Reinforced plastic directional property

The term reinforced plastic (RP), also called composites (more accurately plastic composites), refers to combinations of plastic (matrix) and reinforcing materials that predominantly come in fiber forms such as chopped, continuous, woven and nonwoven fabrics, etc. also in other forms such as powder, flake, etc. They provide significant oriented property and/or cost improvements than the individual components (10, 14, 35, 38, 39-43, 62). 

Thermoplastic RPs (RTPs), even with their relatively lower properties when compared to thermoset RPs (RTSs), consume about 55wt% of all RP products. Practically all of the RTPs are injection molded with very fast cycles using short glass fiber producing highly automated and high performance products. 

Isotropic material In an isotropic material the properties at a given point are the same, independent of the direction in which they are measured (Fig. 3-19). The term isotropic means uniform. As one moves from point-to-point in this type of homogeneous plastic the material s composition remains constant. Also, the smallest samples of material 

Anisotropic material In an anisotropic material the properties vary, depending on the direction in which they are measured. There are various degrees of anisotropy, using different terms such as orthotropic or unidirectional, bidirectional, heterogeneous, and so on (Fig. 3-19). For example, cast plastics or metals tend to be reasonably isotropic. However, plastics that are extruded, injection molded, and rolled plastics and metals tend to develop an orientation in the processing flow direction (machined direction). Thus, they have different properties in the machine and transverse directions, particularly in the case of extruded or rolled materials (plastics, steels, etc.). 

Wood is anisotropic with distinct different properties in three directions. Its highest mechanical properties are in the growth (fiber) direction, with the perpendicular (or second plane) direction having lower properties and the other perpendicular (or third plane) direction having much lower properties. 

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Another important orienting fabricating procedure concerns applying directional properties to reinforced plastics. This subject is reviewed in Chapter 3, DESIGN CONCEPT, Reinforced Plastic Directional Property. 

Figure 3.4 Examples of reinforced plastic directional properties...

An important type of orienting fabricated products concerns applying directional properties to plastics by using fiber reinforcements. Orientation is the alignment of fiber reinforcement within the product that affects mechanical properties. The reinforced plastic (RP) properties increase in the direction of alignment (Figure 3.4). 

The wide choice available in plastics makes it necessary to select not only between TPs, TSs, reinforced plastics (RPs), and elastomers, but also between individual materials within each family of plastic types (Chapters 6 and 7). This selection requires having data suitable for making comparisons which, apart from the availability of data, depends on defining and recognizing the relevant plastics behavior characteristics. There can be, for instance, isotropic (homogeneous) plastics and plastics that can have different directional properties that run from the isotropic to anisotropic. Here, as an example, certain... 

The reinforcement type and form chosen (woven, braided, chopped, etc.) will depend on the performance requirements and the method of processing the RP (Fig. 6-15). Fibers can be oriented in many different patterns to provide the directional properties desired. Depending on their packing arrangement, different reinforcement-to-plastic ratios are obtained (Appendix A. PLASTICS TOOLBOX). 

Molecular orientation results in increased stiffness, strength, and toughness (Table 8-12) as well as resistance to liquid and gas permeation, crazing, microcracks, and others in the direction or plane of the orientation. The orientation of fibers in reinforced plastics causes similar positive influences. Orientation in effect provides a means of tailoring and improving the properties of plastics. 

Orientation of reinforcement The behavior of RPs is dominated by the arrangement and the interaction of the stiff, strong fibers with the less stiff, weaker plastic matrix. The features of the structure and the construction determine the behavior of RPs that is important to the designer. A major advantage is the fact that directional properties can be maximized in the plane of the sheet. As shown in Fig. 8-55 they can be isotropic, orthotropic, etc. Basic design theories of combining actions of plastics and reinforcements... 

Glass fibres dominate this field either as long continuous fibres (several centimetres long), which are hand-laid with the thermoset precursors, e.g., phenolics, epoxy, polyester, styrenics, and finally cured (often called fibre glass reinforcement plastic or polymer (FRP)). With thermoplastic polymers, e.g., PP, short fibres (less than 1 mm) are used. During processing with an extruder, these short fibres orient in the extrusion/draw direction giving anisotropic behaviour (properties perpendicular to the fibre direction are weaker). 

For typical filament winding applications, the fiber reinforcement provides the stiffness and strength required to maintain structural integrity. Thus, material characterization for filament wound structures focuses on characterizing the fiber dominated stiffness and strength properties of the composite. The stiffness of fiber reinforced plastics (FRPs), in the fiber direction, is dominated by the fiber stiffness characteristics. The strength will be influenced by a number of factors, however, and not all of them are related to the fiber, including ... 

This particular ASTM procedure is not directly relevant to wood-thermoplastic composites, and therefore it is only briefly described here. The procedure is recommended for the determination of the tensile properties of thermosetting reinforced plastics. Experience with this test method has been limited to glass-reinforced thermosets. The principal difference with ASTM D 638, using dogbone-shaped specimen, is that ASTM D 5083 uses test specimens of uniform nominal width. 

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