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Development of a new structural prepreg: characterisation of handling, drape and tack properties
Composite Structures 66 (2004) 169–174 www.elsevier.com/locate/compstruct
Development of a new structural prepreg: characterisation of handling, drape and tack properties R. Banks a, A.P. Mouritz a
a,*
, S. John a, F. Coman b, R. Paton
c
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia b Australian Composites Pty. Ltd, 124-132 Cochranes Rd, Moorabbin, Victoria 3189, Australia c CRC for Advanced Composite Structures Ltd., 506 Lorimer Street, Fishermans Bend, Victoria 3207, Australia Available online 9 June 2004
Abstract This paper describes a case-study in the development of a new type of woven glass/epoxy prepreg for use in marine and civil infrastructure applications. Experimental tests were performed on the prepreg at different levels of matrix cure between 1% and 59% to determine the condition that provides the optimum handling, drape and tack properties at room temperature. Bias extension testing was used to characterise the shear resistance and residual stress, bending stiffness tests to assess the drape, and peel tests to determine the tack of the prepregs. A low level of cure (<20%) provided the prepreg with excellent handling and drape properties due to the low complex viscosity of the resin. However, the low cure level resulted in the prepreg having insufficient tack. A high level of cure (59%), on the other hand, provided poor handling, drape and tack properties. The optimum level of cure was found to be approximately 30%, which combined an acceptable level of handling, low residual stress, high drape and good tack. Based on this research, a new type of prepreg is being produced in commercial quantities in Australia for domestic use and overseas export. 2004 Elsevier Ltd. All rights reserved. Keywords: Prepreg; Manufacturing; Tack; Drape; Cure
1. Introduction The Australian composites manufacturing industry is small on a global scale, but is extremely diverse with the production of a range of aerospace, marine, automotive, housing and infrastructure products for local use and overseas export. The market for composite products made in Australia is sizable and this is expected to increase considerably in coming years. A large proportion of composite products are made using prepregs that must be imported from overseas because Australia does not have a prepreg manufacturing capability. The geographic isolation of Australia from prepreg manufacturers in Europe, United States and Japan means that prepregs can be expensive due to high importation costs associated with the requirement for refrigeration during transportation over long distances. The Australian Government awarded a grant in 2000 to a consortium of industry (Speciality Coatings Pty *
Corresponding author. Tel.: +61-3-9925-6269; fax: +61-3-99256108. E-mail address: [email protected] (A.P. Mouritz). 0263-8223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2004.04.034
Ltd), research institution (CRC for Advanced Composite Structures) and university partners (Australian National University, Monash University, RMIT University) to develop the manufacturing capability for a new range of low cost prepreg products. It was believed that attempting to develop a carbon/epoxy prepreg product would not be economically viable due to limited need by the local market. Instead, a strategic decision was made to focus on prepregs reinforced with continuous glass fibres due to existing and developing market opportunities in the Australian boat building and infrastructure industries. The resin matrices selected for the prepreg products were UV-curing polyester, UV-curing vinyl ester, phenolic and epoxy. This paper describes the research and development activities undertaken to determine the level of resin matrix cure that will provide the new glass/epoxy prepreg product with the optimum handling, drape and tack properties. The parameters influencing the handling and drape of a prepreg include fibre architecture and the rheology, surface free energy, volume content and level of cure of the resin matrix. The level of cure is the one parameter that the prepreg manufacturer has explicit
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control over and for this reason the effect of level of cure on the handling, drape and tack of the woven glass/ epoxy prepreg product is experimentally investigated to determine the optimum value for prepreg production. The work presented in this paper is an interesting casestudy of the research and development required to optimise the handling, drape and tack of low cost prepreg products, which is a topic not often described in the scientific literature.
2. Prepreg material and characterisation techniques 2.1. Prepreg Glass/epoxy prepreg was produced with different levels of matrix cure to determine the value that provides the optimum handling, drape and tack properties. The resin used in the prepreg is a solvent-free DGEBA epoxy (Dow Chemicals) premixed with 10% hardener (Ethacure 100). The reinforcement is a plain woven glass fabric with an areal density of 385 g/m2 supplied by Colan. The fabric was impregnated with the liquid resin using a vacuum transfer resin infusion process at room temperature. The fibre volume content of the prepreg was 49 2.5%. After infusion, the prepreg was held at room temperature for different times to vary the level of cure between 1% and 57%, as listed in Table 1. The cure reaction was stopped when the resin reached the desired level by storing the material in a freezer at )15 C. The level of cure was measured using mid-infrared Fourier transform spectroscopy. The rheology of the neat epoxy resin at the different levels of cure was determined at 25 C using a parallel plate rheometer (Rheometrics Scientific Ares) operated at a strain of 1% at rotational speeds between 1 and 19 rad/s. Fig. 1 shows the effect of cure level on the complex viscosity (Eta*) of the epoxy tested at different test speeds. As expected, the complex viscosity increases rapidly with the level of cure. The resin viscosity is independent of the test frequency when the level of cure is below 30%, revealing that the resin behaves as a typical Newtonian fluid. However, at the highest level of cure of 57% the viscosity is clearly dependent on the rotation speed. This indicates a change from Newtonian to non-Newtonian behaviour when the cure level is raised from 30% to 57%.
Fig. 1. Effect of the level of cure on the complex viscosity of the prepreg.
2.2. Experimental methods 2.2.1. Bias extension testing Bias extension tests were performed on the prepreg to determine the effect of level of cure on the intraply shear stiffness and intraply shear relaxation properties. The shear stiffness of the prepreg was measured using a single ply specimen (400 mm long by 160 mm wide) that was clamped with an edge fixture attached to a tensile machine, as shown in Fig. 2. The fibres were aligned at bias angles (±45) from the tensile load direction, and the specimen was loaded at a rate of 120 mm/min. During testing the rotation of the bias fibres under increasing load was measured, and the test was stopped when the prepreg experienced significant buckling or
Table 1 Level of cure of the test prepregs Prepreg number
Level of cure (%)
1 2 3 4 5
1 19 26 29 57
Fig. 2. The bias extension test used to measure the shear resistance and residual shear load of the prepregs.
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distortion. Bias extension tests were also performed on the dry reinforcing fabric. In addition to shear stiffness, the bias extension test was used to measure the residual shear stress of the prepreg after forming. During forming operations it is usual for prepregs to experience a residual (internal) stress after being extensively deformed in shear, and it is important that this stress is small to reduce the problems of spring-back and cracking of the consolidated composite article. The residual stress was measured by preloading the prepreg in the bias extension test to global strains of 5%, 10%, 15%, 20% or 25%. Following this, the decay of the residual load level with time was measured. 2.2.2. Flexural rigidity testing The apparent flexural rigidity of the prepregs was investigated because of the importance of bending stiffness in forming preforms into curved shapes. The flexural rigidity of the prepreg at different cure levels was measured using the ASTM D1388 method, which is shown schematically in Fig. 3. Strips of prepreg (250 mm long · 30 mm wide) were cut in the bias direction because in this condition the resin viscosity has the greatest influence on bending stiffness. The prepreg was supported at one end, and 130 mm was allowed to hang over the edge of a support plate. The overhang length were measured after 30 s, and from this the apparent flexural rigidity of the prepreg was calculated. Five prepreg samples were tested for each level of cure. In addition, the apparent flexural rigidity of the dry fabric reinforcement used in the prepreg was determined. 2.2.3. Prepreg tack testing While techniques have been proposed for measuring prepreg tack, a standardised test method is not available [1,2]. The ASTM D3167 floating roller peel test is often used for measuring the tack of ‘‘pressure sensitive’’ adhesives and has been used by prepreg manufacturers [1], and therefore this method was selected for determining the tack of the prepreg at the different levels of cure. A schematic of the peel test is shown in Fig. 4. Strips of the prepreg measuring 250 mm long and 20 mm
Fig. 4. Schematic of the ASTM D3167 method used to measure the tack of the prepregs.
wide were placed on the surface of a metal adherand. The prepreg was placed in the bias fibre direction (±45) to minimise the bending stiffness. Untreated aluminium was chosen as the adherand because it is used as a tool material, although similar results are expected using steel as the adherand. The prepreg was applied to the adherand with a pressure of 10 kN for 1 min. The tack was measured at room temperature by the force required to peel the prepreg from the adherand at a cross-head speed of 100 mm/min. Five peel tests were performed for each level of cure.
3. Results and discussion 3.1. Shear deformation and relaxation of prepregs The intraply shear resistance of prepregs is an important property controlling the ease of handling and shearing during manufacture. It is essential that the shear resistance is sufficiently low that the prepreg can be easily pulled into shape in a mould by the
Fig. 3. Schematic of the ASTM D1388 method used to measure the apparent flexural rigidity of the prepregs.
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manufacturer of a composite product. A prepreg may need to be sheared to an angle of up to 20 for gently curved components (such as a boat hull), and the angle of shear can be as high as 50 for more complex shaped parts where the prepreg is required to conform into sharp corners (e.g. a flanged hemisphere) [3]. Ergonomic studies by Rodgers et al. [4] have shown that most people are able to exert a force of up to 45 N when pulling a thin sheet (such as a prepreg ply) that is held in a pinch grip. Rogers et al. [4] also found that this force drops to only 30 N when an operator is required to repeat the action many times during a working day. Therefore, the effect of resin cure on the ability of an operator to manually shear the prepreg under these conditions was investigated using the bias extension test method. Fig. 5 presents a linear-log plot of shear angle against bias load resistance for the prepreg at different levels of cure. Also shown is the bias load curve for the dry reinforcing fabric used in the prepreg. It is seen that the load resistance of the prepreg with the highest level of cure increases extremely rapidly with shear angle, and this is due to the high complex viscosity and elastic shear response of the epoxy matrix. This prepreg can only be sheared by a small amount (<7) before the load resistance in this test configuration exceeds the force that an operator can manually apply during forming. The prepregs with lower levels of cure (below 30%) exhibited shear loads that were typically two orders of magnitude lower, due to their lower viscosity and greater ability to undergo shear by viscous (rather than elastic) processes. As expected, the load resistance increased with the level of cure from 1% to 29% at shear angles below 30, although in all cases the loads are within the limits for manual forming. Above a shear angle of 30 there is no significant difference in the four prepregs because the shear resistance is being determined by lock-up of the fabric tows rather than the level of resin cure. It is interesting to note that the load resistance of the prepreg with the lowest levels of cure (i.e. 1% and 19%) was even lower than the dry fabric at shear angles below
Fig. 5. Linear-log curves of force against shear angle for the prepregs.
20. Wang et al. [5] observed similar behaviour in bias extension tests on prepregs performed at elevated temperatures. It is believed that the resin viscosity is so low that it provides a lubricating effect that facilitates the shear rotation of the fibre tows. Based on the bias extension tests, it is apparent that the shear resistance of the prepreg with the highest level of cure (57%) is well above the force that can be applied manually by an operator during preforming. The prepreg can be sheared up to 35 by an operator when the cure level is below 30%. At higher shear angles it would be difficult for an operator to deform the fabric by hand. As mentioned earlier, a problem often encountered during the preforming of prepregs is the generation of a significant residual stress experienced after shear deformation into complex shapes. It is desirable to have the prepreg exhibit negligible residual stress so that the preform retains its shape on the tool surface and the likelihood of spring-back and cracking of the composite product is minimised. Fig. 6 shows the decay in the residual load of the prepreg with different levels of cure over a period of 5 s. In this case the prepreg samples were elongated in bias extension to a strain of 10% and then the reduction to the residual load was measured, although similar results were obtained for pre-strain levels between 5% and 25%. It is seen that the residual load drops rapidly within 1–2 s for the prepregs with cure levels below 30%. Following this initial drop, the residual load decreases very slowly with time, and the residual load level for the prepregs is similar at 4–5 N. The decay curve for the prepreg with the highest level of cure (57%) is extremely high, and remains high even after several minutes. These results show that the prepreg has a low residual shear stress when the level of cure is below 30%, however at a high cure level the stress is very large and therefore would not be acceptable in the manufacture of composite parts.
Fig. 6. Linear-log curves of residual force against time for the prepregs.
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3.2. Bending stiffness of the prepregs Fig. 7 shows the variation in the apparent flexural rigidity of the prepreg with increasing level of cure. Also shown for comparison is the rigidity of the dry woven fabric used as the reinforcement in the prepreg. It is seen that the flexural rigidity rises gradually with level of cure up to 30%, but there is a large increase at the highest cure level. The increased stiffness is caused by the rapid rise in resin viscosity with higher levels of cure. 3.3. Tack properties of the prepregs Tack is an important material property because it determines the ability of the prepreg plies to adhere to each other and the tool surface during lay-up. The tack should not be too low because insufficient adhesion will cause the prepreg to move and slip during preforming. However, when the tack is too high it is difficult to separate ply layers that may have been incorrectly placed. Despite the importance of tack in the lay-up of prepreg, little information has been published on the tack properties of prepreg materials [1,6,7]. The influence of level of cure on the peel fracture energy of the glass/epoxy prepreg is shown in Fig. 8. The peel resistance is very small at low and high cure levels, and the maximum measured value was found at 30% cure. An examination of the fracture surfaces to the peel specimens after testing revealed that at low cure levels (<20%) the matrix viscosity was low enough to fully wetout the adherand. However, the low level of cure resulted in the resin having insufficient strength to provide adequate peel resistance. At the highest level of cure (57%) it was found that the resin was able to bond strongly, but its viscosity was extremely high which caused poor wet-out of the adherand. The level of cure of 30% provided the best balance of wet-out and
Fig. 8. Effect of level of cure on the peel fracture energy of the prepregs.
cohesive strength that resulted in the highest peel resistance for the prepreg. 4. Conclusions The selection of an optimum level of resin cure for the handling, drape and tack properties of a new, low-cost woven glass/epoxy prepreg was evaluated using various laboratory tests, including bias extension testing to evaluate shear stiffness and shear relaxation, flexural rigidity to determine drape, and fracture peel testing to assess tack. It was found that a low level of cure (<20%) provided the prepreg with excellent forming properties, but the material lacked sufficient tack to stay bonded to a tool surface during preforming. At a relatively high level of cure (57%), the complex viscosity of the resin was extremely high which made the prepreg difficult to manually deform in shear and bending. Further problems were the highly cured prepreg retained a high residual shear stress after forming, and inadequate tack because of the inability of the matrix to adequately wetout the tool surface. The optimum level of cure examined in these tests was found to be 30%, which in this condition allows the prepreg to be deformed by hand during lay-up, exhibits little residual stress after shear deformation, and has high adhesive tack strength with a metallic tool surface. Based on the research presented in the paper, the Australian company ‘Speciality Coatings’ are now producing glass/epoxy prepreg with a level of cure of approximately 30%.
Acknowledgements
Fig. 7. Effect of level of cure on the apparent flexural rigidity of the prepregs.
The research was funded by a START grant titled ‘Multilayer technical textiles for high performance rigid composite products’ awarded by the Department of Industry, Science and Resources of the Commonwealth
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of Australia. The research was undertaken as a commercial task with RMIT University and as a research program of the CRC for Advanced Composite Structures Ltd. References [1] Seferis J, Meissonnier J. Development of tack and drape test for prepregs based on viscoelastic principles. SAMPE Q 1988:55–64. [2] Procter P. Tack tester improves composites fabrication. Aviat Week Space Tec 1998.
[3] Arimitsu Y, Chou T-W. Modeling of yarn orientation and stress concentration in fabric shaping process. Proceedings of 6th Japan Int. SAMPE Symposium, 1999, pp. 551–54. [4] Rodgers S, Kiser D, Murphy T, Nielsen W, Rodgers S. Ergonomic Design for People at Work. John Wiley & Sons; 1986. [5] Wang J, Paton R, Page J. Forming properties of themoset fabric prepregs at room and elevated temperatures. CRC-ACS Internal Report, 1997. [6] Gillanders A, Kerr S, Martin T. Determination of prepreg tack. Int J Adhes Adhes 1981:125–34. [7] Putnam J, Haynes B, Seferis J. Prepreg process-structure-property analysis and scale-up for manufacture and performance. J Adv Mater 1996:47–57.
Development of a new structural prepreg: characterisation of handling, drape and tack properties R. Banks a, A.P. Mouritz a
a,*
, S. John a, F. Coman b, R. Paton
c
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia b Australian Composites Pty. Ltd, 124-132 Cochranes Rd, Moorabbin, Victoria 3189, Australia c CRC for Advanced Composite Structures Ltd., 506 Lorimer Street, Fishermans Bend, Victoria 3207, Australia Available online 9 June 2004
Abstract This paper describes a case-study in the development of a new type of woven glass/epoxy prepreg for use in marine and civil infrastructure applications. Experimental tests were performed on the prepreg at different levels of matrix cure between 1% and 59% to determine the condition that provides the optimum handling, drape and tack properties at room temperature. Bias extension testing was used to characterise the shear resistance and residual stress, bending stiffness tests to assess the drape, and peel tests to determine the tack of the prepregs. A low level of cure (<20%) provided the prepreg with excellent handling and drape properties due to the low complex viscosity of the resin. However, the low cure level resulted in the prepreg having insufficient tack. A high level of cure (59%), on the other hand, provided poor handling, drape and tack properties. The optimum level of cure was found to be approximately 30%, which combined an acceptable level of handling, low residual stress, high drape and good tack. Based on this research, a new type of prepreg is being produced in commercial quantities in Australia for domestic use and overseas export. 2004 Elsevier Ltd. All rights reserved. Keywords: Prepreg; Manufacturing; Tack; Drape; Cure
1. Introduction The Australian composites manufacturing industry is small on a global scale, but is extremely diverse with the production of a range of aerospace, marine, automotive, housing and infrastructure products for local use and overseas export. The market for composite products made in Australia is sizable and this is expected to increase considerably in coming years. A large proportion of composite products are made using prepregs that must be imported from overseas because Australia does not have a prepreg manufacturing capability. The geographic isolation of Australia from prepreg manufacturers in Europe, United States and Japan means that prepregs can be expensive due to high importation costs associated with the requirement for refrigeration during transportation over long distances. The Australian Government awarded a grant in 2000 to a consortium of industry (Speciality Coatings Pty *
Corresponding author. Tel.: +61-3-9925-6269; fax: +61-3-99256108. E-mail address: [email protected] (A.P. Mouritz). 0263-8223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2004.04.034
Ltd), research institution (CRC for Advanced Composite Structures) and university partners (Australian National University, Monash University, RMIT University) to develop the manufacturing capability for a new range of low cost prepreg products. It was believed that attempting to develop a carbon/epoxy prepreg product would not be economically viable due to limited need by the local market. Instead, a strategic decision was made to focus on prepregs reinforced with continuous glass fibres due to existing and developing market opportunities in the Australian boat building and infrastructure industries. The resin matrices selected for the prepreg products were UV-curing polyester, UV-curing vinyl ester, phenolic and epoxy. This paper describes the research and development activities undertaken to determine the level of resin matrix cure that will provide the new glass/epoxy prepreg product with the optimum handling, drape and tack properties. The parameters influencing the handling and drape of a prepreg include fibre architecture and the rheology, surface free energy, volume content and level of cure of the resin matrix. The level of cure is the one parameter that the prepreg manufacturer has explicit
170
R. Banks et al. / Composite Structures 66 (2004) 169–174
control over and for this reason the effect of level of cure on the handling, drape and tack of the woven glass/ epoxy prepreg product is experimentally investigated to determine the optimum value for prepreg production. The work presented in this paper is an interesting casestudy of the research and development required to optimise the handling, drape and tack of low cost prepreg products, which is a topic not often described in the scientific literature.
2. Prepreg material and characterisation techniques 2.1. Prepreg Glass/epoxy prepreg was produced with different levels of matrix cure to determine the value that provides the optimum handling, drape and tack properties. The resin used in the prepreg is a solvent-free DGEBA epoxy (Dow Chemicals) premixed with 10% hardener (Ethacure 100). The reinforcement is a plain woven glass fabric with an areal density of 385 g/m2 supplied by Colan. The fabric was impregnated with the liquid resin using a vacuum transfer resin infusion process at room temperature. The fibre volume content of the prepreg was 49 2.5%. After infusion, the prepreg was held at room temperature for different times to vary the level of cure between 1% and 57%, as listed in Table 1. The cure reaction was stopped when the resin reached the desired level by storing the material in a freezer at )15 C. The level of cure was measured using mid-infrared Fourier transform spectroscopy. The rheology of the neat epoxy resin at the different levels of cure was determined at 25 C using a parallel plate rheometer (Rheometrics Scientific Ares) operated at a strain of 1% at rotational speeds between 1 and 19 rad/s. Fig. 1 shows the effect of cure level on the complex viscosity (Eta*) of the epoxy tested at different test speeds. As expected, the complex viscosity increases rapidly with the level of cure. The resin viscosity is independent of the test frequency when the level of cure is below 30%, revealing that the resin behaves as a typical Newtonian fluid. However, at the highest level of cure of 57% the viscosity is clearly dependent on the rotation speed. This indicates a change from Newtonian to non-Newtonian behaviour when the cure level is raised from 30% to 57%.
Fig. 1. Effect of the level of cure on the complex viscosity of the prepreg.
2.2. Experimental methods 2.2.1. Bias extension testing Bias extension tests were performed on the prepreg to determine the effect of level of cure on the intraply shear stiffness and intraply shear relaxation properties. The shear stiffness of the prepreg was measured using a single ply specimen (400 mm long by 160 mm wide) that was clamped with an edge fixture attached to a tensile machine, as shown in Fig. 2. The fibres were aligned at bias angles (±45) from the tensile load direction, and the specimen was loaded at a rate of 120 mm/min. During testing the rotation of the bias fibres under increasing load was measured, and the test was stopped when the prepreg experienced significant buckling or
Table 1 Level of cure of the test prepregs Prepreg number
Level of cure (%)
1 2 3 4 5
1 19 26 29 57
Fig. 2. The bias extension test used to measure the shear resistance and residual shear load of the prepregs.
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171
distortion. Bias extension tests were also performed on the dry reinforcing fabric. In addition to shear stiffness, the bias extension test was used to measure the residual shear stress of the prepreg after forming. During forming operations it is usual for prepregs to experience a residual (internal) stress after being extensively deformed in shear, and it is important that this stress is small to reduce the problems of spring-back and cracking of the consolidated composite article. The residual stress was measured by preloading the prepreg in the bias extension test to global strains of 5%, 10%, 15%, 20% or 25%. Following this, the decay of the residual load level with time was measured. 2.2.2. Flexural rigidity testing The apparent flexural rigidity of the prepregs was investigated because of the importance of bending stiffness in forming preforms into curved shapes. The flexural rigidity of the prepreg at different cure levels was measured using the ASTM D1388 method, which is shown schematically in Fig. 3. Strips of prepreg (250 mm long · 30 mm wide) were cut in the bias direction because in this condition the resin viscosity has the greatest influence on bending stiffness. The prepreg was supported at one end, and 130 mm was allowed to hang over the edge of a support plate. The overhang length were measured after 30 s, and from this the apparent flexural rigidity of the prepreg was calculated. Five prepreg samples were tested for each level of cure. In addition, the apparent flexural rigidity of the dry fabric reinforcement used in the prepreg was determined. 2.2.3. Prepreg tack testing While techniques have been proposed for measuring prepreg tack, a standardised test method is not available [1,2]. The ASTM D3167 floating roller peel test is often used for measuring the tack of ‘‘pressure sensitive’’ adhesives and has been used by prepreg manufacturers [1], and therefore this method was selected for determining the tack of the prepreg at the different levels of cure. A schematic of the peel test is shown in Fig. 4. Strips of the prepreg measuring 250 mm long and 20 mm
Fig. 4. Schematic of the ASTM D3167 method used to measure the tack of the prepregs.
wide were placed on the surface of a metal adherand. The prepreg was placed in the bias fibre direction (±45) to minimise the bending stiffness. Untreated aluminium was chosen as the adherand because it is used as a tool material, although similar results are expected using steel as the adherand. The prepreg was applied to the adherand with a pressure of 10 kN for 1 min. The tack was measured at room temperature by the force required to peel the prepreg from the adherand at a cross-head speed of 100 mm/min. Five peel tests were performed for each level of cure.
3. Results and discussion 3.1. Shear deformation and relaxation of prepregs The intraply shear resistance of prepregs is an important property controlling the ease of handling and shearing during manufacture. It is essential that the shear resistance is sufficiently low that the prepreg can be easily pulled into shape in a mould by the
Fig. 3. Schematic of the ASTM D1388 method used to measure the apparent flexural rigidity of the prepregs.
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manufacturer of a composite product. A prepreg may need to be sheared to an angle of up to 20 for gently curved components (such as a boat hull), and the angle of shear can be as high as 50 for more complex shaped parts where the prepreg is required to conform into sharp corners (e.g. a flanged hemisphere) [3]. Ergonomic studies by Rodgers et al. [4] have shown that most people are able to exert a force of up to 45 N when pulling a thin sheet (such as a prepreg ply) that is held in a pinch grip. Rogers et al. [4] also found that this force drops to only 30 N when an operator is required to repeat the action many times during a working day. Therefore, the effect of resin cure on the ability of an operator to manually shear the prepreg under these conditions was investigated using the bias extension test method. Fig. 5 presents a linear-log plot of shear angle against bias load resistance for the prepreg at different levels of cure. Also shown is the bias load curve for the dry reinforcing fabric used in the prepreg. It is seen that the load resistance of the prepreg with the highest level of cure increases extremely rapidly with shear angle, and this is due to the high complex viscosity and elastic shear response of the epoxy matrix. This prepreg can only be sheared by a small amount (<7) before the load resistance in this test configuration exceeds the force that an operator can manually apply during forming. The prepregs with lower levels of cure (below 30%) exhibited shear loads that were typically two orders of magnitude lower, due to their lower viscosity and greater ability to undergo shear by viscous (rather than elastic) processes. As expected, the load resistance increased with the level of cure from 1% to 29% at shear angles below 30, although in all cases the loads are within the limits for manual forming. Above a shear angle of 30 there is no significant difference in the four prepregs because the shear resistance is being determined by lock-up of the fabric tows rather than the level of resin cure. It is interesting to note that the load resistance of the prepreg with the lowest levels of cure (i.e. 1% and 19%) was even lower than the dry fabric at shear angles below
Fig. 5. Linear-log curves of force against shear angle for the prepregs.
20. Wang et al. [5] observed similar behaviour in bias extension tests on prepregs performed at elevated temperatures. It is believed that the resin viscosity is so low that it provides a lubricating effect that facilitates the shear rotation of the fibre tows. Based on the bias extension tests, it is apparent that the shear resistance of the prepreg with the highest level of cure (57%) is well above the force that can be applied manually by an operator during preforming. The prepreg can be sheared up to 35 by an operator when the cure level is below 30%. At higher shear angles it would be difficult for an operator to deform the fabric by hand. As mentioned earlier, a problem often encountered during the preforming of prepregs is the generation of a significant residual stress experienced after shear deformation into complex shapes. It is desirable to have the prepreg exhibit negligible residual stress so that the preform retains its shape on the tool surface and the likelihood of spring-back and cracking of the composite product is minimised. Fig. 6 shows the decay in the residual load of the prepreg with different levels of cure over a period of 5 s. In this case the prepreg samples were elongated in bias extension to a strain of 10% and then the reduction to the residual load was measured, although similar results were obtained for pre-strain levels between 5% and 25%. It is seen that the residual load drops rapidly within 1–2 s for the prepregs with cure levels below 30%. Following this initial drop, the residual load decreases very slowly with time, and the residual load level for the prepregs is similar at 4–5 N. The decay curve for the prepreg with the highest level of cure (57%) is extremely high, and remains high even after several minutes. These results show that the prepreg has a low residual shear stress when the level of cure is below 30%, however at a high cure level the stress is very large and therefore would not be acceptable in the manufacture of composite parts.
Fig. 6. Linear-log curves of residual force against time for the prepregs.
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3.2. Bending stiffness of the prepregs Fig. 7 shows the variation in the apparent flexural rigidity of the prepreg with increasing level of cure. Also shown for comparison is the rigidity of the dry woven fabric used as the reinforcement in the prepreg. It is seen that the flexural rigidity rises gradually with level of cure up to 30%, but there is a large increase at the highest cure level. The increased stiffness is caused by the rapid rise in resin viscosity with higher levels of cure. 3.3. Tack properties of the prepregs Tack is an important material property because it determines the ability of the prepreg plies to adhere to each other and the tool surface during lay-up. The tack should not be too low because insufficient adhesion will cause the prepreg to move and slip during preforming. However, when the tack is too high it is difficult to separate ply layers that may have been incorrectly placed. Despite the importance of tack in the lay-up of prepreg, little information has been published on the tack properties of prepreg materials [1,6,7]. The influence of level of cure on the peel fracture energy of the glass/epoxy prepreg is shown in Fig. 8. The peel resistance is very small at low and high cure levels, and the maximum measured value was found at 30% cure. An examination of the fracture surfaces to the peel specimens after testing revealed that at low cure levels (<20%) the matrix viscosity was low enough to fully wetout the adherand. However, the low level of cure resulted in the resin having insufficient strength to provide adequate peel resistance. At the highest level of cure (57%) it was found that the resin was able to bond strongly, but its viscosity was extremely high which caused poor wet-out of the adherand. The level of cure of 30% provided the best balance of wet-out and
Fig. 8. Effect of level of cure on the peel fracture energy of the prepregs.
cohesive strength that resulted in the highest peel resistance for the prepreg. 4. Conclusions The selection of an optimum level of resin cure for the handling, drape and tack properties of a new, low-cost woven glass/epoxy prepreg was evaluated using various laboratory tests, including bias extension testing to evaluate shear stiffness and shear relaxation, flexural rigidity to determine drape, and fracture peel testing to assess tack. It was found that a low level of cure (<20%) provided the prepreg with excellent forming properties, but the material lacked sufficient tack to stay bonded to a tool surface during preforming. At a relatively high level of cure (57%), the complex viscosity of the resin was extremely high which made the prepreg difficult to manually deform in shear and bending. Further problems were the highly cured prepreg retained a high residual shear stress after forming, and inadequate tack because of the inability of the matrix to adequately wetout the tool surface. The optimum level of cure examined in these tests was found to be 30%, which in this condition allows the prepreg to be deformed by hand during lay-up, exhibits little residual stress after shear deformation, and has high adhesive tack strength with a metallic tool surface. Based on the research presented in the paper, the Australian company ‘Speciality Coatings’ are now producing glass/epoxy prepreg with a level of cure of approximately 30%.
Acknowledgements
Fig. 7. Effect of level of cure on the apparent flexural rigidity of the prepregs.
The research was funded by a START grant titled ‘Multilayer technical textiles for high performance rigid composite products’ awarded by the Department of Industry, Science and Resources of the Commonwealth
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of Australia. The research was undertaken as a commercial task with RMIT University and as a research program of the CRC for Advanced Composite Structures Ltd. References [1] Seferis J, Meissonnier J. Development of tack and drape test for prepregs based on viscoelastic principles. SAMPE Q 1988:55–64. [2] Procter P. Tack tester improves composites fabrication. Aviat Week Space Tec 1998.
[3] Arimitsu Y, Chou T-W. Modeling of yarn orientation and stress concentration in fabric shaping process. Proceedings of 6th Japan Int. SAMPE Symposium, 1999, pp. 551–54. [4] Rodgers S, Kiser D, Murphy T, Nielsen W, Rodgers S. Ergonomic Design for People at Work. John Wiley & Sons; 1986. [5] Wang J, Paton R, Page J. Forming properties of themoset fabric prepregs at room and elevated temperatures. CRC-ACS Internal Report, 1997. [6] Gillanders A, Kerr S, Martin T. Determination of prepreg tack. Int J Adhes Adhes 1981:125–34. [7] Putnam J, Haynes B, Seferis J. Prepreg process-structure-property analysis and scale-up for manufacture and performance. J Adv Mater 1996:47–57.