Indicative stiffness modulus results

A variety of hot and cold bituminous mixtures have been tested in the Highway Engineering Laboratory of the Aristotle University of Thessaloniki (AUTh) for stiffness determination by indirect tensile method using the NU ITT device. Some of the results obtained are as shown in Table 7.2. 

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FIGURE 6.26 Retrogradation of a 7% amylose solution. The solution was cooled from 90 to 26°C and then kept for the time indicated. Stiffness (as shear modulus) and melting enthalpy (AH) were monitored arbitrary scales. (After results by M. J. Miles et al. Carbohydr. Res. 135 (1985) 271. 

The results indicated that cross-link formation increased the bulk modulus of the system. As noted, cross-linking was a pressure-induced effect that was facilitated by a change in the coordination at zinc when the pressure reached 5 GPa. The observation that stiffening of the film is a pressure-induced phenomenon is consistent with the differences in the measured elastic properties of films found on the tops of the asperities and those found in the valleys between asperities as mentioned above. Basically, in real systems, pressures high enough to form stiff cross-linked films are achieved on top of the asperities, but they are not encountered between the asperities. 

The mechanical response of composites, as shown in these exploratory studies, indicates dependence on the ease with which fracture can occur between fibers, yarns, and plies. Poorly crystallized matrices result in composites that are strong and stiff but with little yield so that failure occurs catastrophically. In contrast, more crystalline matrices seem to be not quite as strong and to have a lower effective modulus, but their increased strain capability ensures that failure is not catastrophic the composited strength decays gradually as further strain is applied. Thus, the energy required for total failure is increased, and the composite with more crystalline matrix is more tolerant of defects or stress risers. 

A more common mechanical method is dynamic mechanical thermal analysis (DMTA). DMTA is also called dynamic mechanical analysis (DMA) or dynamic thermomechanical analysis. An oscillating force is applied to a sample of material and the resulting displacement of the sample is measured. From this the stiffness of the sample can be determined, and the sample modulus can be calculated. A plot of loss modulus as a function of temperature shows a maximum at Tg as shown in Figure 1.35. Figure 1.35 shows a series of blends of high-impact styrene (HIPS) and PPO. As the amount of PPO is increased, Tg increases. The single Tg indicates that these blends are miscible. 

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