4. Conclusions

Issues Possible practical solutions

• The porosities (or voids) considerably degrade the

• High autoclave pressure (in typical. 11 bar) is generally applied to achieve the acceptable porosities (or voids)

• The intra-ply gap (i.e. butt-joint) and overlap splices is a common issue in the production of GLARE parts. It is apparent that the manufacturing–induced defects, such as delamination, or fiber missing result in the part thickness variation and the subsequent stress concen-

• The requirements of ply collation, such as fiber orientation, location and splice gap are normally specified on the engineering drawing, or corresponding process specification. A LPS with low-intensity laser beams may be utilized for precision controls of ply location, especially on tapered or

fabricate the sound part within tight dimensional tolerances. This issue has been particularly found in the integrated GLARE fuselage panels, for example double curved panels manufactured by SFT process. Orthotropic thermal and chemical properties in combination with autoclave production parameters, such as cure temperature and compaction pressure, are detri-

• 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

transverse direction is occasionally observed between two aluminum sheets. This result is attributed to either the loss of vacuum pressure applied consistently, or the absence of curing reaction for epoxy-matrix.

required to compress the materials and squeeze out

• At the same time, high compaction pressure is

shear strength.

trations [63].

contoured parts.

mental to achieve this goal.

curing tool-design.

excess resin.

Spring-back after curing process • One key challenge in GLARE part production is to

Disturbed resin squeeze out after curing • Insignificant resin bleeding, or squeeze-out in the

Table 6. Lesson learned from production of GLARE structures.

contents in GLARE parts.

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

Porosities (or voids)

86 Optimum Composite Structures

Prepreg gap controls

[LPS, source: Virtek]

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

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