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Al fibre metal laminates with different structural characteristics
Composite Structures 225 (2019) 111117
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Performance evaluation of CFRP/Al fibre metal laminates with different structural characteristics Costanzo Bellini, Vittorio Di Cocco, Francesco Iacoviello, Luca Sorrentino
T
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Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, 03043 Cassino, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Fibre metal laminate Carbon fibre Flexural load Structural behaviour
FMLs (fibre metal laminates) are a kind of hybrid material consisting of composite material layers alternating to metal sheets, that present high mechanical characteristics. The aim of the present work deals with the flexural behaviour of CARALL (carbon fibre-reinforced aluminium laminates) specimens; in fact, the influence of both the layer thickness and the adhesion interface between CFRP layer and aluminium sheets was analysed. Four different laminates, bonded with structural adhesive or with prepreg resin only and with one or two metal sheets (but maintaining the composite/metal volumetric ratio at a constant value), were tested according to the three points bending scheme. The experimental tests evidenced that the flexural strength increased in absence of the adhesive layer and with a single metal sheet. Moreover, some micrographs of the fractured specimens were taken to determine the failure mechanism.
1. Introduction
with a stronger bonding was found. Iaccarino et al. [6] performed some experimental tests on FMLs to define the stress-strain curve, the residual strain in correspondence of specific stress level and the stress-shear curve. They proposed a modified classical lamination model and they compared the numerical results with experimental ones, finding a good agreement except for the failure prediction. Dhaliwal and Newaz studied the effect of the metal layers position in the material stacking [7], producing and testing some CARALL specimens with carbon fibre laminate as outside layers. They compared the flexural behaviour of their laminates with that of standard CARALL, that had aluminium layers outside, and found a higher strength. A work has been carried out aiming at analysing the behaviour of laminates with the same composite/metal volume fraction ratio and different layers thicknesses by Wu et al. [8]. These authors studied the flexural behaviour of carbon fibre/ magnesium FMLs, discovering that the flexural modulus linearly decreased with the layer thicknesses, while no differences were observed for the flexural strength. Koziol compared the impact strength of composites with three different reinforcements: stitched plain-wowen fabric, plain-wowen fabric and 3D non-crimp fabric, finding a better behaviour of the first one and the third one in comparison with the second one, since they had higher failure stress and a lower crack growth [9]. Koziol et al. evaluated the influence of carbon nanotubes or graphene addition in the resin of a composite material laminate, and they discovered a shear strength improvement in the latter type of laminate, while the former one remained unaffected [10].
FMLs (fibre metal laminates), that are a type of hybrid material made of composite material laminates alternating to metal sheets, are more and more chosen for applications in several industrial fields, like aeronautics, automotive and sports goods, since they present excellent mechanical characteristics, as high damage tolerance, high strength and low density [1,2]. FLMs present another outstanding characteristic: mechanical properties can be easily adjusted to specific requirements by varying the composite ply orientation, the thickness and the number of layers [3]. Usually, structural frame parts are exposed to bending loads, that represents the most diffused, and consequently most investigated, failure mode. The elements an FML is made of have dissimilar properties, causing a complex damage behaviour; in fact, the composite layers are brittle, while metal layers are ductile. The most diffused failure mechanisms of FLMs are matrix cracking, metal layers plastic deformation, fibres fracture, composite layers delamination and debonding between metal and composite [4]. Lawcock et al. [5] found that the flexural characteristics of CARALL (carbon fibre-reinforced aluminium laminates) depended on the bonding between the composite laminate and the aluminium layers, while the tensile properties were not affected; in fact, a weak bonding can give rise to a decrement of about 10% for the interlaminar shear strength. However, they determined that the bonding strength did not influence the residual strength of a notched specimen, even if a small decrease for a specimen
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Corresponding author. E-mail address: [email protected] (L. Sorrentino).
https://doi.org/10.1016/j.compstruct.2019.111117 Received 20 February 2019; Received in revised form 3 May 2019; Accepted 4 June 2019 Available online 05 June 2019 0263-8223/ © 2019 Elsevier Ltd. All rights reserved.
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laminate thickness was about 5 mm. For evaluating the influence of the adhesive interface between the composite material and the aluminium sheet on the mechanical performance of the hybrid laminates, both FMLs with and without adhesive were produced and tested: for the former ones a structural adhesive, typically used in the aeronautical field, was adopted, while the bonding interface of the latter ones relied on the self-adhesive capacity of the resin being a part of the prepreg material. The metal sheets considered for this work were made of aluminium AA 1100, while the composite material was the SE70/RC303T/1270/ 38%, a prepreg system produced by Gurit® and made of SE70 epoxy resin and a 2x2 twill carbon fabric, and the bonding agent, in case of presence, was the AF163-2k film adhesive. As regards the manufacturing process of the CARALL specimens, four laminates, one for each FML type considered in this work, were produced by the vacuum bag process. It was decided to produce laminates with dimensions equal to 200 × 110 mm2, from which to extract four specimens with dimensions of 160 × 20 mm2. The mould surface was treated with Marbocote, a release agent; then, all the materials required for CARALLs manufacturing, that were the prepreg plies, the aluminium sheets and the adhesive plies, were cut into the right size and stacked as visible in Fig. 2. Then, the stacks were covered with a release film and a breather fabric. Finally, the mould was closed with the vacuum bag and the vacuum was applied and maintained until the thermal cycle end. The parts were polymerized in an autoclave following the thermal cycle suggested by the prepreg manufacturer, that was suitable for the adhesive cure too. In must be remembered that the polymerization cycle is an important aspect of the composite material manufacturing process, that can induce defects if it is not correct [21–23]; in fact, several researches have been carried out aiming at optimization of cure process control [24,25], also through innovative neural-fuzzy approaches [26,27]. This cycle scheduled a heating ramp with a rate of 2 °C/min and a temperature dwell of 100 min at 100 °C. At the end of the curing process, the laminates were taken from the mould and trimmed by a diamond disk saw, obtaining the desired specimens. An illustration of the type D samples is reported in Fig. 3. The bending tests carried out on the specimens to evaluate the flexural characteristic referred to the ASTM D790. This test method considered a three-point loading system applied to a simply supported beam. In particular, the test involved two cylindrical shape supports on which the rectangular cross-section specimen was laid, loaded by a cylindrical punch, halfway between the two supports, as visible in Fig. 4. The span between the support was set to 136 mm, while the loading rate to 6 mm/min. In this work a micrographic inspection was carried out in order to analyse the fracture surface and, consequently, to define the fracture mode. The preparation operations were 3: sample cutting from the specimen, sample mounting and surface polishing. In the first operation, after having chosen the section of interest, which corresponded to the area under the loading nose in the specimen centre, a sample was cut by means of an abrasive disk saw using water as cutting fluid. The second operation was the mounting of the samples in resin panels, which were polymerized in 24 h at room temperature to avoid the thermal alteration of the specimens. The last operation was represented by the sample polishing by a grinder equipped with silicon carbide disks and a felt disk with alumina aqueous suspension, obtaining the samples visible in Fig. 5.
The interface between the metal sheets and the composite laminate is very important since it can influence the mechanical behaviour of the FMLs, especially the interlaminar shear strength. In fact, GonzalesCanche et al. [11] found that the adhesion of the different materials allowed the growth of plastic deformation due to the partial inhibition of the localized strain, postponing the cracking start. The use of epoxy adhesive is suitable for the raising of residual strength, fatigue limit and damage tolerance, and it was observed that the high temperature the material is exposed to during curing cycle did not affect these mechanical properties [12]. Also, the voids formation at the interface negatively influences the structural strength of the FMLs, since it causes a poor bond between the metal and the composite material [13]. Abouhamzeh et al. [14] proposed a modelling procedure suitable for this kind of material, while Jakubczak et al. [15] studied the effect of the thermal ageing on ILSS (interlaminar shear strength) for CARALLs with different aluminium surface preparation and various fibre combinations. They found that the addition of a thin glass ply at the interface between metal sheets and carbon laminates, suitable to avoid galvanic corrosion, influenced neither the ILSS nor the thermal fatigue of the laminates. The effects of annealing and anodizing the aluminium sheets of an FML was studied by Pan et al. [16], that discovered a decrement of mechanical properties due to annealing. In the past, the effect of surface treatment and ageing condition were studied also on composite materials too [17,18]. The aim of the present work concerns the flexural behaviour study of CARALL specimens, analysing the influence of both layer thickness and the adhesion between CFRP layer and aluminium sheet. Moreover, unidirectional reinforcements are usually employed in the design of FMLs, while in this study a carbon fabric was considered as composite material reinforcement. The effect of the stacking characteristic on the laminate mechanical behaviour was studied by several researchers, even if they keep the composite laminate thickness [19] or metal sheet thickness [20] constant and focused the attention on the dynamic characterization. The layer thickness is a significant factor concerning the structural behaviour of FMLs that should be taken into consideration for the product design, even if the study of this topic has been infrequently explored.
2. Material and methods In this work, the effect of the layer thickness and the layer adhesion has been investigated both separately and combined. As reported in Table 1, the full factorial plan of the experimental activity is composed of 2 levels for each of the above-mentioned factors. The analysis was carried out through three points bending tests on CARALL specimens with different stacking and dissimilar bonding strategy. As concerns the stacking sequence, two FMLs with CFRP laminate as external layers were considered; as visible in Fig. 1, one consisted of two layers of CFRP and one of aluminium, while the other one was formed by two layers of aluminium and three layers of carbon fibre. In order to maintain the laminate thickness and the CFRP/aluminium ratio constant, the aluminium sheet thickness of the former typology was equal to 0.6 mm and one composite laminate was composed by six prepreg plies, for a total of 12 plies in the whole FML, while the latter type presented two 0.3 mm thick aluminium foil and three CFRP layers consisting of four prepreg plies. In such a manner, the obtained Table 1 full factorial plan for factor influence analysis.
3. Results and discussion
Factor
# levels
Level value
Number of aluminium sheets Adhesion technology Number of replications Tot number of tested specimens
2 2 4 16
1, 2 with adhesive, without adhesive
The influence of two factors, such as the number of layers and the presence of adhesive, on the flexural properties of a CARALL was studied. In particular, the attention was focused not only on the single factor but also on their combination, leading to a more accurate and deeper interpretation of the material behaviour. The three points bending test results concerned three different mechanical properties: 2
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Fig. 1. Analysed FML stacking sequence.
Fig. 5. FML sample prepared for micrographic analysis.
Fig. 2. Stacking of materials during the manufacturing process of FML.
Fig. 6. Comparison of flexural strength for the different type of CARALL. Fig. 3. An example of finished specimens.
supplementary consistency check, that consisted of a pairwise comparison of populations means: if their difference interval contained zero, the population were not different. Finally, factorial plots were outlined for deeper data analysis. In particular, the main effect plot reports the variation in the average response as a factor changed level, so it is useful to reveal which factor is the most influencing, while the interaction plot is suitable for assessing whether two-way interactions exist. More information on the stress-strain curves can be found in [28]. The flexural strength of the tested laminates, denoted as σf, was determined through the following relation, present in ASTM D790 standard and other papers in the literature [7,29,30]:
σf =
3PL 2bh2
(1)
where P represents the load the specimen undergoes, L is the distance between the supports and h and b are the specimen thickness and width, respectively. As it can be noted in Fig. 6, the flexural strength reached a maximum value of 641.86 MPa and a minimum value of 562.75 MPa for the laminate with one metal sheet and adhesive, while for the similar laminate characterized by the adhesive absence the minimum strength was 644.25 MPa and the maximum one was 734.00 MPa. As regards the CARALL presenting two metal sheets, the flexural strength values ranged between 498.38 MPa and 641.38 MPa for the laminate without adhesive and between 468.88 MPa and 553.30 MPa for that one with the adhesive layers. The CoV (Coefficient
Fig. 4. Three points bending test on an FML specimen.
the maximum flexural strength, the relevant strain and the flexural module, and all of them were analysed through inferential statistical techniques in order to ascertain the presence or absence of significant differences. The observation of ANOVA (analysis of variance) factors and the p-values was chosen as analysis criteria, as commonly done for statistical analysis. Moreover, the Tukey’s method was considered for a 3
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Table 2 ANOVA analysis of experimental data obtained for flexural strength. Source
DF
Seq SS
Contribution
Adj SS
Adj MS
F-value
P-value
n. sheet adhesive n. sheet * adhesive error total
1 1 1 12 15
42087.7 23937.6 330.9 23439.7 78795.9
53.41% 16.42% 0.42% 29.75% 100.00%
42087.7 12937.6 330.9 23439.7
42087.7 12937.6 330.9 1953.3
21.55 6.62 0.17
0.001 0.024 0.688
Fig. 9. Short beam strength comparison for FML with and without adhesive.
Fig. 7. Tukey’s pairwise comparison for flexural strength.
of Variation), defined as in [31], was equal to about 5–7%, and in only one case it overcame 10%, even if only for a very small amount. The obtained strength values were comparable or, in some cases, even higher than that one of aeronautic grade aluminium alloy, that typically is 450 MPa. The results of the ANOVA that was implemented on the experimental results are presented in Table 2. It can be noted that the number of aluminium sheets was the most influencing factor, since it had a contribution of 53.41%, while the effect of the other factor, that is the presence of the adhesive, was far less important (16.42%). The contribution of the interaction between factors was less than 0.5% and it can be neglected. However, for a more significant comparison, the pvalue was calculated too for each factor; the analysis confirmed that both the considered factors influenced the material flexural strength, since their value was less than 0.05, the commonly chosen a-level, instead the interaction term p-value was 0.688 and so it can be disregarded. The results of the Tukey’s method applied to the obtained flexural strength values are presented in Fig. 7. It can be noted that for both the analysed factors the confidence interval did not contain the zero, so the
Fig. 10. Comparison of flexural strain for the different type of CARALL.
previous statement about their influence is confirmed. The main effects plot reported in Fig. 8a denotes the influences the factors exerted on the flexural strength; in particular, the increment of the numbers of metal sheets and the presence of adhesive made the flexural strength decrease. The same conclusion can be drawn from the interaction plot of Fig. 8b, which is also useful to assess the absence of interaction between the two factors, since the lines that tie data are parallel.
Fig. 8. Factorial plots for flexural strength: a) main effects plot, b) interaction plot. 4
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Table 3 ANOVA analysis of experimental data obtained for the flexural strain. Source
DF
Seq SS
Contribution
Adj SS
Adj MS
F-value
P-value
n. sheet adhesive n. sheet * adhesive error total
1 1 1 12 15
0.000006 0.000001 0.000000 0.000016 0.000022
25.03% 2.61% 0.07% 72.29% 100.00%
0.000006 0.000001 0.000000 0.000016
0.000006 0.000001 0.000000 0.000001
4.15 0.43 0.01
0.064 0.523 0.918
Fig. 13. Tukey’s pairwise comparison for flexural modulus.
Fig. 11. Tukey’s pairwise comparison for the flexural strain.
considered, but the same authors acknowledged their results to be highly depending on specimen geometry and load configuration. In fact, the flexural load can generate two different failure behaviour: interfacial delamination and fibre fracture, that depend on shear stress and tensile stress, respectively. Both kind of stress always coexist in three points bending beam, but the preponderance of one with respect to the other depends on the span/thickness ratio: if this is small enough, the interlaminar shear stress is higher, for higher values the tension stresses are predominant [34]. The latter stress can be calculated through the Eq. (1), instead the former one, here indicated as σsbs, can be determined as:
σsbs =
3P 4bh
(2)
That relation can be found in the ASTM D2344 standard and it has been adopted by other authors too [5,30,34]. Considering Eq. (2), the type A specimen, for which a maximum flexural load of about 1500 N was registered, presented a shear strength of about 11 MPa, that was negligible compared to the shear strength of the materials constituting this FML. The flexural and shear strengths are influenced by two antithetic factors: on one hand, the adhesive presence enhances the interfacial bonding and, consequently, improves the FML shear behaviour, while, on the other hand, it reduces the fibre volume content and so the strength of the material. In the light of the abovementioned reflexions, it can be concluded that for the considered specimens the latter factor is predominant and so the strength decay of FML with adhesive is entirely
Fig. 12. Comparison of flexural modulus for the different type of CARALL.
From the previously reported results, it can be concluded that the presence of adhesive was detrimental for the material strength, while the decrease of metal sheet numerousness was beneficial. As concern the former conclusion, it seemed to be in contradiction with literature; in fact, several researchers [5,32,33] discovered an improvement due to the presence of the adhesive, but it must be underlined that this enhancement was found only for lap shear and interlaminar shear tests, while the in-plane mechanical properties, as the tensile strength, were not influenced. In the work of Li et al. [33], the flexural behaviour was Table 4 ANOVA analysis of experimental data obtained for flexural modulus. Source
DF
Seq SS
Contribution
Adj SS
Adj MS
F-value
P-value
n. sheet adhesive n. sheet * adhesive error total
1 1 1 12 15
54,745,058 33,069,339 638,055 37,831,285 126,283,737
43.35% 26.19% 0.51% 29.96% 100.00%
54,745,058 33,069,339 638,055 37,831,285
54,745,058 33,069,339 638,055 3,152,607
17.37 10.49 0.20
0.001 0.007 0.661
5
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Fig. 14. Factorial plots for flexural modulus: a) main effects plot, b) interaction plot.
Fig. 15. Micrography of type A specimen.
for the latter one. In the light of the results found with both kinds of three-point bending tests, it can be concluded that the adhesive enhanced the ILSS resistance but it worsens the flexural strength. As concerns the strain corresponding to the maximum flexural strength for each specimen, it was symbolised with εf and it was calculated through the following relation:
reasonable. In order to strengthen the findings of the previous paragraph, a preliminary experimental campaign of flexural tests was carried out on short beam sample to determine whether the presence of the adhesive affected the ILSS of the FMLs. For this purpose, two different laminates with a single metal sheet were manufactured, one bonded with the adhesive (type A) and the other without (type B). Four specimens were cut from each laminate, whose dimensions were 25 mm × 10 mm, and tested; the experiment consisted of a three-point bending proof, with a span length of 20 mm; in such manner, a span-to-depth ratio of 4 was obtained, that was enough to warrant a shear condition along the specimen thickness. As it can be seen in Fig. 9, the presence of the adhesive resulted in an improvement of the ILSS equal to about 20%; in fact, the type B FMLs had an average strength of 39.26 MPa, while for the type A it was equal to 47.06 MPa. The experimental dispersion was quite limited; in fact, the CoV was 8.57% for the former type and 3.74%
εf =
6Dh L2
(3)
where D represents the maximum deflection at the centre of the specimen. The abovementioned equation can be found in ASTM D790 standard and another work present in the literature [7]. The results relevant to the flexural strain are reported in Fig. 10, where it can be noted that this mechanical characteristic assumed a value between 0.0129 and 0.0146 for the type B laminate, while it ranged between 6
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Fig. 16. Micrography of type B specimen.
0.0116 and 0.0142 for the type A laminate. Instead, the flexural strain interval went from 0.0105 to 0.0138 for the type C laminate and from 0.0110 to 0.0136 for the type D laminate. The CoV was quite reduced for maximum strain too; in fact, it ranged between 5.64% and 12.97%. In Table 3 there are the results of the ANOVA implemented on the experimental results. In this case, neither the number of aluminium sheets nor the presence of the adhesive were significantly affecting, since their contribution was quite low compared to the statistical error. However, for a more significant comparison, the p-value was determined for the all the factors and it resulted higher than the threshold value of 0.05, so it can be concluded that the influence of the considered factor was negligible. The irrelevance of the adhesive presence and number of metal sheets was witnessed by Tukey’s method too. In fact, as visible in Fig. 11, the zero was present in the confidence interval for both the factors analysed in this work. The main effects plot and the interaction plot were not traced for the results relevant to the flexural strain since the ANOVA revealed substantial equality of the strain values for the different specimen type. Finally, the flexural modulus, denoted as Ef, was computed for each of the tested specimens; in particular, the chord modulus definition from the ASTM D790 was considered for this calculation, taking into account the failure instant as the final point and the start of the loaddeflection curve as the initial point:
Ef =
C laminate and from 45.1 GPa to 47.2 GPa for the type D laminate. The data obtained for the flexural modulus presented a CoV lower than those found for both flexural strength and strain; in fact, it ranged from 0.79% to 7.00%. The modulus found for these laminates is lower than that one of aluminium, that is 70 GPa, but it must be remembered that the density of the produced FML is the 70% of bulk aluminium one. Therefore, the specific modulus of both materials, that is the modulus divided by the density, is comparable, since it is 25 MPa/(Kg/m3) in either cases. Table 4 describes the results of the ANOVA that was implemented on the experimental data calculated for the flexural module. The number of aluminium sheets had a contribution of 43.35% on the data variance, so it can be defined as the most influencing factor, while the presence of the adhesive presented a minor contribution, equal to 26.19%. The contribution of the interaction between factors can be neglected since it was less than 0.5%. Even for the flexural modulus analysis, the p-value was calculated, confirming that the influence of both studied factors on the material flexural modulus cannot be ignored, since their p-values were less than 0.05. On the contrary, the pvalue of the interaction was equal to 0.661 and so it can be neglected. For a more accurate analysis, the Tukey’s method was implemented on the flexural modulus data and the results are reported in Fig. 13. Both the analysed factors can be considered affecting the flexural modulus since their confidence interval did not contain the zero. Fig. 14a reports the main effects plot for the flexural modulus, which demonstrates the influences of the studied factors; in fact, the flexural modulus diminished with the number increase of the metal sheets and the adhesive presence. Fig. 14b reports the interaction plot, that confirms the factor importance and demonstrates the absence of interaction between the two factors. From the abovementioned results it can be determined that the
σf εf
(4)
As it can be noted in Fig. 12, that report the data of the flexural modulus for all the tested specimens, it varied from 45.1 GPa to 48.6 GPa for the type A laminate and from 49.5 GPa to 50.4 GPa for the type B laminate, while it ranged from 40.2 GPa to 47.5 GPa for the type 7
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Fig. 17. Micrography of type C specimen.
Fig. 18. Micrography of type D specimen.
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deformations as the aluminium; moreover, the delamination of composite material was present too. As regards the aluminium-composite material contact zone, as it can be seen in Fig. 18b, a thin layer of resin detached from the composite and remained bonded to the aluminium sheet, showing how the metal-composite interface resisted also in this case, as in the type B sample, while the composite material can be considered as the less strong part.
increment of metal sheet numerousness was unfavourable for the modulus, instead the absence of adhesive was valuable. This last finding can be justified in the light of the different material stiffness; in fact, the adhesive Young’s modulus was lower than aluminium and carbon composite ones, so the addition of this more pliable material in the stacking sequence caused the decrease in the flexural modulus. As concerns the influence of the layer count, it must be observed that incrementing this quantity made the composite material shift from the outer zone towards the inner one, that is from higher stress region to lower one (it must be remembered that in a bent beam there is a linear stress distribution, with the null stress zone in the centre and the maximum one in the surface), while the aluminium went from lowstress zone to higher one. Considering that the flexural modulus of the composite material is higher than the aluminium one, it can be explained the reason why the increment of the layer count made the flexural stiffness of the whole FML decrease, as found also by [8]. The tested specimens were analysed by an optical microscope to determine the fracture mode. The micrographs concerning the type A specimens are visible in Fig. 15. In particular, Fig. 15a, taken below the laminate neutral axis, denotes that the fracture mechanism is dominated by the tensile failure of the fibres. In the same figure it can be distinguished the longitudinal fibre bundles, whose breakage is evident, and the orthogonal fibre bundles, identifiable by the circular shape of the single fibres; in fact, it must be remembered that the composite material used to produce the FMLs was composed of twill fabric. Observing the interface zone in Fig. 15b, in which the aluminium is located in the highest part of the micrography, the adhesive in the centre and the composite material in the lowest part, it can be noted that the aluminium sheet-adhesive-composite material system remained undamaged and there are no fractures at both the adhesive-composite material and adhesive-aluminium sheets interfaces; this demonstrates a good adhesive application and an excellent load transferring between the layers of different material. It must be underlined that the aluminium-adhesive interface presented a clear shape, while the compositeadhesive did not, in fact some scattered fibres can be observed. This happened since during cure process the aluminium sheet remained in the solid state, instead adhesive and prepreg resin became a viscous liquid into which some fibre moved. The micrographs of Fig. 16a, relevant to type B specimen, denotes a tensile breakage of the composite material fibres due to the maximum tensile load achievement. Delamination can be noted in Fig. 16b, that is located near the composite-aluminium interface. However, the presence of a thin resin layer attached to the aluminium sheet reveals that the strength of the resin-aluminium connection is higher than the resinfibre one. This finding corroborates the argumentation about the negative effects of the adhesive; in fact, the failure happened inside the composite material, not at the composite-aluminium interface, which is the part the adhesive should improve, if present. Therefore, the beneficial effect of the adhesive did not appear, on the contrary, the negative effect due to its lower strength and stiffness was present. As concerns the type C specimen, that is the one with the lowest flexural stiffness and strength, the tensile failure of the longitudinal fibre and the break of orthogonal bundles of the composite can be noted in Fig. 17a. Moreover, the failure of the aluminium sheet can be noted in Fig. 17b; this sheet was located below the section neutral axis, so in the zone subjected to tensile stress, while the sheet above the neutral axis resulted to be only deformed, without fracture. On the contrary, the metal sheets of type A and type B specimens (laminates with a single aluminium sheet) remained undamaged. It is evident, as in the type A specimens, that the aluminium-adhesive and composite-adhesive interface remained intact, except near the aluminium failure zone, demonstrating an acceptable application of the adhesive and a good distribution of the loads. As reported in Fig. 18a, a complete break of the fibre bundle occurred below the neutral axis for the type D specimen and, as for all the other kind of specimens, it was a fragile failure, since it does not show
4. Conclusion The aim of the presented research was evaluating the influence of both the adhesive presence at the metal-composite interface and the number of metal sheets of FMLs, maintaining the composite/metal volumetric ratio at a constant value. Four different laminates were produced, that were with or without adhesive at the metal-composite interface and with one or two metal sheets, and then the specimens extracted from them were tested according to the three points bending to obtain their flexural properties. From the experimental campaign, it can be concluded that the laminate with a single aluminium sheet bonded with the sole prepreg resin resulted to be the better solution, while the laminate with two metal sheets bonded with the adhesive had the lowest flexural strength. Considering the adhesive AF163-2k, its presence resulted deleterious for both the stiffness and strength of FMLs since the adhesive itself was weaker and more pliant than the other materials in the stack, while the potential beneficial effects due to the interface improvement did not take place, since the shear stresses were very low due to the adopted specimens and load configuration. In fact, the adhesive is useful to enhance the answer to delamination stresses, as demonstrated by the preliminary ILSS tests carried out in this work. A micrographic analysis was carried out in order to determine the failure mechanisms occurring in the flexural tests. This analysis highlighted a tensile failure of the composite for all the type of specimens, moreover delamination was observed in the specimens without the adhesive; however, it must be underlined that the resin remained on the aluminium sheet, so the composite-metal interface remained undamaged, while the break happened in the composite material itself. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of Competing Interest None. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compstruct.2019.111117. References [1] Vermeeren CAJR. An historic overview of the development of fibre metal laminates. Appl Compos Mater 2003;10:189–205. https://doi.org/10.1023/ A:1025533701806. [2] Bellini C, Di Cocco V, Iacoviello F, Sorrentino L. Flexural strength of aluminium carbon/epoxy fibre metal laminates. Mater Des Process Commun 2019;1:e40https://doi.org/10.1002/mdp2.40. [3] Şen I, Alderliesten RC, Benedictus R. Lay-up optimisation of fibre metal laminates based on fatigue crack propagation and residual strength. Compos Struct
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2015;124:77–87. https://doi.org/10.1016/j.compstruct.2014.12.060. [4] Frizzell RM, McCarthy CT, McCarthy MA. An experimental investigation into the progression of damage in pin-loaded fibre metal laminates. Compos Part B Eng 2008;39:907–25. https://doi.org/10.1016/j.compositesb.2008.01.007. [5] Lawcock G, Ye L, Mai YW, Sun CT. The effect of adhesive bonding between aluminum and composite prepreg on the mechanical properties of carbon-fiber-reinforced metal laminates. Compos Sci Technol 1997;57:35–45. https://doi.org/10. 1016/S0266-3538(96)00107-8. [6] Iaccarino P, Langella A, Caprino G. A simplified model to predict the tensile and shear stress-strain behaviour of fibreglass/aluminium laminates. Compos Sci Technol 2007;67:1784–93. https://doi.org/10.1016/j.compscitech.2006.11.005. [7] Dhaliwal GS, Newaz GM. Experimental and numerical investigation of flexural behavior of carbon fiber reinforced aluminum laminates. J Reinf Plast Compos 2016;35:945–56. https://doi.org/10.1177/0731684416632606. [8] Wu X, Pan Y, Wu G, Huang Z, Tian R, Sun S. Flexural behaviour of CFRP/Mg hybrid laminates with different layers thickness. Adv Compos Lett 2017;26:168–72. [9] Koziol M. Evaluation of classic and 3D glass fiber reinforced polymer laminates through circular support drop weight tests. Compos Part B Eng 2019;168:561–71. https://doi.org/10.1016/j.compositesb.2019.03.078. [10] Koziol M, Jesionek M, Szperlich P. Addition of a small amount of multiwalled carbon nanotubes and flaked graphene to epoxy resin. J Reinf Plast Compos 2017;36:640–54. https://doi.org/10.1177/0731684416689144. [11] Gonzalez-Canche NG, Flores-Johnson EA, Carrillo JG. Mechanical characterization of fiber metal laminate based on aramid fiber reinforced polypropylene. Compos Struct 2017;172:259–66. https://doi.org/10.1016/j.compstruct.2017.02.100. [12] Abouhamzeh M, Sinke J, Jansen KMB, Benedictus R. Kinetic and thermo-viscoelastic characterisation of the epoxy adhesive in GLARE. Compos Struct 2015;124:19–28. https://doi.org/10.1016/j.compstruct.2014.12.069. [13] Park SY, Choi WJ, Choi HS. The effects of void contents on the long-term hygrothermal behaviors of glass/epoxy and GLARE laminates. Compos Struct 2010;92:18–24. https://doi.org/10.1016/j.compstruct.2009.06.006. [14] Abouhamzeh M, Sinke J, Jansen KMB, Benedictus R. A new procedure for thermoviscoelastic modelling of composites with general orthotropy and geometry. Compos Struct 2015;133:871–7. https://doi.org/10.1016/j.compstruct.2015.08. 050. [15] Jakubczak P, Bienias J, Surowska B. Interlaminar shear strength of fibre metal laminates after thermal cycles. Compos Struct 2018;206:876–87. https://doi.org/10. 1016/j.compstruct.2018.09.001. [16] Pan L, Ali A, Wang Y, Zheng Z, Lv Y. Characterization of effects of heat treated anodized film on the properties of hygrothermally aged AA5083-based fiber-metal laminates. Compos Struct 2017;167:112–22. https://doi.org/10.1016/j.compstruct. 2017.01.066. [17] Bellini C, Parodo G, Polini W, Sorrentino L. Influence of hydrothermal ageing on single lap bonded CFRP joints. Frat Ed Integrità Strutt 2018;45:173–82. https://doi. org/10.3221/IGF-ESIS.45.15. [18] Sorrentino L, Polini W, Bellini C, Parodo G. Surface treatment of CFRP: influence on single lap joint performances. Int J Adhes Adhes 2018;85:225–33. https://doi.org/ 10.1016/j.ijadhadh.2018.06.008. [19] Pärnänen T, Alderliesten R, Rans C, Brander T, Saarela O. Applicability of AZ31BH24 magnesium in fibre metal laminates – An experimental impact research.
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Compos Part A Appl Sci Manuf 2012;43:1578–86. https://doi.org/10.1016/j. compositesa.2012.04.008. Cortés P, Cantwell WJ. The fracture properties of a fibre-metal laminate based on magnesium alloy. Compos Part B Eng 2005;37:163–70. https://doi.org/10.1016/j. compositesb.2005.06.002. Bellini C, Sorrentino L. Analysis of cure induced deformation of CFRP U-shaped laminates. Compos Struct 2018;197:1–9. https://doi.org/10.1016/j.compstruct. 2018.05.038. Shevtsov S, Zhilyaev IV, Tarasov I, Wu J-K, Snezhina NG. Model-based multiobjective optimization of cure process control for a large CFRP panel. Eng Comput 2018;35:1085–97. https://doi.org/10.1108/EC-10-2017-0375. Sorrentino L, Bellini C, Carrino L, Leone A, Mostarda E, Tersigni L. Cure Process Design to manufacture composite components with variable thickness by a closed die technology. In: 17th Int. Conf. Compos. Mater., Edinburgh; 2009. Shevtsov S, Zhilyaev I, Soloviev A, Parinov I, Dubrov V. Optimization of the composite cure process based on the thermo-kinetic model. Adv Mater Res 2012;569:185–92. https://doi.org/10.4028/www.scientific.net/AMR.569.185. Shevtsov S, Tarasov I, Axenov V, Zhilyaev I, Wu J, Snezhina N. FEM model-based optimal control synthesis for curing a large composite structure with CAD imported geometry. MATEC Web Conf 2017;130. https://doi.org/10.1051/matecconf/ 201713007001. Rubino F, Carlone P, Aleksendrić D, Čirović V, Sorrentino L, Bellini C. Hard and soft computing models of composite curing process looking toward monitoring and control. AIP Conf. Proc. 2016;1769:1–6. https://doi.org/10.1063/1.4963438. Aleksendrić D, Bellini C, Carlone P, Ćirović V, Rubino F, Sorrentino L. Neural-fuzzy optimization of thick composites curing process. Mater Manuf Process 2019;34:262–73. https://doi.org/10.1080/10426914.2018.1512116. Bellini C, Di Cocco V, Iacoviello F, Sorrentino L. Experimental analysis of aluminium carbon/epoxy hybrid laminates under flexural load. Frat Ed Integrità Strutt 2019. in press. Sathyaseelan P, Logesh K, Venketasudhahar M, Dilip Raja N. Experimental and Finite Element Analysis of Fibre Metal Laminates (FML’S) subjected to tensile, flexural and impact loadings with different stacking sequence. Int J Mech Mechatronics Eng 2015;15:23–7. Hamill L, Hofmann DC, Nutt S. Galvanic corrosion and mechanical behavior of fiber metal laminates of metallic glass and carbon fiber composites. Adv Eng Mater 2018;20:1–8. https://doi.org/10.1002/adem.201700711. Rydarowski H, Koziol M. Repeatability of glass fiber reinforced polymer laminate panels manufactured by hand lay-up and vacuum-assisted resin infusion. J Compos Mater 2015;49:573–86. https://doi.org/10.1177/0021998314521259. Hamill L, Nutt S. Adhesion of metallic glass and epoxy in composite-metal bonding. Compos Part B Eng 2018;134:186–92. https://doi.org/10.1016/j.compositesb. 2017.09.044. Li H, Hu Y, Fu X, Zheng X, Liu H, Tao J. Effect of adhesive quantity on failure behavior and mechanical properties of fiber metal laminates based on the aluminum-lithium alloy. Compos Struct 2016;152:687–92. https://doi.org/10.1016/j. compstruct.2016.05.098. Liu C, Du D, Li H, Hu Y, Xu Y, Tian J, et al. Interlaminar failure behavior of GLARE laminates under short-beam three-point-bending load. Compos Part B Eng 2016;97:361–7. https://doi.org/10.1016/j.compositesb.2016.05.003.
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Performance evaluation of CFRP/Al fibre metal laminates with different structural characteristics Costanzo Bellini, Vittorio Di Cocco, Francesco Iacoviello, Luca Sorrentino
T
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Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, 03043 Cassino, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Fibre metal laminate Carbon fibre Flexural load Structural behaviour
FMLs (fibre metal laminates) are a kind of hybrid material consisting of composite material layers alternating to metal sheets, that present high mechanical characteristics. The aim of the present work deals with the flexural behaviour of CARALL (carbon fibre-reinforced aluminium laminates) specimens; in fact, the influence of both the layer thickness and the adhesion interface between CFRP layer and aluminium sheets was analysed. Four different laminates, bonded with structural adhesive or with prepreg resin only and with one or two metal sheets (but maintaining the composite/metal volumetric ratio at a constant value), were tested according to the three points bending scheme. The experimental tests evidenced that the flexural strength increased in absence of the adhesive layer and with a single metal sheet. Moreover, some micrographs of the fractured specimens were taken to determine the failure mechanism.
1. Introduction
with a stronger bonding was found. Iaccarino et al. [6] performed some experimental tests on FMLs to define the stress-strain curve, the residual strain in correspondence of specific stress level and the stress-shear curve. They proposed a modified classical lamination model and they compared the numerical results with experimental ones, finding a good agreement except for the failure prediction. Dhaliwal and Newaz studied the effect of the metal layers position in the material stacking [7], producing and testing some CARALL specimens with carbon fibre laminate as outside layers. They compared the flexural behaviour of their laminates with that of standard CARALL, that had aluminium layers outside, and found a higher strength. A work has been carried out aiming at analysing the behaviour of laminates with the same composite/metal volume fraction ratio and different layers thicknesses by Wu et al. [8]. These authors studied the flexural behaviour of carbon fibre/ magnesium FMLs, discovering that the flexural modulus linearly decreased with the layer thicknesses, while no differences were observed for the flexural strength. Koziol compared the impact strength of composites with three different reinforcements: stitched plain-wowen fabric, plain-wowen fabric and 3D non-crimp fabric, finding a better behaviour of the first one and the third one in comparison with the second one, since they had higher failure stress and a lower crack growth [9]. Koziol et al. evaluated the influence of carbon nanotubes or graphene addition in the resin of a composite material laminate, and they discovered a shear strength improvement in the latter type of laminate, while the former one remained unaffected [10].
FMLs (fibre metal laminates), that are a type of hybrid material made of composite material laminates alternating to metal sheets, are more and more chosen for applications in several industrial fields, like aeronautics, automotive and sports goods, since they present excellent mechanical characteristics, as high damage tolerance, high strength and low density [1,2]. FLMs present another outstanding characteristic: mechanical properties can be easily adjusted to specific requirements by varying the composite ply orientation, the thickness and the number of layers [3]. Usually, structural frame parts are exposed to bending loads, that represents the most diffused, and consequently most investigated, failure mode. The elements an FML is made of have dissimilar properties, causing a complex damage behaviour; in fact, the composite layers are brittle, while metal layers are ductile. The most diffused failure mechanisms of FLMs are matrix cracking, metal layers plastic deformation, fibres fracture, composite layers delamination and debonding between metal and composite [4]. Lawcock et al. [5] found that the flexural characteristics of CARALL (carbon fibre-reinforced aluminium laminates) depended on the bonding between the composite laminate and the aluminium layers, while the tensile properties were not affected; in fact, a weak bonding can give rise to a decrement of about 10% for the interlaminar shear strength. However, they determined that the bonding strength did not influence the residual strength of a notched specimen, even if a small decrease for a specimen
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Corresponding author. E-mail address: [email protected] (L. Sorrentino).
https://doi.org/10.1016/j.compstruct.2019.111117 Received 20 February 2019; Received in revised form 3 May 2019; Accepted 4 June 2019 Available online 05 June 2019 0263-8223/ © 2019 Elsevier Ltd. All rights reserved.
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laminate thickness was about 5 mm. For evaluating the influence of the adhesive interface between the composite material and the aluminium sheet on the mechanical performance of the hybrid laminates, both FMLs with and without adhesive were produced and tested: for the former ones a structural adhesive, typically used in the aeronautical field, was adopted, while the bonding interface of the latter ones relied on the self-adhesive capacity of the resin being a part of the prepreg material. The metal sheets considered for this work were made of aluminium AA 1100, while the composite material was the SE70/RC303T/1270/ 38%, a prepreg system produced by Gurit® and made of SE70 epoxy resin and a 2x2 twill carbon fabric, and the bonding agent, in case of presence, was the AF163-2k film adhesive. As regards the manufacturing process of the CARALL specimens, four laminates, one for each FML type considered in this work, were produced by the vacuum bag process. It was decided to produce laminates with dimensions equal to 200 × 110 mm2, from which to extract four specimens with dimensions of 160 × 20 mm2. The mould surface was treated with Marbocote, a release agent; then, all the materials required for CARALLs manufacturing, that were the prepreg plies, the aluminium sheets and the adhesive plies, were cut into the right size and stacked as visible in Fig. 2. Then, the stacks were covered with a release film and a breather fabric. Finally, the mould was closed with the vacuum bag and the vacuum was applied and maintained until the thermal cycle end. The parts were polymerized in an autoclave following the thermal cycle suggested by the prepreg manufacturer, that was suitable for the adhesive cure too. In must be remembered that the polymerization cycle is an important aspect of the composite material manufacturing process, that can induce defects if it is not correct [21–23]; in fact, several researches have been carried out aiming at optimization of cure process control [24,25], also through innovative neural-fuzzy approaches [26,27]. This cycle scheduled a heating ramp with a rate of 2 °C/min and a temperature dwell of 100 min at 100 °C. At the end of the curing process, the laminates were taken from the mould and trimmed by a diamond disk saw, obtaining the desired specimens. An illustration of the type D samples is reported in Fig. 3. The bending tests carried out on the specimens to evaluate the flexural characteristic referred to the ASTM D790. This test method considered a three-point loading system applied to a simply supported beam. In particular, the test involved two cylindrical shape supports on which the rectangular cross-section specimen was laid, loaded by a cylindrical punch, halfway between the two supports, as visible in Fig. 4. The span between the support was set to 136 mm, while the loading rate to 6 mm/min. In this work a micrographic inspection was carried out in order to analyse the fracture surface and, consequently, to define the fracture mode. The preparation operations were 3: sample cutting from the specimen, sample mounting and surface polishing. In the first operation, after having chosen the section of interest, which corresponded to the area under the loading nose in the specimen centre, a sample was cut by means of an abrasive disk saw using water as cutting fluid. The second operation was the mounting of the samples in resin panels, which were polymerized in 24 h at room temperature to avoid the thermal alteration of the specimens. The last operation was represented by the sample polishing by a grinder equipped with silicon carbide disks and a felt disk with alumina aqueous suspension, obtaining the samples visible in Fig. 5.
The interface between the metal sheets and the composite laminate is very important since it can influence the mechanical behaviour of the FMLs, especially the interlaminar shear strength. In fact, GonzalesCanche et al. [11] found that the adhesion of the different materials allowed the growth of plastic deformation due to the partial inhibition of the localized strain, postponing the cracking start. The use of epoxy adhesive is suitable for the raising of residual strength, fatigue limit and damage tolerance, and it was observed that the high temperature the material is exposed to during curing cycle did not affect these mechanical properties [12]. Also, the voids formation at the interface negatively influences the structural strength of the FMLs, since it causes a poor bond between the metal and the composite material [13]. Abouhamzeh et al. [14] proposed a modelling procedure suitable for this kind of material, while Jakubczak et al. [15] studied the effect of the thermal ageing on ILSS (interlaminar shear strength) for CARALLs with different aluminium surface preparation and various fibre combinations. They found that the addition of a thin glass ply at the interface between metal sheets and carbon laminates, suitable to avoid galvanic corrosion, influenced neither the ILSS nor the thermal fatigue of the laminates. The effects of annealing and anodizing the aluminium sheets of an FML was studied by Pan et al. [16], that discovered a decrement of mechanical properties due to annealing. In the past, the effect of surface treatment and ageing condition were studied also on composite materials too [17,18]. The aim of the present work concerns the flexural behaviour study of CARALL specimens, analysing the influence of both layer thickness and the adhesion between CFRP layer and aluminium sheet. Moreover, unidirectional reinforcements are usually employed in the design of FMLs, while in this study a carbon fabric was considered as composite material reinforcement. The effect of the stacking characteristic on the laminate mechanical behaviour was studied by several researchers, even if they keep the composite laminate thickness [19] or metal sheet thickness [20] constant and focused the attention on the dynamic characterization. The layer thickness is a significant factor concerning the structural behaviour of FMLs that should be taken into consideration for the product design, even if the study of this topic has been infrequently explored.
2. Material and methods In this work, the effect of the layer thickness and the layer adhesion has been investigated both separately and combined. As reported in Table 1, the full factorial plan of the experimental activity is composed of 2 levels for each of the above-mentioned factors. The analysis was carried out through three points bending tests on CARALL specimens with different stacking and dissimilar bonding strategy. As concerns the stacking sequence, two FMLs with CFRP laminate as external layers were considered; as visible in Fig. 1, one consisted of two layers of CFRP and one of aluminium, while the other one was formed by two layers of aluminium and three layers of carbon fibre. In order to maintain the laminate thickness and the CFRP/aluminium ratio constant, the aluminium sheet thickness of the former typology was equal to 0.6 mm and one composite laminate was composed by six prepreg plies, for a total of 12 plies in the whole FML, while the latter type presented two 0.3 mm thick aluminium foil and three CFRP layers consisting of four prepreg plies. In such a manner, the obtained Table 1 full factorial plan for factor influence analysis.
3. Results and discussion
Factor
# levels
Level value
Number of aluminium sheets Adhesion technology Number of replications Tot number of tested specimens
2 2 4 16
1, 2 with adhesive, without adhesive
The influence of two factors, such as the number of layers and the presence of adhesive, on the flexural properties of a CARALL was studied. In particular, the attention was focused not only on the single factor but also on their combination, leading to a more accurate and deeper interpretation of the material behaviour. The three points bending test results concerned three different mechanical properties: 2
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Fig. 1. Analysed FML stacking sequence.
Fig. 5. FML sample prepared for micrographic analysis.
Fig. 2. Stacking of materials during the manufacturing process of FML.
Fig. 6. Comparison of flexural strength for the different type of CARALL. Fig. 3. An example of finished specimens.
supplementary consistency check, that consisted of a pairwise comparison of populations means: if their difference interval contained zero, the population were not different. Finally, factorial plots were outlined for deeper data analysis. In particular, the main effect plot reports the variation in the average response as a factor changed level, so it is useful to reveal which factor is the most influencing, while the interaction plot is suitable for assessing whether two-way interactions exist. More information on the stress-strain curves can be found in [28]. The flexural strength of the tested laminates, denoted as σf, was determined through the following relation, present in ASTM D790 standard and other papers in the literature [7,29,30]:
σf =
3PL 2bh2
(1)
where P represents the load the specimen undergoes, L is the distance between the supports and h and b are the specimen thickness and width, respectively. As it can be noted in Fig. 6, the flexural strength reached a maximum value of 641.86 MPa and a minimum value of 562.75 MPa for the laminate with one metal sheet and adhesive, while for the similar laminate characterized by the adhesive absence the minimum strength was 644.25 MPa and the maximum one was 734.00 MPa. As regards the CARALL presenting two metal sheets, the flexural strength values ranged between 498.38 MPa and 641.38 MPa for the laminate without adhesive and between 468.88 MPa and 553.30 MPa for that one with the adhesive layers. The CoV (Coefficient
Fig. 4. Three points bending test on an FML specimen.
the maximum flexural strength, the relevant strain and the flexural module, and all of them were analysed through inferential statistical techniques in order to ascertain the presence or absence of significant differences. The observation of ANOVA (analysis of variance) factors and the p-values was chosen as analysis criteria, as commonly done for statistical analysis. Moreover, the Tukey’s method was considered for a 3
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Table 2 ANOVA analysis of experimental data obtained for flexural strength. Source
DF
Seq SS
Contribution
Adj SS
Adj MS
F-value
P-value
n. sheet adhesive n. sheet * adhesive error total
1 1 1 12 15
42087.7 23937.6 330.9 23439.7 78795.9
53.41% 16.42% 0.42% 29.75% 100.00%
42087.7 12937.6 330.9 23439.7
42087.7 12937.6 330.9 1953.3
21.55 6.62 0.17
0.001 0.024 0.688
Fig. 9. Short beam strength comparison for FML with and without adhesive.
Fig. 7. Tukey’s pairwise comparison for flexural strength.
of Variation), defined as in [31], was equal to about 5–7%, and in only one case it overcame 10%, even if only for a very small amount. The obtained strength values were comparable or, in some cases, even higher than that one of aeronautic grade aluminium alloy, that typically is 450 MPa. The results of the ANOVA that was implemented on the experimental results are presented in Table 2. It can be noted that the number of aluminium sheets was the most influencing factor, since it had a contribution of 53.41%, while the effect of the other factor, that is the presence of the adhesive, was far less important (16.42%). The contribution of the interaction between factors was less than 0.5% and it can be neglected. However, for a more significant comparison, the pvalue was calculated too for each factor; the analysis confirmed that both the considered factors influenced the material flexural strength, since their value was less than 0.05, the commonly chosen a-level, instead the interaction term p-value was 0.688 and so it can be disregarded. The results of the Tukey’s method applied to the obtained flexural strength values are presented in Fig. 7. It can be noted that for both the analysed factors the confidence interval did not contain the zero, so the
Fig. 10. Comparison of flexural strain for the different type of CARALL.
previous statement about their influence is confirmed. The main effects plot reported in Fig. 8a denotes the influences the factors exerted on the flexural strength; in particular, the increment of the numbers of metal sheets and the presence of adhesive made the flexural strength decrease. The same conclusion can be drawn from the interaction plot of Fig. 8b, which is also useful to assess the absence of interaction between the two factors, since the lines that tie data are parallel.
Fig. 8. Factorial plots for flexural strength: a) main effects plot, b) interaction plot. 4
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Table 3 ANOVA analysis of experimental data obtained for the flexural strain. Source
DF
Seq SS
Contribution
Adj SS
Adj MS
F-value
P-value
n. sheet adhesive n. sheet * adhesive error total
1 1 1 12 15
0.000006 0.000001 0.000000 0.000016 0.000022
25.03% 2.61% 0.07% 72.29% 100.00%
0.000006 0.000001 0.000000 0.000016
0.000006 0.000001 0.000000 0.000001
4.15 0.43 0.01
0.064 0.523 0.918
Fig. 13. Tukey’s pairwise comparison for flexural modulus.
Fig. 11. Tukey’s pairwise comparison for the flexural strain.
considered, but the same authors acknowledged their results to be highly depending on specimen geometry and load configuration. In fact, the flexural load can generate two different failure behaviour: interfacial delamination and fibre fracture, that depend on shear stress and tensile stress, respectively. Both kind of stress always coexist in three points bending beam, but the preponderance of one with respect to the other depends on the span/thickness ratio: if this is small enough, the interlaminar shear stress is higher, for higher values the tension stresses are predominant [34]. The latter stress can be calculated through the Eq. (1), instead the former one, here indicated as σsbs, can be determined as:
σsbs =
3P 4bh
(2)
That relation can be found in the ASTM D2344 standard and it has been adopted by other authors too [5,30,34]. Considering Eq. (2), the type A specimen, for which a maximum flexural load of about 1500 N was registered, presented a shear strength of about 11 MPa, that was negligible compared to the shear strength of the materials constituting this FML. The flexural and shear strengths are influenced by two antithetic factors: on one hand, the adhesive presence enhances the interfacial bonding and, consequently, improves the FML shear behaviour, while, on the other hand, it reduces the fibre volume content and so the strength of the material. In the light of the abovementioned reflexions, it can be concluded that for the considered specimens the latter factor is predominant and so the strength decay of FML with adhesive is entirely
Fig. 12. Comparison of flexural modulus for the different type of CARALL.
From the previously reported results, it can be concluded that the presence of adhesive was detrimental for the material strength, while the decrease of metal sheet numerousness was beneficial. As concern the former conclusion, it seemed to be in contradiction with literature; in fact, several researchers [5,32,33] discovered an improvement due to the presence of the adhesive, but it must be underlined that this enhancement was found only for lap shear and interlaminar shear tests, while the in-plane mechanical properties, as the tensile strength, were not influenced. In the work of Li et al. [33], the flexural behaviour was Table 4 ANOVA analysis of experimental data obtained for flexural modulus. Source
DF
Seq SS
Contribution
Adj SS
Adj MS
F-value
P-value
n. sheet adhesive n. sheet * adhesive error total
1 1 1 12 15
54,745,058 33,069,339 638,055 37,831,285 126,283,737
43.35% 26.19% 0.51% 29.96% 100.00%
54,745,058 33,069,339 638,055 37,831,285
54,745,058 33,069,339 638,055 3,152,607
17.37 10.49 0.20
0.001 0.007 0.661
5
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Fig. 14. Factorial plots for flexural modulus: a) main effects plot, b) interaction plot.
Fig. 15. Micrography of type A specimen.
for the latter one. In the light of the results found with both kinds of three-point bending tests, it can be concluded that the adhesive enhanced the ILSS resistance but it worsens the flexural strength. As concerns the strain corresponding to the maximum flexural strength for each specimen, it was symbolised with εf and it was calculated through the following relation:
reasonable. In order to strengthen the findings of the previous paragraph, a preliminary experimental campaign of flexural tests was carried out on short beam sample to determine whether the presence of the adhesive affected the ILSS of the FMLs. For this purpose, two different laminates with a single metal sheet were manufactured, one bonded with the adhesive (type A) and the other without (type B). Four specimens were cut from each laminate, whose dimensions were 25 mm × 10 mm, and tested; the experiment consisted of a three-point bending proof, with a span length of 20 mm; in such manner, a span-to-depth ratio of 4 was obtained, that was enough to warrant a shear condition along the specimen thickness. As it can be seen in Fig. 9, the presence of the adhesive resulted in an improvement of the ILSS equal to about 20%; in fact, the type B FMLs had an average strength of 39.26 MPa, while for the type A it was equal to 47.06 MPa. The experimental dispersion was quite limited; in fact, the CoV was 8.57% for the former type and 3.74%
εf =
6Dh L2
(3)
where D represents the maximum deflection at the centre of the specimen. The abovementioned equation can be found in ASTM D790 standard and another work present in the literature [7]. The results relevant to the flexural strain are reported in Fig. 10, where it can be noted that this mechanical characteristic assumed a value between 0.0129 and 0.0146 for the type B laminate, while it ranged between 6
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Fig. 16. Micrography of type B specimen.
0.0116 and 0.0142 for the type A laminate. Instead, the flexural strain interval went from 0.0105 to 0.0138 for the type C laminate and from 0.0110 to 0.0136 for the type D laminate. The CoV was quite reduced for maximum strain too; in fact, it ranged between 5.64% and 12.97%. In Table 3 there are the results of the ANOVA implemented on the experimental results. In this case, neither the number of aluminium sheets nor the presence of the adhesive were significantly affecting, since their contribution was quite low compared to the statistical error. However, for a more significant comparison, the p-value was determined for the all the factors and it resulted higher than the threshold value of 0.05, so it can be concluded that the influence of the considered factor was negligible. The irrelevance of the adhesive presence and number of metal sheets was witnessed by Tukey’s method too. In fact, as visible in Fig. 11, the zero was present in the confidence interval for both the factors analysed in this work. The main effects plot and the interaction plot were not traced for the results relevant to the flexural strain since the ANOVA revealed substantial equality of the strain values for the different specimen type. Finally, the flexural modulus, denoted as Ef, was computed for each of the tested specimens; in particular, the chord modulus definition from the ASTM D790 was considered for this calculation, taking into account the failure instant as the final point and the start of the loaddeflection curve as the initial point:
Ef =
C laminate and from 45.1 GPa to 47.2 GPa for the type D laminate. The data obtained for the flexural modulus presented a CoV lower than those found for both flexural strength and strain; in fact, it ranged from 0.79% to 7.00%. The modulus found for these laminates is lower than that one of aluminium, that is 70 GPa, but it must be remembered that the density of the produced FML is the 70% of bulk aluminium one. Therefore, the specific modulus of both materials, that is the modulus divided by the density, is comparable, since it is 25 MPa/(Kg/m3) in either cases. Table 4 describes the results of the ANOVA that was implemented on the experimental data calculated for the flexural module. The number of aluminium sheets had a contribution of 43.35% on the data variance, so it can be defined as the most influencing factor, while the presence of the adhesive presented a minor contribution, equal to 26.19%. The contribution of the interaction between factors can be neglected since it was less than 0.5%. Even for the flexural modulus analysis, the p-value was calculated, confirming that the influence of both studied factors on the material flexural modulus cannot be ignored, since their p-values were less than 0.05. On the contrary, the pvalue of the interaction was equal to 0.661 and so it can be neglected. For a more accurate analysis, the Tukey’s method was implemented on the flexural modulus data and the results are reported in Fig. 13. Both the analysed factors can be considered affecting the flexural modulus since their confidence interval did not contain the zero. Fig. 14a reports the main effects plot for the flexural modulus, which demonstrates the influences of the studied factors; in fact, the flexural modulus diminished with the number increase of the metal sheets and the adhesive presence. Fig. 14b reports the interaction plot, that confirms the factor importance and demonstrates the absence of interaction between the two factors. From the abovementioned results it can be determined that the
σf εf
(4)
As it can be noted in Fig. 12, that report the data of the flexural modulus for all the tested specimens, it varied from 45.1 GPa to 48.6 GPa for the type A laminate and from 49.5 GPa to 50.4 GPa for the type B laminate, while it ranged from 40.2 GPa to 47.5 GPa for the type 7
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Fig. 17. Micrography of type C specimen.
Fig. 18. Micrography of type D specimen.
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deformations as the aluminium; moreover, the delamination of composite material was present too. As regards the aluminium-composite material contact zone, as it can be seen in Fig. 18b, a thin layer of resin detached from the composite and remained bonded to the aluminium sheet, showing how the metal-composite interface resisted also in this case, as in the type B sample, while the composite material can be considered as the less strong part.
increment of metal sheet numerousness was unfavourable for the modulus, instead the absence of adhesive was valuable. This last finding can be justified in the light of the different material stiffness; in fact, the adhesive Young’s modulus was lower than aluminium and carbon composite ones, so the addition of this more pliable material in the stacking sequence caused the decrease in the flexural modulus. As concerns the influence of the layer count, it must be observed that incrementing this quantity made the composite material shift from the outer zone towards the inner one, that is from higher stress region to lower one (it must be remembered that in a bent beam there is a linear stress distribution, with the null stress zone in the centre and the maximum one in the surface), while the aluminium went from lowstress zone to higher one. Considering that the flexural modulus of the composite material is higher than the aluminium one, it can be explained the reason why the increment of the layer count made the flexural stiffness of the whole FML decrease, as found also by [8]. The tested specimens were analysed by an optical microscope to determine the fracture mode. The micrographs concerning the type A specimens are visible in Fig. 15. In particular, Fig. 15a, taken below the laminate neutral axis, denotes that the fracture mechanism is dominated by the tensile failure of the fibres. In the same figure it can be distinguished the longitudinal fibre bundles, whose breakage is evident, and the orthogonal fibre bundles, identifiable by the circular shape of the single fibres; in fact, it must be remembered that the composite material used to produce the FMLs was composed of twill fabric. Observing the interface zone in Fig. 15b, in which the aluminium is located in the highest part of the micrography, the adhesive in the centre and the composite material in the lowest part, it can be noted that the aluminium sheet-adhesive-composite material system remained undamaged and there are no fractures at both the adhesive-composite material and adhesive-aluminium sheets interfaces; this demonstrates a good adhesive application and an excellent load transferring between the layers of different material. It must be underlined that the aluminium-adhesive interface presented a clear shape, while the compositeadhesive did not, in fact some scattered fibres can be observed. This happened since during cure process the aluminium sheet remained in the solid state, instead adhesive and prepreg resin became a viscous liquid into which some fibre moved. The micrographs of Fig. 16a, relevant to type B specimen, denotes a tensile breakage of the composite material fibres due to the maximum tensile load achievement. Delamination can be noted in Fig. 16b, that is located near the composite-aluminium interface. However, the presence of a thin resin layer attached to the aluminium sheet reveals that the strength of the resin-aluminium connection is higher than the resinfibre one. This finding corroborates the argumentation about the negative effects of the adhesive; in fact, the failure happened inside the composite material, not at the composite-aluminium interface, which is the part the adhesive should improve, if present. Therefore, the beneficial effect of the adhesive did not appear, on the contrary, the negative effect due to its lower strength and stiffness was present. As concerns the type C specimen, that is the one with the lowest flexural stiffness and strength, the tensile failure of the longitudinal fibre and the break of orthogonal bundles of the composite can be noted in Fig. 17a. Moreover, the failure of the aluminium sheet can be noted in Fig. 17b; this sheet was located below the section neutral axis, so in the zone subjected to tensile stress, while the sheet above the neutral axis resulted to be only deformed, without fracture. On the contrary, the metal sheets of type A and type B specimens (laminates with a single aluminium sheet) remained undamaged. It is evident, as in the type A specimens, that the aluminium-adhesive and composite-adhesive interface remained intact, except near the aluminium failure zone, demonstrating an acceptable application of the adhesive and a good distribution of the loads. As reported in Fig. 18a, a complete break of the fibre bundle occurred below the neutral axis for the type D specimen and, as for all the other kind of specimens, it was a fragile failure, since it does not show
4. Conclusion The aim of the presented research was evaluating the influence of both the adhesive presence at the metal-composite interface and the number of metal sheets of FMLs, maintaining the composite/metal volumetric ratio at a constant value. Four different laminates were produced, that were with or without adhesive at the metal-composite interface and with one or two metal sheets, and then the specimens extracted from them were tested according to the three points bending to obtain their flexural properties. From the experimental campaign, it can be concluded that the laminate with a single aluminium sheet bonded with the sole prepreg resin resulted to be the better solution, while the laminate with two metal sheets bonded with the adhesive had the lowest flexural strength. Considering the adhesive AF163-2k, its presence resulted deleterious for both the stiffness and strength of FMLs since the adhesive itself was weaker and more pliant than the other materials in the stack, while the potential beneficial effects due to the interface improvement did not take place, since the shear stresses were very low due to the adopted specimens and load configuration. In fact, the adhesive is useful to enhance the answer to delamination stresses, as demonstrated by the preliminary ILSS tests carried out in this work. A micrographic analysis was carried out in order to determine the failure mechanisms occurring in the flexural tests. This analysis highlighted a tensile failure of the composite for all the type of specimens, moreover delamination was observed in the specimens without the adhesive; however, it must be underlined that the resin remained on the aluminium sheet, so the composite-metal interface remained undamaged, while the break happened in the composite material itself. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of Competing Interest None. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compstruct.2019.111117. References [1] Vermeeren CAJR. An historic overview of the development of fibre metal laminates. Appl Compos Mater 2003;10:189–205. https://doi.org/10.1023/ A:1025533701806. [2] Bellini C, Di Cocco V, Iacoviello F, Sorrentino L. Flexural strength of aluminium carbon/epoxy fibre metal laminates. Mater Des Process Commun 2019;1:e40https://doi.org/10.1002/mdp2.40. [3] Şen I, Alderliesten RC, Benedictus R. Lay-up optimisation of fibre metal laminates based on fatigue crack propagation and residual strength. Compos Struct
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