Mechanical properties stress-strain diagram

As an example, for room-temperature applications most metals can be considered to be truly elastic. When stresses beyond the yield point are permitted in the design, permanent deformation is considered to be a function only of applied load and can be determined directly from the stress-strain diagram. The behavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions. Ignorance of these conditions has resulted in the appearance on the market of plastic products that were improperly designed. Fortunately, product performance has been greatly improved as the amount of technical information on the mechanical properties of plastics has increased in the past half century. More importantly, designers have become more familiar with the behavior of plastics rather than... 

Basics Creep data can be very useful to the designer. In the interest of sound design-procedure, the necessary long-term creep information should be obtained on the perspective specific plastic, under the conditions of product usage (Chapter 5, MECHANICAL PROPERTY, Long-Term Stress Relaxation/Creep). In addition to the creep data, a stress-strain diagram under similar conditions should be obtained. The combined information will provide the basis for calculating the predictability of the plastic performance. 

Creep modeling A stress-strain diagram is a significant source of data for a material. In metals, for example, most of the needed data for mechanical property considerations are obtained from a stress-strain diagram. In plastic, however, the viscoelasticity causes an initial deformation at a specific load and temperature and is followed by a continuous increase in strain under identical test conditions until the product is either dimensionally out of tolerance or fails in rupture as a result of excessive deformation. This type of an occurrence can be explained with the aid of the Maxwell model shown in Fig. 2-24. 

We turn our attention now to some practical aspects of mechanical property determinations. The important quantities such as modulus, strength, and ductility are typically summarized in graphical form on a stress-strain diagram. The details of how the experiment is performed and how the stress-strain diagram is generated are described for some common types of applied forces below. 

Figure 6. Example of the relationship between a typical stress-strain diagram and some mechanical properties. Key A, proportional limit B, ultimate strength a, MOR cr., FSPL Acr/Ae (from origin to A), MOE ...

Stress-strain diagrams are commonly used to represent the mechanical properties of polymers. Stress is defined as the average force per unit of cross section. Strain is defined as the length over unstrained (original) length. Figure 2 shows a few examples of such curves, and the polymers associated with them. 

The production of crystals in a polymer by the action of stress, usually in the form of an elongation. It occurs in fiber spinning, or during rubber elongation, and is responsible for enhanced mechanical properties. Simultaneous readings of load and deformation, converted to stress and strain, plotted as ordinates and abscissas, respectively, to obtain a stress-strain diagram. 

The close relation between the composition and the mechanical properties of these polymers is reflected in the stress-strain diagrams measured at 300 K and 348 K (Figs. 47 and 48). Hence, at ambient temperature for the spedfied experimental conditions a distinct increase of initial modulus (11. 45 and 1 MNm ), stress-hysteresis (ratio of area bounded by a strain cycle to the total area underneath the elongation curve 60,80 and 90 %) and extension set (30,65 and 100 %) can be obsened with increasing hard segment content of polyester urethane (a) to (c). 

Some of the principal mechanical properties of poljrmers are shown on the idealized stress-strain diagram of Fig. 12-1. These include ... 

While the water absorbtion of PPO -PA blend is a straight function of the polyamide content (fig.10), the net effect on the mechanical properties shows major synergism as Indicated in the tensile stress-strain diagram (fig. 11) where the dry as molded and 50 % relative humidity conditioned PP0 /PA blends maintains very close properties. A major drop In tensile strength is observed for Its parent polyamide 66. 

If tests are performed at different constant strain rates or temperatures, stress-strain response similar to that shown in Fig. 3.6 is obtained for many polymers. Notice that modulus and intrinsic yield point vary with both rate and temperature. Also, the stress-strain response appears to be nonlinear even at low stress levels. However, caution on the interpretation of the information obtained from such elementary tests is suggested, as it will be shown in a later section that linearity as well as other essential mechanical properties should be deduced from isochronous stress-strain diagrams. 

Polymers typically behave viseo-elastically, that is their mechanical properties are time and temperature dependent. However, the properties mentioned above are measured almost instantaneously and it is assumed that the material behaves elastieally, or more importantly linear elastic if a Young s modulus is considered. In Figure 7.5, typieal stress-strain diagrams are shown for brittle, plastic and highly elastomeric behaviour, as observed in many synthetic polymers. This behaviour is dependent on temperature as well as the amount of plasticizer (or other additives). ... 

Other differences in the mechanical properties of superfibers are most clearly manifested in their operating characteristics. The stress-strain diagram of Kevlar-29 fibers at different temperatures and rates of deformation is shown in Fig. 10.14 [88]. The strength of the fibers decreases by 14% on average with an increase in the rate of deformation from 0.167 to 8000%/sec. The elongation at break virtually does not change. The shape of the curves is almost linear. 

The effectiveness of such an amorphous structure in blocking dislocations can be seen in Figure 14.10 which compares the elastic portions of the stress-strain diagrams of several alloy systems with VlT-001 or Vitreloy. The mechanical properties of this alloy are compared with other high strength alloys in Table 9.1. 

The influence of strain rate, temperature and load level on mechanical properties has also comprehensively been investigated in stress-strain diagrams. An increasing strain rate results in decreasing retarded deformation parts and thus leads to an increase of modulus and yield strength. These effects can be even more pronounced for materials with high viscous deformation parts like PE. 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

Because all tissues are viscoelastic this means that their mechanical properties are time dependent and their behavior is characterized both by properties of elastic solids and those of viscous liquids. The classic method to characterize a viscoelastic material is to observe the decay of the stress required to hold a sample at a fixed strain (stress relaxation) or by the increasing strain required to hold a sample at a fixed stress (creep) as diagrammed in Figure 7.1 and explained further in Figure 7.2. Viscoelastic materials undergo processes that both store (elastic) and dissipate (viscous)... 

Polymers mechanical properties are some from the most important, since even for polynners of different special purpose functions this properties certain level is required [199], However, polymiers structure complexity and due to this such structure quantitative model absence make it difficult to predict polymiers mechanical properties on the whole diagram stress-strain (o-e) length—fi-om elasticity section up to failure. Nevertheless, the development in the last years of fractal analysis methods in respect to polymeric materials [200] and the cluster model of polymers amorphous state structure [106, 107], operating by the local order notion, allows one to solve this problem with precision, sufficient for practical applications [201]. 

It should be noted that the stress-strain curves of filaments and fibers are nonlinear even at smallest strains when comparing their mechanical properties. Such diagrams also show the characteristic behavioral differences between cotton and rayon which are not obvious from Table 38-2 (Figure 38-14). 

Permanent structural changes that occur in a material subjected to fluctuating stress and strain, which cause decay of mechanical properties. See S-N diagram. The ability of a material to plastically deform before fracturing in constant strain amplitude and low-cycle fatigue tests. See S-N diagram. ... 

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