IMPACT PROPERTIES Subject

Mechanical Properties. Properties of typical grades of PBT, either as unfiUed neat resin, glass-fiber fiUed, and FR-grades, are set out in Table 8. This table also includes impact-modified grades which incorporate dispersions of elastomeric particles inside the semicrystalHne polyester matrix. These dispersions act as effective toughening agents which greatly improve impact properties. The mechanisms are not fiiUy understood in all cases. The subject has been discussed in detail (171) and the particular case of impact-modified polyesters such as PBT has also been discussed (172,173). 

The next step in the design procedure is to select the materials. The considerations are the physical properties, tensile and compressive strength, impact properties, temperature resistance, differential expansion environmental resistance, stiffness, and the dynamic properties. In this example, the only factor of major concern is the long-term stiffness since this is a statically loaded product with minimum heat and environmental exposure. While some degree of impact strength is desirable to take occasional abuse, it is not really subjected to any significant impacts. 

Mechanical properties PTFE maintains its tensile, compressive, and impact properties over a broad temperature range. Hence, it can be used continuously at temperatures up to 260 °C, while still possessing a certain compressive plasticity at temperatures near absolute zero. PTFE is quite flexible and does not break when subjected to stresses of 0.7 MPa according to ASTM D 790. Flexural modulus is about 350-650 MPa at room temperature, about 2000 MPa at —80 °C, and about 45 MPa at 260 °C. The Shore D hardness, measured as per ASTM D 2240, has values between D50 and D60. PTFE exhibits plastic memory, that is, if subjected to tensile or compression stresses below the yield point, part of the resulting deformations remain after the discontinuance of the stresses. If the piece is reheated, the induced strains tend to release themselves within the piece, which resumes its original form. 

Fracture and impact properties should be specially mentioned since they are probably the subject of more argument than any other composite property. This arises because of the extreme complexity of composite failure under impact conditions. To use information generated on unidirectional specimens to predict the behaviour of laminates and structures is extremely difficult, if not impossible, but the reader should study Chapter 6 for more information. 

Nylon Cloth Grade with Phenolic Resin Binder. Grade N-1 has excellent electrical properties under high humidity conditions and good impact strength, but is subject to flow or creep under load, especially at temperatures higher than normal. 

For a part to exhibit stmctural stiffness, flexural moduH should be above 2000 N/mm (290,000 psi). Notched l2od impact values should be deterrnined at different thicknesses. Some plastics exhibit different notch sensitivities. For example, PC, 3.2 mm thick, has a notched l2od impact of 800 J/m (15 fdbf/in.) which drops to 100 J/m (1.9 fflbf/in.) at 6.4-mm thickness. On the other hand, one bisphenol A phthalate-based polyarylate resin maintains a 250-J /m (4.7-fdbf/in.) notched l2od impact at both thicknesses. Toughness depends on the stmcture of the part under consideration as well as the plastic employed to make the part. Mechanical properties, like electrical properties, ate also subject to thermal and water-content changes. 

Investigations in the field of shoek eompression of solid materials were originally performed for military purposes. Speeimens sueh as armor were subjected to either projectile impact or explosive detonation, and the severity and character of the resulting damage constituted the experimental data (see, e.g., Helie, 1840). Investigations of this type continue today, and although they certainly have their place, they are now considered more as engineering experiments than scientific research, inasmuch as they do little to illuminate the basic physics and material properties which determine the results of shock-compression events. 

The toughness of a material is a design driver in many structures subjected to impact loading. For those materials that must function under a wide range of temperatures, the temperature dependence of the various material properties is often of primary concern. Other structures are subjected to wear or corrosion, so the resistance of a material to those attacks is an important part of the material choice. Thermal and electrical conductivity can be design drivers for some applications, so materials with proper ranges of behavior for those factors must be chosen. Similarly, the acoustical and thermal insulation characteristics of materials often dictate the choice of materials. 

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