Mechanical energy absorption

The ability to modify their structure and the good cost/ performance ratio makes polyolefins technically and commercially attractive for mechanical energy absorption. This is especially true for High Melt Strength (HMS) PP which allows continuous extrusion foaming. Physical expansion of PP, properties of foamed PP, and application examples are considered in detail, mechanical properties in particular being compared with other polymer foams. 

These two examples illustrate two particular situations where strain rate effects are similar to temperature effects, the spurious hysteretic heating having been isolated. In most cases, strain rate effects (strain hardening, increase of the mechanical properties with speed) are mixed with heating effects due to mechanical energy absorption (softening, decrease of the material properties with temperature). 

The main drawback of epoxy thermosets is its brittleness. To solve this problem, they are commonly modified with reinforcements of different nature (elastomers, thermoplastics, inorganic p>articles), geometry (particles, fibers, platelets) and size (micro and nano) which provide additional mechanical energy absorption mechanisms. The dispersion of a second phase can be obtained using mainly two strategies (Pascault et al., 2002) ... 

A. Addeo, Mechanical Energy Absorption by Plastic Foam, Sitev Forum, Geneve 15-18 May 1990. 

Mechanical energy absorption and unbalanced heat release are related, respectively, to stretching and relaxation of serial and transverse elastic elements. Thus, during isotonic shortening, the heat released per unit length due to ongoing relaxation of transverse elastic elements depends on the load. This heat is quantitatively related to the so-called heat of shortening [86]. 

The basic difference between such a relaxation process and a phase transition such as that discussed in this chapter consists in the frequency dependence of the temperature at which the relaxation processes occur, and in the temperature invariance of phase transition. Moreover, contrary to a relaxation process, which gives rise to a maximum in the mechanical energy absorption at a particular combination of frequency and temperature, a phase transition of the order-disorder type causes just a step change in the energy absorption at... 

The dramatic change in the local movement of polymer chains at Tg leads to large changes in a host of physical properties. These properties include density, specific heat, mechanical modulus, mechanical energy absorption, dielectric coefficients, acoustical properties, viscosity, and the rate of gas or liquid diffusion through the polymer, to name a few. Any of these properties can be used, at least in a crude manner, to determine T ... 

The electromagnetic spectrum measures the absorption of radiation energy as a function of the frequency of the radiation. The loss spectrum measures the absorption of mechanical energy as a function of the frequency of the stress-strain oscillation. 

In silicone adhesives used to bond structural glazing assemblies, the silicone network is made of very long PDMS chains and is filled with silica that improves the elastomeric properties of the adhesive. The strength of such an adhesive is strongly enhanced through various mechanisms of energy absorption. 

Dielectric loss The dielectric loss factor represents energy that is lost to the insulator as a result of its being subjected to alternating current (AC) fields. The effect is caused by the rotation of dipoles in the plastic structure and by the displacement effects in the plastic chain caused by the electrical fields. The frictional effects cause energy absorption and the effect is analogous to the mechanical hysteresis effects except that the motion of the material is field induced instead of mechanically induced. 

In short fiber composites, energy absorption mechanisms, such as interfacial debonding and matrix cracking, most often occur at the fiber ends (Curtis et al., 1978). The damage model proposed by Bader et al. (1979) assumes that short fiber composites fail over a critical cross-section which has been weakened by the accumulation of cracks, since the short fibers bridging this critical zone are unable to carry the load. In fatigue loading, sudden fracture takes place as a direct result from the far-field effect of the composite, rather than due to the near field of the crack tip... 

Several such failure mechanisms may take place simultaneously in a toughened resin, depending on the type of particles, whether liquid rubber or rigid particles, and the matrix material. Each of these mechanisms contributes to the energy absorption of the whole structure. 

Chemical reaction that is induced by the direct absorption of mechanical energy. 

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