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Tham khảo tài liệu 'dynamic mechanical analysis part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | FIGURE 7.1 Methods of obtaining frequency information include a sequential forced frequency runs b free resonance decay and c complex waveforms. All can be used to generate the same types of plots. Viscosity Pa sec 1999 CRC Press LLC Log af Shifted Frequency Fitted data from DMA Multiplex after superposition Data taken from a series of Creep runs and fitted FIGURE 7.2 Frequency data from both a creep test and a DMA frequency scan for a medical-grade polyurethane. Note that the creep data does not lie on the same curve as the frequency data obtained from a DMA run. dynamic scan. This is not always true as shown in Figure 7.2 for polyurethane. However often the nature of the material is simple enough that this does work. 7.2 FREQUENCY AFFECTS ON MATERIALS If you remember back in Chapter 2 we discussed how a fluid or polymer melt response is to strain rate Figure 7.3 rather than to the amount of stress applied. Thu viscosity is one of the main reason why people run frequency scans. As the ftress-strain curves and the creep-recovery runs show Figure 7.4 viscoelastic materials exhibit some degree of flow or unrecoverable deformation. The effect is ftrongest in melts and liquids where frequency vs. viscosity plots are the major application of DMA. Figure 7.5 shows a frequency scan on a viscoelastic material. In this example tho sample is a rubber above the T in fhree-point bending but the trends and principles we discuss will apply to both solids and melts. We have plotted the storage modulus and complex viscosity on log scales against the log of frequency. Let s exgmine the curve and see what it tells US. First of all we should note that in analyzing the Orequency scans we will be looking at trends and changes in the data not for specific peaks or transitions. s toting with the viscosity curve r o we see at low frequency a fairly flat region called the zero shear plateau.iThis is where the polymer exhibits Newtonian behavior end its viscosity is dependent on MW not the