3. Process control methodologies for producing GLARE structures

## 3.1. Part production and quality controls

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

pmetal

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

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

<sup>¼</sup> EG=EM

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 sheet-

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

tal=tlaminate (2)

EGLARE ¼ EM∙MVF þ EGð Þ 1 � MVF (3)

EG=EM <sup>þ</sup> MVF=ð Þ <sup>1</sup> � MVF (4)

MVF <sup>¼</sup> <sup>X</sup><sup>t</sup>

mixtures" available in anisotropic mechanics by using the Eq. (3).

PG PGLARE

GLARE according to MVF can be defined as follows:

(0.612), GLARE4B 3/2–0.4 (0.612) and GLARE5 3/2–0.4 (0.542).

composite interface [6].

MVF value can be calculated as follows:

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

• 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.

[Source: CompositesWorld]

• QC of surface treatment. Surface of aluminum bare sheet should be pre-treated to obtain a proper adhesion strength and durability with the prepreg resin. For this purpose, the milled sheets are transferred via a handling system to the chemical treatment line. The standard surface treatment process consists of solvent degreasing, CAE (Chromic-Sulfuric Acid Etching), CAA (Chromic Acid Anodizing), and followed by organic bond primer. All key process parameters should be checked for each batch of aluminum sheets according to the corresponding Airbus's own specifications, for example deoxidizing/anodic bath temperature, solution chemistry, rinse water purity and so forth. The specific surface treatment procedures of aluminum alloys are going to be explained in detail Section 3.2. Finally, the primed, cut sheets are re-rolled, and covered in a protective black plastic (or paper) bag for storage until needed in fabrication.

• Control of autoclave process. The laid-up parts are vacuum bagged, and then placed into the autoclave to be united by heat and pressure. Autoclave facility shall have instrumentation which autographically records time, temperature, pressure and vacuum where applicable. All gauges shall be controlled and periodically calibrated and certified in accordance with the procedures approved by the QC department. During an autoclave cure cycle, a high compaction pressure (in typical, 11 bar) is normally applied to the GLARE lamination stack at an elevated curing temperature (in typical, 125C) for 3.5 hours. The representative manufacturing-induced defects, such as voids, porosities, should be accurately controlled to prevent internal defects. In addition to the QC activities in the part production of GLARE, there is also required to perform a "final check" prior to the part release. Nondestructive inspection (e.g. ultrasonic C-scanner) and some mechanical tests are generally

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81

• Post processing. The manufacturing and assembly of GLARE structures typically require machining operations, such as milling and drilling. For examples, the GKN Aerospace's Fokker business has been produced a large-sized GLARE panel of 4.5 11.5 m by using a 5 axis milling machine on a movable bed. However, a technique for machining of this multilayered structure has presented more challenges in the aerospace industry than aluminum alloys or composites due to the coupled interaction between composite- and metal-phase cutting. The machining operations should be accomplished to meet the acceptance limit for

the discrepancies as per the engineering drawing, or process specification.

accomplished in the final step of QC.

[Source: CompositesWorld]

[Source: CompositesWorld]

## [Source: Fokker]

• Control of lay-up process. Alternating layers of aluminum bare sheets and prepreg plies are positioned in the right stacking order in accordance with GLARE grade. All the lay-up works should be conducted in a sufficiently clean environment, and the working environment such as temperature and humidity should be also kept below well-defined levels. All cut prepreg plies should be sequentially prepared and collated on the curing tool in the location and orientation as per the engineering drawings, or shop process instruction. An optical LPS (Laser Projection System, Virtek Vision International Inc., Waterloo, ON, Canada) may be capable of attaining the required dimension tolerance.

[Source: Fokker]

• Control of autoclave process. The laid-up parts are vacuum bagged, and then placed into the autoclave to be united by heat and pressure. Autoclave facility shall have instrumentation which autographically records time, temperature, pressure and vacuum where applicable. All gauges shall be controlled and periodically calibrated and certified in accordance with the procedures approved by the QC department. During an autoclave cure cycle, a high compaction pressure (in typical, 11 bar) is normally applied to the GLARE lamination stack at an elevated curing temperature (in typical, 125C) for 3.5 hours. The representative manufacturing-induced defects, such as voids, porosities, should be accurately controlled to prevent internal defects. In addition to the QC activities in the part production of GLARE, there is also required to perform a "final check" prior to the part release. Nondestructive inspection (e.g. ultrasonic C-scanner) and some mechanical tests are generally accomplished in the final step of QC.

[Source: CompositesWorld]

• QC of surface treatment. Surface of aluminum bare sheet should be pre-treated to obtain a proper adhesion strength and durability with the prepreg resin. For this purpose, the milled sheets are transferred via a handling system to the chemical treatment line. The standard surface treatment process consists of solvent degreasing, CAE (Chromic-Sulfuric Acid Etching), CAA (Chromic Acid Anodizing), and followed by organic bond primer. All key process parameters should be checked for each batch of aluminum sheets according to the corresponding Airbus's own specifications, for example deoxidizing/anodic bath temperature, solution chemistry, rinse water purity and so forth. The specific surface treatment procedures of aluminum alloys are going to be explained in detail Section 3.2. Finally, the primed, cut sheets are re-rolled, and covered in a protective black plastic (or

• Control of lay-up process. Alternating layers of aluminum bare sheets and prepreg plies are positioned in the right stacking order in accordance with GLARE grade. All the lay-up works should be conducted in a sufficiently clean environment, and the working environment such as temperature and humidity should be also kept below well-defined levels. All cut prepreg plies should be sequentially prepared and collated on the curing tool in the location and orientation as per the engineering drawings, or shop process instruction. An optical LPS (Laser Projection System, Virtek Vision International Inc., Waterloo, ON,

Canada) may be capable of attaining the required dimension tolerance.

paper) bag for storage until needed in fabrication.

[Source: Fokker]

80 Optimum Composite Structures

[Source: Fokker]

• Post processing. The manufacturing and assembly of GLARE structures typically require machining operations, such as milling and drilling. For examples, the GKN Aerospace's Fokker business has been produced a large-sized GLARE panel of 4.5 11.5 m by using a 5 axis milling machine on a movable bed. However, a technique for machining of this multilayered structure has presented more challenges in the aerospace industry than aluminum alloys or composites due to the coupled interaction between composite- and metal-phase cutting. The machining operations should be accomplished to meet the acceptance limit for the discrepancies as per the engineering drawing, or process specification.

[Source: CompositesWorld]

#### 3.2. Surface treatment of aluminum alloys for producing GLARE structures

A strong bonding interface is one of the key factors for improving the durability of GLAREs. It is apparent that the surface treatment technique can improve the surface energy and wettability of metallic substrates, and is an effective method for enhancing the bonding strength between a metal substrate and a fiber reinforced polymer composite [46]. In addition, the surface treatment can remove the undesirable surface oxides or contaminants on the metallic substrate, and ameliorate the surface composition and microstructure of the metallic substrate [6, 47]. This allows the fiber bridging mechanism and mechanical properties of GLAREs to be improved, and moreover, the crack propagation rate at the aluminum-composite interface can be effectively reduced [48, 49]. Previous research works reported that the surface treatment should be carefully taken into consideration when improving interlaminar shear strength at the aluminum-composite interface [6, 50], environmental durability and low-velocity impact resistance of GLARE. Therefore, the proper production steps should be clearly defined before any production process is implemented. Note that this section is described based on our previous surface treatment studies of aluminum alloys for aircraft structures [46].

Treatments1 CAA [51] PAA [54] BSAA [51] PSA [53]

2.5–3.0 (CrO3) 10 (H3PO4) 5.0–10.0 (H3BO3)/

Voltage (V) 40.0 1.0 10.0 15.0 1.0 18 2.0 Time (min) 35–45 20 18–22 15

> • Control Cl- & F<sup>4</sup> • Filtering required to remove fungus

Racks Al, Ti, Al with Ti-tips Equivalent to CAA Equivalent to CAA Equivalent to CAA

Bio-contaminant organisms, for example fungal (alternaria, fusarium and penicillium species) and bacterial (pseudomo-

Table 4. Anodizing processes for structural adhesion bonding of aluminum alloys (reproduced from Park et al. [46] with

10 18 10 —

Table 5. Comparison of oxide morphology on 2024-T3 bare aluminum alloys (reproduced from Park et al. [46] with

Treatments CAA [51] PAA [55, 56] BSAA [51] PSA [53] Oxide thickness (nm) 4000 200 3000 1500 Pore diameter (nm) 25 32 10 20–25

• Appearances • Solution chemistry • Water purity • Air cleanliness • Voltages • Coating weight

The proprietary materials and exact production steps are slightly dissimilar between organizations.

40.0 2.0 25.0 26.7 2.2 27.0 2.0

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

30.0–50.0 (H2SO4)

• Prone to biological contamination<sup>5</sup> • Use of sodium benzoate or benzoic acid to prevent fungus growth

• Appearances • Solution chemistry • Water purity • Air cleanliness • Voltages

10.0 (H3PO4)/ 10.0 (H2SO4)

83

http://dx.doi.org/10.5772/intechopen.78217

• Control Cl- & F • The installation of preventive devices for fungus growth (e.g.filters and UV lamps)

• Appearances • Solution chemistry • Water purity • Air cleanliness • Voltages

Electrolyte (wt%)

Temperature

Contamination controls

QC issues • Appearances

• Control Cl-<sup>2</sup> & sulfate impurity • Incorporation of BaCO3 powder<sup>3</sup> to remove impurity

• Solution chemistry • Water purity • Air cleanliness • Voltages • Bath temperature

( C)

1

2

3

5

Cl: Chloride ions.

Cell wall thickness

Schematic view of oxide structure [nm] (non to scale)

(nm)

F: Fluorine.

nas species).

BaCO3: Barium carbonate. <sup>4</sup>

permission from Taylor & Francis).

permission from Taylor & Francis).

All the anodizing process are complex multi-stage operations incorporating degreasing and deoxidizing stages, as described in the preceding sections, plus appropriate rinses. Anodizing oxidation in solution of CAA or PAA is the preferred stabilizing treatment for the structural adhesive bonding of aluminum alloys in critical applications such as aircraft structures [51, 52]. However, they typically rely on such hazardous materials as strong acid and hexavalent chromium. The use of chromate is prohibited, or progressively banned in most industries due to its carcinogen activity. For this purpose, non-chromate anodizing such as boric-sulfuric acid anodizing (BSAA), or phosphoric-sulfuric anodizing (PSA), have been developed since the mid-1990s [51, 53], but neither of them have been fully validated for aircraft applications. Typical anodizing processes and their process parameters are listed in Table 4.

The classical porous oxide structure which are produced by anodizing process is likely to be related to capillary forces of primer trying to penetrate into the oxide pores, which in turn increase in mechanical interlocking between anodic oxide and primer [46]. The porous oxide structures can be controllable in accordance with the anodizing processes, as listed in Table 5. This table clearly represents the effects of anodizing processes on the oxide structures in terms of oxide thickness, pore diameter and cell wall thickness. The CAA process was found to give a relatively thick and softer oxide structure than those formed by the other processes [52]. This was established as an effective pretreatment for adhesive bonding with superior durability performance in service [51, 52, 54]. The European aerospace industry is still using this method [51, 52]. However, notwithstanding the remarkable durability data in corrosive environments, the use of chromate treatment process is being restricted due to recent environmental policy.

The PAA process is basically used for the structural adhesive bonding of aluminum and its alloys. The standard process (Boeing's BAC 5555 or ASTM D 3933) has proven to produce the most durable and reactive surface for structural adhesive bonding [57]. The PAA substrates are normally submitted to a Forest Product Laboratory (FPL) etch prior to anodizing, although the non-chromate acid etch (P2) is sometimes used instead. The PAA-treated anodic oxide is highly porous with open cell diameter of approximately 32 nm in height on top of a much The Guidelines of Material Design and Process Control on Hybrid Fiber Metal Laminate for Aircraft Structures http://dx.doi.org/10.5772/intechopen.78217 83


1 The proprietary materials and exact production steps are slightly dissimilar between organizations. 2 Cl: Chloride ions.

3 BaCO3: Barium carbonate. <sup>4</sup>

F: Fluorine.

3.2. Surface treatment of aluminum alloys for producing GLARE structures

82 Optimum Composite Structures

previous surface treatment studies of aluminum alloys for aircraft structures [46].

Typical anodizing processes and their process parameters are listed in Table 4.

All the anodizing process are complex multi-stage operations incorporating degreasing and deoxidizing stages, as described in the preceding sections, plus appropriate rinses. Anodizing oxidation in solution of CAA or PAA is the preferred stabilizing treatment for the structural adhesive bonding of aluminum alloys in critical applications such as aircraft structures [51, 52]. However, they typically rely on such hazardous materials as strong acid and hexavalent chromium. The use of chromate is prohibited, or progressively banned in most industries due to its carcinogen activity. For this purpose, non-chromate anodizing such as boric-sulfuric acid anodizing (BSAA), or phosphoric-sulfuric anodizing (PSA), have been developed since the mid-1990s [51, 53], but neither of them have been fully validated for aircraft applications.

The classical porous oxide structure which are produced by anodizing process is likely to be related to capillary forces of primer trying to penetrate into the oxide pores, which in turn increase in mechanical interlocking between anodic oxide and primer [46]. The porous oxide structures can be controllable in accordance with the anodizing processes, as listed in Table 5. This table clearly represents the effects of anodizing processes on the oxide structures in terms of oxide thickness, pore diameter and cell wall thickness. The CAA process was found to give a relatively thick and softer oxide structure than those formed by the other processes [52]. This was established as an effective pretreatment for adhesive bonding with superior durability performance in service [51, 52, 54]. The European aerospace industry is still using this method [51, 52]. However, notwithstanding the remarkable durability data in corrosive environments, the use of chromate treatment process is being restricted due to recent environmental policy. The PAA process is basically used for the structural adhesive bonding of aluminum and its alloys. The standard process (Boeing's BAC 5555 or ASTM D 3933) has proven to produce the most durable and reactive surface for structural adhesive bonding [57]. The PAA substrates are normally submitted to a Forest Product Laboratory (FPL) etch prior to anodizing, although the non-chromate acid etch (P2) is sometimes used instead. The PAA-treated anodic oxide is highly porous with open cell diameter of approximately 32 nm in height on top of a much

A strong bonding interface is one of the key factors for improving the durability of GLAREs. It is apparent that the surface treatment technique can improve the surface energy and wettability of metallic substrates, and is an effective method for enhancing the bonding strength between a metal substrate and a fiber reinforced polymer composite [46]. In addition, the surface treatment can remove the undesirable surface oxides or contaminants on the metallic substrate, and ameliorate the surface composition and microstructure of the metallic substrate [6, 47]. This allows the fiber bridging mechanism and mechanical properties of GLAREs to be improved, and moreover, the crack propagation rate at the aluminum-composite interface can be effectively reduced [48, 49]. Previous research works reported that the surface treatment should be carefully taken into consideration when improving interlaminar shear strength at the aluminum-composite interface [6, 50], environmental durability and low-velocity impact resistance of GLARE. Therefore, the proper production steps should be clearly defined before any production process is implemented. Note that this section is described based on our

> 5 Bio-contaminant organisms, for example fungal (alternaria, fusarium and penicillium species) and bacterial (pseudomonas species).

> Table 4. Anodizing processes for structural adhesion bonding of aluminum alloys (reproduced from Park et al. [46] with permission from Taylor & Francis).


Table 5. Comparison of oxide morphology on 2024-T3 bare aluminum alloys (reproduced from Park et al. [46] with permission from Taylor & Francis).

thinner barrier layer [54, 55, 58]. The PAA oxide thickness is typically reported in the range from 200 to 400 nm with a much thinner barrier layer of about 10 nm [54, 55]. The physical comparisons between PAA and CAA oxide structures clearly represent the PAA oxide to have a much more open porous structure, which would be more easily penetrated by the subsequent organic bond primer, thereby drawing the organic polymer into the oxide structure to form a very strong interlocking interphase. The PAA oxide structure provides either equivalent or better durability results than the CAA oxide structure in the most experimental trails [59].

requirements, engineering drawings. For this purpose, all available QC factors such as prescribed contractual requirements, available equipment, level of personnel training and documentation systems should be carefully considered. Table 6 describes the specific lesson learned in

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

lay-up process.

Autoclave pressure variations • Pressure controller has to regulate the autoclave pressure to

• The surface treated aluminum sheets should be stored in dust-free area, or be protected to prevent the further contamination. The sealed bags are typically utilized to protect the bond-primed surfaces of aluminum sheets prior to the

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85

• Only materials listed in the process specification shall be used in contact with the aluminum sheets inside the net trim line prior to cure. For this purpose, the consumable processing aid materials used in the parts production

(1) Contact-use materials: approved for use in direct contact with the anodized (or primed) surface of aluminum sheets. (2) Noncontact-use materials: approved for use as aids to processing but shall not contact the anodized (or primed) aluminum sheets inside the trim line prior to autoclave cure.

maintain a uniform pressure level throughout a cure cycle. In addition, the pressure reservoir shall be kept twice the autoclave pressure so as to operate the pneumatic valves

• During the preparation of thin rolled aluminum sheets (0.2–0.5 mm), consisting of unrolling, cutting, surface treatments (i.e. anodizing and bond primering) and consequently rolling up, all aluminum sheets should be prepared without folding, or kinks. The damaged material shall not be used for part production. In the stage of lay-up process, the aluminum sheet should be rolled over the un-cured

ing regions can begin to fold, or wrinkle because of the discontinuity in in-plane strain across the prepreg ply. If the deformation exceeds the limit angle of prepreg material, it is considered to be either fiber wrinkling, or bridging out of

• Pre-cut materials shall be stored in flat, stress-free condition to prevent folding or damage. The damaged material

shall not be used for part production.

should be separated into two categories:

and solenoid valves sufficiently.

prepreg plies.

the material's tolerance.

Folded prepreg during lay-up process • As small regions of prepreg ply are sheared, the surround-

part production and the corresponding practical solution.

Fingerprints or fluid spots on aluminum sheets

[Sealed bags containing prepared rolled aluminum sheets prior to lay-up, source: CompositesWorld]

Folds and kinks of aluminum sheet

[Roll out over prepreg layers, source:

CompositesWorld]

Issues Possible practical solutions

The BSAA process is usually carried out using a mixture of 5–10 wt.% boric acid (H3BO3) and 30–50 wt.% sulfuric acid (H2SO4) at 26.7 2.2C. This process was patented by Boeing as a direct replacement to the CAA process [51, 60]. It is well known that the CAA process produces a chromium mist that is hazardous to health if inhaled. The BSAA is an alternative that eliminates this concern and the need for mist control. The process standard, BAC 5632, involves deoxidizing with tri-acid solutions, consisting of sodium dichromate, sulfuric, and hydrofluoric acid (HF), followed by the application of boric and sulfuric acid anodizing. The parts are then dried in warm air at 75C prior to bond primer application. The anodic film which is produced by the standard BSAA has relatively small pore diameter (10 nm) compared with the conventional CAA film (25 nm), as listed in Table 5. The anodic oxide structure from the BSAA has a paint adhesion that is equal, or superior, to the one formed on CAA [51, 60]. For this purpose, the BSAA process parameters have been modified by the research groups, for example [61]. As a result, the required surface topography and equivalent mechanical stability in strength and durability are only enhanced when the following process variations were instituted: electrolytic phosphoric acid deoxidizer (EPAD) [51]: anodizing bath temperature in the BSAA bath [51, 61] and additional post treatment using a PAD [51].

More recently, a variety of alternative chromate-free electrochemical treatments have been introduced in the context of corrosion protection and adhesive bonding of aluminum and its alloys. The new eco-efficient alternatives developed by Airbus include tartaric-sulfuric acid anodizing (TSA) and PSA. In particular, a significant step towards chromate-free has been achieved by PSA process for adhesively bonded joints. This process, which is utilized for adhesive bonded joints is usually carried out by using a mixture of 10 wt.% phosphoric acid (H3PO4) and 10 wt.% sulfuric acid (H2SO4) at temperatures ranging from 26 to 28C [53]. This process is now ready for the qualification by Airbus. The standard process requires nitric acid deoxidizing prior to PSA treatment. The PSA-treated surface produces an oxide structure of about 1500 nm in thickness with somewhat narrow porous structures in the range from 20 to 25 nm in pore diameter [62]. The PSA process has a reduced process time (in typical, 23 min) and anodizing temperature (27C), compared with the standard CAA [53]. This leads to an improvement in eco-efficiency by decreasing time and energy consumption and offers a capacity increase.

#### 3.3. Lesson learned from serial production of GLARE structures

In-process QCs activities are essential if the fits, forms, functions and requirements designed into a part are to be consistently achieved. In general, the QC systems applied to the part production of GLARE structures should be established based on the company's own specifications, part requirements, engineering drawings. For this purpose, all available QC factors such as prescribed contractual requirements, available equipment, level of personnel training and documentation systems should be carefully considered. Table 6 describes the specific lesson learned in part production and the corresponding practical solution.

thinner barrier layer [54, 55, 58]. The PAA oxide thickness is typically reported in the range from 200 to 400 nm with a much thinner barrier layer of about 10 nm [54, 55]. The physical comparisons between PAA and CAA oxide structures clearly represent the PAA oxide to have a much more open porous structure, which would be more easily penetrated by the subsequent organic bond primer, thereby drawing the organic polymer into the oxide structure to form a very strong interlocking interphase. The PAA oxide structure provides either equivalent or better durability results than the CAA oxide structure in the most experimental trails [59].

84 Optimum Composite Structures

The BSAA process is usually carried out using a mixture of 5–10 wt.% boric acid (H3BO3) and 30–50 wt.% sulfuric acid (H2SO4) at 26.7 2.2C. This process was patented by Boeing as a direct replacement to the CAA process [51, 60]. It is well known that the CAA process produces a chromium mist that is hazardous to health if inhaled. The BSAA is an alternative that eliminates this concern and the need for mist control. The process standard, BAC 5632, involves deoxidizing with tri-acid solutions, consisting of sodium dichromate, sulfuric, and hydrofluoric acid (HF), followed by the application of boric and sulfuric acid anodizing. The parts are then dried in warm air at 75C prior to bond primer application. The anodic film which is produced by the standard BSAA has relatively small pore diameter (10 nm) compared with the conventional CAA film (25 nm), as listed in Table 5. The anodic oxide structure from the BSAA has a paint adhesion that is equal, or superior, to the one formed on CAA [51, 60]. For this purpose, the BSAA process parameters have been modified by the research groups, for example [61]. As a result, the required surface topography and equivalent mechanical stability in strength and durability are only enhanced when the following process variations were instituted: electrolytic phosphoric acid deoxidizer (EPAD) [51]: anodizing bath temperature

More recently, a variety of alternative chromate-free electrochemical treatments have been introduced in the context of corrosion protection and adhesive bonding of aluminum and its alloys. The new eco-efficient alternatives developed by Airbus include tartaric-sulfuric acid anodizing (TSA) and PSA. In particular, a significant step towards chromate-free has been achieved by PSA process for adhesively bonded joints. This process, which is utilized for adhesive bonded joints is usually carried out by using a mixture of 10 wt.% phosphoric acid (H3PO4) and 10 wt.% sulfuric acid (H2SO4) at temperatures ranging from 26 to 28C [53]. This process is now ready for the qualification by Airbus. The standard process requires nitric acid deoxidizing prior to PSA treatment. The PSA-treated surface produces an oxide structure of about 1500 nm in thickness with somewhat narrow porous structures in the range from 20 to 25 nm in pore diameter [62]. The PSA process has a reduced process time (in typical, 23 min) and anodizing temperature (27C), compared with the standard CAA [53]. This leads to an improvement in eco-efficiency by

In-process QCs activities are essential if the fits, forms, functions and requirements designed into a part are to be consistently achieved. In general, the QC systems applied to the part production of GLARE structures should be established based on the company's own specifications, part

in the BSAA bath [51, 61] and additional post treatment using a PAD [51].

decreasing time and energy consumption and offers a capacity increase.

3.3. Lesson learned from serial production of GLARE structures

Fingerprints or fluid spots on aluminum sheets

[Sealed bags containing prepared rolled aluminum sheets prior to lay-up, source: CompositesWorld]

Folds and kinks of aluminum sheet

[Roll out over prepreg layers, source: CompositesWorld]

#### Issues Possible practical solutions

	- (1) Contact-use materials: approved for use in direct contact with the anodized (or primed) surface of aluminum sheets. (2) Noncontact-use materials: approved for use as aids to processing but shall not contact the anodized (or primed) aluminum sheets inside the trim line prior to autoclave cure.
	- During the preparation of thin rolled aluminum sheets (0.2–0.5 mm), consisting of unrolling, cutting, surface treatments (i.e. anodizing and bond primering) and consequently rolling up, all aluminum sheets should be prepared without folding, or kinks. The damaged material shall not be used for part production. In the stage of lay-up process, the aluminum sheet should be rolled over the un-cured prepreg plies.
	- Pre-cut materials shall be stored in flat, stress-free condition to prevent folding or damage. The damaged material shall not be used for part production.

Porosities (or voids)

[NDT defect area with air enclosure, source: Fokker]

#### Prepreg gap controls

[LPS, source: Virtek]

#### Issues Possible practical solutions

• The porosities (or voids) considerably degrade the static strength and fatigue life as a result of insufficient adhesion between aluminum–prepreg and prepreg– prepreg interfaces. Park et al. [6, 35] reported that the reduction in porosity content from 1.30% to 0.69% could account for 46.46% increase in the interlaminar shear strength.

4. Conclusions

Author details

Sang Yoon Park1

South Korea

South Korea

References

The new hybrid material FML has been successfully applied to the commercial aircraft structures by offering weight savings of 10% compared with conventional aluminum and its alloys, together with benefits that include high tensile strength and better F&DT characteristics and high level of fiber safety. A large number of literatures on the practical applications demonstrates that the material properties of FMLs and their additional interlinked advantages make them the ideal option for thin-walled fuselage shells of next single aisle aircrafts. This chapter dealt with the details of technological developments with ongoing research efforts to understand the material property behaviors of FMLs, especially static strength, F&DT properties and long-term durability. In addition, two prediction methods of MVF and CLT have been introduced to predict the corresponding static properties of FMLs respect to the different lay-up patterns. However, to compete with the typical materials used in aerospace engineering, additional efforts should be directed towards producing consistently sound FML structures at affordable costs and ensuring the stringent quality controls for compliance with structural integrity. Recently, the FML manufacturers have continued to make a substantial progress in production technology, which allows for enabling FMLs in high-volume production rates and increasing affordability for aerospace industry. In addition to the consideration of each constituent material's properties, a strong interfacial bonding between metal sheets and composite layers is one of the key factors for the improvement in joint strength and long-term durability of FML structures. Therefore, a proper surface treatment on the metallic substrate is prerequisite for achieving

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long-term service capability through more efficient processing in production.

1 Hyundai Automotive Research and Development Division, Hwaseong-Si, Gyeonggi-Do,

2 Department of Materials Engineering, Korea Aerospace University, Koyang-city, Gyeonggi-Do,

[1] Park Y, O'Kelly ME. Examination of cost-efficient aircraft fleets using empirical operation data in US aviation markets. Journal of Air Transport Management. 2018;69:224-234. DOI:

\* and Won Jong Choi<sup>2</sup>

\*Address all correspondence to: hanavia@empas.com

10.1016/j.jairtraman.2017.02.002

	- Being able to predict the changes in part configuration allows to design curing tool geometries that already compensate for the undesired change in curvature [64]. This approach can lead to a significant cost reduction for the curing tool-design.
	- At the same time, high compaction pressure is required to compress the materials and squeeze out excess resin.

Table 6. Lesson learned from production of GLARE structures.
