Название: Aircraft engineering (Морозова М. А.)

Жанр: Авиационные технологии и управление

Просмотров: 1486

Thermoplastic matrices

In recent years, thermoplastic matrix systems have been introduced. Their major advantages are

• Service temperatures of up to 540 °F (280°C)

• Excellent strain capabilities

• High moisture resistance

• Unlimited shelf-life

• Short processing cycles Disadvantages are

• High processing temperatures

• As yet marginal processing experience

• Lack of drapeability

Thermoplastic matrices are not new to the airframe industry. They have been used for many years for various components, mainly in aircraft fuselage interiors and for other non-structural parts. The engineering thermoplastic resins have  high  continuous  service  temperatures,  from  250°F  to  400°F  (121  to

200°C), high matrix melting temperatures, and high viscosity which leads to higher mold pressure in autoclave operations. Thermoplastic matrices provide better interlaminar fracture toughness combined with acceptable postimpact compression, better resistance to high temperatures and solvents, and have low

moisture sensitivity. The major advantage over thermoset matrices is their shorter fabrication cycle and the fact that a chemical cure does not take place, allowing reprocessing or reconsolidation of a flawed part after manufacture.

Thermoplastics offer potential cost reduction by:

• Reforming capability

• Welding capability

• Eliminating cold refrigeration storage and having unlimited shelf life

• Rapid processing cycles times

• Recyclable scrap

• Being less difficult to drill and machine

Product forms are still being developed and the most recently available prepregs are stiff and boardy. They lack the drape and tack needed for handlea- bility (forms that handle well such as commingled fabrics will be discussed in Materal Forms in Section 2.5). Tack and drape in some thermoplastic prepregs are achieved by the presence of solvent, as is the case with some ployimides.

The «Wet» prepregs compared to «Dry» prepregs are described below: (a) «Wet» Thermoplastic Prepreg Materials

• Wet matrices include Kill, AIX-159, Torlon 696, etc.

• Wet matrices have inherent tack and drape at room temperature

• Wet matrices may react chemically during processing (and in the past have been referred to as "pseudo-thermoplastics")

• Have half life and out time constraints

• Process like thermosets

• Have high volatile content, 12-25\% by weight

• Some require post-cure for maximization of Tg

• Require volatile management during in processing (b) «Dry» Thermop-

lastic

Prepreg Materials

• More difficult to prepreg than «Wet»

• Dry matrices have no inherent tack and drape at room temperature

• Have no shelf life or time constraints

• Melt fusible, no chemical reaction

• Solution or hot melt impregnated

• Amorphous and/or semi-crystalline

One of the most critical factors is lack of an extensive database of perfor- mance properties over service time. Military aircraft structural applications are one of the major drivers to develop thermoplastics for use as high-temperature composite matrices and four major requirements are:

• High temperature capabilities (range 350°F (177°C)) under severe hot/wet environmental conditions

• Better damage tolerance in primary structures

• Easily automated in order to drive down manufacturing process costs

• Lower total part-acquisition and lifetime costs (including material, processing, and supportability)

The characteristics of two major divisions (semi-crystalline and amorph- ous) of thermoplastic matrials are described below:

(a) Semi-crystalline matrices

• Have a definite melting point

• Better resistance to halogenated hydrocarbons and paint strippers

• Gradual loss of properties after Tg (glass transition temperature) is reached

• Density varies slightly depending on degree of crystallinity

• Mechanical properties may vary depending on degree of crystallinity

• Degree of crystallinity dependent on processing

(b) Amorphous Matrices

• No definite melt temperature

• Can be solvated for ease of fabric impregnation

• Free from the problems associated with crystallinity

• More susceptible to methyl chemical paint strippers

Understanding the role of crystallinity in thermoplastics immensely im- proves the success of the engineer in both the design and manufacturing.

Metal carbon and ceramic matrix composites

During the last decades, metal, carbon, and ceramic matrices have not progressed to the same extent as organic matrices because of high cost have li-

mited applications. Most of these composites are still in the research and devel- opment stage and are not widely used on today's airframes.