development of a forming limit diagram for a fibre …

Mechanics of Nano, Micro and Macro Composite Structures Politecnico di Torino, 18-20 June 2012

A. J. M. Ferreira, E. Carrera (Editors)

http://paginas.fe.up.pt/~icnmmcs/

DEVELOPMENT OF A FORMING LIMIT DIAGRAM FOR A FIBRE

METAL LAMINATE BASED ON A GLASS FIBRE REINFORCED

POLYPROPYLENE COMPOSITE

A. Sexton*, W.J. Cantwell

†, S. Kalyanasundaram

*

* Research School of Engineering

Australian National University

North Road, Canberra 0200, Australia

e-mail: [email protected], web page: http://cecs.anu.edu.au

† Centre for Materials and Structures

University of Liverpool

L69 3BX Liverpool, United Kingdom

e-mail: [email protected], web page: http://www.liv.ac.uk/

Key words: Composite structures, Fibre metal laminate, Experimental mechanics,

Formability.

Summary. This paper investigates the forming behaviour of a fibre metal laminate based on

aluminium and a glass fibre reinforced polypropylene composite at room temperature.

Specimens of varying geometry are stretched over a hemispherical punch to elucidate the

deformation behaviour of the laminate in several forming modes. An optical strain measuring

system is used to obtain the surface strain of the specimens throughout the forming process.

These results are compared to the forming of aluminium and a forming limit curve for the

fibre metal laminate is developed. One of the major conclusions from this study is that it is

possible to manufacture fibre metal laminates containing glass fibre composites using

forming techniques generally restricted to more ductile materials such as metals.

1 INTRODUCTION

Evidence of anthropological climate change is fuelling research into advanced materials.

These advanced materials are primarily of interest to the transport industry in an attempt to

reduce the fuel consumption and therefore emissions of aircraft and vehicles. According to the

International Energy Agency (2011), the transport sector is currently responsible for 23% of

energy-related CO2 emissions and it is expected that the transport sectors of emerging

economies will drive all net growth in demand for oil. This means that significant reductions

in greenhouse gas emissions can be achieved by targeting the transport industry for

improvement; alternative fuel, improved engine efficiency and reduction of the vehicle's

weight are some of the methods proposed to reduce fuel consumption. Reducing the weight of

vehicles is seen as a precursor for further improvements in efficiency through the principal of

mass decompounding [1]. Innovative design, which involves the optimisation of components

A. Sexton, W.J. Cantwell and S. Kalyanasundaram.

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to attain higher performance from existing materials, and material substitution, where the

existing material is replaced by a higher performance material are two of the major methods

proposed for mass reduction. In addition to material substitution using alternative metals,

polymer composite materials are also gathering interest. Composite materials have both

advantages and disadvantages with respect to vehicle manufacture; they exhibit superior

specific strength and stiffness than steel but are prohibitively expensive for rapid production

of vehicles. However, the mass reduction using composite materials has been estimated at 20-

35% and 40-65% using glass-fibre and carbon-fibre reinforced composite materials

respectively [2]. Traditionally, thermoset composite materials were used in the production of

components for both automotive and aerospace applications; however, these materials suffer

from long and expensive manufacturing procedures which restrict their use to low volume

applications. The advent of low cost thermoplastic composite materials has led to the study of

the applicability of existing low cost rapid manufacturing techniques, unavailable to

thermoset composites, to thermoplastic composites.

Fibre Metal Laminate (FML) systems are a composite material consisting of alternating

layers of fibre-reinforced composite and metal. The first generation of FML systems utilised

an aramid fibre reinforced thermoset composite called ARALL. This FML system was

developed in the 1970s by TU Delft and Fokker and studies on F-27 wing panels comprised

of this material showed great promise. A reduction in weight of 25% was achieved while

maintaining strength, and loads which would cause failure in monolithic aluminium caused

only minor damage in the FML [3]. However, it was found that the blunt notch strength of the

ARALL FML was critical and premature fatigue cracks appeared at doublers which were

bonded to increase strength. Other deficiencies were also found in the ARALL FML systems,

such as poor bonding between the aramid fibres and matrix material, moisture absorption and

failure of the fibre/matrix interface and fibre failure under tension-compression fatigue

loading [4]. These problems led to the development of FML systems such as GLARE, which

contains a glass-fibre reinforced thermoset composite. Studies showed that the GLARE FML

was not only less expensive than the ARALL FML but also resulted in greater weight

reductions and had superior fatigue resistance. This culminated in the use of GLARE in the

Airbus A380 upper fuselage where it has been shown to possess excellent impact and damage

tolerance properties. The disadvantage of these FML systems is that they utilise thermoset

polymers for the matrix material and suffer from the manufacturing difficulties inherent to

these materials. Thermoplastic polymers offer the ability to form preconsolidated sheets of

material with a manufacturing time comparable to metals. This would allow the use of FML

systems in rapid manufacturing applications.

The most commonly used method for the rapid manufacture of components from sheet

material is stamp forming. Sheet metal forming is a widely used manufacturing process in the

automotive industry which uses a die, blankholder and punch to rapidly create complex

components. Extensive research into the formability of metallic materials led to the

development of the Forming Limit Diagram (FLD) by Keeler and Backofen [5]. The FLD is

used to visualise the state of strain of the surface of a formed part and to evaluate the


3

performance of that material in a stamp forming process. The Forming Limit Curve (FLC) is a

limit on the FLD that defines the transition between safe states of strain and failure. The

failure of metals is determined by the onset of localised necking, therefore the FLD consists

of three regions, the safe region, the necked region and the failed region. A FLD with these

regions is shown in figure 1. However, studies by Morrow et al. [6] showed that certain

composite materials do not exhibit localised necking before failure.

Figure 1 – Forming limit diagram with three regions

Increased interest in composite materials for use in automotive applications has led to

increased research into the formability of thermoplastic composites. Lee et al. [7] and

Venkatesan et al. [8, 9, 10] determined that, by choosing optimal conditions for punch speed

and forming temperature, composites can exhibit formability comparable to metals. Cabrera et

al. [11] investigated the stamp forming of all polypropylene and glass-fibre reinforced

polypropylene composites and found that stretch forming is more desirable than draw forming

because the latter leads to higher forming energy and residual stresses. Recently, studies on

the formability of fibre metal laminates by Mosse et al. [12-15], Gresham et al. [16] and

DharMalingam et al. [17] have shown that it is possible to form FML systems using draw

forming under carefully controlled process parameters. These studies found that lower

blankholder forces and higher temperatures resulted in increased severity of wrinkling in the

flange and sidewall, whereas higher blankholder forces lead to tearing and fracture. Reyes and

Kang [18] performed preliminary investigations into the stretch forming of fibre metal

laminates and found that the deformation was comparable to aluminium of similar thickness

whilst requiring 25% less load. Previous studies by Sexton et al. [19] investigated the

deformation behaviour of a FML based on a self-reinforced polypropylene composite. This

study found that the FML exhibited a greater strain at failure and an increased forming

window. In addition, it was found that the FML system displayed a more even thickness

distribution compared to a similarly formed metal part.

This study evaluates the formability of a fibre metal laminate based on a glass-fibre

reinforced polypropylene composite using a real time photogrammetric strain measurement

system. The aim is to investigate and analyse the evolution of strain to failure and to develop

a FLD and FLC for the FML system. These results will then be compared to the formability

of aluminium specimens of similar geometry.


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2 EXPERIMENTAL WORK

2.1 Materials and Laminate Preparation

The FML system investigated in this study comprises a glass-fibre reinforced

polypropylene composite, Twintex from OCV Reinforcements, and a 5005-O aluminium

alloy in a 2/1 configuration. This means that one 1mm thick composite layer was sandwiched

between two layers of 0.6mm aluminium. Two layers of a 50μm thick hot-melt polypropylene

film adhesive were used to bond the composite and aluminium to achieve a final thickness of

2.2mm for the FML. Adequate adhesion between the materials was ensured through

preparation of the bonding surfaces; the surface of the aluminium was etched in a 5% NaOH

solution for 5 minutes and the surfaces of the adhesive and composite were cleaned using

isopropyl alcohol.

The laminate stacking arrangement is illustrated in figure 2. Laminates of 230mm by

240mm were placed in a hydraulic press and heated to 155°C. Once the temperature was

achieved, a pressure of 1MPa was applied for 5 minutes after which the laminate was rapidly

water cooled. After bonding of the FML, the desired experimental geometries were obtained

using water jet cutting.

Figure 2 - Laminate stacking arrangement

2.2 Experimental Design

Seven specimens of varying width, shown in figure 3a, are stretched over a hemispherical

punch using the method developed by Hecker [20] to obtain all deformation conditions. The

widths used were 25, 50, 75, 100, 125, 150 and 200mm. These widths were chosen as they

provided a large number of specimens for the maximum possible width. Varying the width of

a specimen which is stretched over a hemispherical punch alters the lateral constraint on the

specimen. This means that, for specimens of reduced width, the sides of the specimen are not

subjected to high levels of stretch and are instead allowed to partially “draw” into the die,

which will lead to more negative values of minor strain. This effect is shown in figure 3b.

Using this method, the failure limit of the material can be found for deformation modes

ranging from uniaxial tension to biaxial stretch.


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(a)

(b)

Figure 3 - Examined experimental specimens (a) and the effect of specimen geometry on the deformation

behaviour (b)

2.3 Evaluation of formability

A custom designed 300kN stamp press with a 100mm diameter hemispherical punch and

105mm open die, shown in figure 4a, was used to evaluate the forming of the fibre metal

laminate. A local data acquisition PC controls the feed rate and punch displacement. A

compression load cell measured the punch force and a linear potentiometer provided the

punch displacement. The experiments were conducted at a feed rate of 10mm/s and the depth

at failure was determined by a 2% drop from maximum load. A universal lubricant was used

to reduce friction between the punch and the samples. The formability of the fibre metal

laminate is assessed using the evolution of strain and the strain at failure on the surface of the

laminate. These results will be used to develop a forming limit diagram for the material and to

determine a safe forming limit under different deformation conditions. An open die is used to

facilitate measurement of the strain on the surface of the material throughout the forming

process using the ARAMIS strain measurement system, shown in figure 4b.

(a) (b)

Figure 4 - Experimental specimens (a) and the ARAMIS strain measurement system (b)


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3 RESULTS AND DISCUSSION

3.1 Deformation Modes

The ARAMIS strain measurement system allows determination of the strain for every

point on the surface of the material. This allows the analysis of the strain at identified points

of interest such as regions of high strain, low strain and regions where failure occurs.

Figure 5 shows the evolution of surface strain at the pole of both the FML and aluminium

experimental specimens. These figures elucidate the theory that by altering the geometry of

the specimen it is possible to obtain the different deformation modes. The figures also

highlight three distinct behaviours in the evolution of surface strain at the pole;

• the pre-stretch region,

• initial biaxial stretch, and

• strain path to failure

The pre-stretch behaviour is caused by the lock ring used to securely clamp the specimen

and to prevent drawing of material into the die. The lock ring causes this stretch due to the

bend-unbend it introduces into the specimen. The blankholder force is controlled by six bolts

which are all tightened to the same torque. However, it was found that lower torques were

required for the smaller specimens compared to the larger specimens due to the possibility of

failure at the lock ring. The strain introduced into the specimens as a result of securing the

lock ring was obtained by taking the initial image of the specimen prior to the application of

blankholder force. Therefore, the state of zero strain indicated in the results corresponds to a

completely unloaded specimen.

(a)

(b)

Figure 5 - Evolution of the surface strain at the pole of the experimental FML (a) and aluminium (b) specimens

The deformation behaviour at the pole of the specimens following the pre-stretch is a

region of biaxial stretch. This behaviour is caused by the shape of the punch. In the initial

stages of contact between the punch and the specimen only the region in contact with the

punch deforms. Therefore, for a hemispherical punch, the pole region of the specimen will


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exhibit a deformation mode consistent with biaxial stretch. At a depth of approximately 3-

5mm the strain behaviour again changes. Figure 6 elucidates the nature of the change in

deformation behaviour at this depth which is caused by a change in the evolution of the minor

strain. This change occurs due to the reduced width of the experimental specimens in the

lateral direction which means that after a certain depth there is no more material in that

direction to allow biaxial stretch.

Figure 6 - Evolution of the minor strain in the FML specimens with increasing forming depth

The strain path to failure allows the determination of the strain ratio in the specimens,

which can be used to identify the approximate deformation mode experienced by each

specimen. These strain ratios are shown in table 1.

(a)

Specimen 25mm 50mm 75mm 100mm 125mm 150mm 200mm

Strain Ratio (β) 0.24 -0.06 -0.19 0.01 0.25 0.42 0.9

(b)

Specimen 25mm 50mm 75mm 100mm 125mm 150mm 200mm

Strain Ratio (β) -0.43 -0.2 -0.07 0 0.27 0.6 1

Table 1 - Strain ratio for the FML (a) and aluminium (b) experimental specimens

It can be seen from both figure 5 and table 1 that the FML specimens exhibit vastly

different strain ratios to aluminium specimens of similar geometry. The aluminium specimens

show the validity of using specimens of varying width as proposed by Hecker. The 25mm

aluminium specimen exhibits a strain ratio which is close to the ratio used to define uniaxial

tension, the 100mm specimen deforms in the plane strain mode and the 200mm specimen

displays balanced biaxial stretch. The other aluminium specimens allow the analysis of the

range between these deformation modes. The different behaviour of the FML is more

pronounced in the 25, 50 and 75mm specimens. Instead of demonstrating the range from

uniaxial tension to plane strain, as shown by the equivalent aluminium specimens, there

appears to be no discernible pattern in the strain ratio and deformation behaviour. The

behaviour displayed by the 75mm specimen is of particular interest as it does not follow the

same trend as any of the other FML specimens. In addition to this it also exhibits different

strain behaviour compared to the aluminium specimen. A reason for this behaviour could be


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caused by delamination of the specimen during the forming process as shown in figure 7. The

75mm specimen was the only specimen in which delamination was observed.

Figure 7 - Delamination occurring in the 75mm specimen

The difference in the strain state at failure is due to the aluminium specimens deforming to

a greater depth than the FML specimens. However, this depth does not consider the

aluminium experiencing localised necking in the regions surrounding the punch. In metal

forming studies a material which has commenced localised necking is considered to be failed

because this rapidly leads to material fracture [21], therefore this data alone cannot be used to

determine the depth at failure or the state of strain which causes failure in either of the

material systems. Further analysis of the specimens was undertaken to determine the depth of

failure and the state of strain at the region that failed first.

Figure 8 shows the forming limit diagram for the experimental geometries. These figures

show the overall strain behaviour of the FML and aluminium specimens. It can be seen that

the FML behaviour is not only different at the pole but the strain ratios for all visible points

exhibit differences. The FML specimens appear to partially behave like the monolithic

aluminium but are restricted from the peaked levels of major strain which is apparent in the

aluminium specimens. The highlighted region in figure 8b shows the peaked major strain.

This peaked behaviour occurs in regions to the side of the pole.

Figure 8 - Forming limit diagrams for the FML (a) and aluminium (b) specimens

The evolution of the strain at the pole of the FML specimens shows that the specimens can

exhibit deformation behaviour from plane strain to biaxial stretch. The 100mm and 200mm

specimens, which most accurately represent these deformation modes, were therefore chosen

for further comparison of the surface strain with aluminium. In addition, the 25mm FML

Region of peak strain


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specimen was also selected for comparison as the aluminium specimen reflected the

behaviour of uniaxial tension.

3.2 Strain Evolution

The meridian major strain on the surface of selected FML and aluminium specimens is

shown in figure 9. This information is useful for determining the points at which failure will

occur in specimens as the strain in certain regions will increase more rapidly than other areas

and therefore fail earlier. The evolution of the strain on the surface of the experimental

specimens was captured for every millimetre of forming depth.

(a)

(b)

(c)

Figure 9 - Meridian strain distribution for the experimental specimens exhibiting the uniaxial tension (a), plane

strain (b) and biaxial stretch (c) deformation modes. FML and aluminium strain distributions are on the left and

right respectively

Figure 9 shows the meridian strain distribution along a section through the specimen for


10

the 25mm, 100mm and 200mm specimens at depth increments of 5mm up until failure in the

FML specimens. The section chosen was oriented along the longitudinal axis of the

specimens, which also corresponds with the fibre direction in the composite material. In these

distributions, the centre of the specimen occurs at approximately 50mm along the section and

the edges of the die are at the 0 and 100mm distances. These distributions show that FML

specimens show a slightly more uniform strain distribution than the aluminium specimens.

This is advantageous to the forming of parts as it corresponds to a more even thickness

distribution. However, compared with the meridian strain distribution of a FML based on a

self-reinforced polypropylene composite [19] it can be seen that the stiffer glass fibres and the

single lamina nature of the composite do not allow as much distribution of the strain in the

aluminium layer.

Figure 9a shows the strain distribution for the 25mm specimens. This figure shows the

FML specimen failing at a depth of 18.9mm. This was then compared to the observed onset of

localised necking in the aluminium specimen, which was found to occur at 23.5mm. Figure

9b shows the meridian strain for the 100mm FML and aluminium specimens. In this case the

failure depths of the FML and the aluminium respectively were 21.2mm and 19.6. The

localisation of strain in aluminium specimens, prior to localised necking, can be seen

occurring to the side of the pole region. This is not yet the localised necking which precedes

failure but the increase in strain in these regions shows that the aluminium specimens can very

rapidly lose quality during forming. The 200mm specimens show that, for equivalent forming

depths, the FML and aluminium specimens show approximately the same strain at the pole. It

can be seen, however, that the regions surrounding the pole in the aluminium specimens are

increasing at a faster rate than the pole. This behaviour, even prior to necking, lowers the

quality of the formed part by causing an uneven thickness distribution across the surface of

the part.

3.3 Failure of the experimental specimens

The failure of the experimental specimens was dependent on the specimen geometry. The

larger specimens such as the 100, 125, 150 and 200mm specimens displayed failure which

was visible to the optical measurement system. Similar to metals, the failure occurred in the

region to the side of the pole and along the longitudinal axis. It was observed, however, in the

200mm specimen that failure occurred in both the warp and weft fibre directions leading to

the conclusion that the failure of the laminate was precipitated by fibre breakage. Supporting

this assertion is that, contrary to previous studies on the forming of composite and FML

systems [6, 19], this study found that some localised necking appeared in the FML specimens

prior to the appearance of a tear. The appearance of a neck immediately before tearing

supports the failure of the composite layer. This is because there was no localisation of strain

in this region, compared to the other regions, before the observation of the localised neck.

Figure 10 shows the final three images of the surface of a FML specimen before failure.

These figures show the rapid evolution of a neck to failure in the FML. In the time between

the first and second images, a fraction of a second, the fibres in the composite layer have


11

failed which transfers the entire load to the aluminium layer and causes it to neck and

subsequently tear.

Figure 10 - Appearance of localised neck prior to failure

However, as shown in figure 11, the 25mm and 50mm specimens experienced failure at the

lock ring. Therefore, the development of the FLC from these specimens was determined by

using the pole as a safe point and the regions to the side of the punch as marginal points. This

ensures that no point which might be close to failure is considered safe and therefore

constructs a conservative FLC.

Figure 11 - Failure at the lock ring in the 50mm (left) and 25mm (right) specimens

As stated previously, the larger specimens failed in the region visible to the optical

measurement system. Therefore, information about the exact state of strain at failure could be

determined and “failed” points included on the FLC. This failure, shown in figure 12,

presented as a tear in the aluminium layer and fibre breakage in the composite layer.

.

Figure 12 - Failure behaviour of the larger FML specimens

Neck Failure


12

The forming limit curves constructed for the FML and aluminium are shown in figure 13.

This display is advantageous as it allows the direct comparison of the forming limits of the

FML and aluminium. It is demonstrated in the figure that the FLC of the aluminium is slightly

higher than the FML. The FML FLC agrees with the theory that the glass fibre will fail at a

strain of approximately 4-8%, with the marginal region occurring in the region between these

strain values. The localised necking in the FML specimens can be seen from the failed points

in the figure which are at much higher strain values than can be sustained by a glass fibre

composite. In addition, they display significantly higher strain results than the marginal data

points which were taken prior to an observation of a neck such as in figure 10. The FLC for

the FML shows that, at room temperature, the strain in the glass fibre limits the ability of the

FML to deform using the stamp forming process.

(a)

(b)

Figure 13 – Forming limit curve for the FML (a) and aluminium (b)

4 CONCLUSIONS

The deformation behaviour of a FML based on a glass fibre reinforced polypropylene

composite has been observed, analysed and compared to the forming of aluminium using an

optical strain measurement technique. This technique allowed the comparison of not only the

final state of strain in the formed parts but also the evolution of the strain to failure. This

study has established that fibre metal laminates constructed from high-stiffness, low strain to

failure composites is possible with some restrictions. Firstly, the strain behaviour of the

laminate is different to that exhibited by the monolithic aluminium specimens, with negative

strain ratios difficult to attain. This behaviour could be explained by the limited draw of the

fibres in the lateral direction. Additionally, the strain at failure for the FML is determined by

the maximum strain in the glass fibre, about 4-8%, which is lower than the failure strain of the

aluminium. By orientating the glass fibre at an angle from the longitudinal axis it would be

possible to achieve higher strains in the laminate due to lower strains in the glass fibres. In

addition, a higher temperature could lead to improved forming through softening of the

composite matrix which would allow the fibres in the composite to draw in more easily.

Finally, it was demonstrated that, unlike other composites and fibre metal laminate systems,

this particular FML exhibited necking in the aluminium layer prior to failure. This study

shows that, by optimising some of the forming parameters, it could be possible to process

glass fibre reinforced fibre metal laminates.


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development of a forming limit diagram for a fibre …

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