Laminates fibre content

ICl Development polymide Resin QX-13 and Morganite Modmor Type I (treated) carbon fibre. Unidirectional laminate (S2% v/v fibre content)]. vSource of data ICl Trade Lilerature... 

Another aspect to be considered is the difficulty in producing curved structures with the same fibre content as flat laboratory panels. This effect is shown in Figure 16, at the comer the laminate thickness is larger than at the flat section and fibre content is rather lower. This will affect the bending stiffness of the arm and the predicted failure load. This figure also shows the fillet, which is critical to initiation in the specimens without implanted defects. It is well known that fillets can significantly alter the load path in lap shear joints and increase the failure loads (see [1] and Figure 3 for example). If a fracture mechanics approach is to be applied this effect must be considered. Some recent studies on stress intensity factors for such cases may allow this to be addressed [22]. 

In the spray-up process, a mould is made, just as in the hand-lay-up process. Continuous roving is fed into a chopper gun, which as its name suggests chops the fibres and sprays them onto the mould resin is also sprayed at the same time. The fibre and resin streams meet each other on the mould surface. The laminate is then rolled to remove as much air as possible. The fibre content achieved with this process is typically between 25-35 wt%. This method is rarely used by itself for corrosion-resistant vessels but is used in combination with filament winding for larger vessels. 

As pressure is not applied to the laminate during the curing process, high fibre content cannot be achieved and therefore only modest but in many cases adequate properties are possible. The use of woven (or knitted) rovings provides enhanced properties compared with chopped strand mat. Hand lay-up is, for example, the process used to produce the hull components for glass FRP minesweepers, demonstrating the scale of the production that can be achieved. 

Cure of the laminate takes place at atmospheric pressure, which helps to make the process simple and inexpensive. However, a high level of fibre content is not possible and hence high mechanical performance properties are not achievable. This does not mean they are unacceptable it simply results in thicker laminates than those from processes which can achieve high fibre content. The extra material cost is balanced against low capital cost, etc. 

While early studies by the group at the University of Leeds focused on melt-spun UHMWPE fibres, the hot compaction process was also subsequently applied to commercial UHMWPE gel-spun fibres [124,125]. It was reported that for some PE fibres, the optimum hot compaction temperature was only 1-2 C less than the temperature at which substantial crystalline melting occurred [117]. At these temperatures, approximately 30% of the oriented fibre content could be lost to melting, and so would be expected to strongly affect the mechanical properties of the final composite. A comparison of the hot compaction of different commercial UHMWPE assemblies, such as fabrics and felts, with commercial UHMWPE laminates was later presented by Morye et al. [126] and Puente Orench et al. [127]. 

Methods of preparation of the laminates depend on the partieular grade of polyimide resin used but in one process the polyimide precursor is dissolved in acetone and this solution is used to impregnate the glass or carbon fibre and thus produce a pre-preg . The pre-preg is dried and then pre-cured at about 200°C for about 3 hours. This operation reduces the volatile content and also modifies the flow properties to make them more suitable for the subsequent... 

Because of the lack of solubility in the usual solvents, aniline-formaldehyde laminates are made by a pre-mix method. In this process the aniline hydrochloride-formaldehyde product is run into a bath of paper pulp rather than of caustic soda. Soda is then added to precipitate the resin on to the paper fibres. The pulp is then passed through a paper-making machine to give a paper with a 50% resin content. 

Reinforcement Matrix Laminate configuration Fibre volume content Fabrication method Producer 5 layers Devoid DBT 800 glass fibre fabric NORPOL 20M 80 polyester 5 layers Devoid fabric with 50% of the fibres oriented in 0° direction and 50% in 45° direction. The 0° direction is parallel to the x-direction 42% (estimated from laminate weight) Hand lay up, room temperature curing Karlskronavarvet AB, Karlskrona, Sweden... 

Especially in thick laminates with low fibre volume content, the absolute distance of a specific layer from the laminate surface may vary significantly. Therefore, the recommendation that the fibre orientation on the bond surface is to coincide with the principal loading direction is sometimes hard to fulfil. Furthermore, it is to be noted that machining may damage the fibres on the step shear surface. 

In order to confirm theoretical estimates of stiffness necessary for a finite element analysis, experimental determination of laminate properties are required. Measurement of laminate thickness and fibre and resin content as well as mechanical properties are also required. 

High fibre volumes can be accurately achieved with low void content for very thick laminates (100 mm)... 

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