J. Sinke

MANUFACTURING OF GLARE PARTS AND STRUCTURES

GLARE is a hybrid material consisting of alternating layers of metal sheets and composite layers, requiring special attention when manufacturing of parts and structures is concerned. On one hand the applicable manufacturing processes for GLARE are limited, on the other hand, due to the constituents and composition of the laminate, it offers new opportunities for production. One of the opportunities is the manufacture of very large skin panels by lay-up techniques. Lay-up techniques are common for full composites, but uncommon for metallic structures. Nevertheless, large GLARE skin panels are made by lay-up processes. In addition, the sequences of forming and laminating processes, that can be selected, offer manufacturing options that are not applicable to metals or full composites. With respect to conventional manufacturing processes, the possibilities for Fibre Metal Laminates in general, are limited. The limits are partly due to the different failure modes, partly due to the properties of the constituents in the laminate. For machining processes: the wear of the cutting tools during machining operations of GLARE stems from the abrasive nature of the glass fibres. For the forming processes: the limited formability, expressed by a small failure strain, is related to the glass fibres. However, although these manufacturing issues may restrict the use of manufacturing processes for FMLs, application of these laminates in aircraft is not hindered.

Maintenance of Glare Structures and Glare as Riveted or Bonded Repair Material

Aircraft structures constructed from new and advanced materials will become more common in the near future, starting with the use of the Fibre Metal Laminate Glare in large parts of the Airbus A-380 fuselage. These materials are primarily used because of their excellent damage tolerance properties. However, questions about maintenance and repair of such structures need to be answered before such new materials can be used. These questions include whether new and advanced materials can be repaired in a conventional way, which would not only be preferable from the operators point of view (no change in tools, maintenance procedures, and personnel training), but also from the manufacturers point of view (Structural Repair Manuals similar to aluminium structures). A Glare demonstrator panel has been designed and applied to an Airbus A-310 and research into the repairability of Glare has been performed to answer these questions. Apart from looking into the repairability of Glare structures, the material itself is also investigated as material for bonded repair patches. Bonded repair many times proves to be a more viable solution than conventional riveted repair due to its more efficient load transfer. Important aspects of bonded (Glare) repair are under investigation to show that bonded patch repair is not only working for the ageing aircraft of several Air Forces around the world, but is also a promising candidate for safe and cost-effective repairs to ageing and new (incidental damage) aircraft of commercial operators. This research is conducted cooperatively by Delft University of Technology and the United States Air Force Academy and has led to two real-life repairs on a C-5A “Galaxy”.

Some Inspection Methods for Quality Control and In-service Inspection of GLARE

Quality control of materials and structures is an important issue, also for GLARE. During the manufacturing stage the processes and materials should be monitored and checked frequently in order to obtain a qualified product. During the operation of the aircraft, frequent monitoring and inspections are performed to maintain the quality at a prescribed level. Therefore, in-service inspection methods are applied, and when necessary repair activities are conducted. For the quality control of the GLARE panels and components during manufacturing, the C-scan method proves to be an effective tool. For in-service inspection the Eddy Current Method is one of the suitable options. In this paper a brief overview is presented of both methods and their application on GLARE products.

Out-of-autoclave manufacturing of GLARE panels using resistance heating:

Autoclave manufacturing of fibre metal laminates, such as GLARE, is an expensive process. Therefore, there is an increasing interest to find cost-effective out-of-autoclave manufacturing processes without diminishing the laminate quality. The aim of this study is to evaluate the quality of fibre metal laminate panels adhesively bonded and cured using resistance heating. Three manufacturing processes are compared for different layups with an embedded steel mesh at the mid-plane: autoclave curing, resistance bonding of two (autoclave-cured) panels and complete out-of-autoclave resistance curing of panels. Interlaminar shear strength tests and optical microscopy analysis showed that resistance bonding is a promising technique, leading to results comparable to autoclave curing. Resistance curing led to an interlaminar shear strength decrease of 30–60%. A study of the correlation between degree of cure and distance from the mesh revealed the potential of resistance bonding to be used for flexible embedded mesh geometries and on-site repairs.

Detection and Evaluation of Pre-Preg Gaps and Overlaps in Glare Laminates

Gaps and overlaps between pre-preg plies represent common flaws in composite materials that can be introduced easily in an automated fibre placement manufacturing process and are potentially detrimental for the mechanical performances of the final laminates. Whereas gaps and overlaps have been addressed for full composite material, the topic has not been extended to a hybrid composite material such as Glare, a member of the family of Fibre Metal Laminates (FMLs). In this paper/research, the manufacturing, the detection, and the optical evaluation of intraply gaps and overlaps in Glare laminates are investigated. As part of an initial assessment study on the effect of gaps and overlaps on Glare, only the most critical lay-up has been considered. The experimental investigation started with the manufacturing of specimens having gaps and overlaps with different widths, followed by a non-destructive ultrasonic-inspection. An optical evaluation of the gaps and overlaps was performed by means of microscope image analysis of the cross sections of the specimens. The results from the non-destructive evaluations show the effectiveness of the ultrasonic detection of gaps and overlaps both in position, shape, width, and severity. The optical inspections confirm the accuracy of the non-destructive evaluation also adding useful insights about the geometrical features due to the presence of gaps and overlaps in the final Glare laminates. All the results justify the need for a further investigation on the effect of gaps and overlaps on the mechanical properties.