Metal 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... 

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. 

In conclusion regarding creep testing, it can be stated that creep data and a stress-strain diagram indicate whether plain plastic properties can lead to practical product dimensions or whether a RP has to be substituted to keep the design within the desired proportions. For long-term product use under continuous load, plastic materials have to consider creep with much greater care than would be the case with metals. 

Figure 5.77 Comparison of idealized stress-strain diagrams for metals, amorphous polymers, and elastomers.

Figure 1.9. Deformation of typical elastoplastic materials (hardened metal and polymer) in the stress-strain diagram (after Guy, 1976).

A typical stress-strain diagram for a metal is shown in Fig. 11. This metal follows Hook s law up to a proportional limit ox yield strength) of 2 x 109 Pa. The elastic limit, above which the metal undergoes plastic deformation, which is not recoverable when the stress is removed, is close to the proportional limit. The maximum stress that the metal can support is the ultimate strength (or tensile strength) of the metal, which occurs at the maximum extension of the material. 

Nonmetallic substances show a wide variety of stress-strain diagrams, with each type related to the bonding of the particular material. One type, that for an elastomeric rubber, is shown in Fig. 12. Compared to a metal, the rubber can support much larger strains and only much smaller stresses. It follows Hook s law only as a limit at very small displacements. However, displacement is elastic well outside of this range. 

Figure 11 A typical stress-strain diagram for a metal.

The appearance of a permanent set is said to mark a yield point, which indicates the upper limit of usefiilness for any material. Unlike some metals, in particular, the ferrous alloys, the drop-of-beam effect and a sharp knee in the stress-strain diagram are not exhibited by plastics. An arbitrary yield point is usually assigned to them. Typical of these arbitrary values is the 0.2% or the 1% offset yield stress (Figure 3.3a). 

Engineering materials are generally referred to as metallic and nonmetallic (ceramics and high pol5nners) materials, which are further classified as ductile or brittle. As shown in the stress-strain diagram in Figure 1.1, the strain of ductile materials is 100-1000 times larger than that of brittle materials. The... 

If the failure strain in the matrix is larger than in the fibre, the fibres fracture before the matrix fails. This is frequently the case in composites with metallic or polymeric matrix. Figure 9.3 shows the resulting stress-strain diagram. It is assumed that the matrix yields plastically before the fibre breaks. The material deforms elastically until the matrix yields. On further increasing the strain, the strengthening fibres fracture, and the stress-strain curve drops to a small stress value that lies below that of the pure matrix material because of the reduced volume. Eventually, failure by fracturing of the matrix occurs. The fracture strain is smaller than in a pure matrix material. This is due... 

Figure 3-6. An example of tensile stress-strain diagrams for some metals.

Above their Tg, semicrystalline polymers at low strains have a metal-like behavior as may be seen in Figure 9.8. The linear portion of their stress-strain diagram results from... 

Stress-strain diagram comparing the performance of a bulk metal glass with other high strength materials. Data for these plots were taken from Table 9.1. 

Prestressing steel (see typical stress-strain diagram in Fig. 3) is used in external tendons (posttensioning) in the form of strands or wires or bundle of monostrands (inside corrugated metal or plastic ducts or inside smooth steel or plastic pipes) in concrete stmctures. 

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

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