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The comparison of effects of hygrothermal conditioning on mechanical properties of fibre metal laminates and fibre reinforced polymers
Accepted Manuscript The comparison of effects of hygrothermal conditioning on mechanical properties of fibre metal laminates and fibre reinforced polymers Krzysztof Majerski, Barbara Surowska, Jaroslaw Bienias PII:
S1359-8368(17)30634-0
DOI:
10.1016/j.compositesb.2018.01.002
Reference:
JCOMB 5493
To appear in:
Composites Part B
Received Date: 9 March 2017 Revised Date:
15 November 2017
Accepted Date: 12 January 2018
Please cite this article as: Majerski K, Surowska B, Bienias J, The comparison of effects of hygrothermal conditioning on mechanical properties of fibre metal laminates and fibre reinforced polymers, Composites Part B (2018), doi: 10.1016/j.compositesb.2018.01.002. 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.
ACCEPTED MANUSCRIPT Keywords: Fibre-metal laminates, environmental conditioning, mechanical properties, moisture absorption, carbon fibre, glass fibres.
Abstract
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The purpose of this article is to present the effects of hygrothermal conditioning of fibre-metal laminates and conventional fibre-reinforced polymer composites on some mechanical properties of these materials. The study was carried out by long-term conditioning of tested materials at elevated temperature (60° C)
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and relative humidity (99% ). The equilibrium of moisture absorption for both fibre-metal laminates and fibre-reinforced polymer composites was determined. The mechanical properties have been investigated
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by tensile strength and interlaminar shear strength tests. The obtained results show that moisture absorption of fibre-metal laminates is significantly lower that of a fibre reinforced polymer composites. After conditioning, the loss of strength properties has been identified. The tensile strength of fibre-metal laminates decreases by 1–15% and the interlaminar shear strength decreases by 9–11%, depending on the configuration. The damage analysis revealed that exposure to environmental conditions has an
1.
Introduction
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impact on the nature of the damage of the tested laminates.
Over the past decades, fibre-reinforced polymer (FRP) composites have become very popular in
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many technical fields, especially in the aircraft industry [1]. A wide range of beneficial properties determines the use of this type of materials for light primary structures [2–5]. The specific structure of
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composite materials makes not only the mechanical properties, but also characteristic of damage process becomes the subject of detailed research [6,7]. On the other hand, however, FRP composites are sensitive to environmental factors such as temperature and humidity. It is commonly known that the impact of the environment can result in the reduction of the performance properties of polymer composites [8,9]. Different mechanisms have been proposed to describe the phenomenon of moisture absorption by FRP composites [10]. FRP composites absorb moisture mainly through diffusion in the matrix area, which, according to Fick's law, proceeds from the surface to the centre of a laminate. Moisture absorption may also take place through diffusion by osmotic swelling limited by polymer creep, and osmotic
ACCEPTED MANUSCRIPT phenomena due to the presence of micropores, channels and other defects in the polymer [9,11]. As a result of moisture absorption, the polymer matrix of a laminate can undergo the processes of plasticisation and hydrolysis, which, in turn, can cause changes in the mechanical, thermophysical and chemical properties [12]. In consequence of moisture absorption, FRP composites also undergo dimensional
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expansion. A significant portion of absorption takes place in the matrix area, whereas fibres absorb moisture in minimal quantities. The nature of this process causes an uneven volumetric expansion of components, which leads to the induction of stresses in the laminate volume. A particularly dangerous phenomenon related to the moisture absorption by polymer composites is the weakening of the bond at
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the fibre-matrix interface, which reduces an effective transmission of stresses from the matrix to the fibres and contributes to further deterioration of the properties of the laminate [13,14]. All processes related to
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the effects of a humid environment usually lead to a decrease of the stiffness and glass transition temperature of polymer. Deterioration is found mainly in the properties controlled by the matrix, e.g., interlaminar shear strength, compressive strength, fatigue tolerance and impact resistance [9,10,13,15]. Fibre-metal Laminates (FML) are materials that have a significant potential for application in the aircraft industry [16,17]. FMLs are hybrid composites consisting of alternating thin layers of metal and a
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FRP composite, which are adhesively bonded. FMLs have the beneficial properties of both metals and polymer composites. In general, FMLs are characterised by low density, impact resistance, high static and fatigue strength, resistance to flame and corrosion [18–22]. Due to their structure, FMLs also exhibit
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considerable resistance to the influence of environmental factors. In the case of FMLs, the influence of moisture is limited since its penetration is possible only through uncovered free edges or holes. Nevertheless, FMLs can be susceptible not only to the weakening of the bonding at the fibre-matrix
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interface in the composite layer, but also to degradation at the metal-composite interface, e.g., as a result of the hydration of the oxide layer [23,24]. The issue of the effects of environmental conditions on FMLs has been discussed in the literature.
In a series of studies, Botelho et al. analyse the changes of the properties of FMLs caused by the
influence of elevated temperature and moisture. Their study [25] presents results of the testing of the mass increase of Glare laminates in a 3/2 configuration, which absorbed less than 0.15% of moisture before reaching the saturation point after 6 weeks. Botelho et al. also studied the strength of GLARE [26] and CARAL [27] laminates using the Iosipescu shear test. The obtained results indicate that the shear strength
ACCEPTED MANUSCRIPT of the tested Glare and Caral laminates is not significantly reduced under the influence of environmental conditions. Moreover, these laminates are characterised by a very low tendency to absorb moisture in comparison to conventional FRP composites. At the same time, a fractographic analysis confirmed the occurrence of matrix plasticisation in the edge areas, which points to a concentration of diffusing
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molecules in these areas. In another study, Botelho et al. [23] observed no significant changes of the tensile strength of Glare 3/2 laminates after conditioning and registered a small (3%) decrease in their compressive strength. Botelho et al. [24] obtained similar results for Caral laminates.
On the other hand, according to Zhong et al.[28], the strength values of Glare laminates decrease
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significantly after exposure to a high humidity environment or after immersion in water. After 90 days, the strength of Glare laminates was at 64% of the original strength. In addition, Zhong et al. observed that
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the moisture absorbed by the matrix of the composite layers can cause similar degradation processes as in the case of conventional FRP composits, which will be invisible from the outside. Both the matrix and the metal-composite interface play a significant role in the transmission of stresses through the laminate. Degradation of these components leads to the reduction of tensile strength [29,30]. Ypma et al. [31] showed that the conditioning of Glare laminates (3000h, 80° C, 85% RH) results in
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a decrease in the values of blunt notch strength and interlaminar shear strength (ILSS) by 15%. Lopes et al. [32] studied the impact of the content of porosity on the level of moisture absorption. The tests showed that increasing the porosity of a Glare laminate over 0.5% results in an increased intensity of the
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processes of moisture absorption by the edges of the laminate and a decrease of ILSS determined by the short-beam method.
Regrettably, the available information on the effects of environmental factors on FMLs in different
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configurations is incomplete, and the available test results point to considerable discrepancies. The reason is probably due to the use of different types of fibres, resin systems with different properties and carrying out the process of conditioning using different temperature and moisture values. Due to the complex structure, the use of different components and the presence of many interface areas, it is essential to carry out further research on the influence of environmental factors on the moisture absorption process and the mechanical properties of FMLs. The aim of this paper is to assess the influence of hygrothermal conditioning on selected properties of hybrid fibre-metal laminates with different material configurations. This article presents tests results on
ACCEPTED MANUSCRIPT moisture absorption and the changes of selected mechanical properties: tension strength and ILSS determined by the short-beam method, before and after the process of conditioning. Moreover, the effects of environmental conditions on the nature of the damage of the tested materials have been identified.
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2. Experimental procedure 2.1. Materials and methods
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The laminates used in this study were Aluminium/GFRP (AlG) and Aluminium/CFRP (AlC) laminates. The FMLs were manufactured by stacking alternating sheets of 2024-T3 aluminium alloy
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(thickness of 0.3 mm) and unidirectional R-glass or AS7-carbon fibre reinforced M12 epoxy prepregs (Hexcel, USA). The content of fibres in a composite was about 60%. A [0/90] fibre orientation was used in all of the tested systems. Metal sheets were subjected to surface treatment in the form of anodising in chromic acid (CAA). In order to ensure proper adhesion to the polymer composite, the produced oxide layer was coated with primer containing a corrosion inhibitor (EC 3924B, 3 M, USA). The plates for the
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research were produced in the Department of Materials Engineering, Lublin University of Technology in an autoclave process (Scholz Maschinenbau, Germany). The laminate curing process was carried out with the following parameters: heating rate of 2°C/min to 135°C and curing at this temperature for a period of 2 h. The pressure and the vacuum used were 0.5 MPa and 0.08 MPa, respectively. After curing, the
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laminates were inspected by the ultrasonic method (Olympus Ominscan MX2). Conventional GFRPs and CFRPs made of the same materials as the composite layers in FMLs were used as comparative materials.
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The manufactured panels were cut into specimens using a water-cooled abrasive disc cutter.
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configurations of the tested laminates are shown in Table 1.
Table 1 Configuration of laminates prepared for testing Total
Laminate
Material
Lay-up configuration
thickness (t)
F
average [mm] GFRP 1
R-glass/epoxy
[0/90]2
1.1
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R-glass/epoxy
[0/90]4
2.15
CFRP 1
AS7 carbon/epoxy
[0/90]4
1.08
CFRP 2
AS7 carbon /epoxy
[0/90]8
1.9
AlG 1
Al 0.3mm/R-glass/epoxy
[2/1]
1.08 [3/2]
AlG 2
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(Al/0/90/Al)
1.92
Al 0.3mm/R-glass/epoxy
FML
(Al/0/90/Al/90/0/Al) [2/1] AlC 1
1.1
Al 0.3mm /AS7/epoxy
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(Al/0/0/90/90/Al) [3/2]
AlC 2
Al 0.3mm /AS7/epoxy
1.84
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(Al/0/0/90/90/Al/90/90/0/0/Al)
2.2. Hygrothermal conditioning
The kinetics of moisture absorption and the state of equilibrium of the tested laminates were
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determined in accordance with ASTM D 5229 for 100x100xt mm control samples. At the initial stage, the specimens were subjected to drying at 70±1° C for a period of 10 days in a laboratory drying oven (SLW 53 ECO) in order to remove moisture and to determine the initial state to the hygrothermal conditioning process. The specimens were then conditioned in a climatic test chamber
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(ASCOT CC450) xp at the temperature of 60°C and 99% relative humidity to the point of reaching moisture equilibrium. The duration of this part of the test was 60 days, which made it possible to reach
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moisture equilibrium control samples. Changes in the mass of the control samples were registered periodically at 24 hour intervals with a laboratory scale (Radwag WAS 220/X) to an accuracy of 0.0001g.
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2.3. Mechanical testing
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Fig. 1. Samples for mechanical testing in environmental chamber
The laminates were tested for tensile strength and ILSS using the short-beam method. The tests of mechanical properties were carried out on both reference specimens and on specimens after conditioning
Tensile test
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in elevated temperature and moisture.
Rectangular-shaped 20x200xt mm specimens were used for the tests. The GFRP and CFRP
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specimens have been equipped with additional protective linings in the gripping part. The tensile test was carried out with the (Instron 8801) testing system and the Instron axial extensometer with a 50 mm base.
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The testing crosshead speed was set at 2 mm/min. During the test, the strength limit (Rm), the Young's modulus (E) and the elongation at break were determined.
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Fig. 2. Strength testing stand with tensile test specimen
The interlaminar shear test
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Interlaminar shear strength was determined in a short beam shear test. The tests were carried out on the (Instron 8801) testing system. The testing speed was set at 1.3 mm/min. The dimensions of the
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specimens and the support span length were adjusted to the laminate thickness (h) according to ASTM 2344. The width of the specimens (b) was: b = 5h, and the length (l) was: l = 10h. The support span length (s) was: s = 4h. The specimens and support span dimensions were selected carefully to ensure the maximum contribution of the shear stresses during the test. The test was conducted in reference to ASTM 2344, however, thin walled specimens (less than 2mm thick) were also used while maintaining the balance between bending and crushing due to compression effects. The test stand layout and specimens dimensions are shown in the figure 3. Interlaminar shear strength was determined by the following equation:
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(1)
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where:
ILSS = short-beam strength, [MPa],
b = measured specimen widh, [mm],
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h = measured specimen thickness, [mm].
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Pmax = maximum load observed during the test, [N],
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Fig. 3 Specimen dimensions and load diagram of the samples during the 3-point bending (ILSS method)
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2.4. Damage assessment The nature of the damage was assessed on the basis of meso-scale observations made on a
stereoscopic microscope (Nikon SMZ 1500) and micro-scale observations on a scanning electron microscope (FEI Nova Nano SEM).
3. Results As a result of carrying out the process of drying at the first stage of conditioning, the state of equilibrium was reached after 10 days. The FRP composite specimens lost from 0.08 to 0.12 % of their mass, and the FML specimens lost from 0.02 to 0.03% of their mass. The mass loss of FMLs is about 10
ACCEPTED MANUSCRIPT times lower than that of FRP composites. The results indicate a lower content of moisture in the tested materials and show that the dynamics of the diffusion process is significantly lower in FMLs than in the case of conventional FRPs. At the second stage of conditioning, an analysis was conducted of the kinetics of the process of
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moisture absorption by the specimens which were placed in an environmental chamber after the process of drying. Figure 4 shows the changes in the specimens mass during the process of conditioning in the
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environmental chamber.
Fig. 4 Changes in the specimens mass during the process of conditioning in the environmental chamber
Significant differences in the level and kinetics of moisture absorption were observed in the tested materials by analysing the individual curves obtained during the conditioning. Generally, based on the shape of the curves representing moisture absorption, two major phases can be distinguished: an area of intensive absorption at the beginning of the test and an area of the stabilisation of the moisture level. Depending on the type and thickness of a laminate, the transition between these phases takes place at a
ACCEPTED MANUSCRIPT different point in time – for FRP laminates after about 8–10 days, and for FMLs after only 2–3 days. The differences in the mass increase values of particular types of laminates are visible already during the first days of seasoning and become greater until the moisture level is stabilised. The abrupt mass increase during the first days of seasoning is caused by a considerable moisture potential gradient occurring
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between the environment and the composite layer. The mass of conventional FRP composites increased by 0.8–1.2%, and the mass of FMLs increased by 0.1–0.2%, depending on the configuration. The differences in the quantity of absorbed moisture may be related to the different diameter of fibres, their type and the volumetric quantity of the
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matrix, whereas the moisture absorption by FMLs is limited because the metal layers constitute a major barrier for the diffusion process and moisture can penetrate into the laminate only through uncovered
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edges. The kinetics of the diffusion process is dependent on the temperature and the capacity of the material to absorb moisture. In this case, the capacity of FMLs to absorb moisture is limited by a reduced area of absorption. The curves given in Figure 1 have the shape of an exponential function and the changes in the dynamics of the mass increase in time are much more visible for FRP composites as they
Tensile strength
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are characterised by greater relative moisture absorption.
The results of the static tensile test indicate that the specimens subjected to the process of
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conditioning in elevated temperature and moisture are characterised by a decrease of tensile strength (Fig. 5), Young's modulus (Fig. 6) and elongation at break (Fig. 7). This tendency is found in the group of
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conventional FRP composites and in fibre-metal laminates in the tested configurations.
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Fig. 5 Tensile strength of tested laminates before and after conditioning
As far as the group of GFRP composites is concerned, specimens with different thickness yielded convergent results and constant exposure to environmental conditions resulted in a significant 30– 32% decrease of tensile strength with a simultaneous preservation of the downward trend of the remaining parameters, i.e., up to 11% for Young's modulus and up to 27% for elongation at break. In the
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group of carbon-epoxy composites, a difference of 7–11% in the value of tensile strength was observed at a simultaneously higher level of output strength than in the case of glass-epoxy composites. The stiffness of carbon-epoxy composites decreases under the influence of environmental factors by up to 6% and the
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elongation upon fracture by up to 9%. The process of conditioning has a considerable impact on the values of the parameters set for FRP composites, which may be related to the plasticisation of the matrix
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and the degradation of the bonding at the fibre-matrix interface [26].
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Fig. 6 Young's modulus value of tested laminates before and after conditioning
Fig. 7 Value of elongation at break of tested laminates before and after conditioning
Tensile strength and Young's modulus of AlG fibre-metal laminates decrease after the process of conditioning by up to 15% and 5%, respectively. The differences observed between particular
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downward trend – from 2 to 5% for Young's modulus and from 6 to 12% for elongation at break. The comparison of composite laminates and metal fibre laminates shows that composite laminates are characterised by a greater decrease of the set strength parameters and, in consequence, they are more susceptible to the influence of environmental factors. Both the level of absorbed moisture and
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the decrease of mechanical parameters indicate that metal layers cause a considerable decrease of the degradation of composite layers. In the case of axial loads in relation to the laminate structure, reinforcing
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fibres play a decisive role in the transmission of stresses. The saturation of the matrix with moisture can contribute to an anomalous distribution of stresses and, in consequence, to the decrease of tensile strength.
ILSS test
Interlaminar shear strength determined in the short beam shear test has a wide application for
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comparative testing, also of FMLs [13,33,34]. The conducted short beam shear tests clearly point to a decrease of the ILSS value for all groups of the tested laminates under the influence of exposure to
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environmental factors (Fig. 8).
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Fig. 8 Results of the short beam shear test
GFRP laminates exhibit a decrease of ILSS by 12–23%, and CFRP laminates by 20–27%. The
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shear strength of FMLs after conditioning is 9–11% lower. In every case, configurations with greater thickness are characterised by lower strength. During the bending of the short beam, the proportion of shear stresses is relatively high, which is why the damage is concentrated mainly in the interlaminar areas. The influence of moisture can both weaken the polymer matrix and affect the quality of the
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bonding at the metal-composite interface.
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Damage analysis
Figures 6 and 7 show examples of the damage morphology of FRP and FML specimens
observed on the lateral surfaces using a stereoscopic microscope. The main damages observed in the group of fibre-metal laminates are numerous delaminations at
the metal-composite interface. The specimens after the process of conditioning exhibit a higher level of degradation.
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Fig. 9 Lateral surface of AlG specimens after the ILSS test
The cracks observed both in AlG laminates (Fig. 9a-d) and in AlC laminates (Fig. 10a-d) are similar in nature. Cracks are visible at the metal-composite interface and on the surface of the interface of
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composite layers with a [0/90] orientation. Transverse cracks in the layer with a [90] orientation join the delaminated areas at the individual interfaces (Fig. 10d). The metal layers of laminates with a 2/1
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configuration have traces of considerable plastic deformations.
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Fig. 10 Lateral surface of AlC specimens after the ILSS test More detailed observations with the use of SEM (Fig. 11) reveal the nature of the damage of the individual components of the laminate. It was observed that the damage at the metal-composite interface
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is adhesive and cohesive in nature. Between the delaminations located at different levels, there are characteristic diagonal cracks found in [90] composite layers, which result from shear stress (Fig. 11a). In
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layers with a [0] orientation, cracks of reinforcing fibres were found near the outer [0] metal layers (Fig. 8b), in particular in the case of AlG laminates.
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Fig. 11 Lateral surface of AlC specimens after the ILSS test: a) AlC, b) AlG Observation of the fractures of laminates after the tensile strength test revealed particularly significant morphological differences in the nature of the fractures of composite layers in AlC laminates. The surfaces of the fractures of the composite layers of an AlC laminate with a [0] orientation are shown in Figure 12. Considerable differences in the position of the tips of the fractured composite fibres were
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observed, which indicates a distinct crack growth mechanism. The fractures of the reference specimens are brittle in nature with a small number of pulled fibres. The fracture morphology of the specimens after conditioning in environmental conditions is much more complex. The deterioration of the properties of the bonding at the fibre-matrix interface can cause a change of the direction of the crack propagation and
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the initiation of cracks delaminating fibres from the matrix. After the crack delaminates the fibre along a greater length, the fibre is damaged. The microcrack then once again changes the propagation direction
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and reaches another fibre. The result of this mechanism is a formation of a very complex fracture surface. Observations of the surface of the inner metal layers (Fig. 12c) do not reveal any considerable
variations between conditioned and unconditioned specimens. In every case, the damage was observed to be mixed adhesive and cohesive in nature. It appears that the degradation of the matrix and the fibre-matrix interface is the primary factor determining the decrease of tensile strength as a result of the influence of environmental conditioning.
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Fig. 12 Fractured surface of an AlC specimens after the tensile strength test – 1) reference, b), c) after conditioning
Conclusions
In this article, the effects of environmental conditioning on selected properties and the nature of the damage of FMLs and conventional FRP composites have been presented. The conducted tests indicate that a long-term influence of environmental factors in the form of elevated temperature and moisture significantly contributes to the deterioration of mechanical properties and can affect the nature of the damage of the tested FMLs and FRPs.
ACCEPTED MANUSCRIPT In comparison to conventional FRPs, fibre-metal laminates exhibit considerably higher resistance to moisture absorption from the environment, which is related to the presence of outer metal layers that constitute a barrier for the diffusion process. In the case of FMLs, the processes of moisture absorption can take place only in the areas of unprotected edges of the composite layers of a laminate.
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Static tensile strength tests indicate that the effects of environmental factors cause a decrease in the strength, stiffness and deformations upon fracture of the tested materials. The decrease of the tensile strength of FMLs after conditioning ranges from 1 to 15% and is considerably lower than that of FRPs, which ranges from 7 to 30%, depending on the configuration.
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The results of ILSS testing obtained for both FMLs and FRPs after the process of conditioning show decreased interlaminar shear strength. A decrease by 9–11% was found in FMLs and by up to 27%
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in the case of FRPs.
The conducted analysis of the damage of laminates indicates that conditioning results in the intensification of the degradation process on the surface of the fibre-matrix interface, which is the primary damage factor that can contribute to the deterioration of their mechanical properties.
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Acknowledgements
The research project was financed by the National Science Center of Poland pursuant to decision No.
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hygrothermally conditioned glare laminate. Int J Eng Sci 2007;45:163–72. Botelho EC, Almeida RS, Pardini LC, Rezende MC. Influence of hygrothermal conditioning on the elastic properties of carall laminates. Appl Compos Mater 2007;14:209–22. Botelho EC, Pardini LC, Rezende MC. Hygrothermal effects on damping behavior of metal/glass fiber/epoxy hybrid composites. Mater Sci Eng A 2005;399:190–8. Botelho EC, Rezende MC, Pardini LC. Hygrothermal Effects Evaluation Using the Iosipescu Shear Test for Glare Laminates. J Brazilian Soc Mech Sci Eng 2008;30:213–20. Botelho EC, Pardini LC, Rezende MC. Evaluation of hygrothermal effects on the shear properties of Carall composites. Mater Sci Eng A 2007;452–453:292–301. Zhong Y, Joshi SC. Response of hygrothermally aged GLARE 4A laminates under static and cyclic loadings. Mater Des 2015;87:138–48. Vlot A, Gunnink W. Fibre Metal Laminates: an Introduction. vol. 1. 2001. Borgonje B, Ypma MS. Long term behaviour of glare. Appl Compos Mater 2003;10:243–55. Ypma M, Borgonje B. Influence of Moisture and Temperature on Glare Material Properties and Glare Structures n.d. Lopes CS, Remmers JJC, Gürdal Z. Influence of Porosity on the Interlaminar Shear Strength of Fibre-Metal Laminates. Key Eng Mater 2008;383:35–52. 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. 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.
S1359-8368(17)30634-0
DOI:
10.1016/j.compositesb.2018.01.002
Reference:
JCOMB 5493
To appear in:
Composites Part B
Received Date: 9 March 2017 Revised Date:
15 November 2017
Accepted Date: 12 January 2018
Please cite this article as: Majerski K, Surowska B, Bienias J, The comparison of effects of hygrothermal conditioning on mechanical properties of fibre metal laminates and fibre reinforced polymers, Composites Part B (2018), doi: 10.1016/j.compositesb.2018.01.002. 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.
ACCEPTED MANUSCRIPT Keywords: Fibre-metal laminates, environmental conditioning, mechanical properties, moisture absorption, carbon fibre, glass fibres.
Abstract
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The purpose of this article is to present the effects of hygrothermal conditioning of fibre-metal laminates and conventional fibre-reinforced polymer composites on some mechanical properties of these materials. The study was carried out by long-term conditioning of tested materials at elevated temperature (60° C)
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and relative humidity (99% ). The equilibrium of moisture absorption for both fibre-metal laminates and fibre-reinforced polymer composites was determined. The mechanical properties have been investigated
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by tensile strength and interlaminar shear strength tests. The obtained results show that moisture absorption of fibre-metal laminates is significantly lower that of a fibre reinforced polymer composites. After conditioning, the loss of strength properties has been identified. The tensile strength of fibre-metal laminates decreases by 1–15% and the interlaminar shear strength decreases by 9–11%, depending on the configuration. The damage analysis revealed that exposure to environmental conditions has an
1.
Introduction
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impact on the nature of the damage of the tested laminates.
Over the past decades, fibre-reinforced polymer (FRP) composites have become very popular in
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many technical fields, especially in the aircraft industry [1]. A wide range of beneficial properties determines the use of this type of materials for light primary structures [2–5]. The specific structure of
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composite materials makes not only the mechanical properties, but also characteristic of damage process becomes the subject of detailed research [6,7]. On the other hand, however, FRP composites are sensitive to environmental factors such as temperature and humidity. It is commonly known that the impact of the environment can result in the reduction of the performance properties of polymer composites [8,9]. Different mechanisms have been proposed to describe the phenomenon of moisture absorption by FRP composites [10]. FRP composites absorb moisture mainly through diffusion in the matrix area, which, according to Fick's law, proceeds from the surface to the centre of a laminate. Moisture absorption may also take place through diffusion by osmotic swelling limited by polymer creep, and osmotic
ACCEPTED MANUSCRIPT phenomena due to the presence of micropores, channels and other defects in the polymer [9,11]. As a result of moisture absorption, the polymer matrix of a laminate can undergo the processes of plasticisation and hydrolysis, which, in turn, can cause changes in the mechanical, thermophysical and chemical properties [12]. In consequence of moisture absorption, FRP composites also undergo dimensional
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expansion. A significant portion of absorption takes place in the matrix area, whereas fibres absorb moisture in minimal quantities. The nature of this process causes an uneven volumetric expansion of components, which leads to the induction of stresses in the laminate volume. A particularly dangerous phenomenon related to the moisture absorption by polymer composites is the weakening of the bond at
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the fibre-matrix interface, which reduces an effective transmission of stresses from the matrix to the fibres and contributes to further deterioration of the properties of the laminate [13,14]. All processes related to
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the effects of a humid environment usually lead to a decrease of the stiffness and glass transition temperature of polymer. Deterioration is found mainly in the properties controlled by the matrix, e.g., interlaminar shear strength, compressive strength, fatigue tolerance and impact resistance [9,10,13,15]. Fibre-metal Laminates (FML) are materials that have a significant potential for application in the aircraft industry [16,17]. FMLs are hybrid composites consisting of alternating thin layers of metal and a
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FRP composite, which are adhesively bonded. FMLs have the beneficial properties of both metals and polymer composites. In general, FMLs are characterised by low density, impact resistance, high static and fatigue strength, resistance to flame and corrosion [18–22]. Due to their structure, FMLs also exhibit
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considerable resistance to the influence of environmental factors. In the case of FMLs, the influence of moisture is limited since its penetration is possible only through uncovered free edges or holes. Nevertheless, FMLs can be susceptible not only to the weakening of the bonding at the fibre-matrix
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interface in the composite layer, but also to degradation at the metal-composite interface, e.g., as a result of the hydration of the oxide layer [23,24]. The issue of the effects of environmental conditions on FMLs has been discussed in the literature.
In a series of studies, Botelho et al. analyse the changes of the properties of FMLs caused by the
influence of elevated temperature and moisture. Their study [25] presents results of the testing of the mass increase of Glare laminates in a 3/2 configuration, which absorbed less than 0.15% of moisture before reaching the saturation point after 6 weeks. Botelho et al. also studied the strength of GLARE [26] and CARAL [27] laminates using the Iosipescu shear test. The obtained results indicate that the shear strength
ACCEPTED MANUSCRIPT of the tested Glare and Caral laminates is not significantly reduced under the influence of environmental conditions. Moreover, these laminates are characterised by a very low tendency to absorb moisture in comparison to conventional FRP composites. At the same time, a fractographic analysis confirmed the occurrence of matrix plasticisation in the edge areas, which points to a concentration of diffusing
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molecules in these areas. In another study, Botelho et al. [23] observed no significant changes of the tensile strength of Glare 3/2 laminates after conditioning and registered a small (3%) decrease in their compressive strength. Botelho et al. [24] obtained similar results for Caral laminates.
On the other hand, according to Zhong et al.[28], the strength values of Glare laminates decrease
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significantly after exposure to a high humidity environment or after immersion in water. After 90 days, the strength of Glare laminates was at 64% of the original strength. In addition, Zhong et al. observed that
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the moisture absorbed by the matrix of the composite layers can cause similar degradation processes as in the case of conventional FRP composits, which will be invisible from the outside. Both the matrix and the metal-composite interface play a significant role in the transmission of stresses through the laminate. Degradation of these components leads to the reduction of tensile strength [29,30]. Ypma et al. [31] showed that the conditioning of Glare laminates (3000h, 80° C, 85% RH) results in
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a decrease in the values of blunt notch strength and interlaminar shear strength (ILSS) by 15%. Lopes et al. [32] studied the impact of the content of porosity on the level of moisture absorption. The tests showed that increasing the porosity of a Glare laminate over 0.5% results in an increased intensity of the
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processes of moisture absorption by the edges of the laminate and a decrease of ILSS determined by the short-beam method.
Regrettably, the available information on the effects of environmental factors on FMLs in different
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configurations is incomplete, and the available test results point to considerable discrepancies. The reason is probably due to the use of different types of fibres, resin systems with different properties and carrying out the process of conditioning using different temperature and moisture values. Due to the complex structure, the use of different components and the presence of many interface areas, it is essential to carry out further research on the influence of environmental factors on the moisture absorption process and the mechanical properties of FMLs. The aim of this paper is to assess the influence of hygrothermal conditioning on selected properties of hybrid fibre-metal laminates with different material configurations. This article presents tests results on
ACCEPTED MANUSCRIPT moisture absorption and the changes of selected mechanical properties: tension strength and ILSS determined by the short-beam method, before and after the process of conditioning. Moreover, the effects of environmental conditions on the nature of the damage of the tested materials have been identified.
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2. Experimental procedure 2.1. Materials and methods
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The laminates used in this study were Aluminium/GFRP (AlG) and Aluminium/CFRP (AlC) laminates. The FMLs were manufactured by stacking alternating sheets of 2024-T3 aluminium alloy
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(thickness of 0.3 mm) and unidirectional R-glass or AS7-carbon fibre reinforced M12 epoxy prepregs (Hexcel, USA). The content of fibres in a composite was about 60%. A [0/90] fibre orientation was used in all of the tested systems. Metal sheets were subjected to surface treatment in the form of anodising in chromic acid (CAA). In order to ensure proper adhesion to the polymer composite, the produced oxide layer was coated with primer containing a corrosion inhibitor (EC 3924B, 3 M, USA). The plates for the
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research were produced in the Department of Materials Engineering, Lublin University of Technology in an autoclave process (Scholz Maschinenbau, Germany). The laminate curing process was carried out with the following parameters: heating rate of 2°C/min to 135°C and curing at this temperature for a period of 2 h. The pressure and the vacuum used were 0.5 MPa and 0.08 MPa, respectively. After curing, the
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laminates were inspected by the ultrasonic method (Olympus Ominscan MX2). Conventional GFRPs and CFRPs made of the same materials as the composite layers in FMLs were used as comparative materials.
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The manufactured panels were cut into specimens using a water-cooled abrasive disc cutter.
The
configurations of the tested laminates are shown in Table 1.
Table 1 Configuration of laminates prepared for testing Total
Laminate
Material
Lay-up configuration
thickness (t)
F
average [mm] GFRP 1
R-glass/epoxy
[0/90]2
1.1
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R-glass/epoxy
[0/90]4
2.15
CFRP 1
AS7 carbon/epoxy
[0/90]4
1.08
CFRP 2
AS7 carbon /epoxy
[0/90]8
1.9
AlG 1
Al 0.3mm/R-glass/epoxy
[2/1]
1.08 [3/2]
AlG 2
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(Al/0/90/Al)
1.92
Al 0.3mm/R-glass/epoxy
FML
(Al/0/90/Al/90/0/Al) [2/1] AlC 1
1.1
Al 0.3mm /AS7/epoxy
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(Al/0/0/90/90/Al) [3/2]
AlC 2
Al 0.3mm /AS7/epoxy
1.84
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(Al/0/0/90/90/Al/90/90/0/0/Al)
2.2. Hygrothermal conditioning
The kinetics of moisture absorption and the state of equilibrium of the tested laminates were
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determined in accordance with ASTM D 5229 for 100x100xt mm control samples. At the initial stage, the specimens were subjected to drying at 70±1° C for a period of 10 days in a laboratory drying oven (SLW 53 ECO) in order to remove moisture and to determine the initial state to the hygrothermal conditioning process. The specimens were then conditioned in a climatic test chamber
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(ASCOT CC450) xp at the temperature of 60°C and 99% relative humidity to the point of reaching moisture equilibrium. The duration of this part of the test was 60 days, which made it possible to reach
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moisture equilibrium control samples. Changes in the mass of the control samples were registered periodically at 24 hour intervals with a laboratory scale (Radwag WAS 220/X) to an accuracy of 0.0001g.
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2.3. Mechanical testing
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Fig. 1. Samples for mechanical testing in environmental chamber
The laminates were tested for tensile strength and ILSS using the short-beam method. The tests of mechanical properties were carried out on both reference specimens and on specimens after conditioning
Tensile test
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in elevated temperature and moisture.
Rectangular-shaped 20x200xt mm specimens were used for the tests. The GFRP and CFRP
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specimens have been equipped with additional protective linings in the gripping part. The tensile test was carried out with the (Instron 8801) testing system and the Instron axial extensometer with a 50 mm base.
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The testing crosshead speed was set at 2 mm/min. During the test, the strength limit (Rm), the Young's modulus (E) and the elongation at break were determined.
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Fig. 2. Strength testing stand with tensile test specimen
The interlaminar shear test
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Interlaminar shear strength was determined in a short beam shear test. The tests were carried out on the (Instron 8801) testing system. The testing speed was set at 1.3 mm/min. The dimensions of the
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specimens and the support span length were adjusted to the laminate thickness (h) according to ASTM 2344. The width of the specimens (b) was: b = 5h, and the length (l) was: l = 10h. The support span length (s) was: s = 4h. The specimens and support span dimensions were selected carefully to ensure the maximum contribution of the shear stresses during the test. The test was conducted in reference to ASTM 2344, however, thin walled specimens (less than 2mm thick) were also used while maintaining the balance between bending and crushing due to compression effects. The test stand layout and specimens dimensions are shown in the figure 3. Interlaminar shear strength was determined by the following equation:
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(1)
ℎ
where:
ILSS = short-beam strength, [MPa],
b = measured specimen widh, [mm],
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h = measured specimen thickness, [mm].
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Pmax = maximum load observed during the test, [N],
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Fig. 3 Specimen dimensions and load diagram of the samples during the 3-point bending (ILSS method)
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2.4. Damage assessment The nature of the damage was assessed on the basis of meso-scale observations made on a
stereoscopic microscope (Nikon SMZ 1500) and micro-scale observations on a scanning electron microscope (FEI Nova Nano SEM).
3. Results As a result of carrying out the process of drying at the first stage of conditioning, the state of equilibrium was reached after 10 days. The FRP composite specimens lost from 0.08 to 0.12 % of their mass, and the FML specimens lost from 0.02 to 0.03% of their mass. The mass loss of FMLs is about 10
ACCEPTED MANUSCRIPT times lower than that of FRP composites. The results indicate a lower content of moisture in the tested materials and show that the dynamics of the diffusion process is significantly lower in FMLs than in the case of conventional FRPs. At the second stage of conditioning, an analysis was conducted of the kinetics of the process of
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moisture absorption by the specimens which were placed in an environmental chamber after the process of drying. Figure 4 shows the changes in the specimens mass during the process of conditioning in the
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environmental chamber.
Fig. 4 Changes in the specimens mass during the process of conditioning in the environmental chamber
Significant differences in the level and kinetics of moisture absorption were observed in the tested materials by analysing the individual curves obtained during the conditioning. Generally, based on the shape of the curves representing moisture absorption, two major phases can be distinguished: an area of intensive absorption at the beginning of the test and an area of the stabilisation of the moisture level. Depending on the type and thickness of a laminate, the transition between these phases takes place at a
ACCEPTED MANUSCRIPT different point in time – for FRP laminates after about 8–10 days, and for FMLs after only 2–3 days. The differences in the mass increase values of particular types of laminates are visible already during the first days of seasoning and become greater until the moisture level is stabilised. The abrupt mass increase during the first days of seasoning is caused by a considerable moisture potential gradient occurring
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between the environment and the composite layer. The mass of conventional FRP composites increased by 0.8–1.2%, and the mass of FMLs increased by 0.1–0.2%, depending on the configuration. The differences in the quantity of absorbed moisture may be related to the different diameter of fibres, their type and the volumetric quantity of the
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matrix, whereas the moisture absorption by FMLs is limited because the metal layers constitute a major barrier for the diffusion process and moisture can penetrate into the laminate only through uncovered
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edges. The kinetics of the diffusion process is dependent on the temperature and the capacity of the material to absorb moisture. In this case, the capacity of FMLs to absorb moisture is limited by a reduced area of absorption. The curves given in Figure 1 have the shape of an exponential function and the changes in the dynamics of the mass increase in time are much more visible for FRP composites as they
Tensile strength
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are characterised by greater relative moisture absorption.
The results of the static tensile test indicate that the specimens subjected to the process of
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conditioning in elevated temperature and moisture are characterised by a decrease of tensile strength (Fig. 5), Young's modulus (Fig. 6) and elongation at break (Fig. 7). This tendency is found in the group of
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conventional FRP composites and in fibre-metal laminates in the tested configurations.
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Fig. 5 Tensile strength of tested laminates before and after conditioning
As far as the group of GFRP composites is concerned, specimens with different thickness yielded convergent results and constant exposure to environmental conditions resulted in a significant 30– 32% decrease of tensile strength with a simultaneous preservation of the downward trend of the remaining parameters, i.e., up to 11% for Young's modulus and up to 27% for elongation at break. In the
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group of carbon-epoxy composites, a difference of 7–11% in the value of tensile strength was observed at a simultaneously higher level of output strength than in the case of glass-epoxy composites. The stiffness of carbon-epoxy composites decreases under the influence of environmental factors by up to 6% and the
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elongation upon fracture by up to 9%. The process of conditioning has a considerable impact on the values of the parameters set for FRP composites, which may be related to the plasticisation of the matrix
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and the degradation of the bonding at the fibre-matrix interface [26].
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Fig. 6 Young's modulus value of tested laminates before and after conditioning
Fig. 7 Value of elongation at break of tested laminates before and after conditioning
Tensile strength and Young's modulus of AlG fibre-metal laminates decrease after the process of conditioning by up to 15% and 5%, respectively. The differences observed between particular
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downward trend – from 2 to 5% for Young's modulus and from 6 to 12% for elongation at break. The comparison of composite laminates and metal fibre laminates shows that composite laminates are characterised by a greater decrease of the set strength parameters and, in consequence, they are more susceptible to the influence of environmental factors. Both the level of absorbed moisture and
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the decrease of mechanical parameters indicate that metal layers cause a considerable decrease of the degradation of composite layers. In the case of axial loads in relation to the laminate structure, reinforcing
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fibres play a decisive role in the transmission of stresses. The saturation of the matrix with moisture can contribute to an anomalous distribution of stresses and, in consequence, to the decrease of tensile strength.
ILSS test
Interlaminar shear strength determined in the short beam shear test has a wide application for
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comparative testing, also of FMLs [13,33,34]. The conducted short beam shear tests clearly point to a decrease of the ILSS value for all groups of the tested laminates under the influence of exposure to
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environmental factors (Fig. 8).
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Fig. 8 Results of the short beam shear test
GFRP laminates exhibit a decrease of ILSS by 12–23%, and CFRP laminates by 20–27%. The
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shear strength of FMLs after conditioning is 9–11% lower. In every case, configurations with greater thickness are characterised by lower strength. During the bending of the short beam, the proportion of shear stresses is relatively high, which is why the damage is concentrated mainly in the interlaminar areas. The influence of moisture can both weaken the polymer matrix and affect the quality of the
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bonding at the metal-composite interface.
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Damage analysis
Figures 6 and 7 show examples of the damage morphology of FRP and FML specimens
observed on the lateral surfaces using a stereoscopic microscope. The main damages observed in the group of fibre-metal laminates are numerous delaminations at
the metal-composite interface. The specimens after the process of conditioning exhibit a higher level of degradation.
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Fig. 9 Lateral surface of AlG specimens after the ILSS test
The cracks observed both in AlG laminates (Fig. 9a-d) and in AlC laminates (Fig. 10a-d) are similar in nature. Cracks are visible at the metal-composite interface and on the surface of the interface of
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composite layers with a [0/90] orientation. Transverse cracks in the layer with a [90] orientation join the delaminated areas at the individual interfaces (Fig. 10d). The metal layers of laminates with a 2/1
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configuration have traces of considerable plastic deformations.
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Fig. 10 Lateral surface of AlC specimens after the ILSS test More detailed observations with the use of SEM (Fig. 11) reveal the nature of the damage of the individual components of the laminate. It was observed that the damage at the metal-composite interface
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is adhesive and cohesive in nature. Between the delaminations located at different levels, there are characteristic diagonal cracks found in [90] composite layers, which result from shear stress (Fig. 11a). In
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layers with a [0] orientation, cracks of reinforcing fibres were found near the outer [0] metal layers (Fig. 8b), in particular in the case of AlG laminates.
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Fig. 11 Lateral surface of AlC specimens after the ILSS test: a) AlC, b) AlG Observation of the fractures of laminates after the tensile strength test revealed particularly significant morphological differences in the nature of the fractures of composite layers in AlC laminates. The surfaces of the fractures of the composite layers of an AlC laminate with a [0] orientation are shown in Figure 12. Considerable differences in the position of the tips of the fractured composite fibres were
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observed, which indicates a distinct crack growth mechanism. The fractures of the reference specimens are brittle in nature with a small number of pulled fibres. The fracture morphology of the specimens after conditioning in environmental conditions is much more complex. The deterioration of the properties of the bonding at the fibre-matrix interface can cause a change of the direction of the crack propagation and
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the initiation of cracks delaminating fibres from the matrix. After the crack delaminates the fibre along a greater length, the fibre is damaged. The microcrack then once again changes the propagation direction
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and reaches another fibre. The result of this mechanism is a formation of a very complex fracture surface. Observations of the surface of the inner metal layers (Fig. 12c) do not reveal any considerable
variations between conditioned and unconditioned specimens. In every case, the damage was observed to be mixed adhesive and cohesive in nature. It appears that the degradation of the matrix and the fibre-matrix interface is the primary factor determining the decrease of tensile strength as a result of the influence of environmental conditioning.
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Fig. 12 Fractured surface of an AlC specimens after the tensile strength test – 1) reference, b), c) after conditioning
Conclusions
In this article, the effects of environmental conditioning on selected properties and the nature of the damage of FMLs and conventional FRP composites have been presented. The conducted tests indicate that a long-term influence of environmental factors in the form of elevated temperature and moisture significantly contributes to the deterioration of mechanical properties and can affect the nature of the damage of the tested FMLs and FRPs.
ACCEPTED MANUSCRIPT In comparison to conventional FRPs, fibre-metal laminates exhibit considerably higher resistance to moisture absorption from the environment, which is related to the presence of outer metal layers that constitute a barrier for the diffusion process. In the case of FMLs, the processes of moisture absorption can take place only in the areas of unprotected edges of the composite layers of a laminate.
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Static tensile strength tests indicate that the effects of environmental factors cause a decrease in the strength, stiffness and deformations upon fracture of the tested materials. The decrease of the tensile strength of FMLs after conditioning ranges from 1 to 15% and is considerably lower than that of FRPs, which ranges from 7 to 30%, depending on the configuration.
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The results of ILSS testing obtained for both FMLs and FRPs after the process of conditioning show decreased interlaminar shear strength. A decrease by 9–11% was found in FMLs and by up to 27%
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in the case of FRPs.
The conducted analysis of the damage of laminates indicates that conditioning results in the intensification of the degradation process on the surface of the fibre-matrix interface, which is the primary damage factor that can contribute to the deterioration of their mechanical properties.
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Acknowledgements
The research project was financed by the National Science Center of Poland pursuant to decision No.
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