Tensile stress necking

FFs that are parameterized for high-pressure conditions can still lead to behavior that differs from that observed in experiments. For instance, it is common practice to treat the interatomic interactions with Lennard-Jones (LJ) potentials. Although this method is convenient from a computational standpoint, it is known that LJ potentials do not reproduce experimentally observed behavior such as necking, where a material attempts to minimize surface area and will break under large tensile stresses. Many other examples exist where particular types of FFs cannot reproduce properties of materials, and once again, we emphasize that one should ensure that the FF used in the simulation is sufficiently accurate. 

The tensile stress-strain response of the homopolymer, and of two rubber modified grades of polystyrene, is shown in Fig. 1. The principal mode of deformation is crazing and all three materials exhibit a craze yield stress. However, there is no evidence of localized necking in any of the three materials. The craze yield stress decreases and the elongation to fracture, and the toughness, increase significantly with increase in rubber content. 

Although the uniaxial tension test is the one most widely used, it has two drawbacks when it is used to provide information on the yielding of polymers. First, the tensile stress applied can lead to brittle fracture before yield takes place, and second, yield occurs in an inhomogeneous way due to the formation of a neck accompanying the tensile test. In any case, given that the section of sample decreases as the stress increases, cj cy . 

The plateau value of the tensile stress is almost independent of the stretching rate. After neck propagation, the gel becomes fairly soft and sustains large elongation, up to an elongation strain of around 20. 

The simplest type of stress that can be applied to a polymer is a tensile stress, so the behaviour of polymers under such a stress is described before more general forms of stress are considered. This leads directly to a discussion of necking and cold drawing. 

Figure 11-14. Schematic representation of the tensile stress aw as a function of strain e at constant temperature for an elastomer E, a partially crystalline thermoplast T, and a hard-elastic thermoplast HT. The ductile region is la-II-III. The necking effect shown below the diagram is typical of normal thermoplasts, but does not occur with elastomers or hard-elastic thermpolasts. The diagram is not drawn to scale for example, elastomers show a much larger elongation at break than do thermoplasts.

The response of Ti3SiC2 to tensile stresses is a strong function of temperature and strain rate. Belotv the BPT, they are britde, but above they can be quite plastic, with strains to failure up to 25% in some cases, especially at low strain rates (Figure 7.20) [161]. The deformation occurs without necking, and the majority of the failure strain is due to damage accumulation in the form of cavitations, pores, and microcracks [142,148,162,163]. 

After a hard sample (high PSf) receives a load, brittle fracture occurs without any appreciable deformation and by rapid crack propagation, as shown in Figure 13.45(a). In addition, the direction of crack motion is very nearly perpendicular to the direction of load tensile stress. In the case of a ductile sample (high SPNR), it is shown to neck down to a point fracture, showing almost 100% reduction in area. 

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