On mechanical properties

Saunders S R J, Evans FI E and Stringer J A (eds) Workshop on Mechanical Properties of Protective Oxide Scales. Materials at High Temperatures vo 12 (Teddington)... 

The kinetic nature of the glass transition should be clear from the last chapter, where we first identified this transition by a change in the mechanical properties of a sample in very rapid deformations. In that chapter we concluded that molecular motion could simply not keep up with these high-frequency deformations. The complementarity between time and temperature enters the picture in this way. At lower temperatures the motion of molecules becomes more sluggish and equivalent effects on mechanical properties are produced by cooling as by frequency variations. We shall return to an examination of this time-temperature equivalency in Sec. 4.10. First, however, it will be profitable to consider the possibility of a thermodynamic description of the transition which occurs at Tg. 

H. J. Hoffman, Moisture Effects on Mechanical Properties of Solid Propellants, CPTR 84-29, CPIA Pubhcations, Johns Hopkins University, Laurel, Md., Feb. 1984. 

Table 2. Effects of Molecular Weight and Branching on Mechanical Properties of Polyethylenes...

E = no adverse reaction, Htde or no absorption, htde or no effect on mechanical properties G = some effect, some absorption causing slight sweUing, and reduction in mechanical properties and NR = not recommended, material adversely affected in a short time. 

Copolymerisation also affects morphology under other crystallisation conditions. Copolymers ia the form of cast or molded sheets are much more transparent because of the small spheruHte size. In extreme cases, crystallinity cannot be detected optically, but its effect on mechanical properties is pronounced. Before crystallisation, films are soft and mbbery, with low modulus and high elongation. After crystallisation, they are leathery and tough, with higher modulus and lower elongation. 

Wood preservatives ate appHed either from an oil system, such as creosote, petroleum solutions of pentachlorophenol, or copper naphthanate, or a water system. Oil treatments ate relatively inert with wood material, and thus, have Htde effect on mechanical properties. However, most oil treatments require simultaneous thermal treatments, which ate specifically limited in treating standards to preclude strength losses (24). 

C. C. Gerhards andj. M. McMiUen, eds.. Proceedings of the Kesearch Conference on High-Temperature Dying Effects on Mechanical Properties of Softwood Dumber, U.S. Department of Agriculture, Eorest Service, Eorest Products Laboratory, Madison, Wise., 1976. 

J. E. Wiaandy, "Effects of Waterborne Preservative Treatment on Mechanical Properties A Review," ia Proceedings of the 91 st Annual Meeting of the American Wood-Preservers Association, New York, May 21—24, 1995, Vol. 91, AWPA, Woodstock, Md., 1995, pp. 17—33. 

In (2.19), F has been replaced by P because force and pressure are identical for a one-dimensional system. In (2.20), S/m has been replaced by E, the specific internal energy (energy per unit mass). Note that all of these relations are independent of the physical nature of the system of beads and depend only on mechanical properties of the system. These equations are equivalent to (2.1)-(2.3) for the case where Pg = 0. As we saw in the previous section, they are quite general and play a fundamental role in shock-compression studies. 

So far we have concentrated on mechanical properties at room temperature. Many structures - particularly those associated with energy conversion, like turbines, reactors, steam and chemical plant - operate at much higher temperatures. 

As with other plastics materials, temperature has a considerable effect on mechanical properties. This is clearly illustrated in Figure 13.5 in the case of stress to break and elongation at break. Even at 20°C unfilled PTFE has a measurable creep with compression loads as low as 3001bf/in (2.1 MPa). 

It will be observed that molecular weight has little effect on mechanical properties but does influence the flow temperature. 

The figures given in the table are obtained on mouldings relatively free from orientation and tested under closely controlled conditions of temperature, testing rate, and humidity. Changes in these conditions or the use of additives may profoundly affect these properties. Details of the influence of these factors on mechanical properties have been published in the trade literature but Figures 18.11-18.14 have been included to illustrate some salient features. 

The type of reinforeement. In the case of fabric reinforcement, factors such as cloth weight and crimp will have a large effect on mechanical properties. 

An intriguing recent review of size effects in materials due to microstructural and dimensional constraints with a focus on mechanical properties, including those of multilayers, is by Arzt (1998). 

A compatible blend should have good processing properties along with a smooth surface and cross-section. If needed, further tests on mechanical properties should be carried out on testing samples made from 2-mm films produced by compression molding. 

In nonrigid ionomers, such as elastomers in which the Tg is situated below ambient temperature, even greater changes can be produced in tensile properties by increase of ion content. As one example, it has been found that in K-salts of a block copolymer, based on butyl acrylate and sulfonated polystyrene, both the tensile strength and the toughness show a dramatic increase as the ion content is raised to about 6 mol% [10]. Also, in Zn-salts of a butyl acrylate/acrylic acid polymer, the tensile strength as a function of the acrylic acid content was observed to rise from a low value of about 3 MPa for the acid copolymer to a maximum value of about 15 MPa for the ionomer having acrylic acid content of 5 wt% [II]. Other examples of the influence of ion content on mechanical properties of ionomers are cited in a recent review article [7],... 

For partially crystalline ionomers, such as those based on copolymers of ethylene and methacrylic acid, even time or aging at room temperature can have an effect on mechanical properties. For example, upon aging at 23°C, the modulus of the acid form of the copolymer increased 28%, while in the ionomer form, the increase ranged up to 130%, with the specific gain in modulus depending on the degree of conversion and on the counterion that was present [17]. 

Table 4 Effect of Compatibilizer Loading on Mechanical Properties...

Reference No. (BS 3468 1974) Typical composition (%) Heat resistance Effect on mechanical properties Overall... 

Table 18. Effect of molding method on mechanical properties of mica-filled thermoplastics (Vf = 0.5) [375]...

Table 6-17 Example of carbon black on mechanical properties of an ABS...

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