Dynamic mechanical tests experimental

Die Tg can be determined readily only by observing the temperature at which a significant change takes place in a specific electric, mechanical, or physical property. Moreover, the observed temperature can vary significantly, depending on the specific property chosen for observation and on details of the experimental technique (for example, the rate of heating, or frequency). Therefore, the observed Tg should be considered to be only an estimate. The most reliable estimates are normally obtained from the loss peak observed in dynamic mechanical tests or from dilatometric data (ASTM D-20). 

Dynamic Mechanical Spectroscopy. As pointed out above, the heterogeneous morphology, observed in the present study, implies network flaws (weak connections between the primary and secondary microgels). However, the low cyclic strains applied in dynamic mechanical tests detect only the effect of the basic network structure and not the network flaws. Therefore, as observed experimentally (, ) dynamic mechanical spectroscopy should not indicate heterogeneity in the samples. 

The Takayanagi model was developed to account for the viscoelastic relaxation behaviour of two phase polymers, as recorded by dynamic mechanical testing. " It was then extended to treat both isotropic and oriented semi-crystalline polymers. The model does not deal with the development of mechanical anisotropy on drawing, but attempts to account for the viscoelastic behaviour of either an isotropic or a highly oriented polymer in terms of the response of components representing the crystalline and amorphous phases. Hopefully, comparisons between the predictions of the model and experimental results may throw light on the molecular processes occurring. 

Dynamic mechanical testing allows the use of a variety of instrument types and a wide range of experimental conditions. The temperature may range from practically obtainable subambient up to levels where thermal degradation occurs, the frequencies typically from 0.01 to 1,000 Hz. The results should be examined for possible self-resonances. The elastic modulus of the material to be examined may range from 0.1 Jcm to 100Jcm depending on type of polymer, temperature, and frequency. 

Table 5.3 lists the principal experimental methods used in dynamic mechanical testing. Of the experiments considered below, the thermal scan mode (method 1) is the technique most commonly used by thermal analysts. Here typical applications in quality control or processing look for differences in material batches, thermal history, different grades, reactivity, and other characteristics. The stepped isotherm (or step isothermal) experiment (method 2) is used mainly in studies involving detailed mechanical property determination for structural analysis, vibration damping applications, and for determining time-temperature superposition master curves. Method 3 (fast scan or single isotherm) is application specific. 

The dynamic mechanical tests over a wide temperature range are very sensitive to the physical and chemical structure of polymers and composites. They allow the study of glass transitions or secondary transitions and yield information about the morphology of polymers. Experimental results of dynamic tensile tests (DMTA) conducted on nanocomposites are shown in Table 2 for selected temperatures (20 °C, 100 °C, 220 °C, and glass transition, T ). 

The viscoelastic behavior of polymer fibers is complex. Experimentally, it is important to perform simple laboratory tests from which information relevant to actual in-use conditions can be obtained. The viscoelastic characterization of polymer fibers often consists of condncting mechanical tests that are similar to those discussed in Chapter 15, but are modified so as to enable the observation of the time dependency of fibers response. Three most important viscoelastic tests are creep, stress relaxation, and dynamic mechanical testing. 

There are several other comparable rheological experimental methods involving linear viscoelastic behavior. Among them are creep tests (constant stress), dynamic mechanical fatigue tests (forced periodic oscillation), and torsion pendulum tests (free oscillation). Viscoelastic data obtained from any of these techniques must be consistent data from the others. 

Dynamic mechanical measurements for elastomers that cover wide ranges of frequency and temperature are rather scarce. Payne and Scott [12] carried out extensive measurements of /a and /x" for unvulcanized natural mbber as a function of test frequency (Figure 1.8). He showed that the experimental relations at different temperatures could be superposed to yield master curves, as shown in Figure 1.9, using the WLF frequency-temperature equivalence, Equation 1.11. The same shift factors, log Ox. were used for both experimental quantities, /x and /x". Successful superposition in both cases confirms that the dependence of the viscoelastic properties of rubber on frequency and temperature arises from changes in the rate of Brownian motion of molecular segments with temperature. 

Becanse there are many factors involved in the dynamic mechanical compression of polyolefin foams, the Taguchi method was employed in a Perkin Elmer DM A7 dynamic mechanical analyser to establish a method to improve the measurement process. The signal-to-noise ratio was measured to determine how the variability could be improved. Control and noise factors were evaluated and levels chosen, with details being tabulated. Appendix A describes some of the factors. Tests were conducted on two closed cell foams. NA2006 foam is 48 kg/cu m LDPE and NEE3306 foam is 32 kg/cu m EVA. Different factors were shown to influence results for E and tan delta but an optimum combination is proposed for the simultaneous measurement of both properties. The results were less variable as frequency was increased. Small differences in the dynamic response of different materials should be measurable because of the low variability in the experimental results. 18 refs. 

The complex sorption behavior of the water in amine-epoxy thermosets is discussed and related to depression of the mechanical properties. The hypothesized sorption modes and the corresponding mechanisms of plasticization are discussed on the basis of experimental vapor and liquid sorption tests, differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and dynamic mechanical analysis. In particular, two different types of epoxy materials have been chosen low-performance systems of diglycidyl ether of bisphenol-A (DGEBA) cured with linear amines, and high-performance formulations based on aromatic amine-cured tetraglycidyldiamino diphenylmethane (TGDDM) which are commonly used as matrices for carbon fiber composites. 

The phenomenon of such bimodal lifetime distribution proposed for reaction 1 on the basis of direct quasiclassical trajectory calculations were tested experimentally with the reaction of diaza-[2.2.1]bicycloheptane to [2.1. Ojbicyclopentane [Equation (2)].6 8 Experimental study on reaction 2 showed that the exo isomer 5x is formed favorably over the endo isomer 5n by about 3 1 in the gas phase. One explanation for the preferential formation of 5x invokes a competitive concerted and stepwise mechanism the concerted pathway directly from 4 to 5 gives 5x with the inversion of configuration at the carbon from which N2 is departing, whereas the stepwise pathway goes through the radical intermediate and leads to both 5x and 5n in equal amount. Alternatively, the product stereochemistry can be rationalized by dynamic matching between the entrance channel to the cyclopentane-1,3-diyl radical intermediate and the exit channel to bicyclo[2.1. Ojpentane as was assumed for reaction 2. 

Whilst these difficulties do not invalidate application of molecular mechanics methods to such systems, they do mean that the interpretation of the results must be different from what is appropiate for small-molecule systems. For these reasons, the real value of molecular modeling of macromolecule systems emerges when the models are used to make predictions that can be tested experimentally or when the modeling is used as an adjunct to the interpretation of experiments. Alternatively, the relatively crude molecular mechanics models, while not of quantitative value, are an excellent aid to the visualization of problems not readily accessible in any other way. Molecular dynamics is needed, especially for large molecules, to scan the energy surface and find low-energy minima. The combination of computational studies with experimental data can help to assign the structure. 

The main experimental methodology used is to directly characterize the tensile properties of CNTs/polymer composites by conventional pull tests (e.g. with Instron tensile testers). Similarly, dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA) were also applied to investigate the tensile strength and tensile modulus. With these tensile tests, the ultimate tensile strength, tensile modulus and elongation to break of composites can be determined from the tensile strain-stress curve. 

L. R. Deobald and A. S. Kobayashi A Bar Impact Tester for Dynamic Fracture Testing of Ceramics and Ceramic Composites, Experimental Mechanics, 32, 109-116 (1992). 

ASTM (1994) E1640-94. Standard Test Method for Assignment of the Glass Transition Temperature by Dynamic Mechanical Analysis, American Society for Testing and Materials, Philadelphia, PA. Tomblin, )., Salah, L., and Ng, Y. (2001) Determination of Temperature/Moisture Sensitive Composite Properties. DOT-FAA Report DOT/FAA/AR-01/40, Office of Aviation Research, Washington, DC. Bai, Y., Post, N.L, Lesko, J.J., and Keller, T. (2008) Experimental investigations on temperature-dependent thermophysical and mechanical properties of pultmded GFRP composites. Thermochim. Acta, 469, 28-35. 

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