JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 43

as +45, 0, -45 – are commonly stitched into a single ply, resulting in a typical thickness of 0.3-0.5 mm. For economic reasons, the range of multi-axial fabric constructions that are purchased and certified is usually limited. The increased thickness limits the possibility of designing a highly-optimized structure; since thickness changes in large increments, intermediate values are not possible. Typical requirements for balanced and symmetrical lay-ups are another reason for large incremental changes in part thickness – since dropping one (thick) ply is accompanied by dropping another (thick) ply (see Fig. 2). In some cases, it was found impossible to produce laminate symmetry within other part design constraints. New rules for ply drop-off design thus had to be developed. Fabrics are supplied in finite width rolls, sometimes necessitating splicing within a ply. Overlap splices were not practical with the thick multi-axial plies. Butt splicing a multi-axial reinforcement required permitting fibre discontinuities that are not allowed by existing rules for conventional reinforcements. The multi-axial nature of the usual NCF fabrics means that fibre continuity is not possible in all directions at splices. This requires substantiation that the adopted splicing rules satisfy part structural requirements. These new constraints often necessitate detail element substantiation testing. Moreover, when cutting and laying up the multidirectional reinforcements, ply alignment is defined by the surface layer of the ply. Despite specifying the allowable angular discrepancies permitted between layers in each ply, the precise orientation of non-visible layers within plies cannot be assured completely. This is thought to be a source of the higher property variability measured for NCF reinforcements compared to conventional material forms. This higher variability necessitates a statistical knockdown when deriving design allowable values from the test data base.

Fig. 2: Symmetric and balanced lay-up and ply drops with multi-axial NCF

very stiff construction to prevent tool deformation during the process, or external restraint systems need to be used to oppose the injection forces. This becomes particularly acute as part size increases. As for all tooling, thermal expansion differences between the tool (usually steel) and the composite have to be taken into account, but for RTM this is complicated by the behaviour of internal tool details. The necessity to extract the tools from the cured part introduces constraints on design concepts. Tool extraction limits thickness variations in the parts (local pad-ups for hard points and at part ends). When using NCF plies, each ply drop-off produces a substantial step change in local thickness, unlike the more gradual slopes associated with conventional reinforcements. In some cases, the size of these steps was sufficient to prevent tool detail extraction. This necessitated local relaxation of the ply drop-off procedures.

Pultrusion
The continuous pultrusion process is used industrially for the production of fibre-reinforced profiles with constant cross sections. The pultrusion process is not new, and has been routinely used for consumer products, electrical equipment, transport and civil engineering applications for many years [11]. In recent years, the process has been refined and developed for aerospace use, improving product capabilities and properties [12]. Briefly, the process is usually based on pulling fibres or a fibre preform through a shaped heated die, while introducing resin before the fibres enter the die. Its economic advantages are quite obvious – a continuous process that can manufacture profiles with various cross sections in virtually limitless lengths and at high rates. However, the use of pultrusion in aircraft structures is still very limited due to various reasons. The resins used are relatively brittle compared to prepreg and even RTM resins. They show impact sensitivity and make drilling and fastener application difficult. Different design rules are required for fastener edge distances. In addition to this drawback, the use of pultrusion in composite structures poses some design constraints that limit their applicability. The main issue is that the standard No74 July 2012 /

RTM process constraints
RTM is very attractive from an economical viewpoint, mainly due to the possibility of manufacturing large complex structures in a single process. For example, control surfaces can be manufactured in a single cure process that includes the external skins, internal spars and ribs [6]. However, this process also incorporates many limitations and drawbacks when compared to conventional prepreg design that have to be considered during the design stage. RTM resins typically have a lower toughness compared to prepreg resins; however, resin manufacturers are actively developing new RTM resins with improved toughness. The main constraint with RTM is the complex tooling required. Due to high resin injection pressures, the tools must have a

jec composites magazine 43



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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