FABRICATION OF FLEXIBLE EP-FG COMPOSITE AUTOMOBILE SHELL STRUCTURE USING COMPRESSION MOULDING PROCESS
Husam Kareem Mohsin Al-Jothery, Puteri Sri Melor Megat Yusoff, Thar M. Badri Albarody and Vikneshwaran K. Balakrishnan
Department of Mechanical Engineering, Universiti Teknologi PETRONAS
E-mail: [email protected]
ABSTRACT
Corrugations are believed to enhance the mechanical properties of a shell structure. In this study, the theory of corrugation is applied to enhance the flexibility of epoxy- fibreglass material which is being used widely in the automotive industry. It could fulfil the requirement of using a flexible composite material for automobile shell structure for a more crashworthy design. Two different corrugated profiles were studied in this project which are the damped curve and pyramidal groove profiles. Rubber moulds were fabricated and used for the corrugated epoxy-fibreglass composite preparation. A compression moulding machine was used to form the corrugations on the composite material as it can deliver high pressure and temperature to cure the laminate. In this study, the compression moulding process was carried out under constant parameters: temperature, pressure and holding time. Results obtained from the tensile test of the corrugated epoxy-fibreglass composite sheets were compared with the tensile properties of a flat epoxy-fibreglass composite sheet to determine the effect of corrugations in terms of flexibility. The pyramidal groove corrugation managed to enhance the flexibility of the epoxy-fibreglass composite sheet, however there was a significant decrease in yield load, and no notable changes in tensile strength, yield strength and elongation.
On the other hand, the flexibility achieved by the sample with damped curve corrugation is higher than the flat sample but lower than the flexibility possessed by the pyramidal groove sample. Elongation of the damped curve sample prior to fracture is two times higher compared to the elongation achieved by both pyramidal groove and flat samples. In conclusion, the epoxy-fibreglass composite sheet with the damped curve corrugation profile is the best, as it exhibits the desired increase in flexibility with minimal loss in terms of tensile strength, yield load and yield strength.
Keywords: corrugation; thermoset composite; compression moulding; tensile test; damped curve; the pyramidal groove
INTRODUCTION
In the 1890s, the automotive industry was embarked where hundreds of manufacturers were involved in pioneering the horseless carriage. In the automotive production, the main process is stamping, which consists of a sequence of sheet-metal forming manufacturing processes, for instance, blanking, embossing, bending, flanging and coining [1]. The main goal of the stamping process in the automotive
industry is to process parts of the vehicle body such as a door, hood, roof, fender, etc. The cold metal sheet is the material that is commonly used in stamping for the automotive shell structure [2].
Conventional structure for the automobile is developed to maintain a single shape throughout their design life with the complete range of loading patterns that the designer anticipates. The design of such structures is typically governed initially by efficiency, then by material expense, fabrication
expense or weight depending upon the application.
In general, the majority of structures can be classified either as truss/lattice-type structures or as continuous shells which is focused in this study. Shells can give considerable weight benefits because they offer a surface area with that of carrying loads, for instance, observing their application in many modern-day automobiles, changing the separation of the load- bearing chassis frame and non-load-bearing body, as utilized by the majority of producers prior to the 1960s [3].
Traditionally, thin-walled shells have been developing to carry external loads efficiently and rigidly, using a suitable design of their global geometry. This study aims to extend the capabilities of conventional shell structures and expand the applications by utilizing the benefits of corrugations. Introduction of the local pattern (corrugations, dimples, folds, etc.) to otherwise isotropic thin-walled sheets, the global mechanical properties of the sheets can be favourably modified [4]. Specifically, the corrugated shell structure can deliver a combination of lightweight with high strength, offering a broad scope for the designer to tailor the stiffness properties to the required behaviour [5]. Exploiting their expanded stiffness at insignificant cost of weight, corrugated sheets were utilized as the external skin for a few aircraft designs [6].
A square array of alternating conical protrusions has a significant feature as it can be deformed into both single and compound curvatures with minimal loss of load-carrying capacity of the core [7]. Another relevant textured core material was introduced based on a doubly-corrugated sheet which can be corrugated from a flat sheet without stretching of the base material: a corrugated core [8]. Shimansky and Lele [9] researched the stretching stiffness across corrugations of corrugated shells with varying thickness and amplitude. Stiffness is noticeably decreased compared to the flat plate, and in this case, deformation occurs due to the bending and stretching of the material. From the findings, it is discovered that thin shells with large corrugation amplitudes deform when it is stretched due to local bending in corrugations and the effect of local stretching can be
neglected. Moreover, the corrugated patterns enable the sheets to locally expand and contract thereby change their global Gaussian curvature without any stretching at material level [10].
Although the intricacy and automation of the fabrication process have since increased considerably, the fundamental approach of producing iron and aluminium corrugated sheets has not altered. To permit the in-depth, sharp-edged profiles used today, several rollers are utilized, that slowly develop the corrugation profile which the technique is viewed in 1899, which relates particularly to an approach for making corrugated paper The method relies on a ductile product; thus, paper is soaked to make it ductile: making use of several rollers then restricts the contortion to prevent the paper from tearing [11].
Wadley [12] has summarised work done by others in a similar field and outlined three methods required in creating corrugated sheets:
i. Corrugations pulled out of the plane of a flat sheet is particularly suitable in developing expanded honeycombs.
ii. Rolling through a gear press is quick, and no limitation is set on the sheet length across the corrugations.
iii. Corrugations pressed individually is a slower
process than gear press though enables a fullest possible array of corrugation profiles.
This paper discusses the effect of corrugations on the behaviour of thermoset composite sheet in terms of flexibility. Two corrugated profiles are focused which are a damped curve and pyramidal groove profiles.
METHOD
A. Corrugated Design Development
Two corrugated designs were selected and developed for the thermoset composite specimen which will undergo mechanical testing and analysis. The selected designs were inspired by the egg-box shell structure and sin wave pattern. However, the two designs could not be proceeded to fabricate the specimens due to cost constraint in developing the mould.
Figure 1 Damped curve 3D model design
Figure 2 Pyramidal groove 3D model design
Table 1 Specification of the design
Type of corrugation Specifications
Damped curve i. Damped curve
= (2*sin(x))/(x^0.4) ii. Width = 100 mm iii. Length = 100 mm iv. Thickness = 0.3 mm
Pyramidal groove i. Pyramidal groove
ii. Width = 100 mm iii. Length = 100 mm iv. Thickness = 0.3 mm
So, alternate designs were developed for the specimen and different technique was used to fabricate the mould which will be explained further in this report. Modelling of the specimen designs were done by using SOLIDWORKS 2016 as shown in Figures 1 and 2. Besides, all specifications of those two corrugated designs (damped curve and pyramidal groove) state in Table 1.
B. Experimental Setup
The experimental set-up for this study requires a compression moulding machine and mould. The compression moulding machine is able to deliver 25 tons of pressure at maximum, and it has a maximum of 250℃ of two heated platens.
The fabricated moulds will be compressed by the platen where the thermoset composite material is placed in between the mould beforehand. The sample produced will be prepared for the tensile test to investigate the performance of the material. In this experiment, a tensile testing machine that has the capability to deliver a maximum load of 10 kN will be used.
Table 2 Parameters of compression moulding process of thermoset composite
Parameters Thermoset
Die temperature 200℃
Stamping pressure 20 tonnes
Hold time 1 min for each specimen
Material temperature No preheating
Table 3 Parameters of compression moulding process of mould
Parameters Mould
Die temperature 200℃
Stamping pressure 20 tonnes
Hold time 15 minutes
Material temperature No preheating C. Material Selection
The material to be used for this application is a thermoset composite material which is epoxy reinforced with s-type fibreglass. The material is a prepreg with a woven type arrangement.
Thermoset composite material was selected due to its combination of strength, weight and durability.
Table 2 shows the parameters that were kept constant during the compression moulding process of the composite sheets.
D. Mould Fabrication
Moulds were fabricated by using rubber composite material. Compression moulding machine is utilized as well in this process. Two 100 mm x 100 mm square rubber composite with 20 mm thickness were prepared. Aluminium sheet of the same size except for the thickness of 1mm is cut and prepared based on the corrugated profiles studied which are a damped curve and pyramidal groove. The rubber piece was placed into the cavity of a mould which has the same size and beneath it, between the mould and rubber lies the corrugated aluminium sheet as shown in Figures 3 and 4. Compression moulding machine is
used to press the rubber material against the aluminium sheet with sufficient temperature and pressure to ensure the rubber piece acquires the corrugated profiles of the aluminium sheets, as shown in Figure 5. Parameters of the compression moulding process are shown in Table 3.
The rubber piece is left to be cooled at room temperature after the compression process where it hardens, thus suitable to be used as a mould.
a b
Figure 3 a) Damped curve mould, b) Pyramidal groove mould
a b
Figure 4 a) Damped curve sheet, b) Pyramidal groove sheet
E. Fabrication of Corrugated Thermoset Composite Sheet
The method used to fabricate the corrugated thermoset composite sheet is quite similar to the fabrication process of the mould. The prepared prepreg epoxy-glass laminate was placed onto the
corrugated aluminium sheet, and the rubber mould is placed above. Compression moulding process takes place where the parameters set were kept constant throughout the process to ensure all the samples are cured equally to avoid differing in properties.
After the process, the laminate is left to be cooled at room temperature, 26°C and it hardens to the desired corrugated profiles.
F. Preparation of Specimen for Tensile Test
For the tensile test, 5 specimens were prepared for each corrugated pattern. The specimens was cut according to the specimen dimension specified in ASTM D638, as shown in Figures 5 and 6. Specimen was cut by using a laser cutting machine to ensure the
dimensions are accurate and there are no flaws in the specimen which could affect the tensile properties during testing. With 5 sets of the same pattern for each sample chosen, the average result of the test is then calculated and analysed.
Figure 5 Type IV damped curve samples (ASTM D638)
Figure 6 Type IV pyramidal groove samples (ASTM D638)
RESULTS AND DISCUSSIONS
The tensile test was carried out for all the corrugated thermoset composite samples, and the results were obtained. For each corrugated pattern, 5 samples were prepared for the test. Based on ASTM D636 standard,
the test speed of 5.00 mm/min was used to determine the tensile properties of the samples subjected to failure. The thickness and gage width are constant for all the samples which are 0.3 mm and 6 mm. The results obtained are discussed and analysed which
will illustrate the effect of respective corrugation on the behaviour of the thermoset composite samples in terms of elastic modulus, tensile strength, yield load, yield strength and elongation. Results used for the flat thermoset composite sheet is obtained from a study done in [13], shown in Table 6.
A. Behaviour of Flat Thermoset Composite Sheet towards Tensile Test
Based on Table 4, the average tensile properties of 4 samples are obtained for a flat thermoset composite sheet. As shown in Figure 7, the elongation of the flat thermoset composite sheet relative to the load is plotted. The maximum load achieved is 66 kgf where the elongation is 0.54 mm. The graph increases linearly which shows that the sample undergoes linear elastic deformation before it experiences plastic deformation. Fracture point of the flat laminar is at 22 kgf at 0.6 mm elongation. It shows that the flat sheet undergoes very short elongation before failure; thus it has a brittle behaviour.
Table 4 Tensile properties of flat samples [13]
Tensile Properties Average value
Max. Load (N) 742.5
Elastic Modulus (MPa) 14554.3
Tensile Strength (MPa) 162.34
Yield Load (MPa) 612.71
Yield Strength (MPa) 134
Elongation (mm) 0.61
Figure 7 Graph of Load vs. Elongation for flat samples
B. Effect of Damped Curve Profile on Thermoset Composite Sheet
Table 5 shows the tensile properties obtained for the sample with damped curve profile and the average value of 5 samples. Elastic modulus of the damped curve sample is 11.63 GPa which is comparable to the flat sample which possesses an elastic modulus of 14.55 GPa and the percentage difference is 20%.
For tensile strength, the flat sample exhibits a tensile strength of 162.34 MPa while the damped curve sample is 152.9 MPa. The difference in tensile strength between the two samples is only 5.8 %. This shows that applying corrugation on the thermoset composite sheet does not significantly affect the stiffness and tensile strength of the material.
Table 5 Tensile properties of damped curve samples
Tensile Properties Average value
Max. Load (N) 275.2
Elastic Modulus (MPa) 11627.5
Tensile Strength (MPa) 152.9
Yield Load (MPa) 200.94
Yield Strength (MPa) 111.63
Elongation (mm) 1.69
Figure 8 Graph of Load vs. Elongation for damped curve samples Moving to the yield load, the value obtained for
the flat sample is 612.71 MPa, which is significantly higher compared to damped curve sample that has a value of 200.94 MPa. Yield strength achieved by both samples does not have a drastic difference as for the flat sample, it is 134 MPa and damped curve sample is 111.63 MPa. However, the elongation of the damped curve sample achieved a remarkable elongation of 1.69 mm, which is more than twice the elongation of the flat sample that is 0.61 mm. This shows that the damped curve sample is very much ductile than the flat sample. In Figure 8, it can be seen that the sample
experienced very high elongation prior to fracture under non-linear elastic deformation. The immediate fracture occurred after the maximum load is achieved which is 28 kgf, 42% lower than the maximum load achieved by the flat sample. Load at the fracture point was 25 kgf with an elongation of 1.84 mm. Despite the notable difference in yield load, the damped corrugation has huge impact on the behaviour of thermoset composite sheet as it enhances the flexibility and ductility at the cost of a reasonable sacrifice in terms of tensile strength, yield load and yield strength.
C. Effect of Pyramidal Groove on Thermoset Composite Sheet
Table 6 shows the tensile properties obtained for the sample with pyramidal groove profile and the average value of 5 samples. The elastic modulus shown by the pyramidal groove sample is 4.68 GPa, which is notably low compared to the flat sample that has an elastic modulus of 14.55 GPa. This shows that the pyramidal groove sample has very low stiffness compared to the flat sample. Tensile strength of the pyramidal groove is 149.9 MPa whereas the flat sample exhibit a tensile strength of 162.34 MPa. The difference in tensile strength between the two samples is only 7.7 %. It can be said that the application of pyramidal groove profile on the thermoset composite sheet does not have major impact on the tensile strength.
Table 6 Tensile properties of pyramidal groove samples
Tensile Properties Average value
Max. Load (N) 270
Elastic Modulus (MPa) 4682.8
Tensile Strength (MPa) 149.93
Yield Load (MPa) 242.65
Yield Strength (MPa) 134.8
Elongation (mm) 0.78
Figure 9 Graph of Load vs. Elongation for pyramidal groove samples
Focusing on the yield load, the flat sample has a remarkably high value which is 612.71 MPa while the yield load of the pyramidal groove sample is only 242.64 MPa. Yield strength exhibited by the pyramidal groove sample is 134.8 MPa, the difference is insignificant when compared to the flat sample that possesses a yield strength of 134 MPa. In terms of elongation, the values obtained for both the samples do not have much difference, 0.78 mm for pyramidal groove sample and 0.61 mm for the flat sample.
Thus, it is clear that the tensile properties portrayed by the pyramidal groove sample are almost similar to the flat sample except for the yield load. In Figure 9, it can be noticed that the sample experience very low elongation prior to fracture under linear elastic
deformation. After the maximum load is achieved which is 25 kgf, the pyramidal groove experience failure with a gradual decrease in load. This clearly shows that the behaviour of the pyramidal groove sample under load is comparable to the flat sample.
Based on the interpretation, the pyramidal groove corrugation managed to enhance the flexibility of the thermoset composite sheet; however there was a significant decrease in yield load, and no notable changes in tensile strength, yield strength and elongation.
Comparison of tensile properties between damped curve sample and pyramidal groove is made as well to obtain a better understanding of the behaviour of the epoxy-fibreglass composite sheet towards different corrugated profiles. Elastic modulus of the damped curve and pyramidal groove sample is 11.62 GPa and 4.68 GPa respectively. Damped curve sample exhibits 60% higher elastic modulus than pyramidal groove sample which shows that the damped curve sample is stiffer. In terms of tensile strength, the difference between both samples are not significant as for the damped curve sample, it is 152.89 MPa while for the pyramidal groove sample is 149.9 MPa. Yield load for the pyramidal groove sample is 242.64 MPa whereas for the damped curve sample is 200.94 MPa. Moving to the yield strength, the damped curve sample exhibits 111.63 MPa while the pyramidal groove sample exhibits 134.80 MPa. This shows that the yield load and yield strength of the damped curve sample are 17.2 % lower than the pyramidal groove sample.
However, the crucial aspect that distinguishes the damped curve sample with the pyramidal groove sample is the amount of elongation it achieved prior to failure. Elongation of the damped curve sample is 1.69 mm, which is more than twice the elongation achieved by the pyramidal groove sample that is 0.78 mm. This demonstrates that the damped curve sample is very ductile compared to the pyramidal groove sample.
Based on the data analysed, it is evident that the corrugated profile decreases the stiffness of a material, which is similar to the finding by [9]. The ductility of the material also increases when the corrugation
is applied. This is because the material undergoes deformation by bending and stretching due to the corrugation when the load is applied, causing the material to further elongate. From the comparison made, the epoxy-fibreglass sheet with the damped curve corrugation is the best, as it exhibits high flexibility with minimal loss in terms of tensile strength and yield strength compared to the pyramidal groove epoxy-fibreglass sheet. The stiffness of the pyramidal groove sheet is also lower when compared to the damped curve sheet. The distinguishing factor, in this case, is the elongation achieved by the damped curve sample which is substantially higher. This is due to the high number of hinge points developed in the damped curve sample, which enhances the bending of the material during deformation.
This finding can be used as a reference by automobile manufacturers that intend to use a more flexible composite material to fabricate the shell structure for a crashworthy design. For parts that require high strength, the number of layers of the composite can be increased which can deliver outstanding mechanical properties, potentially eliminating the usage of metals such as aluminium and steel. Furthermore, it is observed that the behaviour of the damped curve sample under load is quite similar to the behaviour of rubber material, as shown in Figure 8. This shows that there is a high potential for the damped curve epoxy- fibreglass composite to replace rubber materials in specific applications like the cable grommets and high-temperature seals found in automobiles, considering the resistance to elevated temperatures.
CONCLUSION AND RECOMMENDATIONS
The tensile properties of the epoxy-fibreglass composite sheet with two different corrugated profiles which are a damped curve and pyramidal groove were thoroughly studied. Pyramidal groove corrugation managed to increase the flexibility of the sample where the elastic modulus obtained is 4.68 GPa compared to the flat sample which is 14.55 GPa.
However, other properties of the pyramidal groove sample do not have significant changes due to the
corrugation applied based on the comparison done with the flat sample except for the yield load. On the other hand, the damped curve sample exhibits outstanding properties in terms of flexibility, which is the focus of this study. Elastic modulus achieved by the damped curve sample is 11.63 GPa, which proves that it is more flexible than the flat sample that has an elastic modulus of 14.55 GPa.
In terms of elongation, damped curve sample stands out among the rest of the samples as it managed to elongate up to 1.69 mm prior to fracture, which is more than twice the elongation achieved by the flat and pyramidal groove samples. Thus, it is proved that despite the notable decrease in yield load, the damped curve epoxy-fibreglass composite sheet happens to exhibit very good flexibility with minimal loss in terms of tensile strength and yield strength. In future work, it is recommended to increase the number of layers for the epoxy-fibreglass sheet to develop a flexible material with high strength that can be used in a variety of applications involving metals such as steel and aluminium. Other than that, variation in the type corrugated profiles being experimented has to be increased to enhance the understanding of material behaviour towards corrugation. It also enables to tailor the mechanical properties of a material according to the desired application by using a specific corrugated profile.
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