Anisotropy ultrasonic properties

Laser induced phonon spectroscopy has been used to study the ultrasonic properties of nematic sidechain LCEs [47, 48], It has been shown that both the ultrasonic velocity and the attenuation show a strong anisotropy and it was concluded that the coupling between density changes and reorientation-al motion of the sidechains takes place on a time scale of 10" -10" s [47]. 

In addition to the anisotropy and heterogeneity described above, wood is a highly attenuative, dispersive material. A brief summary of relevant ultrasonic properties is presented here. A more complete coverage can be found in comprehensive treatments such as Bucur [1995]. 

Elastic Properties [1.30,1.31,1.35]. In regard to elasticity, at least below room temperature, tungsten behaves nearly isotropically the anisotropy coefficient at 24 °C is = 1.010 [1.35]. The elastic constants for polycrystalline tungsten at 20 °C are given below. Their temperature dependence as well as the respective values for singlecrystal elastic constants are shown in Fig. 1.10 [1.40], based on ultrasonic measurements [1.30,1.31]. 

Treatments of deteriorated wood, which are often indispensable, especially for waterlogged wood, must be tailored to the particular properties and state of deterioration of each object (44). Therefore, it becomes highly desirable to find appropriate methods of nondestructive evaluation. Some preliminary experiments with ultrasonic pulse measurements appeared to be sensitive to the loss in degree of anisotropy accompanying loss of crystalline structure in cellulose (45), However, most published efforts appear to have been focused entirely on hardness measurements 41, 46-48),... 

It is interesting to note that haversian bones, whether human or bovine, have both their compressive and shear anisotropy factors considerably lower than the respective values for plexiform bone. Thus, not only is plexiform bone both stiffer and more rigid than haversian bone, it is also more anisotropic. These two scalar anisotropy quantities also provide a means of assessing whether there is the possibility either of systematic errors in the measurements or artifacts in the modeling of the elastic properties of hard tissues. This is determined when the values of Ac (%) and/or As (%) are much greater than the close range of lower values obtained by calculations on a variety of different ultrasonic measurements (Table 47.5). A possible example of this is the value of As (%) = 7.88 calculated from the mechanical testing data of Knets [1978], Table 47.2. 

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