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Effect of the extreme conditions on the tensile impact strength of GFRP composites
Accepted Manuscript Effect of the extreme conditions on the tensile impact strength of GFRP composites P.N.B. Reis, M.A. Neto, A.M. Amaro PII: DOI: Reference:
S0263-8223(17)32751-4 https://doi.org/10.1016/j.compstruct.2018.01.001 COST 9241
To appear in:
Composite Structures
Received Date: Accepted Date:
27 August 2017 2 January 2018
Please cite this article as: Reis, P.N.B., Neto, M.A., Amaro, A.M., Effect of the extreme conditions on the tensile impact strength of GFRP composites, Composite Structures (2018), doi: https://doi.org/10.1016/j.compstruct. 2018.01.001
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Effect of the extreme conditions on the tensile impact strength of GFRP composites P.N.B. Reis1, M.A. Neto2, A.M. Amaro2* 1
C-MAST, Depart. of Electromechanical Engineering, University of Beira Interior, Covilhã, Portugal
2
CEMMPRE, Depart. of Mechanical Engineering, University of Coimbra, Coimbra, Portugal
Abstract During operational conditions, extreme environments combined with unexpected high loading rates can induce severe damages or, inclusively, premature fails. Therefore, this paper intends to study the effect of different hostile solutions on the longitudinal impact strength in order to establish design criterions. For this purpose, samples of GFRP composites were immersed into alkaline and acid solutions as well as distilled water. Variables like exposure time, temperature and concentration of the solution were analysed in detail. The effect of pre-damages was also studied for different pre-loads, where the severity of the damages introduced was quantified by acoustic emission. It was possible to conclude that, independently of the solution, the exposure time and temperature were determinant to decrease the tensile impact strength. Finally, the magnitude of the initial damage has a significant influence on the impact resistance.
Keywords: Polymer-matrix composites (PMCs); Glass fibres; Environmental Degradation; Mechanical properties.
*corresponding author
1. Introduction In the engineering world is consensual that the composite materials are, every day, replacing the traditional metals. This is consequence of their high stiffness and strength, low weight and adjustable properties. However, the main disadvantages are yet the poor compression strength and the poor transverse properties. In such context, the lower through-thickness direction make these materials particularly susceptible to low-velocity impact events because they can occur easily in-service or during maintenance activities, promoting damages that are difficult to detect [1, 2] and responsible by significant reductions of the mechanical properties [3-6]. Therefore, it is not strange the several studies that can be found in the open literature about the characterization of composite laminates subjected to low-velocity impact events on the throughthickness direction [7-17] or strategies to improve their impact performance [18-25]. In terms of longitudinal tensile impact strength, there are not so many studies because literature focuses mainly on the dependence of some mechanical properties with the strain rate. For example, Jacob et al. [26] made a review of the work published about the strain rate effects on the tensile, shear, compressive and bending properties of FRP composites. Basically, in all these works, it was analysed the effect of the strain rate on the strength, modulus and strain of those materials, and it is possible to conclude that, generally, the tensile strength and modulus increase with the increasing loading rate while the strain decreases [27-29]. On the other hand, since these studies are mainly focused to evaluate the mechanical properties in function of the strain rate, the tests are carried out at room temperature and under controlled conditions. However, in many applications, the loading at high strain rate can be combined with extreme environmental conditions. Therefore, the main goal of this work is to study their effect on the tensile impact strength of GFRP composites, where the analysis will be done in energetic purposes instead of specific mechanical parameters, like in the previous studies. The hygrothermal effect will be studied, because if the water is responsible by irreversible damages and when it is combined with temperature promotes severe degradations in terms of
mechanical properties. This degradation is directly related with the amount of moisture absorbed, and it is consequence of the chemical changes of the matrix as well as the debonding at the fibre/matrix interface [30]. According to Apicella et al. [31-33] there are three absorption modes: bulk dissolution of water in polymer network; moisture absorption onto the surface of vacuoles which define the excess of free volume of the glassy structure; and hydrogen bonding between polymer hydrophilic groups and water. Once inside the matrix, water will act as plasticizer (spacing the polymer chains) and, consequently, the composite becomes softer because the matrix becomes pliable [30]. However, higher temperatures increase the diffusion rates of moisture and generally accelerate the aging [34]. In such conditions, Liao et al. [35] observed that moisture absorption induces larger strains than high temperature exposure for unidirectional polymer composites, because the coefficient of moisture expansion is higher than the coefficient of thermal expansion. On the other hand, composite materials are replacing the stainless or coated steel in the chemical industry, building and/or infrastructures, where hostile environments are present. Literature reports some studies about GFRP composites under stress corrosion showing that, in these conditions, the failure is often catastrophic and involves the propagation of sharp cracks in a brittle way [36-41]. These sharp cracks initiate and propagate through the material, as a direct consequence of the weakening of the glass fibres. Therefore, the failures are characterized by planar fracture surfaces with little fibre pull-out, normal to the line of maximum stress [37]. Therefore, this subject should be target of the scientific community and, in this context, the present study intends to analyse the effect of alkaline and acid environments on the longitudinal impact strength. Finally, the effect of the pre-damages will be also analysed because, according with the authors’ knowledge, there are very few studies in the open literature about this subject. For example, Amaro et al. [42] observed that the magnitude of the damages promoted by bending loads has a strong influence on the impact strength obtained along the through-thickness direction.
2. Material and experimental procedure Glass fibre Prepreg TEXIPREG® ET443 (EE190 ET443 Glass Fabric PREPREG from SEAL, Legnano, Italy) was used to produce composite laminates with different stacking sequences. According with the manufacturer recommendations, the fabrication process involved different steps: make the hermetic bag and apply 0.05 MPa vacuum; heat up to 130º C at a 3 ºC/min rate; apply a pressure of 0.5 MPa when a temperature of 130 ºC is reached; maintaining pressure and temperature for 60 min; cool down to room temperature (25 ºC) maintaining pressure and finally get the part out from the mould. The plates were manufactured in a useful size of 300×300×t [mm], where t is 1.2 mm for [0 2,902]s laminates and 2.1 mm for [02,902]2s, [08,908] and [02, 45 2, 452, 902]s laminates. Finally, the specimens used in the experiments were obtained from these thin plates with the geometry shown in Figure 1. Rectangular specimens of 100×12×2.1 [mm] were also obtained for the transversal impacts. Figure 1 Tensile tests were performed according to ASTM D 3039, using a Shimadzu universal testing machine, model AutographAG-X, equipped with a load cell of 5 kN. For each condition, 5 samples were tested at room temperature and at 2 mm/min. Relatively to the tensile impact tests, they were carried out according to ISO 8256. An impact machine Instron-Ceast 9050 and a hammer with 25 J of energy was used for this purpose. Figure 2 shows details of the equipment. Finally, the transverse impact tests were performed in the same impact machine according to ISO 179, and a hammer with 5 J of energy was used. Five specimens were used for each condition and tested at room temperature. Figure 2 Different hostile conditions were studied in order to obtain the residual tensile impact strength. The effect of pre-damages was analysed in [02,452,-452,90 2]s laminates and, for this purpose, the specimens were submitted to pre-loads around 20%, 40%, 60% and 80% of the ultimate tensile strength. In order to quantify the severity of the pre-damages introduced, a Marandy acoustic
emission analyzer (model MR1004) was used to provide the amplitude and the number of ringdown counts for each acoustic emission (AE) event. The total ringdown count for each event is the number of times of the signal amplitude exceeds the threshold level, and the amplitude detector unit sorts the AE events into twenty-five levels, each one with a bandwidth of 2.4 dB. More details about the equipment can be found in [43]. In order to study the hygrothermal effect on the tensile impact strength, specimens obtained from the [0, 90]8 laminates were immersed into distilled water at 3 °C, 40 °C and 80 °C, and conveniently conditioned during 20, 40 and 60 days. With similar purpose, the effect of alkaline and acid solutions was studied. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions were used to immerse the specimens because, according to Amaro et al. [14, 44], they affect significantly the impact performance of GFRP composites. Variables like exposure time, concentration of the solution and temperature were analysed in detail. For this end, specimens obtained from the [02, 90 2]S laminates were submerged during 15, 30 and 60 days into solutions of HCl and NaOH at room temperature and concentration of 10%. Similar specimens were submerged during 30 days into solutions of HCl and NaOH, at room temperature and with concentrations of 10%, 20% and 30%, in order to compare the impact strength relatively to the one obtained with control samples. Finally, the effect of temperature was investigated in specimens submerged during 15 days into solutions with concentration of 20% and conveniently conditioned at temperatures of 8 °C, 25 °C and 40 °C.
3. Results and discussion Static tests were performed for better understanding of the stacking sequence effect on the tensile impact strength. In this context, Figure 3 presents the typical stress-displacement curves and respective ultimate tensile strength for all configurations. The symbols correspond to the average values and the bands to the maximum and minimum values obtained.
Figure 3 It is possible to observe that the tensile strength depends significantly of the fibre orientation, which is in perfect agreement with the literature [45-50]. As expected, the [02, 45 2, -452, 902]S laminates present the lowest ultimate strength, as consequence of the lower number of fibres oriented on the load direction. Comparatively to the [0, 90]8 laminates, a decreasing around 21.7% can be found, while this difference is only about 5.5% for [02, 902]2S laminates. The failure mechanisms were also analysed and, as shown in Figure 4, two types of failure modes were identified. Relatively to the [0, 90]8 and [02, 90 2]2S laminates, the failure is dominated by the fracture of axially aligned fibres, while for the [02, 452, -452, 90 2]S laminates the damage is associated with matrix and matrix-fibre interface cracking. In fact, they represent the two main types of failure reported by the literature, where laminates with sufficient fibres oriented in the load direction exhibit fibre dominated failure mode, while laminates with off-axis fibres present a matrix dominated failure mode. [46-48]. Figure 4 Figure 5 shows the effect of the stacking sequence on the tensile impact strength and, for comparison, on the transverse impact strength. It is recognised that the out plane properties are yet a constraint in application of these materials in primary structures. Therefore, this comparison intends to quantify the difference between the longitudinal and transverse impact strength in order to establish adequate design criterions. Figure 5 Similar to the ultimate tensile strength, the impact strength is also dependent of the stacking sequence. The highest tensile impact strength was obtained for [0, 90]8 laminates, with an average value around 946.1 kJ/m2, and a decreasing around 20.6% and 11.9% can be found for [02, 452, -452, 90 2]s and [02, 90 2]2s laminates, respectively. Comparatively to the values obtained from the tensile tests, the decreasing observed between [0, 90]8 and [02, 452, -45 2, 902]s laminates is very similar (20.6% and 21.7%), but an increase around twice more (11.9% and 5.5%) occurs
between [0, 90]8 and [02, 902]2s laminates. Independently of the same number of layers with fibres oriented in the load direction, the stacking sequence of [02, 902]2s shows to be more sensible to the strain rate effects as consequence of its lay-up. Finally, in terms of damage mechanisms, Figure 6 confirms the same two types of failure modes occurred on the static tests: the matrix dominated failure in terms of [02, 45 2, -45 2, 902]s laminates and fibre dominated failure mode for the other ones. While the first type of failure is a rate/frequency-dependent phenomenon due to the viscous matrix behaviour, the last one is independent of the rate/frequency of loading [46]. On the other hand, when the values obtained in transverse mode are compared with the ones obtained longitudinally, a significant decreasing of the impact strength can be observed independently of the lay-up. For example, decreases of 88.9%, 87% and 89.7% can be found, respectively, for [0, 90]8, [0 2, 902]2s and [02, 452, -452, 902]s laminates. Other evidence related with the [0, 90]8 and [02, 90 2]2s laminates is, contrary to observed in tensile mode, the same value of transverse impact strength. These results agree with the literature, where the poor mechanical performance of the composite laminates is justified by the lack of through thickness reinforcement [51]. Therefore, as shows Figure 7, the damage mechanisms are completely different relatively to the tensile mode (Fig. 6). For all laminates, the main damage occurred by the fibres fracture in the tensile side followed by delaminations between layers. However, the [02, 452, -452, 902]s laminates present higher delaminations, which promote practically the total collapse of the sample (see Fig. 7c). Figure 6 Figure 7 In order to analyse the effect of pre-damages on the residual tensile impact strength, [02,452,452,90 2]s laminates were submitted to tensile pre-loads correspondent to 20%, 40 %, 60% and 80% of the respective ultimate tensile strength. Figure 8 illustrates the severity of the damage introduced in terms of “Ringdown Counting” versus tensile stress. This is a typical curve
obtained from the tensile tests, where the total “Ringdown Counting” for each event is the number of times that the signal amplitude exceeds the threshold level. In present work the threshold for “Ringdown Counting” was fixed at 18.45 mV. More details about this parameter can be found in [43]. For better understanding, an additional partial curve is shown, where the y axis is 100 times lower comparatively to the one of total curve. Figure 8 It is possible to observe that the AE activity starts just after the first loading and it increases significantly for higher loads. The first events that occur are related with the matrix plastic deformation and cracking, crack propagation and fibre/matrix interfacial debonding, which are damages characterized by low amplitude levels [43, 52-54]. On the other hand, delaminations are responsible by middle amplitude events and the high amplitude events are resultant of fibre breakage [43, 52-54]. Figure 9 After the damages induced, the specimens were submitted to tensile impact loads and Figure 9 present the results obtained in comparison with the ones for the control samples (0%). As expected, the magnitude of the pre-load affects strongly the longitudinal impact strength. For example, relatively to the control samples, a decreasing around 22.2% is observed for pre-loads of 20% and this value reaches about 53.1% for magnitudes of 80%. In fact, as shown in Fig. 8, higher pre-loads promote higher damages and, consequently, the strength and stiffness of the laminates decreases. Similar tendency was observed by the authors, where the magnitude of the initial damage promoted by bending loads affected substantially the transverse impact fatigue life [42]. Finally, when the pre-load is known, it is possible to estimate the residual tensile impact strength by the equation 1 with a correlation coefficient of 0.991.
RTIS = 0.0258 PS2 - 6.7315 PS + 737.31
(1)
Where RTIS is the residual tensile impact strength and PS the value of pre-stress applied, in percentage, relatively to the ultimate tensile strength of the laminate. Figure 10 shows the hygrothermal effect on the tensile impact strength where, for this purpose, the specimens were completely submerged in distilled water at 3ºC, 40ºC and 80ºC. Figure 10 It is possible to observe that, independently of the temperature value, the tensile impact strength decreases with the exposure time. Simultaneously, it is also evident that the temperature affects substantially the impact strength, which in both cases agrees with the open literature [30, 55, 56]. For example, comparatively to the control samples, the tensile impact strength decreases about 12%, 28.3% and 33% after 20, 40 and 60 days of immersion in water at 3ºC, respectively. When immersed in water at 40ºC these values are 40.4%, 45% and 51.5%, respectively. Finally, comparing the values obtained at 80ºC they are, respectively, around 53.4%, 61.2% and 64.4% lower. In fact, according to Ray [57], the degradation that occurs in a composite is directly related with the amount of moisture that is absorbed, which is made through diffusion and/or capillary processes [58-60]. Consequently, debonding at the fibre/matrix interface can occurs as well as changes in the thermophysical, mechanical, and chemical characteristics of the matrix by plasticization and hydrolysis [57, 61, 62]. On the other hand, being the moisture absorbed mainly by the resin and the diffusion a phenomenon thermally activated, higher temperatures accelerate the short-term diffusion and increases the diffusion coefficient [63]. Therefore, it is possible to conclude that the exposure time and temperature are parameters determinant on the low performance observed. With similar purpose, the effect of alkaline and acid solutions was studied. Figure 11 shows the effect of the exposure time for specimens completely immersed in HCl and NaOH, both with a concentration of 10%. Figure 11
Independently of the solution, the exposure time presents a significant effect on the tensile impact strength, which is in perfect agreement with the literature [14, 44, 64]. For example, considering the laminates submerged in hydrochloric acid, the impact strength decreases relatively to the control samples around 1.4%, 4.5% and 7.2% after 15, 30 and 60 days of exposure, respectively. However, this decreasing is significantly higher when the samples are in contact with sodium hydroxide, reaching, in this case, values about 4.8%, 16.1% and 28.6%, respectively. This difference, around 3.5 times higher, reveals that the alkaline solution is much more corrosive than the acid solution used, leading, in this case, to the worst results [14, 65, 66]. According to Mahmoud et al. [67], the decreasing of the mechanical properties observed is explained by the absorption, penetration and reaction that occur between solution/matrix and/or fibres. In a composite with similar constituents, Amaro et al. [14] obtained the absorption curves and, after 36 days of exposure, the weight gains were about 4.5%. Consequently, the matrix expansion promotes pits and/or occurrence of microstresses in the composite [68-70]. Simultaneously, the surface topology was analysed through roughness measurements and scanning electron microscopy (SEM) evidencing the existence of micro-cracks [14]. In fact, after long exposure time occurs blisters, which may start growing by swelling until final collapse [68]. Finally, in terms of matrix, Amaro et al. [14] reported also that the exposure to those solutions promote a decreasing of the Young’s modulus. However, if the matrix contributes for the low impact performance, the degradation of the fibre/matrix interface observed by Amaro et al. [14] is considered by Stamenovic et al. [65] as the main cause of the lower load carrying capacity. In such context, the fibre itself is attacked and some cracks are expected on the fibre surface [68]. Figure 12 shows the effect of the corrosive solution and respective concentration on the tensile impact strength. As expected, independently of the solution, higher concentrations promote a decreasing of the impact resistance. Relatively to the control samples, the hydrochloric acid is responsible by decreases about 4.5%, 10.7% and 13%, while for NaOH they are around 16.1%, 19.9% and 24.3%, respectively, for concentrations of 10%, 20 % and 30%. One more time, it is
possible to observe that the alkaline solution is responsible by the worst results, as consequence of its high corrosive effect on the composite material. These results agree with the study developed by Mortas et al. [64] and the decreasing observed is consequence of the absorption, penetration and reaction that occurs between the solutions and the composite’s constituents, as described previously. However, as reported by Stamenovic et al. [65], the influence of alkaline solution on mechanical properties increases with the pH value. For example, Mortas et al. [64] observed that the Young’s modulus decreases, independently of the solutions, but the decreasing observed is more expressive in terms of solutions’ concentration. Figure 12 Finally, the effect of the temperature was also analysed and Figure 13 presents the results obtained. The specimens were submerged during 15 days at 8ºC, 25ºC and 40ºC into both solutions with a concentration of 20 wt.%. Figure 13 Is evident the effect of the temperature where, for both solutions, its increasing promote higher decreasing of the impact strength. For example, in comparison to the control samples, the temperature of 8ºC is responsible by a marginal decreasing about 0.6% for the HCl solution and 1.2% for the NaOH solution. However, when the temperature increases to 25ºC these values increase to 2.9% and 5.4%, respectively, and for 40ºC they are around 7.1% and 13.2%, evidencing a clear tendency to higher decreases of the impact resistance. This is a consequence of the solutions being essentially absorbed by the resin and the temperature to affect its absorption [63]. As referred previously, diffusion is a thermally activated process and an increase of the temperature accelerates short-term diffusion.
4. Conclusions This work studied the effect of the extreme conditions on the tensile impact strength of GFRP composites. For this purpose, composite laminates were immersed into alkaline and acid
solutions with different exposure times, temperatures and concentrations. It was possible conclude that, independently of the solution, the time of immersion has a strong influence on the longitudinal impact degradation of those materials. However, the alkaline solution is more aggressive than the acid solution, promoting the lowest values. In terms of temperature, its increasing is responsible by higher decrease of the longitudinal impact resistance, and similar behaviour was found for higher values of concentration. In terms of hygrothermal effect, it was observed that the tensile impact strength decreases with the exposure time, but, for the same exposure time, the impact response is substantially affected by the temperature. This conclusion evidences that the amount of absorbed moisture is determinant on the impact degradation observed. Finally, the magnitude of the initial damage has a significant influence on the impact resistance.
Acknowledgement This research is sponsored by the project UID/EMS/00285/2013.
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Figure 1 - Geometry and dimensions of the specimens.
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Figure 2 - Impact machine Instron-Ceast 9050: a) Global view of the machine; b) Details of the apparatus used in the longitudinal impact tests; c) Details of the apparatus used in the transverse impact tests.
Stacking sequence [0, 90]8
[02, 902]2S
[02, 452, -452, 90 2]S 500
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Figure 4 - Typical failures for: a) [0,90]8, b) [0 2,902]2s e c) [0 2,452,-452,90 2]s.
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Lay-up Figure 5 - Values of the tensile and transversal impact strength.
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1 mm
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Figure 6 - Typical failures obtained from the tensile impact tests for: a) [0,90]8, b) [02,902]2s e c) [02,452,-452,902]s.
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1 mm
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Figure 7 - Typical failures obtained from the transversal impact tests for: a) [0,90]8, b) [0 2,902]2s e c) [02,452,-452,902]s.
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Stress [MPa] Figure 8 – Typical “Ringdown Counting” versus tensile stress curve for [02,452,-45 2,902]s laminates.
Tensile impact strength [kJ/m2]
900 800 700 600 500 400 300 200 100 0 -20
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40 60 80 Value of the pre-stress applied [%]
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Figure 9 - Tensile impact strength in function of the pre-load for [02,45 2,-45 2,902]s laminates.
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1200 3 ºC
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Figure 10 - Hygrothermal effects on the tensile impact strength for [0, 90]8 laminates.
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10% HCl
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10% NaOH 0 -10
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Exposure time [days] Figure 11 - Effect of the exposure time and solution type on the tensile impact strength for [02, 902]S laminates.
Tensile impact strength [kJ/m2]
800
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HCl
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NaOH 0 -10
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Solutions' concentration [%] Figure 12 - Effect of the solutions’ concentration on the tensile impact strength for [02, 90 2]S laminates exposed during 30 days.
Tensile impact strength [kJ/m2]
800
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HCl - 15 days
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NaOH - 15 days 0 -10
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Figure 13 - Effect of the solutions’ temperature on the tensile impact strength for [02, 902]S laminates.
S0263-8223(17)32751-4 https://doi.org/10.1016/j.compstruct.2018.01.001 COST 9241
To appear in:
Composite Structures
Received Date: Accepted Date:
27 August 2017 2 January 2018
Please cite this article as: Reis, P.N.B., Neto, M.A., Amaro, A.M., Effect of the extreme conditions on the tensile impact strength of GFRP composites, Composite Structures (2018), doi: https://doi.org/10.1016/j.compstruct. 2018.01.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of the extreme conditions on the tensile impact strength of GFRP composites P.N.B. Reis1, M.A. Neto2, A.M. Amaro2* 1
C-MAST, Depart. of Electromechanical Engineering, University of Beira Interior, Covilhã, Portugal
2
CEMMPRE, Depart. of Mechanical Engineering, University of Coimbra, Coimbra, Portugal
Abstract During operational conditions, extreme environments combined with unexpected high loading rates can induce severe damages or, inclusively, premature fails. Therefore, this paper intends to study the effect of different hostile solutions on the longitudinal impact strength in order to establish design criterions. For this purpose, samples of GFRP composites were immersed into alkaline and acid solutions as well as distilled water. Variables like exposure time, temperature and concentration of the solution were analysed in detail. The effect of pre-damages was also studied for different pre-loads, where the severity of the damages introduced was quantified by acoustic emission. It was possible to conclude that, independently of the solution, the exposure time and temperature were determinant to decrease the tensile impact strength. Finally, the magnitude of the initial damage has a significant influence on the impact resistance.
Keywords: Polymer-matrix composites (PMCs); Glass fibres; Environmental Degradation; Mechanical properties.
*corresponding author
1. Introduction In the engineering world is consensual that the composite materials are, every day, replacing the traditional metals. This is consequence of their high stiffness and strength, low weight and adjustable properties. However, the main disadvantages are yet the poor compression strength and the poor transverse properties. In such context, the lower through-thickness direction make these materials particularly susceptible to low-velocity impact events because they can occur easily in-service or during maintenance activities, promoting damages that are difficult to detect [1, 2] and responsible by significant reductions of the mechanical properties [3-6]. Therefore, it is not strange the several studies that can be found in the open literature about the characterization of composite laminates subjected to low-velocity impact events on the throughthickness direction [7-17] or strategies to improve their impact performance [18-25]. In terms of longitudinal tensile impact strength, there are not so many studies because literature focuses mainly on the dependence of some mechanical properties with the strain rate. For example, Jacob et al. [26] made a review of the work published about the strain rate effects on the tensile, shear, compressive and bending properties of FRP composites. Basically, in all these works, it was analysed the effect of the strain rate on the strength, modulus and strain of those materials, and it is possible to conclude that, generally, the tensile strength and modulus increase with the increasing loading rate while the strain decreases [27-29]. On the other hand, since these studies are mainly focused to evaluate the mechanical properties in function of the strain rate, the tests are carried out at room temperature and under controlled conditions. However, in many applications, the loading at high strain rate can be combined with extreme environmental conditions. Therefore, the main goal of this work is to study their effect on the tensile impact strength of GFRP composites, where the analysis will be done in energetic purposes instead of specific mechanical parameters, like in the previous studies. The hygrothermal effect will be studied, because if the water is responsible by irreversible damages and when it is combined with temperature promotes severe degradations in terms of
mechanical properties. This degradation is directly related with the amount of moisture absorbed, and it is consequence of the chemical changes of the matrix as well as the debonding at the fibre/matrix interface [30]. According to Apicella et al. [31-33] there are three absorption modes: bulk dissolution of water in polymer network; moisture absorption onto the surface of vacuoles which define the excess of free volume of the glassy structure; and hydrogen bonding between polymer hydrophilic groups and water. Once inside the matrix, water will act as plasticizer (spacing the polymer chains) and, consequently, the composite becomes softer because the matrix becomes pliable [30]. However, higher temperatures increase the diffusion rates of moisture and generally accelerate the aging [34]. In such conditions, Liao et al. [35] observed that moisture absorption induces larger strains than high temperature exposure for unidirectional polymer composites, because the coefficient of moisture expansion is higher than the coefficient of thermal expansion. On the other hand, composite materials are replacing the stainless or coated steel in the chemical industry, building and/or infrastructures, where hostile environments are present. Literature reports some studies about GFRP composites under stress corrosion showing that, in these conditions, the failure is often catastrophic and involves the propagation of sharp cracks in a brittle way [36-41]. These sharp cracks initiate and propagate through the material, as a direct consequence of the weakening of the glass fibres. Therefore, the failures are characterized by planar fracture surfaces with little fibre pull-out, normal to the line of maximum stress [37]. Therefore, this subject should be target of the scientific community and, in this context, the present study intends to analyse the effect of alkaline and acid environments on the longitudinal impact strength. Finally, the effect of the pre-damages will be also analysed because, according with the authors’ knowledge, there are very few studies in the open literature about this subject. For example, Amaro et al. [42] observed that the magnitude of the damages promoted by bending loads has a strong influence on the impact strength obtained along the through-thickness direction.
2. Material and experimental procedure Glass fibre Prepreg TEXIPREG® ET443 (EE190 ET443 Glass Fabric PREPREG from SEAL, Legnano, Italy) was used to produce composite laminates with different stacking sequences. According with the manufacturer recommendations, the fabrication process involved different steps: make the hermetic bag and apply 0.05 MPa vacuum; heat up to 130º C at a 3 ºC/min rate; apply a pressure of 0.5 MPa when a temperature of 130 ºC is reached; maintaining pressure and temperature for 60 min; cool down to room temperature (25 ºC) maintaining pressure and finally get the part out from the mould. The plates were manufactured in a useful size of 300×300×t [mm], where t is 1.2 mm for [0 2,902]s laminates and 2.1 mm for [02,902]2s, [08,908] and [02, 45 2, 452, 902]s laminates. Finally, the specimens used in the experiments were obtained from these thin plates with the geometry shown in Figure 1. Rectangular specimens of 100×12×2.1 [mm] were also obtained for the transversal impacts. Figure 1 Tensile tests were performed according to ASTM D 3039, using a Shimadzu universal testing machine, model AutographAG-X, equipped with a load cell of 5 kN. For each condition, 5 samples were tested at room temperature and at 2 mm/min. Relatively to the tensile impact tests, they were carried out according to ISO 8256. An impact machine Instron-Ceast 9050 and a hammer with 25 J of energy was used for this purpose. Figure 2 shows details of the equipment. Finally, the transverse impact tests were performed in the same impact machine according to ISO 179, and a hammer with 5 J of energy was used. Five specimens were used for each condition and tested at room temperature. Figure 2 Different hostile conditions were studied in order to obtain the residual tensile impact strength. The effect of pre-damages was analysed in [02,452,-452,90 2]s laminates and, for this purpose, the specimens were submitted to pre-loads around 20%, 40%, 60% and 80% of the ultimate tensile strength. In order to quantify the severity of the pre-damages introduced, a Marandy acoustic
emission analyzer (model MR1004) was used to provide the amplitude and the number of ringdown counts for each acoustic emission (AE) event. The total ringdown count for each event is the number of times of the signal amplitude exceeds the threshold level, and the amplitude detector unit sorts the AE events into twenty-five levels, each one with a bandwidth of 2.4 dB. More details about the equipment can be found in [43]. In order to study the hygrothermal effect on the tensile impact strength, specimens obtained from the [0, 90]8 laminates were immersed into distilled water at 3 °C, 40 °C and 80 °C, and conveniently conditioned during 20, 40 and 60 days. With similar purpose, the effect of alkaline and acid solutions was studied. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions were used to immerse the specimens because, according to Amaro et al. [14, 44], they affect significantly the impact performance of GFRP composites. Variables like exposure time, concentration of the solution and temperature were analysed in detail. For this end, specimens obtained from the [02, 90 2]S laminates were submerged during 15, 30 and 60 days into solutions of HCl and NaOH at room temperature and concentration of 10%. Similar specimens were submerged during 30 days into solutions of HCl and NaOH, at room temperature and with concentrations of 10%, 20% and 30%, in order to compare the impact strength relatively to the one obtained with control samples. Finally, the effect of temperature was investigated in specimens submerged during 15 days into solutions with concentration of 20% and conveniently conditioned at temperatures of 8 °C, 25 °C and 40 °C.
3. Results and discussion Static tests were performed for better understanding of the stacking sequence effect on the tensile impact strength. In this context, Figure 3 presents the typical stress-displacement curves and respective ultimate tensile strength for all configurations. The symbols correspond to the average values and the bands to the maximum and minimum values obtained.
Figure 3 It is possible to observe that the tensile strength depends significantly of the fibre orientation, which is in perfect agreement with the literature [45-50]. As expected, the [02, 45 2, -452, 902]S laminates present the lowest ultimate strength, as consequence of the lower number of fibres oriented on the load direction. Comparatively to the [0, 90]8 laminates, a decreasing around 21.7% can be found, while this difference is only about 5.5% for [02, 902]2S laminates. The failure mechanisms were also analysed and, as shown in Figure 4, two types of failure modes were identified. Relatively to the [0, 90]8 and [02, 90 2]2S laminates, the failure is dominated by the fracture of axially aligned fibres, while for the [02, 452, -452, 90 2]S laminates the damage is associated with matrix and matrix-fibre interface cracking. In fact, they represent the two main types of failure reported by the literature, where laminates with sufficient fibres oriented in the load direction exhibit fibre dominated failure mode, while laminates with off-axis fibres present a matrix dominated failure mode. [46-48]. Figure 4 Figure 5 shows the effect of the stacking sequence on the tensile impact strength and, for comparison, on the transverse impact strength. It is recognised that the out plane properties are yet a constraint in application of these materials in primary structures. Therefore, this comparison intends to quantify the difference between the longitudinal and transverse impact strength in order to establish adequate design criterions. Figure 5 Similar to the ultimate tensile strength, the impact strength is also dependent of the stacking sequence. The highest tensile impact strength was obtained for [0, 90]8 laminates, with an average value around 946.1 kJ/m2, and a decreasing around 20.6% and 11.9% can be found for [02, 452, -452, 90 2]s and [02, 90 2]2s laminates, respectively. Comparatively to the values obtained from the tensile tests, the decreasing observed between [0, 90]8 and [02, 452, -45 2, 902]s laminates is very similar (20.6% and 21.7%), but an increase around twice more (11.9% and 5.5%) occurs
between [0, 90]8 and [02, 902]2s laminates. Independently of the same number of layers with fibres oriented in the load direction, the stacking sequence of [02, 902]2s shows to be more sensible to the strain rate effects as consequence of its lay-up. Finally, in terms of damage mechanisms, Figure 6 confirms the same two types of failure modes occurred on the static tests: the matrix dominated failure in terms of [02, 45 2, -45 2, 902]s laminates and fibre dominated failure mode for the other ones. While the first type of failure is a rate/frequency-dependent phenomenon due to the viscous matrix behaviour, the last one is independent of the rate/frequency of loading [46]. On the other hand, when the values obtained in transverse mode are compared with the ones obtained longitudinally, a significant decreasing of the impact strength can be observed independently of the lay-up. For example, decreases of 88.9%, 87% and 89.7% can be found, respectively, for [0, 90]8, [0 2, 902]2s and [02, 452, -452, 902]s laminates. Other evidence related with the [0, 90]8 and [02, 90 2]2s laminates is, contrary to observed in tensile mode, the same value of transverse impact strength. These results agree with the literature, where the poor mechanical performance of the composite laminates is justified by the lack of through thickness reinforcement [51]. Therefore, as shows Figure 7, the damage mechanisms are completely different relatively to the tensile mode (Fig. 6). For all laminates, the main damage occurred by the fibres fracture in the tensile side followed by delaminations between layers. However, the [02, 452, -452, 902]s laminates present higher delaminations, which promote practically the total collapse of the sample (see Fig. 7c). Figure 6 Figure 7 In order to analyse the effect of pre-damages on the residual tensile impact strength, [02,452,452,90 2]s laminates were submitted to tensile pre-loads correspondent to 20%, 40 %, 60% and 80% of the respective ultimate tensile strength. Figure 8 illustrates the severity of the damage introduced in terms of “Ringdown Counting” versus tensile stress. This is a typical curve
obtained from the tensile tests, where the total “Ringdown Counting” for each event is the number of times that the signal amplitude exceeds the threshold level. In present work the threshold for “Ringdown Counting” was fixed at 18.45 mV. More details about this parameter can be found in [43]. For better understanding, an additional partial curve is shown, where the y axis is 100 times lower comparatively to the one of total curve. Figure 8 It is possible to observe that the AE activity starts just after the first loading and it increases significantly for higher loads. The first events that occur are related with the matrix plastic deformation and cracking, crack propagation and fibre/matrix interfacial debonding, which are damages characterized by low amplitude levels [43, 52-54]. On the other hand, delaminations are responsible by middle amplitude events and the high amplitude events are resultant of fibre breakage [43, 52-54]. Figure 9 After the damages induced, the specimens were submitted to tensile impact loads and Figure 9 present the results obtained in comparison with the ones for the control samples (0%). As expected, the magnitude of the pre-load affects strongly the longitudinal impact strength. For example, relatively to the control samples, a decreasing around 22.2% is observed for pre-loads of 20% and this value reaches about 53.1% for magnitudes of 80%. In fact, as shown in Fig. 8, higher pre-loads promote higher damages and, consequently, the strength and stiffness of the laminates decreases. Similar tendency was observed by the authors, where the magnitude of the initial damage promoted by bending loads affected substantially the transverse impact fatigue life [42]. Finally, when the pre-load is known, it is possible to estimate the residual tensile impact strength by the equation 1 with a correlation coefficient of 0.991.
RTIS = 0.0258 PS2 - 6.7315 PS + 737.31
(1)
Where RTIS is the residual tensile impact strength and PS the value of pre-stress applied, in percentage, relatively to the ultimate tensile strength of the laminate. Figure 10 shows the hygrothermal effect on the tensile impact strength where, for this purpose, the specimens were completely submerged in distilled water at 3ºC, 40ºC and 80ºC. Figure 10 It is possible to observe that, independently of the temperature value, the tensile impact strength decreases with the exposure time. Simultaneously, it is also evident that the temperature affects substantially the impact strength, which in both cases agrees with the open literature [30, 55, 56]. For example, comparatively to the control samples, the tensile impact strength decreases about 12%, 28.3% and 33% after 20, 40 and 60 days of immersion in water at 3ºC, respectively. When immersed in water at 40ºC these values are 40.4%, 45% and 51.5%, respectively. Finally, comparing the values obtained at 80ºC they are, respectively, around 53.4%, 61.2% and 64.4% lower. In fact, according to Ray [57], the degradation that occurs in a composite is directly related with the amount of moisture that is absorbed, which is made through diffusion and/or capillary processes [58-60]. Consequently, debonding at the fibre/matrix interface can occurs as well as changes in the thermophysical, mechanical, and chemical characteristics of the matrix by plasticization and hydrolysis [57, 61, 62]. On the other hand, being the moisture absorbed mainly by the resin and the diffusion a phenomenon thermally activated, higher temperatures accelerate the short-term diffusion and increases the diffusion coefficient [63]. Therefore, it is possible to conclude that the exposure time and temperature are parameters determinant on the low performance observed. With similar purpose, the effect of alkaline and acid solutions was studied. Figure 11 shows the effect of the exposure time for specimens completely immersed in HCl and NaOH, both with a concentration of 10%. Figure 11
Independently of the solution, the exposure time presents a significant effect on the tensile impact strength, which is in perfect agreement with the literature [14, 44, 64]. For example, considering the laminates submerged in hydrochloric acid, the impact strength decreases relatively to the control samples around 1.4%, 4.5% and 7.2% after 15, 30 and 60 days of exposure, respectively. However, this decreasing is significantly higher when the samples are in contact with sodium hydroxide, reaching, in this case, values about 4.8%, 16.1% and 28.6%, respectively. This difference, around 3.5 times higher, reveals that the alkaline solution is much more corrosive than the acid solution used, leading, in this case, to the worst results [14, 65, 66]. According to Mahmoud et al. [67], the decreasing of the mechanical properties observed is explained by the absorption, penetration and reaction that occur between solution/matrix and/or fibres. In a composite with similar constituents, Amaro et al. [14] obtained the absorption curves and, after 36 days of exposure, the weight gains were about 4.5%. Consequently, the matrix expansion promotes pits and/or occurrence of microstresses in the composite [68-70]. Simultaneously, the surface topology was analysed through roughness measurements and scanning electron microscopy (SEM) evidencing the existence of micro-cracks [14]. In fact, after long exposure time occurs blisters, which may start growing by swelling until final collapse [68]. Finally, in terms of matrix, Amaro et al. [14] reported also that the exposure to those solutions promote a decreasing of the Young’s modulus. However, if the matrix contributes for the low impact performance, the degradation of the fibre/matrix interface observed by Amaro et al. [14] is considered by Stamenovic et al. [65] as the main cause of the lower load carrying capacity. In such context, the fibre itself is attacked and some cracks are expected on the fibre surface [68]. Figure 12 shows the effect of the corrosive solution and respective concentration on the tensile impact strength. As expected, independently of the solution, higher concentrations promote a decreasing of the impact resistance. Relatively to the control samples, the hydrochloric acid is responsible by decreases about 4.5%, 10.7% and 13%, while for NaOH they are around 16.1%, 19.9% and 24.3%, respectively, for concentrations of 10%, 20 % and 30%. One more time, it is
possible to observe that the alkaline solution is responsible by the worst results, as consequence of its high corrosive effect on the composite material. These results agree with the study developed by Mortas et al. [64] and the decreasing observed is consequence of the absorption, penetration and reaction that occurs between the solutions and the composite’s constituents, as described previously. However, as reported by Stamenovic et al. [65], the influence of alkaline solution on mechanical properties increases with the pH value. For example, Mortas et al. [64] observed that the Young’s modulus decreases, independently of the solutions, but the decreasing observed is more expressive in terms of solutions’ concentration. Figure 12 Finally, the effect of the temperature was also analysed and Figure 13 presents the results obtained. The specimens were submerged during 15 days at 8ºC, 25ºC and 40ºC into both solutions with a concentration of 20 wt.%. Figure 13 Is evident the effect of the temperature where, for both solutions, its increasing promote higher decreasing of the impact strength. For example, in comparison to the control samples, the temperature of 8ºC is responsible by a marginal decreasing about 0.6% for the HCl solution and 1.2% for the NaOH solution. However, when the temperature increases to 25ºC these values increase to 2.9% and 5.4%, respectively, and for 40ºC they are around 7.1% and 13.2%, evidencing a clear tendency to higher decreases of the impact resistance. This is a consequence of the solutions being essentially absorbed by the resin and the temperature to affect its absorption [63]. As referred previously, diffusion is a thermally activated process and an increase of the temperature accelerates short-term diffusion.
4. Conclusions This work studied the effect of the extreme conditions on the tensile impact strength of GFRP composites. For this purpose, composite laminates were immersed into alkaline and acid
solutions with different exposure times, temperatures and concentrations. It was possible conclude that, independently of the solution, the time of immersion has a strong influence on the longitudinal impact degradation of those materials. However, the alkaline solution is more aggressive than the acid solution, promoting the lowest values. In terms of temperature, its increasing is responsible by higher decrease of the longitudinal impact resistance, and similar behaviour was found for higher values of concentration. In terms of hygrothermal effect, it was observed that the tensile impact strength decreases with the exposure time, but, for the same exposure time, the impact response is substantially affected by the temperature. This conclusion evidences that the amount of absorbed moisture is determinant on the impact degradation observed. Finally, the magnitude of the initial damage has a significant influence on the impact resistance.
Acknowledgement This research is sponsored by the project UID/EMS/00285/2013.
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Figure 1 - Geometry and dimensions of the specimens.
a)
b)
c)
Figure 2 - Impact machine Instron-Ceast 9050: a) Global view of the machine; b) Details of the apparatus used in the longitudinal impact tests; c) Details of the apparatus used in the transverse impact tests.
Stacking sequence [0, 90]8
[02, 902]2S
[02, 452, -452, 90 2]S 500
[0, 90]8
400
400
[02, 902]2S 300
300
[02, 45 2, -452, 90 2]S 200
200
100
100
0 0
0.5
1
1.5
2
2.5
3
Displacement [mm] Figure 3 - Stress-displacement curves and respective ultimate stress.
a)
b)
1 mm
1 mm
c)
1 mm
Figure 4 - Typical failures for: a) [0,90]8, b) [0 2,902]2s e c) [0 2,452,-452,90 2]s.
Maximum tensile stress [MPa]
Tensile stress [MPa]
500
240
1000
200
800
160
600
120
400
80
200
40
0
Transversal impact strength [kJ/m2]
Tensile impact strength [kJ/m2]
1200
0 0 [0, 90]0.5 8
1 [02, 902]1.5 2S
[02,245 2, -4522.5 , 902]S
3
Lay-up Figure 5 - Values of the tensile and transversal impact strength.
a)
1 mm
c)
b)
1 mm
1 mm
Figure 6 - Typical failures obtained from the tensile impact tests for: a) [0,90]8, b) [02,902]2s e c) [02,452,-452,902]s.
a)
b)
1 mm
c)
1 mm
1 mm
Figure 7 - Typical failures obtained from the transversal impact tests for: a) [0,90]8, b) [0 2,902]2s e c) [02,452,-452,902]s.
5000
50 Partial curve with magnification in the y axis
4000
80%
60% 3000
40
30
40%
2000
Number of events
Number of events
Total curve
20
20%
1000
10
0
0 0
50
100
150
200
250
300
350
400
Stress [MPa] Figure 8 – Typical “Ringdown Counting” versus tensile stress curve for [02,452,-45 2,902]s laminates.
Tensile impact strength [kJ/m2]
900 800 700 600 500 400 300 200 100 0 -20
0
20
40 60 80 Value of the pre-stress applied [%]
100
Figure 9 - Tensile impact strength in function of the pre-load for [02,45 2,-45 2,902]s laminates.
Tensile impact strength [kJ/m2]
1200 3 ºC
1000
40 ºC 80 ºC
800 600 400 200 0 -20
0
20
40
60 80 Exposure time [days]
100
Figure 10 - Hygrothermal effects on the tensile impact strength for [0, 90]8 laminates.
Tensile impact strength [kJ/m2]
800
600
400
10% HCl
200
10% NaOH 0 -10
0
10
20
30
40
50
60
70
Exposure time [days] Figure 11 - Effect of the exposure time and solution type on the tensile impact strength for [02, 902]S laminates.
Tensile impact strength [kJ/m2]
800
600
400
HCl
200
NaOH 0 -10
0
10
20
30
40
Solutions' concentration [%] Figure 12 - Effect of the solutions’ concentration on the tensile impact strength for [02, 90 2]S laminates exposed during 30 days.
Tensile impact strength [kJ/m2]
800
600
400
HCl - 15 days
200
NaOH - 15 days 0 -10
0
10
20
30 40 Temperature [ºC]
50
Figure 13 - Effect of the solutions’ temperature on the tensile impact strength for [02, 902]S laminates.