Test geometries

The energy release rate (G) represents adherence and is attributed to a multiplicative combination of interfacial and bulk effects. The interface contributions to the overall adherence are captured by the adhesion energy (Go), which is assumed to be rate-independent and equal to the thermodynamic work of adhesion (IVa)-Additional dissipation occurring within the elastomer is contained in the bulk viscoelastic loss function 0, which is dependent on the crack growth velocity (v) and on temperature (T). The function 0 is therefore substrate surface independent, but test geometry dependent. 

Any rheometric technique involves the simultaneous assessment of force, and deformation and/or rate as a function of temperature. Through the appropriate rheometrical equations, such basic measurements are converted into quantities of rheological interest, for instance, shear or extensional stress and rate in isothermal condition. The rheometrical equations are established by considering the test geometry and type of flow involved, with respect to several hypotheses dealing with the nature of the fluid and the boundary conditions the fluid is generally assumed to be homogeneous and incompressible, and ideal boundaries are considered, for instance, no wall slip. 

The diametral compressive strength has been used to estimate the tensile strength of certain AB cements (Smith, 1968). In this test, the load is applied diametrically across a cylinder of cement. Theoretical consideration of the test geometry shows that for a perfectly brittle material the failure that occurs is tensile in character. The difficulty in applying this test to AB cements is that they are not sufficiently brittle for this to hold true. In particular, the zinc polycarboxylate and glass-ionomer cements show sufficient plastic character to make the relationship between diametral compressive and tensile strength vary between AB cements of different types like the compressive strength test, this test is valid only as a means of comparison between similar materials (Darvell, 1990). 

In order to ascertain these properties in a reproducible manner, very specific test geometry must be used since it is necessary to know the stress distribution at predefined, induced cracks of known length. For example, the measured tensile or compressive strength of a series of glass bar... 

If a disklike specimen is sheared between two end plates by rotation of one over the other to obtain the shear modulus, then at any moderate twist angle the strain (and strain rate) vary along the radius, so only an effective shear modulus is obtained. For better results the upper plate is replaced with a cone of very small angle. Figure 4 shows fche cone-and-plate and two other possible test geometries for making shear measurements. 

Explosives Sensitivity Data. Card-gap and projectile sensitivity, data are presented by Watson (Ref 1) for a wide variety of expl compns tested at the USBurMines laboratories in more or less standard test geometries. The results of both tests are in good agreement in that they provide the same sensitivity ordering fbr different subclasses of expls. Least sensitive were homogeneous liquids that did not exhibit a tendency, to undergo low-velocity detonation, AN-FO (Ammonium Nitrate-Fuel Oil), and most cast military expls. Of intermediate sensitivity were pressed and powdered military expls, cast Pentolite, permissible and nonpetmissible water-based expls, and one commercial two-component expl. The most sensitive were permissible and nonpermissible Dynamites and expls susceptible to low-velocity detonations Refs I) R.W. Watson, 1 Card-Gap and Projectile Impact Sensitivity Measurements, A Compilation , USBurMines Information Circular 1C 8605(1973)... 

It is always very useful to be able to predict at what level of external stress and in which directions the macroscopic yielding will occur under different loading geometry. Mathematically, the aim is to find functions of all stress components which reach their critical values equal to some material properties for all different test geometries. This is mathematically equivalent to derivation of some plastic instability conditions commonly termed as the yield criterion. Historically, the yield criteria derived for metals were appHed to polymers and, later, these criteria have been modified as the knowledge of the differences in deformation behavior of polymers compared to metals has been acquired [20,25,114,115]. 

Multiaxial test geometries and test conditions may be analyzed with reference to three orthogonal principal stresses as shown in Figure 15. 

Various test geometries may be used to determine values of the fracture energy, G,c, and stress-intensity factor, KIc, at the onset of crack growth and the more common ones are illustrated in Fig. 1. 

Fig. 7. Schematic representation of examples of test geometries used. Open systems channel, rotating disc (Hoyt 1972, 1986). Closed systems pipe flow, couette- or searle-systems (Kulicke 1986)...

There are three principal problems relating to the specimen, and test geometry. These are the end effect due to gripping the sample, the alignment of the specimen in the test equipment, and the shape of the specimen itself. We shall deal with each of these separately. 

Wall-slip is not an easy phenomenon to detect. Although in principle, the velocity profile should reveal whether or not the fluid velocity is zero at the stationary wall, in reality determining the velocity profile with sufficient resolution near the wall is very difficult. So alternate means, e.g., checking for viscosity variation with appropriate changes in the test geometry, are also widely used in practice. 

As a first test, geometries and vibrational spectra of known complexes such as Ni(C0)4, Fe(C0)5, Cr(C0)6 were optimized and their IR frequencies calculated. Table 1 reports the calculated CO and MC bond lengths and the vco ofNi(CO)5, Cr(C0)6 and Fe(CO)s in comparison with experimental data. 

The usefulness and limitations of standard testing methods should be understood clearly before a testing program is established. The end user must choose the test geometry, procedure, and methodology that best serve the application. To do this, one needs to understand the differences in the various test methods and the outside parameters that will affect the data. Once the advantages and limitations of the various standard tests are understood, the end user may find it necessary to devise his or her own methods to test specific combinations of loads and environments that are anticipated. 

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