JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 44

design
pultrusion process can only produce straight profiles with a constant cross section. This means that pultrusion cannot be used for typical wing or stabilizer spars, since these usually have a tapered shape. Its application to stringers for stiffened skins (e.g., T or Z shapes) is limited to surfaces that have straight ruled surfaces. An attempt to use pultrusion to produce stringers for a typical aerodynamic surface (wing or stabilizer skins) will require a converging layout, since parallel lines on the surface are not straight. This results in a sub-optimal design from a weight aspect. Theoretically, pultrusion can be used for fuselage stringers, except in the rear sections where curved stringers are required. However, this application has additional limitations. Besides the high impact resistance required for the fuselage skin, the fuselage stringers usually require local cross section changes, such as “duck feet” near frame attachments, which cannot be achieved using pultrusion (unless a large section is removed by machining on most of the length, which is clearly not economical). Another issue arises at part ends, such as production breaks, or other main joints. The classical design of this critical detail usually involves increasing the skin thickness, and having the stringer “climb” (joggle) on this thickness variation (Fig. 3). This too cannot be achieved using conventional pultrusion. Methods are being developed to allow post-forming of pultruded parts that could, in principle, accommodate joggled designs, but these are not yet commercial. This issue can be resolved by novel designs of this detail but this requires an additional effort to verify the integrity of these new concepts in the development stage. In any event, despite these limitations, the economic potential for pultrusion is so attractive that there is a substantial motivation for using it. the honeycomb core cells fill with resin during the process. The usual solution consists in using closed-cell polymeric foam cores for sandwich structures. These cores are more costly than honeycombs, and they tend to absorb resin at the interface, which increases the product weight. Initial designs with these cores used the same edge bevel angles as defined for honeycomb cores in co-cured prepreg skin autoclave manufacture. With this bevel angle, it was impossible to prevent ply bridging at the base of the bevelled section in any part geometry more complex than a single edge on a flat plate. The solution was to change the core bevel angle rules to provide a more gradual slope at core edges. This was particularly important for the radiused corners of cores in bi-curved part geometries.

Lay-up automation
The introduction of automated tape laying systems has generated cost savings due to shorter manufacturing times and the reduced amount of skilled manual labour required for the overall lay-up process. Automated lay-up improves directional repeatability and reduces raw material waste. It also opens new fibre steering possibilities, allowing a higher degree of structure optimization. However, it is important to note that incorporating automated machines for ply lay-up involves new design rules too. Some machines have certain limitations that affect the design. Tape laying equipment is limited in capability due to the dimensions of the machine, in particular the tape laying head. The head’s degrees of freedom of movement and absolute dimensions restrict the part geometries that can be designed. Moreover, the prepreg tapes used in automated methods are usually narrower than those used for manual lay-up. A laminate produced with automated methods will contain more splice areas than a manually produced one. This reduces mechanical properties due to the resulting restriction of the fibre volume fraction. With automated methods, the lay-up accuracy is such that the permissible lay-up discrepancies that were historically specified for manual lay-up methods do not apply. The use of automated lay-up systems requires the generation of new misalignment, gap and overlap limits specific to the manufacturing technique.

Sandwich core design
It is well known that honeycomb cores cannot be used in liquid composite moulding processes. Unless the core is encapsulated (which is problematic for infusion processes),

Certification
The aircraft certification process is a major issue for all new technologies. Certification authorities are by nature very conservative, and correctly so. The introduction of new technology involves increased efforts to certify the structure since certification authorities are more cautious and certification requirements stricter than with proven technologies. For any new type of material, process or structure, it is essential to involve certification authorities at an early stage of the development process. At this stage,

Fig. 3: Stringer end with pultrusion

44 jec composites magazine / No74 July 2012



JEC COMPOSITES MAGAZINE - Issue #74 - July 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #74 - July 2012

Cover
Edito
Point of view: Closer to the shop floor
Contents
Agenda of Events
COMPANY & BUSINESS
- In Brief
- Glass fibre
APPLICATIONS
- Pipes & tanks
- Building
MARKET
- Human resources
- Glass fibre
- Turkey
FEATURE AUTOMOTIVE
- Braiding
- Lightening
- D-LFT
- Sustainability
- RTM
- RIM
RESEARCH & DEVELOPMENT
- Design
- Fatigue behaviour
TECHNOLOGY & INNOVATIONS
- Wind
- Reinforcement
Index
In the world
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Cover
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 2
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Edito
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Point of view: Closer to the shop floor
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 5
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Contents
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 7
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Agenda of Events
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - In Brief
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Glass fibre
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 11
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 12
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Pipes & tanks
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Building
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 15
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 16
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 17
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Human resources
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Glass fibre
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 20
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 21
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Turkey
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 23
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Braiding
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 25
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Lightening
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 27
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 28
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 29
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - D-LFT
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 31
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 32
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 33
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Sustainability
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 35
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 36
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - RTM
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 38
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 39
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - RIM
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Design
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 42
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 43
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 44
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 45
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Fatigue behaviour
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 47
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 48
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 49
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 50
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Wind
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 52
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 53
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Reinforcement
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 55
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 56
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Index
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - In the world
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 59
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 60
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