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Double cantilever beam testing of repaired carbon fibre composites
Composites: Part A 31 (2000) 603–608 www.elsevier.com/locate/compositesa
Double cantilever beam testing of repaired carbon fibre composites M.G. Bader a,*, I. Hamerton b, J.N. Hay b,1, M. Kemp c, S. Winchester b a b
Materials Science and Engineering, School of Engineering, University of Surrey, Guildford GU2 5XH, UK Department of Chemistry, School of Physics and Chemistry, University of Surrey, Guildford GU2 5XH, UK c Structural Materials Centre, Defence Evaluation and Research Agency, Farnborough GU14 OLX, UK Received 22 January 1998; received in revised form 1 October 1999; accepted 18 October 1999
Abstract Low-temperature cure adhesive systems have been evaluated for repair of composites. Pre-cracked DCB fracture toughness specimens were repaired using an ethylcyanoacrylate (ECA) system and an epoxy-based system cured with 4,4 0 -methylene-bis(aminocyclohexane) (PACM), and the post-repair fracture toughness measured. The results showed that both groups of repaired specimens exhibited higher GIc than the original specimens. However, the ECA-repaired specimens showed a greater tendency to fracture in a “stick-slip” fashion compared to specimens repaired with epoxy. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Carbon–epoxy composite; B. Adhesion
1. Introduction The use of composite materials is becoming increasingly more widespread in military and civil aircraft, which has, in turn, created a need for repair procedures when damage occurs. On impact, carbon fibre/epoxy laminates may sustain damage in the form of delaminations and matrix cracks, even though there may be limited visible damage to the outer surface or the fibres. This type of damage is often referred to as “barely visible impact damage” [1,2] and can be caused by dropped tools or stones thrown up from the runway. Repair of the damage can take the form of patching [3–5] or resin injection [6–8] into the damaged area. The primary aim of the present work was to compare the fracture toughness (GIc) of repaired double cantilever beam (DCB) specimens with that of the original material and to examine whether the repair of interlaminar fracture is viable. A secondary aim was to compare the use of two different types of adhesive, one based on ethylcyanoacrylate (ECA) and the other on epoxy resin. The work investigates which is the more effective adhesive for low temperature
* Corresponding author. Tel.: ⫹44-1483-259312; fax: ⫹44-1483259508. 1 Tel: ⫹44-1483-879586; fax: ⫹44-1483-876851. E-mail addresses: [email protected] (M.G. Bader), j.hay@ surrey.ac.uk (J.N. Hay).
repair in terms of ease of application, reliability and strength.
2. Background 2.1. ECA polymerisation The use of an ECA-based adhesive for repairing composites is novel. ECA-based adhesives are one-part adhesives, which do not require a crosslinking agent. An ECA molecule is highly polar, due principally to the electron withdrawing effect of the cyano group, drawing charge through the double bond away from the end carbon, making it susceptible to nucleophilic attack, hence the polymerisation of ECA is facile (see Fig. 1) and rapid (occurring in a matter of seconds). The ECA polymerisation can be readily initiated in the area of the composite matrix (cured epoxy resin) by water, metal impurities or anions present. However, ECA does have disadvantages: limited thermal stability of the polyethylcyanoacrylate (PECA) and the inability of large volumes of ECA to polymerise. ECAbased adhesives begin to depolymerise at 100⬚C and are completely depolymerised at around 200⬚C [9,10] although this will vary slightly depending on adhesive additives and conditions (e.g. humidity). Furthermore, ECA-based adhesives tend to polymerise only in close proximity to the surfaces of the material to be joined, hence the bond thickness should not exceed 0.15 mm for complete
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Fig. 1. Polymerisation of ECA (A ⫺ anionic species).
polymerisation of the ECA monomer to occur. The properties of ECA make it ideal for repair; its low viscosity allows it to penetrate the small fracture crevices and then to undergo rapid polymerisation.
2.2. Epoxy cure The epoxy resin used in this study is based on short oligomers of the diglycidyl ether of bisphenol A (DGEBA). The average number of monomer units that make up the oligomers is reflected in the Epoxy Equivalent (EE): the number of moles of epoxy groups in a given mass of resin calculated for each batch of resin. The main reactions between the epoxy and a primary amine are shown in Fig. 2A and B. Further reactions can also take place with other hydroxyl groups produced as the reaction proceeds. To facilitate these reactions the DGEBA oligomers and PACM are heated (to 150⬚C) to form a crosslinked polymer network.
3. Experimental 3.1. Materials Standard commercial ECA, DGBA and PACM products were used. Specimens for testing were manufactured from carbon fibre epoxy unidirectional prepreg T300-924 (6k) with an overall resin volume fraction of 34%.
Fig. 2. Reaction of epoxy with primary amine (A) and secondary amine group (B).
3.2. Fabrication, preparation and testing of DCB specimens The double cantilever beam test (Fig. 3) is a method of testing the toughness of a laminate under mode I (symmetrical tensile force) delamination and DCB specimens were prepared to ESIS specifications [11]. Polytetrafluoroethylene (PTFE) film ( ⬇ 20 mm in thickness) was used as the crack initiator to aid crack initiation and reduce unstable crack propagation. The composite specimens are stiff in the plane of the sheet and as such behave as linear elastic materials, hence the analysis can be based on Linear Elastic Fracture Mechanics (LEFM) [12–16]. The analysis involves the reduction of the data from the force versus displacement plot (produced experimentally) to the fracture energy versus displacement plot using the equation: GIc
nPd 2Ba
1
where n is the gradient of compliance plot; P the force applied; d the displacement; B the specimen width; a the crack length and GIc the mode I delamination fracture energy. Compliance is the ratio of crack displacement (the total crack length) over stress applied. The gradient of the compliance plot is calculated using regression statistics taken from the graph of log compliance versus log of total crack length. Specimens were manufactured in a single 600 × 600 mm 2 laminate comprising 24 plies (unidirectional) with PTFE
Fig. 3. DCB specimen undergoing mode I delamination.
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605
Fig. 4. Schematics illustrating continuous and stick-slip propagation: a0 is the crack initiation and an the crack propagation marker.
crack initiator strips embedded at mid-plane. The laminate was cured at 180⬚C for 1 h in an autoclave. Once cured and cooled the composite was scanned using ultrasound and found to be free of voids. The composite was then cut into specimens 20 × 125 mm 2 with a thickness of 3.0 mm, each incorporating a 50 mm long strip of PTFE film. Piano hinges were attached using a two part epoxy adhesive. The specimens were tested in accordance with the ESIS guidelines [11] using a crosshead speed of 2 mm min ⫺1. Crack length was measured visually, with crack initiation and every subsequent 5 mm of propagation recorded. The fracture was allowed to propagate for 55 mm before the test machine was set in reverse to allow the crack to close. Once this initial cracking was completed the specimens were divided into two sets. One set was repaired with ECA and the other set by the epoxy. The epoxy and PACM were mixed in a one to one ratio of epoxy group to labile hydrogen, based on the known EE of this batch of epoxy (187 g mol ⫺1) which works out as a mass ratio of 1:5 (PACM:epoxy). Each adhesive was applied by slowly prising open the specimen to expose the fracture surfaces. The adhesive was then spread evenly over one of the fracture surfaces
using a scalpel. The specimens were allowed to close slowly before being clamped with spring clips to distribute the pressure evenly. Excess adhesive was wiped off and the specimens were left to cure: the ECA was cured at ambient temperature for a minimum of 24 h and the epoxy was cured in an oven at 150⬚C for 1 h. The sides of the repaired specimens were lightly sanded to remove any adhesive that was bridging across the crack. The second round of DCB testing was then conducted on the repaired specimens.
4. Results and analysis 4.1. Original specimens For every DCB specimen in this study the gradients of the GIc versus crack length plots were positive indicating that as the crack length increased more energy was required to continue fracturing the surface. If the gradients were negative it would imply that after crack initiation the crack would fracture spontaneously. Crack propagation can occur continuously or it may be
Fig. 5. Fracture energy against crack length for the original specimens and the specimens repaired with ECA, where A original specimens and W ECArepaired specimens (first points are the initiation values taken from the end of the insert).
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Fig. 6. Typical photograph of a stick-slip fracture surface of ECA-repaired specimens.
unstable. In continuous crack propagation the force gradually declines as displacement increases (Fig. 4A). However, unstable or “stick-slip” propagation occurs intermittently exhibiting force values appropriate to both crack initiation and crack arrest for a given displacement (Fig. 4B). In the original specimens continuous fracture occurred. With the exception of the crack initiation points the plots were highly reproducible, which was reflected in a goodness-of-fit (r 2) close to unity (0.935) for the linear plot of GIc calculated from the crack propagation points (see Fig. 5). The crack initiation points were excluded as they were not representative of crack propagation. This is possibly due to the use of a relatively thick PTFE film and the consequent ‘resin wedge’ in the crack initiation region. The environment at the edge of the PTFE film where the crack initiates differs from the matrix where the crack propagates in that there may be voids or excess cured resin built up at the edge which varies between different specimens. The slopes of the G versus a curves, for the original specimens used in the ECA and the epoxy studies, were 74 and 58 kJ m ⫺3, respectively. Note that although a thinner (ca. 13 mm) foil is used conventionally as the crack starter, in these tests the comparison between the original and repaired specimen toughness values should still be valid. 4.2. ECA-repaired specimens The ECA-repaired specimens were found to propagate in an unstable fashion as shown by the force–displacement plot (Fig. 4B) and by the appearance of the fracture surface, a typical photograph of which is shown in Fig. 6. Reduction of the force–displacement data of the ECA-repaired specimens was conducted by taking the force–displacement values for every initiation point of the unstable propagation
Fig. 7. Typical photograph of a hybrid fracture surface of epoxy-repaired specimens.
including crack initiation. These values were substituted into Eq. (1) with n 3: The GIc values measured for the ECA-repaired specimens cover a greater range (3–27 kJ m ⫺2) (Fig. 5) compared with the original specimens (4–8 kJ m ⫺2). This can be quantified by comparing the statistics of the linear regressions to the data. The r 2 of the regression of the data for both the ECArepaired specimens and the original specimens are 0.8355 and 0.935, respectively, indicating that in both cases the use of the regression as a means of representing the data is valid. The slope of the regression line of ECA-repaired specimens was calculated at 493 kJ m ⫺3 compared with 86 kJ m ⫺3 for the original specimens, implying that the ECA-repaired specimens have an interlaminar fracture energy approximately five-fold greater than the original specimens. No evidence was observed for fibre bridging in fractured specimens of either the original epoxy composites or any of the repaired samples. However, the testing of the ECA-repaired specimens is less reproducible than the original specimens. Catastrophic failure of two of the ECA-repaired specimens occurred when on initiation the crack propagated along the entire length of the specimens. This extreme form of unstable crack growth meant that two sets of data were missing. The situation was further exacerbated by the unstable crack growth in the remaining specimens that resulted in between two and four measurements being collected. On inspection, in areas of the fracture surface corresponding to rapid uncontrolled fracture, unpolymerised ECA was found. Conversely, in areas corresponding to the “sticking”, the ECA was found to have polymerised completely. Hence, it has been hypothesised that the fracture was halted at the regions where the ECA had polymerised completely and that on further fracture initiation the crack travelled rapidly along a region of incompletely polymerised ECA until the crack reached another region of polymerised ECA where the process would be repeated. Given the potential for strong repair the main disadvantage of ECA is that it is susceptible to unstable crack growth that could result in catastrophic failure if it were to be used in interlaminar fracture repair without some means of ensuring complete polymerisation of the ECA. 4.3. Epoxy-repaired specimens Unstable crack growth also occurred in the testing of the epoxy-repaired specimens. Four specimens were tested of which three were affected by the unstable crack growth although to a lesser degree than the ECA-repaired specimens. The unstable crack growth occurred at random intervals during the crack propagation leading to a hybrid of continuous and unstable crack growth (Fig. 4C). An example of the hybrid fracture surface can also be seen in Fig. 7 showing the regions of continuous and unstable crack growth. After initiation the crack would propagate normally up to the point where the bond between the two beams of the
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Fig. 8. Fracture energy against crack length for the original specimens and the specimens repaired with epoxy, where A original specimens and W epoxyrepaired specimens (first points are the initiation values taken from the end of the insert).
specimen would yield and the crack would propagate rapidly for about 15 mm after which the crack would return to propagating continuously until the next region of unstable crack propagation occurred. When unstable crack propagation took place, the GIc of the epoxy-repaired specimens was generally lower for a given displacement, ranging from 2.8 to 7.0 kJ m ⫺2 (Fig. 8) compared to the original specimens which were between 3.0 and 7.0 kJ m ⫺2, but in the epoxyrepaired specimen that propagated continuously the repair was stronger, ranging from 3.6 to 9.8 kJ m ⫺2. This discrepancy in the crack propagation of the epoxy-repaired
specimens was reflected in the linear regression calculated. The regression calculated represented the data less effectively than the original data, and is exemplified by the value r 2 0:1955; which is much lower than the r2 0:8694 obtained for the original specimens. However, the regression can be used for comparison with the original specimens as it gives the average performance of the epoxy-repaired specimens. The epoxy-repaired specimens were less fracture resistant than the original specimens, with corresponding slopes, for the GIc versus crack length plots, of 58 kJ m ⫺3 and 74 kJ m ⫺3, respectively.
Fig. 9. Fracture energy against crack length for the original specimens, specimens repaired with epoxy and specimens repaired with ECA, where S original specimens, W ECA-repaired specimens and A epoxy-repaired specimens.
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Given the unusual pattern of continuous and unstable crack growth along a single fracture, the anomalies have been attributed to varying thickness of the adhesive and the possibility of voids (e.g. air bubbles) originating from the initial application of the epoxy and imperfect crack alignment on the sandwiching of the specimen beams while the adhesive was curing. 4.4. Comparison of ECA and epoxy-repaired specimens In comparing the relative fracture toughness (GIc) of the original, ECA-repaired and epoxy-repaired specimens, using the regressions calculated (Fig. 9), it is apparent that the ECA-repaired specimens have a five-fold greater GIc than either the original or epoxy-repaired specimens. However the fracturing of the ECA-repaired specimens was less reproducible than the epoxy-repaired specimens. 5. Conclusions Two sets of specimens were repaired using an ECA-based adhesive and an epoxy-based adhesive. DCB testing was carried out and the GIc was calculated. The GIc values of the two sets of repaired specimens were compared first with the original specimens and secondly with each other. Overall, the ECA-repaired specimens proved to have a higher fracture energy (493 kJ m ⫺3) than either the original (74–86 kJ m ⫺3) or epoxy-repaired (58 kJ m ⫺3) specimens. However, the ECA-repaired specimens were affected by unstable crack growth which influenced reliability of the DCB testing adversely compared to the original specimens. The epoxy-repaired specimens were also affected by unstable crack growth which lowered the interlaminar fracture energy when compared with the original specimens. Although the ECA-repaired specimens were more fracture resistant than the original specimens, the unstable crack growth resulted in catastrophic failure of two of the specimens. This was caused by incompletely polymerised ECA
and hence further research is required to ensure that the ECA polymerises completely. In the case of the epoxyrepaired specimens, the unstable crack growth adversely affected the fracture energy value obtained. Where the crack propagated in a stable fashion, the fracture energy was greater than the original specimens, but where the crack propagated in an unstable fashion the GIc was less than the original specimens. The presence of unstable crack growth was attributed to voids in the interlaminar zone and hence further work is required to exclude these. Acknowledgements This work was funded by the MOD and the Structural Materials Centre, DERA. References [1] Adsit NR, Waszczak JP. ASTM STP 1979;674:101–17. [2] Starnes JH, Rhodes MD, Williams JG. ASTM STP 1979;696:145–71. [3] Dodiuk H, Buchman L, Liran I, Kenig S. J Adhesion 1993;40:127– 38. [4] Jones R, Bartholomeusz R, Kaye R, Roberts J. Theor Appl Fract Mech 1994;21:41–9. [5] Paul J, Jones R. Engng Fract Mech 1992;41:127–41. [6] Dehm S, Wurzel D. J Aircraft 1989;26:476–81. [7] Russell AJ, Bowers CP. AGARD Conference Proceedings, 530. 1992. p. 27.1–10. [8] Russell AJ, Ferguson JS. AGARD Conference Proceedings, 550. 1994. p. 14.1–8. [9] Chorbadjiev KG, Novakov CHP. Eur Polym J 1991;27:1009–15. [10] Matsui K. Int J Adhesion Adhesives 1990;10:277–84. [11] Davies P. ESIS, 1992. [12] Hashemi S, Kinloch AJ, Williams JG. Compos Sci Technol 1990;37:429–62. [13] Hashemi S, Kinloch AJ, Williams JG. Proc R Soc 1990;427:173–99. [14] Okada A, Dyson IN, Kinloch AJ. J Mater Sci 1995;30:2305–12. [15] Blackman B, Dear JP, Kinloch AJ, Osiyemi S. J Mater Sci Lett 1991;10:253–6. [16] Hashemi S, Kinloch AJ, Williams JG. J Mater Sci Lett 1989;8:125–9.
Double cantilever beam testing of repaired carbon fibre composites M.G. Bader a,*, I. Hamerton b, J.N. Hay b,1, M. Kemp c, S. Winchester b a b
Materials Science and Engineering, School of Engineering, University of Surrey, Guildford GU2 5XH, UK Department of Chemistry, School of Physics and Chemistry, University of Surrey, Guildford GU2 5XH, UK c Structural Materials Centre, Defence Evaluation and Research Agency, Farnborough GU14 OLX, UK Received 22 January 1998; received in revised form 1 October 1999; accepted 18 October 1999
Abstract Low-temperature cure adhesive systems have been evaluated for repair of composites. Pre-cracked DCB fracture toughness specimens were repaired using an ethylcyanoacrylate (ECA) system and an epoxy-based system cured with 4,4 0 -methylene-bis(aminocyclohexane) (PACM), and the post-repair fracture toughness measured. The results showed that both groups of repaired specimens exhibited higher GIc than the original specimens. However, the ECA-repaired specimens showed a greater tendency to fracture in a “stick-slip” fashion compared to specimens repaired with epoxy. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Carbon–epoxy composite; B. Adhesion
1. Introduction The use of composite materials is becoming increasingly more widespread in military and civil aircraft, which has, in turn, created a need for repair procedures when damage occurs. On impact, carbon fibre/epoxy laminates may sustain damage in the form of delaminations and matrix cracks, even though there may be limited visible damage to the outer surface or the fibres. This type of damage is often referred to as “barely visible impact damage” [1,2] and can be caused by dropped tools or stones thrown up from the runway. Repair of the damage can take the form of patching [3–5] or resin injection [6–8] into the damaged area. The primary aim of the present work was to compare the fracture toughness (GIc) of repaired double cantilever beam (DCB) specimens with that of the original material and to examine whether the repair of interlaminar fracture is viable. A secondary aim was to compare the use of two different types of adhesive, one based on ethylcyanoacrylate (ECA) and the other on epoxy resin. The work investigates which is the more effective adhesive for low temperature
* Corresponding author. Tel.: ⫹44-1483-259312; fax: ⫹44-1483259508. 1 Tel: ⫹44-1483-879586; fax: ⫹44-1483-876851. E-mail addresses: [email protected] (M.G. Bader), j.hay@ surrey.ac.uk (J.N. Hay).
repair in terms of ease of application, reliability and strength.
2. Background 2.1. ECA polymerisation The use of an ECA-based adhesive for repairing composites is novel. ECA-based adhesives are one-part adhesives, which do not require a crosslinking agent. An ECA molecule is highly polar, due principally to the electron withdrawing effect of the cyano group, drawing charge through the double bond away from the end carbon, making it susceptible to nucleophilic attack, hence the polymerisation of ECA is facile (see Fig. 1) and rapid (occurring in a matter of seconds). The ECA polymerisation can be readily initiated in the area of the composite matrix (cured epoxy resin) by water, metal impurities or anions present. However, ECA does have disadvantages: limited thermal stability of the polyethylcyanoacrylate (PECA) and the inability of large volumes of ECA to polymerise. ECAbased adhesives begin to depolymerise at 100⬚C and are completely depolymerised at around 200⬚C [9,10] although this will vary slightly depending on adhesive additives and conditions (e.g. humidity). Furthermore, ECA-based adhesives tend to polymerise only in close proximity to the surfaces of the material to be joined, hence the bond thickness should not exceed 0.15 mm for complete
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Fig. 1. Polymerisation of ECA (A ⫺ anionic species).
polymerisation of the ECA monomer to occur. The properties of ECA make it ideal for repair; its low viscosity allows it to penetrate the small fracture crevices and then to undergo rapid polymerisation.
2.2. Epoxy cure The epoxy resin used in this study is based on short oligomers of the diglycidyl ether of bisphenol A (DGEBA). The average number of monomer units that make up the oligomers is reflected in the Epoxy Equivalent (EE): the number of moles of epoxy groups in a given mass of resin calculated for each batch of resin. The main reactions between the epoxy and a primary amine are shown in Fig. 2A and B. Further reactions can also take place with other hydroxyl groups produced as the reaction proceeds. To facilitate these reactions the DGEBA oligomers and PACM are heated (to 150⬚C) to form a crosslinked polymer network.
3. Experimental 3.1. Materials Standard commercial ECA, DGBA and PACM products were used. Specimens for testing were manufactured from carbon fibre epoxy unidirectional prepreg T300-924 (6k) with an overall resin volume fraction of 34%.
Fig. 2. Reaction of epoxy with primary amine (A) and secondary amine group (B).
3.2. Fabrication, preparation and testing of DCB specimens The double cantilever beam test (Fig. 3) is a method of testing the toughness of a laminate under mode I (symmetrical tensile force) delamination and DCB specimens were prepared to ESIS specifications [11]. Polytetrafluoroethylene (PTFE) film ( ⬇ 20 mm in thickness) was used as the crack initiator to aid crack initiation and reduce unstable crack propagation. The composite specimens are stiff in the plane of the sheet and as such behave as linear elastic materials, hence the analysis can be based on Linear Elastic Fracture Mechanics (LEFM) [12–16]. The analysis involves the reduction of the data from the force versus displacement plot (produced experimentally) to the fracture energy versus displacement plot using the equation: GIc
nPd 2Ba
1
where n is the gradient of compliance plot; P the force applied; d the displacement; B the specimen width; a the crack length and GIc the mode I delamination fracture energy. Compliance is the ratio of crack displacement (the total crack length) over stress applied. The gradient of the compliance plot is calculated using regression statistics taken from the graph of log compliance versus log of total crack length. Specimens were manufactured in a single 600 × 600 mm 2 laminate comprising 24 plies (unidirectional) with PTFE
Fig. 3. DCB specimen undergoing mode I delamination.
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Fig. 4. Schematics illustrating continuous and stick-slip propagation: a0 is the crack initiation and an the crack propagation marker.
crack initiator strips embedded at mid-plane. The laminate was cured at 180⬚C for 1 h in an autoclave. Once cured and cooled the composite was scanned using ultrasound and found to be free of voids. The composite was then cut into specimens 20 × 125 mm 2 with a thickness of 3.0 mm, each incorporating a 50 mm long strip of PTFE film. Piano hinges were attached using a two part epoxy adhesive. The specimens were tested in accordance with the ESIS guidelines [11] using a crosshead speed of 2 mm min ⫺1. Crack length was measured visually, with crack initiation and every subsequent 5 mm of propagation recorded. The fracture was allowed to propagate for 55 mm before the test machine was set in reverse to allow the crack to close. Once this initial cracking was completed the specimens were divided into two sets. One set was repaired with ECA and the other set by the epoxy. The epoxy and PACM were mixed in a one to one ratio of epoxy group to labile hydrogen, based on the known EE of this batch of epoxy (187 g mol ⫺1) which works out as a mass ratio of 1:5 (PACM:epoxy). Each adhesive was applied by slowly prising open the specimen to expose the fracture surfaces. The adhesive was then spread evenly over one of the fracture surfaces
using a scalpel. The specimens were allowed to close slowly before being clamped with spring clips to distribute the pressure evenly. Excess adhesive was wiped off and the specimens were left to cure: the ECA was cured at ambient temperature for a minimum of 24 h and the epoxy was cured in an oven at 150⬚C for 1 h. The sides of the repaired specimens were lightly sanded to remove any adhesive that was bridging across the crack. The second round of DCB testing was then conducted on the repaired specimens.
4. Results and analysis 4.1. Original specimens For every DCB specimen in this study the gradients of the GIc versus crack length plots were positive indicating that as the crack length increased more energy was required to continue fracturing the surface. If the gradients were negative it would imply that after crack initiation the crack would fracture spontaneously. Crack propagation can occur continuously or it may be
Fig. 5. Fracture energy against crack length for the original specimens and the specimens repaired with ECA, where A original specimens and W ECArepaired specimens (first points are the initiation values taken from the end of the insert).
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Fig. 6. Typical photograph of a stick-slip fracture surface of ECA-repaired specimens.
unstable. In continuous crack propagation the force gradually declines as displacement increases (Fig. 4A). However, unstable or “stick-slip” propagation occurs intermittently exhibiting force values appropriate to both crack initiation and crack arrest for a given displacement (Fig. 4B). In the original specimens continuous fracture occurred. With the exception of the crack initiation points the plots were highly reproducible, which was reflected in a goodness-of-fit (r 2) close to unity (0.935) for the linear plot of GIc calculated from the crack propagation points (see Fig. 5). The crack initiation points were excluded as they were not representative of crack propagation. This is possibly due to the use of a relatively thick PTFE film and the consequent ‘resin wedge’ in the crack initiation region. The environment at the edge of the PTFE film where the crack initiates differs from the matrix where the crack propagates in that there may be voids or excess cured resin built up at the edge which varies between different specimens. The slopes of the G versus a curves, for the original specimens used in the ECA and the epoxy studies, were 74 and 58 kJ m ⫺3, respectively. Note that although a thinner (ca. 13 mm) foil is used conventionally as the crack starter, in these tests the comparison between the original and repaired specimen toughness values should still be valid. 4.2. ECA-repaired specimens The ECA-repaired specimens were found to propagate in an unstable fashion as shown by the force–displacement plot (Fig. 4B) and by the appearance of the fracture surface, a typical photograph of which is shown in Fig. 6. Reduction of the force–displacement data of the ECA-repaired specimens was conducted by taking the force–displacement values for every initiation point of the unstable propagation
Fig. 7. Typical photograph of a hybrid fracture surface of epoxy-repaired specimens.
including crack initiation. These values were substituted into Eq. (1) with n 3: The GIc values measured for the ECA-repaired specimens cover a greater range (3–27 kJ m ⫺2) (Fig. 5) compared with the original specimens (4–8 kJ m ⫺2). This can be quantified by comparing the statistics of the linear regressions to the data. The r 2 of the regression of the data for both the ECArepaired specimens and the original specimens are 0.8355 and 0.935, respectively, indicating that in both cases the use of the regression as a means of representing the data is valid. The slope of the regression line of ECA-repaired specimens was calculated at 493 kJ m ⫺3 compared with 86 kJ m ⫺3 for the original specimens, implying that the ECA-repaired specimens have an interlaminar fracture energy approximately five-fold greater than the original specimens. No evidence was observed for fibre bridging in fractured specimens of either the original epoxy composites or any of the repaired samples. However, the testing of the ECA-repaired specimens is less reproducible than the original specimens. Catastrophic failure of two of the ECA-repaired specimens occurred when on initiation the crack propagated along the entire length of the specimens. This extreme form of unstable crack growth meant that two sets of data were missing. The situation was further exacerbated by the unstable crack growth in the remaining specimens that resulted in between two and four measurements being collected. On inspection, in areas of the fracture surface corresponding to rapid uncontrolled fracture, unpolymerised ECA was found. Conversely, in areas corresponding to the “sticking”, the ECA was found to have polymerised completely. Hence, it has been hypothesised that the fracture was halted at the regions where the ECA had polymerised completely and that on further fracture initiation the crack travelled rapidly along a region of incompletely polymerised ECA until the crack reached another region of polymerised ECA where the process would be repeated. Given the potential for strong repair the main disadvantage of ECA is that it is susceptible to unstable crack growth that could result in catastrophic failure if it were to be used in interlaminar fracture repair without some means of ensuring complete polymerisation of the ECA. 4.3. Epoxy-repaired specimens Unstable crack growth also occurred in the testing of the epoxy-repaired specimens. Four specimens were tested of which three were affected by the unstable crack growth although to a lesser degree than the ECA-repaired specimens. The unstable crack growth occurred at random intervals during the crack propagation leading to a hybrid of continuous and unstable crack growth (Fig. 4C). An example of the hybrid fracture surface can also be seen in Fig. 7 showing the regions of continuous and unstable crack growth. After initiation the crack would propagate normally up to the point where the bond between the two beams of the
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Fig. 8. Fracture energy against crack length for the original specimens and the specimens repaired with epoxy, where A original specimens and W epoxyrepaired specimens (first points are the initiation values taken from the end of the insert).
specimen would yield and the crack would propagate rapidly for about 15 mm after which the crack would return to propagating continuously until the next region of unstable crack propagation occurred. When unstable crack propagation took place, the GIc of the epoxy-repaired specimens was generally lower for a given displacement, ranging from 2.8 to 7.0 kJ m ⫺2 (Fig. 8) compared to the original specimens which were between 3.0 and 7.0 kJ m ⫺2, but in the epoxyrepaired specimen that propagated continuously the repair was stronger, ranging from 3.6 to 9.8 kJ m ⫺2. This discrepancy in the crack propagation of the epoxy-repaired
specimens was reflected in the linear regression calculated. The regression calculated represented the data less effectively than the original data, and is exemplified by the value r 2 0:1955; which is much lower than the r2 0:8694 obtained for the original specimens. However, the regression can be used for comparison with the original specimens as it gives the average performance of the epoxy-repaired specimens. The epoxy-repaired specimens were less fracture resistant than the original specimens, with corresponding slopes, for the GIc versus crack length plots, of 58 kJ m ⫺3 and 74 kJ m ⫺3, respectively.
Fig. 9. Fracture energy against crack length for the original specimens, specimens repaired with epoxy and specimens repaired with ECA, where S original specimens, W ECA-repaired specimens and A epoxy-repaired specimens.
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Given the unusual pattern of continuous and unstable crack growth along a single fracture, the anomalies have been attributed to varying thickness of the adhesive and the possibility of voids (e.g. air bubbles) originating from the initial application of the epoxy and imperfect crack alignment on the sandwiching of the specimen beams while the adhesive was curing. 4.4. Comparison of ECA and epoxy-repaired specimens In comparing the relative fracture toughness (GIc) of the original, ECA-repaired and epoxy-repaired specimens, using the regressions calculated (Fig. 9), it is apparent that the ECA-repaired specimens have a five-fold greater GIc than either the original or epoxy-repaired specimens. However the fracturing of the ECA-repaired specimens was less reproducible than the epoxy-repaired specimens. 5. Conclusions Two sets of specimens were repaired using an ECA-based adhesive and an epoxy-based adhesive. DCB testing was carried out and the GIc was calculated. The GIc values of the two sets of repaired specimens were compared first with the original specimens and secondly with each other. Overall, the ECA-repaired specimens proved to have a higher fracture energy (493 kJ m ⫺3) than either the original (74–86 kJ m ⫺3) or epoxy-repaired (58 kJ m ⫺3) specimens. However, the ECA-repaired specimens were affected by unstable crack growth which influenced reliability of the DCB testing adversely compared to the original specimens. The epoxy-repaired specimens were also affected by unstable crack growth which lowered the interlaminar fracture energy when compared with the original specimens. Although the ECA-repaired specimens were more fracture resistant than the original specimens, the unstable crack growth resulted in catastrophic failure of two of the specimens. This was caused by incompletely polymerised ECA
and hence further research is required to ensure that the ECA polymerises completely. In the case of the epoxyrepaired specimens, the unstable crack growth adversely affected the fracture energy value obtained. Where the crack propagated in a stable fashion, the fracture energy was greater than the original specimens, but where the crack propagated in an unstable fashion the GIc was less than the original specimens. The presence of unstable crack growth was attributed to voids in the interlaminar zone and hence further work is required to exclude these. Acknowledgements This work was funded by the MOD and the Structural Materials Centre, DERA. References [1] Adsit NR, Waszczak JP. ASTM STP 1979;674:101–17. [2] Starnes JH, Rhodes MD, Williams JG. ASTM STP 1979;696:145–71. [3] Dodiuk H, Buchman L, Liran I, Kenig S. J Adhesion 1993;40:127– 38. [4] Jones R, Bartholomeusz R, Kaye R, Roberts J. Theor Appl Fract Mech 1994;21:41–9. [5] Paul J, Jones R. Engng Fract Mech 1992;41:127–41. [6] Dehm S, Wurzel D. J Aircraft 1989;26:476–81. [7] Russell AJ, Bowers CP. AGARD Conference Proceedings, 530. 1992. p. 27.1–10. [8] Russell AJ, Ferguson JS. AGARD Conference Proceedings, 550. 1994. p. 14.1–8. [9] Chorbadjiev KG, Novakov CHP. Eur Polym J 1991;27:1009–15. [10] Matsui K. Int J Adhesion Adhesives 1990;10:277–84. [11] Davies P. ESIS, 1992. [12] Hashemi S, Kinloch AJ, Williams JG. Compos Sci Technol 1990;37:429–62. [13] Hashemi S, Kinloch AJ, Williams JG. Proc R Soc 1990;427:173–99. [14] Okada A, Dyson IN, Kinloch AJ. J Mater Sci 1995;30:2305–12. [15] Blackman B, Dear JP, Kinloch AJ, Osiyemi S. J Mater Sci Lett 1991;10:253–6. [16] Hashemi S, Kinloch AJ, Williams JG. J Mater Sci Lett 1989;8:125–9.