Rheometry

Two types of rheometers used as laboratory research tools have historically been commercially available: controlled-strain or controlled-stress instruments. In the former, strain—represented by у for shear deformation—is defined as the input test parameter and stress a is the output response based on the geometry of the test material; in the latter, a force (or load) is the controlled parameter and the corresponding deformation is measured. The current state-of-the-art instrumentation allows for both types of control from a single instrument, handles a wide range of samples, and provides a footprint that requires mini­mal laboratory space.

Rheological instruments have the capability to characterize the stress – strain behavior, transient properties such as creep and stress relaxation, and dynamic mechanical behavior of elastomeric materials undergoing oscillatory (cyclic) deformation. The modes of deformation include tensile, compression, bending, torsion, and shear. Therefore, rheometry would appear a suitable test method overall for characterizing the properties of cured elastomers including sealants and adhesives.

Rheometers capable of oscillatory testing provide a tool for assessing the durability of a material in terms of fatigue resistance. Cyclic testing can be conducted under (1) controlled strain (deformation) at low frequencies to simu­late joint movement due to thermal expansion differentials or at intermediate frequencies (1 to 103 Hz) corresponding to seismic events, and (2) controlled stress at frequencies that model hurricane-force wind loads or design pres­sures. Although a sinusoidal waveform, shown schematically in Fig. 1, is the default standard for defining a dynamic deformation cycle, arbitrary wave­forms can be programmed in some instruments including the (1 ) triangular waveform in ASTM C719, (2) square waveform found to relate to real joint movements at constant temperature [8], and (3) trapezoidal waveform being recommended in current RILEM durability test methods [9]. A sinusoidal waveform may also be viewed as a suitable representation of both daily and monthly temperature cycles. Figure 1 depicts an output sinusoidal response lagging the input pulse if the material being tested was not completely elastic.

A rheological instrument [10] can be accessorized or lend itself to be custom-modified to expose a test specimen to a controlled (oxidizing, inert, or pressurized) atmosphere either at a constant temperature or a dynamic range

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FIG. 1—A sinusoidal waveform used in cyclic testing in shear deformation mode where the output response will lag the input pulse if the test material was not completely elastic.

anywhere from -170° C to 1000° C, controlled humidity between 5 ° C to 80° C, total immersion in fluid media, or ultraviolet or visible radiation energy. Rhe­ometers that accommodate disposable geometry fixtures provide the option of defining a desired size for the test specimen, constructing substrates from dif­ferent materials, or allowing test specimens to be cured off-line. Using smaller fixtures will shorten the time required to fully cure and condition one-part RTV sealants, for example.

Experimental