Previous Definition (Dynamic Properties)- (Group: Materials) In many engineering applications, materials are subjected to dynamic loadings. These may include (1) sudden loads (impacts) or loads that vary rapidly in magnitude; (2) repeated loading and unloading ; or (3) frequent changes in the mode of loading , such as from tension to compression. For such operating conditions , the engineer must be concerned with properties other than those determined by the static tests. (MATERIALS AND PROCESSES IN MANUFACTURING 7th edition E.PAUL DEGARMO P.51)
New Definition (Dynamic Properties)- (Group: Materials) (better) : A material that under sinusoidal stress has some amount of strain at the peak of the sine wave and an angle defining the lag between the stress sine wave and the strain sine wave. All of the other properties for the DMA are calculated from these data. We can first calculate the storage or elastic modulus, E¢. This value is a measure of how elastic the material is and ideally is equivalent to Young’s modulus. This is not true in the real world for several reasons. First, Young’s modulus is normally calculated over a range of stresses and strains, as it is the slope of a line, while the E¢ comes from what can be considered a point on the line. Secondly, the tests are very different, as in the stress–strain test, one material is constantly stretched, whereas it is oscillated in the dynamic test. If we were to bounce a ball, as shown in Figure 4.3, the storage modulus (also called the elastic modulus, the in-phase modulus, and the real modulus) could be related to the amount of energy the ball gives back (how high it bounces). E¢ is calculated as follows:
Once we have calculated the basic properties all the other properties are calculated from them. Table 4.1 shows the calculation of the remaining properties from DMA. Note that complex viscosity has a dependence on the frequency in the denominator so that at 1 Hz, complex viscosity will overlap the complex modulus. Also note that converting E into G or the reverse requires the use of Poisson’s ratio, n. Out of these remaining properties, the most commonly used is the complex viscosity,h*. The biggest reason is that data from frequency scans give a viscosity vs. shear rate (or frequency) curve that can be obtained much faster than by other methods.
DYNAMIC MECHANICAL ANALYSIS Kevin P. Menard CRC Press Boca Raton London New York Washington, D.C.
A Practical Introduction 1999 CRC Press LLC. Chapter 4 (4.2 CALCULATING VARIOUS DYNAMIC PROPERTIES)
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