabric when formed into a die. If this is the case,

171

500

I

V

0

l

v

I

175 mm

I

square

Diaphragms

I

sheets only

400

Z

-

300

x -0 .c 2E

l

200

10.0

properties of the composite. Without a knowledge of these properties it is impossible to redict failure precisely. By increasing the forming speed and blank size, and reducing the forming temperature the punch load is increased. This response is tantamount to an increase in energy input, and to avoid wrinkling the maximum depth of forming must be reduced. Regions of severe deformation are easily identified through arrow diagrams and strain contour maps. These provide useful information about where material can be removed from a blank to reduce the likelihood of gross buckling. Then by altering the initial size and shape of a blank accordingly, better quality parts can be produced.

REFERENCES 25 Punch

Figure 11 at different

l

l

172

displacement

Punch load vs displacement speeds. Width = 175 mm,

(mm)

for PLYTRON sheets formed temperature = 175°C

over the majority of the sheet the principal strain directions must change during forming as the fibres rotate. Such deformation violates one of the assumptions of the grid strain analysis technique. However, it is not clear at this stage whether such an infringement significantly affects the results. Flange buckling is associated with a large compressive strain gradient across a part, and is not strongly dependent on the strain magnitudes developed during forming. It actually depends on the energy necessary to draw the blank into the die. No single forming parameter may be used to define a critical buckling condition, since the onset of gross buckling must depend upon the constitutive

1 2 3

4

Sowerby, R., Cbu, E. and Duncan, J.L. ‘Determination of large strains in metal forming’ J Srrain Analysis 17 (1982) pp 955101 Zbang, Z.T. and Duncan, J.L. ‘Developments in nodal strain analysis of sheet forming’ Int J Mech Sci 32 (1990) pp 717-727 Martin, T.A., Bbattacharyya, D. and Pipes, R.B. ‘Computer-aided grid strain analysis in fibre reinforced thermoplastic sheet forming’ In Computer Aided Design in Composite Material Technology III ed. S.G. Advani et al (Computational Mechanics Publications, 1992) pp 143-163 Burt, C., Bhattacharyya, D. and Martin, T.A. ‘The product quality and formability of fibre reinforced thermoplastic sheets’ In Proc 8th Annual Meeting of the Polymer Processing Sot (New Dehli, 1992) pp 2877288

AUTHORS T.A. Martin and D. Bhattacharyya are with the Department of Mechanical Engineering, University of Auckland, Auckland, New Zealand. R.B. Pipes is in the Office of the Provost, University of Delaware, Newark, DE 19716, USA. Correspondence should be addressed to D. Bhattacharyya.

Composites Manufacturing No. 3 1992