The rigid-bond test

The use of more sophisticated scattering models, in which bonding effects on the charge density are taken into account, discussed in chapter 3, leads to a significant improvement in the results of the rigid-bond test. An example, based on a low-temperature analysis of p-nitropyridine-N-oxide, is given in Table 2.3. 

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Some criteria have been suggested in order to decide when a thermal motion analysis can be expected to yield meaningful results. Two of them, the rigid-bond test and the rigid-molecule test, investigate whether or not bonds or portions of molecules are considered to move in a physically reasonable way. They will be described in turn. 

The introduction of standard aspherical-atom scattering factors leads to a very significant improvement in Hirshfeld s rigid bond test. The results are a beautiful confirmation of Hirshfeld s (1992) statement that an accurate set of nuclear coordinates (and thermal parameters ) and a detailed map of the electron density can be obtained, via X-ray diffraction, only jointly and simultaneously, never separately or independently . 

De Hoff PH, Anusavice KJ, Wang Z (1995) Three-dimensional finite element analysis of the shear bond test. Dent Mater 11(2) 126-131 Lee CH, Kobayashi S (1973) New solution to rigid-plastic deformation problems using matrix methods. Trans ASME J Eng Ind 95(3) 865-873 Lewis RW, Ravindran K (2000) Finite element simulation of metal casting. Int J Numer Methods Eng 47(l-3) 29-59 (Special Issue Richard H. Gallagher Memorial Issue)... 

Muller et al. focused on polybead molecules in the united atom approximation as a test system these are chains formed by spherical methylene beads connected by rigid bonds of length 1.53 A. The angle between successive bonds of a chain is also fixed at 112°. The torsion angles around the chain backbone are restricted to three rotational isomeric states, the trans (t) and gauche states (g+ and g ). The three-fold torsional potential energy function introduced [142] in a study of butane was used to calculate the RIS correlation matrix. Second order interactions , reflected in the so-called pentane effect, which almost excludes the consecutive combination of g+g- states (and vice-versa) are taken into account. In analogy to the polyethylene molecule, a standard RIS-model [143] was used to account for the pentane effect. 

Triaxial tests usually involve more elaborate testing equipment and more precise measuring techniques than those normally used for uniaxial and biaxial measurements. In the poker chip test (33, 59) the faces of a thin circular disc are bonded to rigid plates (Figure 18). The speci-... 

It is also necessary for the rigid parts of the pendulum to be sufficiently stiff to avoid spurious vibrations, for the impact to occur at the centre of percussion and for corrections to be made for friction if necessary. The test piece must either be bonded to a backing plate or very firmly clamped by mechanical means or vacuum. 

Very strong bonding between polyurethane and metal is needed in items such as polyurethane mounts. When metal is used as reinforcement or for location and stabilization, a very good bond is needed to form the bridge between the rigid metal and the elastic properties of the polyurethane. Ideally, a test that could be performed on the finished product is desirable, but in practice this is often not the case. 

Polyvinyl Chloride (PVC) With PVC plasticizer migration to the adhesive bond line can cause difficulties, especially in the softer, highly plasticized materials. Adhesives must be tested for their ability to resist the plasticizer. Nitrile-rubber adhesives are resistant to plasticizers. Polyurethanes and neoprenes are also used. Even rigid PVC contains up to 5% plasticizer. Most vinyl materials are fairly easy to... 

Figure 8-4 may be regarded as a direct test of this relation. We see from Eq. (8-22) that the rigidity tends to decrease with the square of bond length, as does Fj, and drops with decreasing covalency. 

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