Energy-absorbing mechanism

Independently of the strain rate and the test temperature, the toughness of the material depends on its ability to absorb or dissipate energy, and this requires chain mobility. In the glassy state the energy-absorbing mechanism is related to the p relaxation. Therefore, impact resistance... 

Figure 7.7. Total, elastic, and viscous stress-strain curves for uncrosslinked self-assembled type I collagen fibers.Total (open squares), elastic (filled diamonds), and viscous (filled squares) stress-strain curves for self-assembled uncrosslinked collagen fibers obtained from incremental stress-strain measurements at a strain rate of 10%/min. The fibers were tested immediately after manufacture and were not aged at room temperature. Error bars represent one standard deviation of the mean value for total and viscous stress components. Standard deviations for the elastic stress components are similar to those shown for the total stress but are omitted to present a clearer plot. The straight line for the elastic stress-strain curve closely overlaps the line for the viscous stress-strain curve. Note that the viscous stress-strain curve is above the elastic curve suggesting that viscous sliding is the predominant energy absorbing mechanism for uncrosslinked collagen fibers.

This is a simplification of the process occurring in a curing resin-hardener system and a detailed discussion may be found in Pascault et al (2002), Williams et al (1997) and Inoue (1995). The main parameter that it is important to control in the reactive phase separation is the diameter of the elastomer particle. This is because the toughness of the resulting network is controlled by the energy-absorbing mechanisms such as particle cavitation and rubber bridging of cracks. Also of importance is the limitation of the effect of the rubber dispersed phase on the critical properties of the cured epoxy resin such as the stiffness and Tg. This will be affected by the extent to which the rubber dissolves in the matrix-rich phase. 

Energy absorbing mechanisms in composite structural elements... 

As with tensile and impact behavior of rubber-toughened plastics, a major energy-absorbing mechanism appears to be crazing. Thus, at least qualitatively, low-frequency fatigue behavior of rubber-modified plastics appears to involve the same phenomena as are seen in tensile and impact loadings. 

More than 15 toughening mechanisms have been described for rubbery toughening agents added to epoxies but crazing is not a likely energy absorbing mechanism in these materials. Thirteen... 

Many elastomers have been blended with PBT, but the greatest Interest appears to have developed in the core/shell acrylates [25], [26]. These additives reduce stiffness proportionally with their concentration, but do not much depress heat deflection temperature of the base resin. They do make a major change in notch sensitivity, causing a propagating crack to lose Itself In a welter of crazes and other energy absorbing mechanisms. 

This favors crystallization of the folded chain lamellar type (49), Furthermore the extended chain-helical transformation (alpha beta) offers a unique energy absorbing mechanism which can lead to tough thermoplastic materials. The possibility of grafting reactions on elastomeric backbones has shown how these polyesters can transform ordinary elastomers into thermoplastic elastomers (50, 51). 

From an end use perspective, the objective is simply to convert a stiff but brittle plastic (typified by polystyrene) having useful all-round properties into a tougher material with little increase in cost. From a material science perspective, the modification needs to provide a new energy-absorbing mechanism without giving up other inherent and desirable properties. ... 

---