2. Material property behaviors of GLAREs

## 2.1. Mechanical behaviors of GLAREs for aircraft structures

GLAREs boast a large number of favorable characteristics, such as low density, static strength, better F&DT properties, high impact and flame resistances, as shown in Figure 3 [22–24]. More descriptions on advantages of GLAREs are provided as follows:

[28] reported that GLARE3–3/2 exhibit almost constant slow crack-growth when it is subjected to constant-amplitude fatigue loading as shown in Figure 4. Such low fatigue crack growth rate can lead to the minimal scheduled inspection downtime of aircraft. • Blunt notch strength: The notched residual strength is also an important design parameter since the geometrical notches (e.g. cutouts to serve as doors and windows) are inevitable in the design of fuselage shells. Although the GLARE presents a relatively high notch sensitivity compared with ductile aluminum alloys, and the use of high ultimate strength S2 unidirectional glass fiber (in typical, 4890 MPa) makes it superior to ARALL in notch strength [26]. Hagenbeek et al. [29] proposed a numerical simulation model for predicting blunt notch strength by considering the metal volume fraction based on Norris failure criterion, and they reported that such approach has been shown to be effective for use in predicting multi-axial blunt notch strengths (i.e. biaxial and shear components) of GLARE.

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• High impact resistance: Impact resistance is actually a significant advantage of GLARE, especially when compared to either aluminum alloys or CFRP [30]; Figure 5 compares the respective impact energy absorbing capacities based on the through-the-thickness cracking (i.e. puncture energy). Obviously, GLARE3–3/2 shows higher impact resistance to cracking than aluminum alloy. This result may be attributed due to the localized fiber fracture and the extensive shear failure in the outer aluminum sheets [31, 32]. In addition, a high strain rate strengthening phenomenon which occurs in the glass fibers, combined with their relatively high failure strain contribute to increase in the impact resistance of

• Burn-through capabilities: To meet the airworthiness standard of a max. 90 seconds evacuation time (JAR/FAR 25.803: Emergency evacuation), a structural integrity of fuselage is of major importance in order to prolong a safe environment of the passengers in the event of a post-crash fire scenario. Apparently, the GLARE shows high thermal insulation

GLARE, rather than other FMLs, such as ARALL and CARALL [33].

Figure 4. Fatigue crack growth [14].


Figure 3. GLARE vs. aluminum alloy comparison ratio.

[28] reported that GLARE3–3/2 exhibit almost constant slow crack-growth when it is subjected to constant-amplitude fatigue loading as shown in Figure 4. Such low fatigue crack growth rate can lead to the minimal scheduled inspection downtime of aircraft.


Figure 4. Fatigue crack growth [14].

2. Material property behaviors of GLAREs

72 Optimum Composite Structures

range from 2.38 to 2.52 g/cm<sup>3</sup>

Figure 3. GLARE vs. aluminum alloy comparison ratio.

2.1. Mechanical behaviors of GLAREs for aircraft structures

descriptions on advantages of GLAREs are provided as follows:

.

provide the balanced mechanical properties in both directions [26].

GLAREs boast a large number of favorable characteristics, such as low density, static strength, better F&DT properties, high impact and flame resistances, as shown in Figure 3 [22–24]. More

• Lightweight: High static strength of GLARE2 in 0 fiber direction contributes to weight saving over the aluminum alloys by roughly 6% in the design based on bending stiffness, and by 17% in the design based on yield strength, respectively [25]. For example, the use of GLAREs on A380 fuselage shells achieves a weight saving of up to 794 kg (10%) compared with the aluminum alloys [26]. The typical density of standard GLAREs is the

• High strength: It is apparent that the GLAREs reinforced with unidirectional glass fiber have anisotropic properties. This glass fiber contributes to increase in static strength and elastic modulus in the longitudinal direction along which the glass fiber is oriented. On the other hands, the aluminum sheets control overall mechanical properties of GLAREs in the transverse direction. As a result, the unidirectional GLAREs (e.g. GLARE1 and GLARE2) exhibit the high ultimate tensile strength compared with the aluminum alloys in the longitudinal direction, and it contributes to weight reduction in the case of tensiondominated structural components. In contrast, the transverse strength is somewhat lower than those of aluminum alloys. To overcome this limitation, the cross-ply GLAREs (e.g. GLARE3 and GLARE5) and angle-ply GLARE (e.g. GLARE6) have been introduced to

• High fatigue resistance: The superior fatigue resistance is a result of fiber bridging mechanism whereby the intact fiber layers provide an alternative load path around the cracked metal layers, eventually reducing local stress in front of crack tip [27]. Vogelesang et al.

2.2. GLARE grades

Figure 7. Schematic view of GLARE 3/2.

Grade Thickness (mm)

Grade Metal layers Prepreg layers Typical

2024-T3 0.2–0.5 90/90 0.25

2024-T3 0.2–0.5 90/0/90 0.375

Table 2. Classification of GLARE for aircraft structures [5, 12, 39].

Orientation Thickness (mm)

direction (stiffeners) GLARE2 2024-T3 0.2–0.5 0/0 0.25

GLARE6 2024-T3 0.2–0.5 45/+45 0.5 2.52 • Shear, off-axis properties

in mm corresponds to the total thickness of composite layers in between two aluminum layers. (b) The rolling direction (axial) is defined as 0, and the transverse rolling direction is defined as 90.

Another beneficial feature of GLARE is that the number and orientation of composite layers can be selected to best suit different applications, and such material features make it attractive for structural applications [37]. For the certification of GLAREs for aircraft structures, several lay-up patterns are already defined as a standard grade: the schematic view of GLARE 3/2 is shown in Figure 7. This approach is useful to define the specific lay-up pattern used in the structural design [38]. Nowadays, the standard GLAREs are being produced in six different grades as listed in Table 2 [39]. All grades are classified according to the type of lay-up pattern where the composite layers consists of unidirectional S2 glass fiber (AGY Holding Corp., USA)

The Guidelines of Material Design and Process Control on Hybrid Fiber Metal Laminate for Aircraft Structures

density (g/cm<sup>3</sup> )

GLARE1 7475-T761 0.3–0.4 0/0 0.25 2.52 • Unidirectional loaded parts with rolling

GLARE3 2024-T3 0.2–0.5 0/90 0.25 2.52 • Bi-axially loaded parts with 1:1 of princi-

GLARE4 2024-T3 0.2–0.5 0/90/0 0.375 2.52 • Bi-axially loaded parts with 2:1 of princi-

GLARE5 2024-T3 0.2–0.5 0/90/90/0 0.5 2.38 • Impact critical areas (floors & cargo liners)

(a) The number of orientations is equal to the number of unidirectional prepreg ply in each composite layer. The thickness

Characteristics

lage skins)

direction aluminum sheet in loading

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ple stresses (fuselage skins, bulkheads)

ple stresses with aluminum sheet in main or perpendicular loading direction (fuse-

Figure 5. Comparison of low-velocity impact performance [15].

performance, and subsequently contributes to enhancing structural integrity in fuselage shells as shown in Figure 6. Owing to high melting temperature of S2 glass fiber (in typical, 1466C), only the outer aluminum sheet starts to melt and separates from the other layers. As a result, the unexposed side of GLARE panel would remain relatively intact where the unexposed side temperature was just below 400C.

• Long-term hygrothermal behaviors: In general, the significant changes in moisture absorption are not observed by GLAREs, which confirmed well to the shielding effect of the outer aluminum sheet in this material [35, 36]. However, in the case of thermal cycling exposure, the decrease rate of GLAREs is 1–7% higher than those of glass fiber-reinforced composites. This reduction is attributed to the large coefficient of thermal expansion (CTE) difference between their constituent materials [35].

Figure 6. GLARE fire resistance comparing to aluminum alloy [34].

#### 2.2. GLARE grades

Another beneficial feature of GLARE is that the number and orientation of composite layers can be selected to best suit different applications, and such material features make it attractive for structural applications [37]. For the certification of GLAREs for aircraft structures, several lay-up patterns are already defined as a standard grade: the schematic view of GLARE 3/2 is shown in Figure 7. This approach is useful to define the specific lay-up pattern used in the structural design [38]. Nowadays, the standard GLAREs are being produced in six different grades as listed in Table 2 [39]. All grades are classified according to the type of lay-up pattern where the composite layers consists of unidirectional S2 glass fiber (AGY Holding Corp., USA)

Figure 7. Schematic view of GLARE 3/2.

performance, and subsequently contributes to enhancing structural integrity in fuselage shells as shown in Figure 6. Owing to high melting temperature of S2 glass fiber (in typical, 1466C), only the outer aluminum sheet starts to melt and separates from the other layers. As a result, the unexposed side of GLARE panel would remain relatively

• Long-term hygrothermal behaviors: In general, the significant changes in moisture absorption are not observed by GLAREs, which confirmed well to the shielding effect of the outer aluminum sheet in this material [35, 36]. However, in the case of thermal cycling exposure, the decrease rate of GLAREs is 1–7% higher than those of glass fiber-reinforced composites. This reduction is attributed to the large coefficient of thermal expansion (CTE)

intact where the unexposed side temperature was just below 400C.

difference between their constituent materials [35].

Figure 6. GLARE fire resistance comparing to aluminum alloy [34].

Figure 5. Comparison of low-velocity impact performance [15].

74 Optimum Composite Structures


(a) The number of orientations is equal to the number of unidirectional prepreg ply in each composite layer. The thickness in mm corresponds to the total thickness of composite layers in between two aluminum layers.

(b) The rolling direction (axial) is defined as 0, and the transverse rolling direction is defined as 90.

Table 2. Classification of GLARE for aircraft structures [5, 12, 39].

and FM®94 modified epoxy (Cytec-Solvay Group, USA). Nominal fiber volume fraction and ply thickness of prepreg are 59% and 0.125 mm, respectively [39].

A special coding convention is used to describe the different GLARE grades and specify their lay-up patterns. Symbolically, a general configuration is represented as follows [5]:

$$\text{GIARE}\,\text{N}\_{\text{G}} = \text{N}\_{\text{dl}}/\text{N}\_{\text{gl}} - \text{t}\_{\text{al}} \tag{1}$$

For example, alternating layers (i.e. aluminum sheets and unidirectional S2 glass/epoxy prepreg plies) are laid up over the curing tool, in which forms single, or double curvature shape [5, 42]. The splices are then staggered with respect to each other, while the adhesive layers are continuous. This interlaminar doubler solution can offer the local thickness variations in the skin panel [43]. Furthermore, this design concept can allow tailor-made skin panels of any size, not limited by the standard width of aluminum sheet rolls (in typical, 1.5 m). Now, the practical limitation of panel sized is only defined by autoclave size. The thickness variations in the skin panel are generally utilized for compliance with the fail-safe design requirements and the cost-effective part production for integrating the fuselage structures between skin panels, longitudinal stringers and circumstance frames. However, the difficulties in the production of splice GLARE panels in two bonding cycles demand for a feasible production solution, which allows for completing a splice joint including doublers through co-curing process. For this purpose, a SFT (Self-Forming Technique) can provide a smart solution to produce the required doublers without an additional cure cycle for bonding the doublers over the base GLARE panel. Such an inter-laminar panel highlights the advantages of using a SFT process as follows: (1) no dimensional tolerance issue for overlap in double curvature panel, (2) the evacuation of entrapped air or volatiles in composite layers through splice (adhesive squeeze-out). It therefore allows for the increased fuselage panels width with reducing the

• Glass layers locally between two aluminum sheets

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Splice in skin panel or doublers Additional glass fiber layers

• Internal stress level in double curved panels • Embedded at frame locations

• Spliced, go through depending on length/orientation • Adjust properties to loading condition

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• e.g. GLARE4 to GLARE3 • Aluminum sheet locally at frame station

Inter-laminar doublers Fiber oriented and lay-up

Transition of GLARE type Additional layers:

Table 3. GLARE design features–"giant tool box" (reproduced from Pleitner [41]).

• Lamination of aluminum sheet width

For the standard GLARE grades qualified, their in-plane static properties can be defined by simple prediction based on MVF, which can reduce the additional experimental testing for material qualification. A terminology, MVF, reflects the relative contribution of aluminums

additional joints, structural weight and production cost [5].

2.4. Metal volume fraction (MVF)

where; NG is the number indicating GLARE grade, Nal is the number of aluminum layers, Ngl is the number of composite layers (Ngl = Nal – <sup>1</sup>) and tal is the thickness of each separate aluminum sheet (in typical, 0.25–0.5 mm). Each composite layer in turn consists of a certain number of unidirectional prepreg plies in 0�/90�/�45� directions. For example, each composite layer in GLARE4 consists of two unidirectional prepreg plies in oriented at 0 and 90�with respect to the rolling direction of aluminum sheets. Thereafter, GLARE4B-3/2 comprises three cross-plies within a composite layer, for example two layers in 90� and one layer in 0� direction. The fraction of unidirectional fibers in the rolling direction is twice much than that in the perpendicular direction.

#### 2.3. Design philosophy for GLARE structures

An introduction of new materials for aircraft structures took place in evolution steps which suggests a realistic application of the innovative design philosophy, eventually leading to optimization of design concept. The innovative design concept of GLAREs on A380 fuselage shells is shown in Figure 8 [40]. The structural efficiencies, such as damage tolerance and residual notch strength are much better served by incorporating the local variations in skin panel thickness with adhesively bonded joints. In early stage of technology development, GLARE structures were produced only as a flat panel. The innovations in structural design have been developed to overcome the joining problem and is termed the splice joint. The first splice concept consisted of butted aluminum sheets with the composite layer bridging the splices (e.g. butt joint). However, this concept is not recommended for structural applications because of premature failure in the butts. To overcome this limitation, several designs of splice concepts where two aluminum sheets are positioned with a slight over-lap forming a single metal sheet layer are introduced, as shown in Table 3 [41].

Figure 8. Construction and production possibility with the optimized GLARE panel (reproduced from Wischmann [40]).

Table 3. GLARE design features–"giant tool box" (reproduced from Pleitner [41]).

For example, alternating layers (i.e. aluminum sheets and unidirectional S2 glass/epoxy prepreg plies) are laid up over the curing tool, in which forms single, or double curvature shape [5, 42]. The splices are then staggered with respect to each other, while the adhesive layers are continuous. This interlaminar doubler solution can offer the local thickness variations in the skin panel [43]. Furthermore, this design concept can allow tailor-made skin panels of any size, not limited by the standard width of aluminum sheet rolls (in typical, 1.5 m). Now, the practical limitation of panel sized is only defined by autoclave size. The thickness variations in the skin panel are generally utilized for compliance with the fail-safe design requirements and the cost-effective part production for integrating the fuselage structures between skin panels, longitudinal stringers and circumstance frames. However, the difficulties in the production of splice GLARE panels in two bonding cycles demand for a feasible production solution, which allows for completing a splice joint including doublers through co-curing process. For this purpose, a SFT (Self-Forming Technique) can provide a smart solution to produce the required doublers without an additional cure cycle for bonding the doublers over the base GLARE panel. Such an inter-laminar panel highlights the advantages of using a SFT process as follows: (1) no dimensional tolerance issue for overlap in double curvature panel, (2) the evacuation of entrapped air or volatiles in composite layers through splice (adhesive squeeze-out). It therefore allows for the increased fuselage panels width with reducing the additional joints, structural weight and production cost [5].

#### 2.4. Metal volume fraction (MVF)

and FM®94 modified epoxy (Cytec-Solvay Group, USA). Nominal fiber volume fraction and

A special coding convention is used to describe the different GLARE grades and specify their

where; NG is the number indicating GLARE grade, Nal is the number of aluminum layers, Ngl is the number of composite layers (Ngl = Nal – <sup>1</sup>) and tal is the thickness of each separate aluminum sheet (in typical, 0.25–0.5 mm). Each composite layer in turn consists of a certain number of unidirectional prepreg plies in 0�/90�/�45� directions. For example, each composite layer in GLARE4 consists of two unidirectional prepreg plies in oriented at 0 and 90�with respect to the rolling direction of aluminum sheets. Thereafter, GLARE4B-3/2 comprises three cross-plies within a composite layer, for example two layers in 90� and one layer in 0� direction. The fraction of unidirectional fibers in the rolling direction is twice much than that in the perpendicular direction.

An introduction of new materials for aircraft structures took place in evolution steps which suggests a realistic application of the innovative design philosophy, eventually leading to optimization of design concept. The innovative design concept of GLAREs on A380 fuselage shells is shown in Figure 8 [40]. The structural efficiencies, such as damage tolerance and residual notch strength are much better served by incorporating the local variations in skin panel thickness with adhesively bonded joints. In early stage of technology development, GLARE structures were produced only as a flat panel. The innovations in structural design have been developed to overcome the joining problem and is termed the splice joint. The first splice concept consisted of butted aluminum sheets with the composite layer bridging the splices (e.g. butt joint). However, this concept is not recommended for structural applications because of premature failure in the butts. To overcome this limitation, several designs of splice concepts where two aluminum sheets are positioned with a slight over-lap forming a single

Figure 8. Construction and production possibility with the optimized GLARE panel (reproduced from Wischmann [40]).

GLARE NG ¼ Nal=Ngl � tal (1)

lay-up patterns. Symbolically, a general configuration is represented as follows [5]:

ply thickness of prepreg are 59% and 0.125 mm, respectively [39].

76 Optimum Composite Structures

2.3. Design philosophy for GLARE structures

metal sheet layer are introduced, as shown in Table 3 [41].

For the standard GLARE grades qualified, their in-plane static properties can be defined by simple prediction based on MVF, which can reduce the additional experimental testing for material qualification. A terminology, MVF, reflects the relative contribution of aluminums properties to the properties of GLARE [5]. As a result, the MVF approach is useful for the prediction of static strength properties for GLARE as found in the literatures [5, 44, 45]. The MVF value can be calculated as follows:

$$\text{MVF} = \sum\_{p\_{\text{metal}}}^{t} \mathbf{t}\_{\text{al}} / \mathbf{t}\_{\text{laminate}} \tag{2}$$

3. Process control methodologies for producing GLARE structures

Basically, the production process for making GLARE structures is similar to the traditional production of metallic bonded structures and composite laminates. Before the parts are released, the part's quality should be assured through a reliable quality control (QC) method. For this purpose, the stringent QCs procedures shall be developed and applied to the part production of GLAREs. At this time, the QCs system includes all procedures that ensure the raw material quality, in-process control monitoring and verification of fitness for part acceptance. At each production stage, the key process parameters should be also standardized with

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• QC of raw materials. GLARE manufacturer starts with the preparation with rolls of thin aluminum bare sheet (in typical, 0.3–0.4 mm). A custom-built machine decoils the thin aluminum sheet from rolls, and flattens the sheet and cuts it to lengths of up to 11 m for large skin panels. Next, the cut sheets are milled in accordance with the engineering drawings. At this time, all the aluminum sheets and unidirectional prepreg plies should be controlled by raw materials inspection specifications, and some specific properties should be controlled: (1) rolling direction, straightness, waviness and surface roughness for aluminum sheets; (2) fiber direction, prepreg bridging, or wrinkles and shelf-life requirements (e.g. storage life and mechanical life) for prepreg plies. This QC activity is basically the same as the traditional production of sheet metal forming, or composites. All prepreg shall be cut over a clean, non-contaminated surface with clean, sharp knives, or digital cutting machine to minimize distortion and splitting. The pre-cut materials (i.e. kit) should be stored in flat or stress-free condition to prevent folding or further damage. Unless otherwise specified by the engineering drawings, all the prepreg size should have a suitable trim at required locations to keep irregular edges out of the final part dimension.

3.1. Part production and quality controls

[Source: CompositesWorld]

the specified production tolerances as the follows [4, 5, 41]:

where; tal is the thickness of each separate aluminums sheet, tlaminate is the total thickness of GLARE panel and pmetal is the number of aluminums sheets [5]. The typical MVF values of the standard GLARE grades are valid in a range between 0.55 and 0.70. The material property of GLARE having any MVF can be calculated by using a linear relation which follows the "rule of mixtures" available in anisotropic mechanics by using the Eq. (3).

$$E\_{\rm GLARE} = E\_M \cdot MVF + E\_G (1 - MVF) \tag{3}$$

where; EGLARE is the elasticity of GLARE and EM and EG are the elasticity of aluminum sheet and composite layers, respectively. The load transfer ratio for composite layers (PG/PGLARE) in GLARE according to MVF can be defined as follows:

$$\frac{P\_G}{P\_{\text{GLARE}}} = \frac{E\_G/E\_M}{E\_G/E\_M + MVF/(1 - MVF)}\tag{4}$$

The load transfer ratio for composite layers in GLARE according to MVF can be predicted as shown in Figure 9. It is worth noting that the load transfer ratio of composite layer in GLARE exponentially decreases with the fraction of aluminum sheets. As the fraction of aluminum sheets in GLARE decreases, more shear load can be dissipated through the aluminum sheetcomposite interface [6].

Figure 9. Plot of load transfer ratio for glass/epoxy layers in GLARE according to MVF for various modulus ratios of EG/ EM: The corresponding GLARE grades of • GLARE2A 3/2–0.4 (0.703), GLARE3 3/2–0.4 (0.703), GLARE4A 3/2–0.4 (0.612), GLARE4B 3/2–0.4 (0.612) and GLARE5 3/2–0.4 (0.542).
