Stress/strain characteristics

Areas where geotextdes are used as reinforcement include embankment constmction, reinforced sod wad. constmction, and slope improvement. In reinforcement, the physical properties of importance are primarily related to strength, that is, a combination of the stress—strain characteristics of the material. 

As a starting point it is useful to plot the relationship between shear stress and shear rate as shown in Fig. 5.1 since this is similar to the stress-strain characteristics for a solid. However, in practice it is often more convenient to rearrange the variables and plot viscosity against strain rate as shown in Fig. 5.2. Logarithmic scales are common so that several decades of stress and viscosity can be included. Fig. 5.2 also illustrates the effect of temperature on the viscosity of polymer melts. 

MFI of the composition to that of the matrix, as a function of the filler concentration. It can be seen that, as the concentration of a particular filler increases, the index increases too for one matrix but decreases for another, and varies by a curve with an extremum for a third one. Even for one and the same polymerfiller system and a fixed concentration of filler, the stress-strain characteristics, such as ultimate stress, may, depending on the testing conditions (temperature, rate of deformation, etc.) be either higher or lower than in the reference polymer sample [36],... 

Agglomeration also has a strong impact on the stress/strain characteristics of composites. This may be exemplified by the data of Table 6 borrowed from [215]. 

Standard ASTM D 638 states that it is correct to apply the term modulus of elasticity to describe the stiffness or rigidity of a plastic where its stress-strain characteristics depend on such factors as the stress or strain rate, the temperature, and its previous history... 

The designer can use several approaches to prevent hysteresis failure. The first is material selection. The stiffer the material is, the smaller the strain is for a given stress level and the lower the hysteresis loss per cycle. Some materials are additionally fairly linear in stress-strain characteristics and have smaller hysteresis loops. These would be preferred in dynamic loading applications. 

Designing products is usually performed based on experience since most products only require a practical approach (Fig. 1-4). Experience is also used in producing new and complex shaped products usually with the required analytical evaluation that involves stress-strain characteristics of the plastic materials. Testing of prototypes and/or preliminary production products to meet performance requirements is a very viable approach used by many. 

FIGURE 4.3 Stress-strain characteristics of ethylene-vinyl acetate (EVA)-expanded graphite (EG) nanocomposites at different loadings. (From George, J.J. and Bhowmick, A.K., J. Mater. Sci., 43, 702, 2008. Courtesy of Springer.)... 

Typical compositions of polymeric GMs are depicted in Table 26.3. As the table shows, the membranes contain various admixtures such as oils and fillers that are added to aid the manufacturing of the FML but may affect future performance. In addition, many polymer FMLs will cure once installed, and the strength and elongation characteristics of certain FMLs will change with time. It is important therefore to select polymers for FML construction with care. Chemical compatibility, manufacturing considerations, stress-strain characteristics, survivability, and permeability are some of the key issues that must be considered. 

ISO 844 2001 Rigid cellular plastics - Determination of compression properties ISO 3386-1 1986 Polymeric materials, cellular flexible - Determination of stress-strain characteristics in compression - Part 1 Low-density materials ISO 3386-2 1997 Flexible cellular polymeric materials - Determination of stress-strain characteristics in compression - Part 2 High-density materials ISO 5893 2002 Rubber and plastics test equipment - Tensile, flexural and compression types (constant rate of traverse) - Specification ISO 7743 2004 Rubber, vulcanized or thermoplastic - Determination of compression stress-strain properties... 

Stress-strain characteristics. Linear chain polymers are quite flexible and subject to creep or stretch. Branching or rings in the backbone have a stiffening effect. For example ... 

Whilst it is generally held that an extensometer is necessary, it would be rather less expensive if elongation of dumbbells could be obtained from crosshead movement. Tay and Teoh76 devised a numerical scheme whereby the stress strain characteristics could be derived from measured load versus total elongation data from a finite element analysis of the dumb-bell shape. Their method was shown to work to within 10% of values measured with an infra red extensometer for two fairly soft plastics and a silicone rubber. To be effective, the tensile test must be carried out with grips which essentially prevent any slippage and it is, of course, necessary to have the computing facility set up to carry out the analysis. 

The stress-strain characteristics of the two types are shown in Fig. 7 which indicates that the curves to failure are almost linear. It has been suggested 81 > that for a large part of the curve, the strain is mainly brought about by elongation of the crystal lattice... 

Our last example of the mechanical properties of a protein is that of keratin found in the top layer of skin. The stratum corneum in skin is almost exclusively made up of different keratins that have an a-helical structure. The helices do not run continuously along the molecule so the structure is not ideal. However, the stress-strain characteristics are shown in Figure 6.4 and demonstrate that at low moisture content the stress-strain curve for keratins in skin is approximately linear with a UTS of about 1.8 GPa and a modulus of about 120 MPa. These values are between the values reported for elastin and silk, which is consistent with the axial rise per amino acid being 0.15 nm for the a helix. Thus the a helix with an intermediate value of the axial rise per amino acid residue has an intermediate value of the... 

In order to evaluate the variation in the stress-strain characteristics for a given bamboo culm, a Pennsylvania bamboo was used for this study. All Pennsylvania bamboo was obtained in the vicinity of Lehigh University. The diameter of the bamboo culm varies from 0.5 to 2 in. (1.27-5.08 cm). The length is approximately 10 to 15 ft (3.04-4.56 m) and is classified as A tecta bamboo (10). Figure 5 shows the bamboo specimens used for the compression tests. Two strain gages were placed on the... 

In addition to those standardized tests, two other test methods, monotonic creep and microhardness, have been developed by Hough and Wright [48]. In the monotonic creep test, the strain response to a constant stress rate is monitored. The deviation of the stress-strain characteristics in air and in the fluid of interest is taken to be the initiation of ESC. This method is shown to differentiate to a high resolution between polymers, and in the short term, the ESCR of polymer/fluid pairs that exhibit mild/weak interactions can be distinguished. The microhardness method, in which a pyramidal diamond indentor is pressed into the surface of the polymer component at a known load and for a known time, has the potential for mass screening of plastic/fluid compatibility, including extraction as well as absorption, and should be of interest to polymer suppliers. 

The textural implications of the above characteristics of the stress-strain relationships are not always clear. When one examines a bread loaf or a roll with the fingers to evaluate its freshness, it seems obvious that the perceived mechanical stimulus is associated with the first region of the curve. Yet, in mastication, the compact s resistance to tearing probably plays a more significant role than the first and second stages of the compression. At the point where the bread crumb is tom, however, the specimen may have already been wetted by saliva so that the relationship between the stress-strain characteristics of the dry sponge and its perceived textural properties is usually obscured. 

If we now look at the stress/strain characteristics of polymers (Figure 13-20), shown purely schematically just to give you a feel... 

Polymeric materials show a wide range of stress-strain characteristics. One characteristic of polymers that is markedly different from metals and ceramics is that their mechanical properties are highly time- and temperature-dependent. An elastomer or a rubbery polymer shows a stress-strain curve that is nonlinear. 

Stress-strain curves for PDMS, containing different types of Aerosil, are compared in Fig. 14. As can be seen, the total interfacial area between Aerosil particles and elastomer matrix and/or the amount of filler and its adsorption ability are of great importance for the improvement of stress-strain characteristics,... 

A molecular model is proposed for the explanation of the temperature dependence of stress-strain characteristics such as, e.g., the modulus, the stress-softening and the tensile streilgth at break for filled PDMS. The model emphasizes the importance of the following molecular parameters ... 

The stress-strain characteristics of alkali-treated wools are variable (Satlow, 1959). Probably two main reactions affect the stiffness of the fibers conversion of disulfide bonds to thioether linkages and disruption of cystine residues to form a-aminoacrylic acid residues (Cuthbertson and Phillips, 1945). 

Fig. 4. Different stress-strain characteristics of siloxane-urea block copolymers with different silicone contents.

From previous statistical studies on similar systems which Include the combination of errors Involved in sample preparation and measurement of static compressive stress-strain characteristics, the following reproducibility results were obtained (4,5, ) ... 

---