Low-strain tests

Low-strain tests that require borings include down-hole test, up-hole test, cross hole test and suspension logger test. Among these methods, cross-hole test is widely used to measure the... 

Fig. 8.95 Time to failure ratios from constant-deflection rate tests and normalised threshold stresses

At sufficiently low strain, most polymer materials exhibit a linear viscoelastic response and, once the appropriate strain amplitude has been determined through a preliminary strain sweep test, valid frequency sweep tests can be performed. Filled mbber compounds however hardly exhibit a linear viscoelastic response when submitted to harmonic strains and the current practice consists in testing such materials at the lowest permitted strain for satisfactory reproducibility an approach that obviously provides apparent material properties, at best. From a fundamental point of view, for instance in terms of material sciences, such measurements have a limited meaning because theoretical relationships that relate material structure to properties have so far been established only in the linear viscoelastic domain. Nevertheless, experience proves that apparent test results can be well reproducible and related to a number of other viscoelastic effects, including certain processing phenomena. 

Standard geotechnical test reports address typical static properties of soil such as shear strength and bearing capacity but may not provide dynamic properties unless they are specifically requested. In these situations, it is necessary to use the static properties. Dynamic soil properties which are reported may be based on low strain amplitude tests which may or may not be applicable to the situation of interest. Soils reports will generally provide vertical and lateral stiffness values for the foundation type recommended. These can be used along with ultimate bearing capacities to perform a dynamic response calculation of the foundation for the applied blast load. 

Dynamic properties are more relevant than the more usual quasi-static stress-strain tests for any application where the dynamic response is important. For example, the dynamic modulus at low strain may not undergo the same proportionate change as the quasi-static tensile modulus. Dynamic properties are not measured as frequently as they should be simply because of high apparatus costs. However, the introduction of dynamic thermomechanical analysis (DMTA) has greatly widened the availability of dynamic property measurement. 

ASTM E399 Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials , 1999 Annual Book of ASTM Standard Volume 3.01 Metals-Mechanical Testing Elevated and Low-Temperature Tests Metallography, American Society for Testing and Materials, 1999. 

PP bead foams were subjected to oblique impacts (167), in which the material was compressed and sheared. This strain combination could occur when a cycle helmet hit a road surface. The results were compared with simple shear tests at low strain rates and to uniaxial compressive tests at impact strain rates. The observed shear hardening was greatest when there was no imposed density increase and practically zero when the angle of impact was less than 15 degrees. The shear hardening appeared to be a unique function of the main tensile extension ratio and was a polymer contribution, whereas the volumetric hardening was due to the isothermal compression of the cell gas. Eoam material models for FEA needed to be reformulated to consider the physics of the hardening mechanisms, so their... 

The constant-jaw displacement rate test mode is most frequently used. It meets most of the requirements for mass testing such as in the quality control situation, but various test analysis methods have made it quite suitable for design as well as research purposes. Displacement rates from 0.2 to 20 inches/min. are normally used, but some testing equipment provides reasonably controlled rates upwards of 10,000 in./min. Extremely low rate tests are time consuming, of course, but some specialized equipment has been designed to produce strain rates down to 8 X 10 7 inches/min. (1, 4). 

One of the simplest criteria specific to the internal port cracking failure mode is based on the uniaxial strain capability in simple tension. Since the material properties are known to be strain rate- and temperature-dependent, tests are conducted under various conditions, and a failure strain boundary is generated. Strain at rupture is plotted against a variable such as reduced time, and any strain requirement which falls outside of the boundary will lead to rupture, and any condition inside will be considered safe. Ad hoc criteria have been proposed, such as that of Landel (55) in which the failure strain eL is defined as the ratio of the maximum true stress to the initial modulus, where the true stress is defined as the product of the extension ratio and the engineering stress —i.e., breaks down at low strain rates and higher temperatures. Milloway and Wiegand (68) suggested that motor strain should be less than half of the uniaxial tensile strain at failure at 0.74 min.-1. This criterion was based on 41 small motor tests. 

It is unfortunate that test methods for soft plastics and for rubbers, although very similar, are not identical, for example differences in tensile stress strain, tear and hardness methods. If they were aligned, much of debate about which method to use would be eliminated. For some properties, there is a distinct difference in approach. For example, glass transition temperature is frequently determined for plastics whilst various low temperature tests have been specifically developed for rubbers. 

In principle, the shear modulus could be measured using test pieces strained in torsion and in engineering practice components, such as torsion discs and bushes, do operate in this mode. However, it is not common practice to test rubber in this manner except as a low temperature test (see Chapter 15) when a strip test piece is twisted by means of a torsion wire. The instrument traditionally used is not really accurate enough for precise measurement of modulus at room temperature but it would seem reasonable to suppose that an accurate instrument could be devised. 

One approach using deformation in tension is worthy of note. When the deformation at low temperatures is applied repeatedly the apparent modulus becomes lower until an equilibrium level is reached. Eagles and Fletcher20 described a dynamic low temperature test in which the test piece is continuously cycled in tension and the force monitored so that both initial and equilibrium moduli can be calculated. Furthermore, tests could be made at different applied strains. This method could undoubtedly provide more comprehensive precise data but, despite claims of better reproducibility, it was not adopted as a standard method, principally because it involved rather... 

In principle, any of the low temperature tests can be used to study crystallisation effects by conditioning the test pieces at the low temperature for much longer times than is usual. In fact, most of the standard methods include a clause to the effect that the method can be used in this way. In the temperature retraction test, it is suggested that the greater degrees of applied elongation are used when the effects of crystallisation are to be considered, because crystallisation is more rapid in the strained state. 

The theory of sqeezing flow rheometry assumes that the sample is nonelastic. Tests on viscoelastic samples should therefore be carried out at low strain rates, to minimize elastic response, and results should be reported as apparent elongational viscosity. 

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