Seminar On GLARE
Seminar On GLARE
Seminar On GLARE
CHAPTER 1
INTRODUCTION
GLARE (Glass Laminated Aluminium Reinforced Epoxy) is a new class of fibre metal laminates for advanced aerospace structural applications. Fibre metal laminates (FML) are good materials for these applications due to their high specific mechanical properties especially fatigue resistance. Fibre metal laminates are hybrid composites consisting of alternating thin layers of metals and fibre-reinforced epoxy prepreg .FML offer many advantages when compared to metallic alloys especially where high
strength and stiffness to weight ratio is concerned. With all these advantages, FML structures have gained widespread use in the aerospace industry during the last decades. The FMLs with glass fibres and aluminium are known with their trade name as GLARE. Beginning of the 21st century marked the use of GLARE for the upper fuselage skin structures of Airbus A380.This is the first structural application of GLARE laminate in a commercial airline. Each A380 will have about 380m2 of GLARE. An overall weight reduction of 794kg was obtained by usage of glare, which is 10% less dense than aluminium. It has proven superior in fatigue, damage and fire resistance. GLARE may also be used in the leading edge of wings and tails of the Airbus A380 due to its outstanding impact performance. A patent on glare was filed by AKZO in 1987.A partnership between AKZO and ALCOA started to operate in 1991 to produce and commercialize glare.
Glare materials are commercialized in six different standard grades. They are all based on unidirectional glass fibres embedded with epoxy adhesive resulting in prepregs with a nominal fibre volume fraction of 60%. During fabrication of composites the prepregs are laid-up in different fibre orientations between aluminium alloy sheets, resulting in different standard GLARE grades
CHAPTER 2
GLARE Grade
Sub Category
Aluminium Gra de
Prepreg orientation
2.2 Coding
A laminate coding system is used to specify laminates from the Table 1. For instance: GLARE 2B-4/3-0.4, means a GLARE laminate with fibre orientation according to GLARE 2B. Have 4 layers of Aluminium and 3 layers of fibre/epoxy composite. Each aluminium layer is 0.4 mm thick
CHAPTER 3
The most common process used to produce FML laminates, as for polymeric composite materials, involves the use of autoclave processing. The overall generic scenario for the production of FML composite aerospace components involves about five major activities: 1. Preparation of tools and materials. During this step, the aluminium layer surfaces are pre-treated by chromic acid or phosphoric acid, in order to improve the bond between the adhesive system and the used aluminium alloy. 2. Material deposition, including cutting, lay-up and debunking. 3. Cure preparation, including the tool cleaning and the part transferring in some cases, and the vacuum bag preparation in all cases. 4. Cure, including the flow-consolidation process, the chemical curing reactions, as well as the bond between fibre/metal layers. 5. Inspection, usually by ultrasound, X ray, visual techniques and mechanical tests.
CHAPTER 4
In GLARE there is a combination of high stiffness and strength from the composite layer and good impact properties from aluminium, resulting in a great performance for space applications. The tensile strength of GLARE composite is 380 MPa and the ultimate failure strength occurs at a strain of ~ 1.9%. as observed from the graph 1.
The failure process of GLARE laminates is quite complicated and there are multi-fracture modes involved in the failure of GLARE laminates such as matrix cracks, fibre-matrix debonding, fibre fracture, fibre/matrix interfacial shear failure, and inter delamination of laminates. For longitudinal tensile loading, fibre pull-out and interfacematrix shear mode are commonly observed in the fibre-epoxy layer of FML. In addition, the aluminium layer prevents multiple global longitudinal splits. Under transverse tensile loading, matrix failure and matrix-fibre interface debonding/fibre splitting are the main fracture modes in the fibre-epoxy layer of FML. More efforts are being carried out by various laboratories for detail study of the tensile behaviour of the composite.
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The ultimate compressive stress for GLARE occurred at a strain of approximately 19.9%. The development of damage microstructure within fibre/metal laminates during compression is investigated mainly by scanning electron microscopy technique. SEM micrographs revealed that the damage in the FML laminates under compression load occurred mainly between the reinforcement and the fibre.
Glass Laminated Aluminium Reinforced Epoxy [GLARE] shear properties for these materials is to induce a pure shear stress state in the gauge section of a constant magnitude. This is a special concern for composites because they exhibit high anisotropy and structural heterogeneity. In general, the ideal shear test must be simple enough to perform, require small and easily fabricated specimens, enable measuring of very reproducible values for both shear modulus and shear strength at simple data procedure. The interlaminar shear strength value for GLARE is approximately 40MPa.
Graph 3 represents a typical vibration damping representative curve of the Glare. The curve shows an exponential decay of maximum peak amplitudes as a function of time. Elastic modulus of composites obtained by experimental measurements differs from values obtained from the theoretical calculations (micromechanics approach), because ideal bonding between fibre/matrix interface, perfect alignment of fibres and absence of voids and other defects are considered in the last. For the FML composites there is an additional factor related to the influence of surface treatment on the aluminium foil, which is not considered also in the theoretical calculations. The experimental modulus values of aluminium 2024-T3, Caral and Glare composites result in a decrease of 5%, 10% and 9%, respectively.
Glass Laminated Aluminium Reinforced Epoxy [GLARE] fibre volume fraction, and fibre surface treatment; fastener parameters such as fastener type (bolt, pin, screw, rivet etc.), fastener size, clamping force hole size, and tolerance; and design parameters such as laminate thickness, geometry (pitch, edge distance, hole pattern, etc.), joint type (single lap, single cover butt, etc.), load direction, and loading rate. Table II presents the bearing yield strength and bearing ultimate strength of pintype and bolt-type bearing joints for various GLARE laminates tested at room temperature. It is obvious that the bolt-type bearing with lateral restraint is superior to the pin-type bearing without lateral restraint. The relatively low bearing strength of GLARE in the pin-type bearing is attributed to delamination buckling. This permits buckling of the individual aluminium layers, under the action of bearing loads, in a zone that extends beyond the original delamination (i.e., also induces strains in the prepreg layers). The bolt-type bearing fixture inhibits out-of-plane displacement and no delamination buckling was observed even though some limited delamination occurred.
Pin Type Bearings Bearing yield Laminates strength Bearing ultimate strength MPa GLARE 2 GLARE 3 GLARE 4 NA NA NA MPa 549 537 510
Bolt Type Bearings Bearing yield strength Bearing ultimate strength MPa 530 546 518 MPa 709 789 658
Thus far, research on the bearing behaviour of fibre metal laminates is far behind that on other mechanical properties such as fatigue, impact, or notched strength. Work is still needed to verify the bearing failure mode, predict the bearing strength, and identify the different effects of some important parameters such as joint size, edge distance, thickness, laminate lay-up, width, joint type, fastener type, temperature, and environment on the bearing behaviour of fibre metal laminates.
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Fracture resistance is the property of a material to resist the growth of crack produced in it. Experiments conducted on the GLARE laminates to investigate the crack growth resistance behaviour results clearly the superior fracture behaviour of GLARE over 2024-T3 aluminium alloy fibres can effectively prevent fibre failure from occurring before the aluminium fails. Hence, the fracture toughness value for GLARE is significantly higher. Evidently, the facture toughness of GLARE laminates is controlled by various toughening mechanisms including metal/prepreg layer interfacial debonding, stress redistribution after crack initiation, and the fracture behaviour of metal and prepreg layers. Generally, it was found that fibre metal laminates with fatigue cracks have higher fracture toughness than laminates with a saw cut due to the unbroken fibres in the wake of the crack and the delamination zone around the crack, which effectively enlarge the strain length of the fibres. In the GLARE 2 laminate, static delamination occurs between the prepreg and aluminium layer at loads close to fracture. The delamination propagates along the fibre direction and results in a large crack opening of the aluminium layers. Final fracture was initiated as fibre fracture near the fatigue crack tip while fibres in the wake of the fatigue crack remain intact. Fracture in GLARE 3 was initiated in the fibres near the hole at the center of the fatigue crack. This results in a larger crack opening and subsequently fibre fracture from the center toward the tips of the fatigue crack. Very little data has been openly published on the crack-growth properties of GLARE, especially for GLARE 4 and 5. More work is needed to characterize the fracture resistance of GLARE laminates with various cracking geometries.
The notched residual strength is an important design consideration since geometrical notches cannot normally be avoided in an aircraft. Like most fibrereinforced composite materials, GLARE is highly notch sensitive in comparison with its monolithic aluminium alloy. However, the advantage of high ultimate strength and high strain-to- fracture of glass fibres makes GLARE laminates superior to other fibre metal laminates such as ARALL in notch strength. The factors that can affect the notched
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Glass Laminated Aluminium Reinforced Epoxy [GLARE] residual strength of GLARE laminates include the volume fraction and properties of the constituents, the fibre direction, and the nature of the flaw present. In this study, the notched residual strength of GLARE laminates with two main notch types has been investigated. Those types are the circular cut-out or blunt notch and the saw-cut or crack like defect (sharp notch). The stress concentration associated with the crack-like defect is much higher than that associated with the blunt notch. As a result, it is expected that cracked specimens would exhibit lower strength than those specimens with a circular cut-out of equal size.
However, it was found that delamination is always present for GLARE laminates if the starting defect is a crack or saw cut, and this delamination could possibly postpone fi ber failure and thus increase the notched strength. Interestingly, the blunt notch strength of GLARE appears to increase with increasing notch size. This is attributed to the occurrence of static delamination in the glass-fibre reinforced laminates, which level off the stress distribution and delay fibre failure in the vicinity of the hole.
Notched strength modelling and stress analysis of the crack tip zone is difficult because of the complexity of the actual damage process, which involves matrix micro cracking, fibre bridging or breakage, delamination, and plastic zone development in the aluminium layer. Several models have been proposed in recent years to predict the notched strength and describe the crack-tip damage zone of fibre-reinforced laminate composites. However, the application of these models to GLARE needs to be further investigated or confirmed, and numerical modelling is required.
The fatigue properties of GLARE have been extensively evaluated and investigated by several authors under both constant load amplitude and realistic flight simulation. Graph 4 compares the fatigue performance of two GLARE variants and monolithic 2024-T3 under Graph 4 growth rate increases rapidly with increasing crack length, the GLARE laminates exhibit almost constant slow crack-growth behaviour. Under realistic loading conditions, GLARE laminates exhibit crack-growth rates 10 to 100 times slower than their monolithic aluminium constituents. GLARE excels in all
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Glass Laminated Aluminium Reinforced Epoxy [GLARE] types of fatigue-critical aircraft loading situations. Figure 2 shows a schematic of the fatigue crack mechanism responsible for the outstanding fatigue crack-growth characteristics of laminates. The phenomenon known as crack bridging by intact fibres imposes a significant restraint on crack opening. Furthermore, the fibres in the cracked area transmit a substantial amount of the load through the cracked area. As a result, there is a large reduction in the stress-intensity factor.
Several models have been proposed to predict the fatigue crack-growth behaviour of GLARE laminates. Researchers in the Delft University developed a model for calculating the fatigue crack-growth behaviour of centrally cracked specimens of fibre metal laminates, taking into account the stress intensity factor at the crack tip in the metallic part and the energy-release rate for delamination between the metallic sheet and a fibre-adhesive layer. But the bridging stress along the crack face is assumed to be uniform. Actually, a uniform bridging stress only exists in a centrally cracked specimen
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Glass Laminated Aluminium Reinforced Epoxy [GLARE] with an elliptical delamination and no saw cut, while the observed delamination is in most cases irregular, sometimes closer to a triangle.
According to the traditional method of analysis, the bridging traction must first be determined for predicting fatigue crack-growth rates and lives in GLARE under cyclic loading, but bridging traction is strongly affected by the delamination shape and size, the adhesive shear deformation and saw-cut size, etc. Characterization of the delamination shape and growth of GLARE under cyclic loading is a difficult issue that is not yet well understood. In addition, the existing models for predicting crack growth still have a large error with experimental data. Thus, there is a growing need for a more practical model of predicting crack growth in the GLARE laminates as a function of GLARE lay-up, maximum stress, and crack geometries.
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The post-impact fatigue performance and residual strength of GLARE laminates outperform a typical fibre composites and monolithic aluminium. Table 3 shows the post-impact fatigue test results of GLARE and aluminium 2024-T3. Although the crack initiation time in aluminium is much longer, the crack propagates through the thickness rapidly, which results in a catastrophically premature failure. Compression after impact tests revealed that the reduction of compressive strength due to impact for GLARE is similar to 2024-T3. GLARE 4-5/4-0.5 (3.2 mm thickness) and 2024-T3 aluminium (3.8 mm thickness) showed a similar compression strength after impact (i.e., 2% strength reduction due to a dent and 10% strength reduction due to a through crack).No delamination buckling, which is critical for composite materials, was observed on the GLARE laminates.
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Materials
Cycles to initiation
Cycles to failure
54 46 36 58
So far, research into the post impact behaviour of GLARE laminates has been very limited. A broad program to investigate the post-impact behaviour including fatigue and residual strength will have to be established. The development of a damagepropagation model to predict the post-impact behaviour of these laminates is also required.
The environmental durability of GLARE laminates, including moisture absorption and corrosion resistance, has been investigated. Like fibrous polymer composites, the fibre-adhesive layer in GLARE laminates is susceptible to moisture absorption controlled by temperature and humidity, though moisture absorption is very limited due to the protective aluminium layers. Moisture in the glass fibre-adhesive layers of GLARE increases the ease of delamination between the prepreg and metal layers. The effects are more pronounced in distilled water or salt solution than in humid air and more significant at high temperatures. Consequently, fatigue crack initiation/growth and blunt-notch strength may become inferior.
The GLARE laminate exhibits excellent corrosion resistance since all aluminium sheets used are anodized and coated with a corrosion-inhibiting primer prior to the bonding process. Through the- thickness corrosion is prevented by the
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Glass Laminated Aluminium Reinforced Epoxy [GLARE] fibre/epoxy layer, which serves as a barrier. Accelerated corrosion tests of the GLARE laminate found the only corrosion attack was in the outer (0.4 mm thick) aluminium layer. The corrosion resistance of the thin 2024-T3 sheet layers of GLARE was shown to be superior to that of a thicker (4 mm thick) panel of the same alloy. No stresscorrosion problems were observed for GLARE laminates during stress-corrosion tests.
A more extensive environmental durability study is needed to determine the effect of prolonged exposure to high temperature and moisture on the mechanical behaviour of GLARE laminates. Particularly, the effect of initial flaw and damage due to impact also needs to be taken into account.
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ADVANTAGES OF GLARE
Fibre metal laminates take advantages of metal and fibre-reinforced composites, providing superior mechanical properties to the conventional lamina consisting only of fibre-reinforced lamina or monolithic aluminium alloys. Summarizes all the advantages of fibre metal laminates depending on previous studies.
The major disadvantage associated with epoxy based fibre metal laminates is the long processing cycle to cure the polymer matrix in the composite plies. This problem increases the cycle time of whole production and decreases productivity. This increases labour costs and overall cost of FMLs.
The advantages of fibre metal laminate based on above study are summarized here.
MECHANICAL BEHAVIOUR High Fatigue resistance High strength High Fracture Toughness High Impact Resistance High Energy Absorption Capacity
DURABILITY Excellent Moisture Resistance Excellent Corrosion Resistance Lower Material Degradation
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345
325
855
789
524
546
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CHAPTER 7
CONCLUSION
Due to its outstanding fatigue resistance, high specific-static properties, excellent impact resistance, good residual blunt-notch strength, flame resistance, corrosion properties, ease of manufacture and repair, GLARE is a promising material for fuselage skin structures of the new generation high-capacity aircrafts. GLARE laminates seem poised to play a much greater role in the primary structure of pressurized transport fuselages.
The R&D activities to date have covered the mechanical properties of GLARE, however, insufficient information about the mechanical behaviour of GLARE is available in published literature. Many areas are open to future investigation, especially for the cross-ply configuration of GLARE. In addition, some areas must be further verified by more detailed testing. More research and testing in basic mechanical behaviour such as in plane shear strength, bearing strength, fatigue behaviour and crack growth rates, notch sensitivity, impact behaviour, delamination, and damage characterization are necessary. Also, the influence of long-term environmental exposure, especially under a combined influence of moisture and temperature, on the damage tolerance and durability of GLARE laminate needs to be better understood. In addition, an analytical certification based on analytical and numerical models validated by experiment needs to be established. Such a certification would facilitate greater utilization of GLARE in future aircraft structures.
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REFERENCES
1. Guocai Wu and J.-M. Yang , The Mechanical Behaviour of GLARE Laminates for Aircraft Structures ,Journal Of Materials ,Vol. 1, 2005 , January ,pp.72-79. 2. Tamer Sinmazcelik , Egemen Avcu , Mustafa Ozgur Bora, Onur Coban, A review: Fibre metal laminates, background, bonding types and applied test methods", Materials and Design,Vol.32,2011,pp. 3671-3685. 3. Edson Cocchieri Botelho, Rogrio Almeida Silva, Luiz Cludio Pardinia, Mirabel Cerqueira Rezendea. A Review on the Development and Properties of Continuous Fiber/epoxy/aluminum Hybrid Composites for Aircraft Structures,Materials Research, Vol. 9,No. 3, pp.247-256.
4. Mohammad Alemi Ardakani, Akbar Afaghi Khatibi, Seyed Asadollah Ghazavi , A study on the manufacturing of Glass-Fiber-Reinforced Aluminum Laminates and the effect of interfacial adhesive bonding on the impact behaviour.,Society for experimental mechanics ,June, 2008.
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