Mechanical properties stress concentration factor

Stress Concentrators and Stress Concentration Factor. Just as all materials, polymer-based materials exhibit structure imperfections of various kinds. They appear already in processing, during handling in transport, as well as in service. Because of the presence of crazes, scratches, cracks, and other imperfections, mechanical properties of real polymeric materials are not as good as they theoretically could be. In this section we deal particularly with stress concentrators such as cracks (which appear although we did not want them) and notches (which are made on purpose to have well-defined cracks). 

The maximum radial stress as a function of the mechanical properties of the components and the geometry are calculated by finite-element modeling. The critical load must be determined experimentally. It may be observed by a kink in the force-displacement curve, by direct optical observation, or by acoustic emission analysis. The calculated stress concentration factors for different curvatures of the interface are given in Table 1. 

Weldline is a serious problem in injection moldings which causes visual defects and reduction of mechanical properties. The main factors leading to the reduction are considered to be poor intermolecular entanglement at the weldline interface, molecular orientation indnced by the fountain flow, and the stress concentration effect of surface V-notch and so on [1-6]. However, in conparison to other factors, there are less papers regarding molecular orientation in weldline region. Only a few methods for detecting molecular orientation have so far been reported, for exanple, inlfared dichroism [7] and observation of birefringence [8]. However,... 

The influence of a stress concentrator on the mechanical properties of injected plastic parts was studied. Polystyrene plaques with different dimensions of a triangular concentrator were injected. Melt temperature, injection and holding/packing pressures and injection speed were modified in order to determine their influence on the stress concentrator factor (Kl). The experimental results were compared with the simulation ones. It was verified that Kt depends on geometrical parameters and process conditions of the injected plastic parts. 

Owing to hydrogen embrittlement, the mechanical properties of metallic and nonmetal-lic materials of containment systems may degrade and fail resulting in leaks. Hydrogen embrittlement depends on many factors such as environmental temperature and pressure, purity of metal, concentration and exposure time to hydrogen, stress state, physical and mechanical properties, microstructure, surface conditions, and the nature of the crack front of material [23]. 

This approach tends to be limited to high strength alloys since these often have mechanical properties that are closest to the ideal required and because of their engineering importance. The type of specimem employed takes into account the stress concentration arising from the presence of a crack in a specimen and employs a measured component K, the stress intensity factor, which is obtained from the applied stress a X c1/2, where c is the crack depth. It has units MN m 3/2. If such specimens are now tested as a function of time-to-failure, the results obtained are of the kind shown in Figure 2. Again, the question arises of a threshold which is such specimens is termed where the subscript I refers to the loading mode( 5). The whole term represents that value of K below... 

Recycled EVA/GRT powder blends of three particle sizes of greater than 200 turn, 200-500 turn, and greater than 500 p,m with concentrations up to 70 wt.% were prepared by using a Brabender mixer (Mujal-Rosas et al., 2011). The stress-strain behavior showed that upon the addition of smaller particles to the matrix up to 10%, the Young s modulus of the blends increased, while other mechanical properties reduced. At the higher concentration of GRT, all mechanical properties decreased. However, conductivity, permittivity, and dielectric loss factor of blends increased with the powder concentration. 

The uniformity and stability of nanotube dispersion in polymer matrices are most important parameters for the performance of composites. A good dispersion leads to efficient load transfer concentration centres in composites and to uniform stress distribution. Many scientists have reviewed the dispersion and functionalisation techniques of CNT for polymer-based nanocomposites, as well as their effects on the properties of CNT/polymer nanocomposites. They demonstrated that the control of these two factors led to uniform dispersion. Overall, the results showed that a proper dispersion enhanced a variety of mechanical properties of nanocomposites. 

One of the most important factors affecting composite properties is the volume percentage of fibres in a composite. Two composites made of the same resin and volume of fibres but different fibre densities will have identical mechanical properties. Also, closely packed fibres decrease fibre spacing, resulting in higher local stress concentrations in the material, and reduce complete penetration of resin into the interstices. 

Studies on randomness of filler distribution in polymethylacrylate nanocomposite are interesting. In this experiment, siUca particles were formed both before and after matrix polymerization. The results indicated that the concentration of silica was a controlling factor in the stress-strain relationship rather than the uniformity of particle distribution. Also, there was no anisotropy of mechanical properties regardless of the sequence of filler formation. This outcome cannot be expected to be duplicated in all other systems. For example, when nickel coated fibers were used in an EMI shielding application." When compounded with polycarbonate resin, fibers had a much worse performance than when a diy blend was prepared first and then incorporated into the polymer (Figure 7.1). In this case, pre-blending protected the fiber from breakage. 

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