Quasistatic tests

The elastic modulus by this dynamic method is slightly higher than by load extension, a quasistatic method. Tests involving elastic deformations, where either stress or strain is held constant, are called static tests. In quasistatic tests, stress or strain is changed slowly with time, and in dynamic tests, stress and/or strain are varied rapidly with time. 

Surprisingly, this geometry is not commonly found for quasistatic tests on plastics but has been standardized for dynamic tests. 

Figure 10.17 Subfloor of frame structure showing final damage at end of each test, (a) Quasistatic test, (b) dynamic 1, (c) dynamic 2.

FIGURE 24.4. The machine for quasistatic testing—inciuding tension, compression, 3-point bending and/or 4-point bending. 

In Fig. 9.1, the temperature dependences of elasticity modulus E for the studied HDPE have been adduced. As one can see, at comparable testing temperatures E value in case of quasistatic tests is about twice smaller, than in impact ones. Let us note that this distinction is not due to tests type. As it has been noted above, for HDPE with the same ciystalUnity degree at r=293 K E value can reach 1252 MPa [10]. Let us consider the physical grounds of this discrepancy. The value of fractal dimension of polymer stiucture, which is its main characteristic, can be determined by several methods appUcation. The first from them uses the following equation [11] ... 

Calculation according to the equations (6) and (7) has shown that in case of HDPE quasistatic tests value is close to zero and in case of impact tests % =0.400-0.146 within the range of 7 =231-333K. Now reinforcement degree of HDPE, considered as hybrid nanocomposite, can be expressed as the ratio EIE. In Fig. 9.3, the dependence EIE zlr) has been adduced (such form of dependence was chosen for... 

In Fig. 14.1, the temperature dependences of elasticity modulus E for the studied HDPE have been adduced. As one can see, at comparable testing temperatures E value in case of quasistatic tests is about twice smaller, than in impact ones. Let us note, tiiat this distinction is not due to tests type. As it has... 

In Table 14.1 the comparison of values, determined by the three indicated methods has been adduced (K change with temperature was estimated according to the data of Ref [7]). As one can see, if in case of impact tests the calculation according to all three indicated methods gives coordinated results, then for quasistatic tests estimation according to the Eqs. (1) and (2) gives elearly understated revalues, especially with appreciation of possible variation of this dimension for nonporous solids (2 <2.95 [11]). 

In Fig. 2.8, the dependences E(yJ and EJvJ for HDPE, received in quasistatic tests, are adduced. The principally different picture in comparison with PC and PAr behavior zXTrelaxation processes occurrence is expressed by the plot EJy slope change in comparison with E(y. If equality to zero of values E and E at = 0 is... 

The Eq. (12.1) assumes that the apphed in microhardness tests pressure under the indentor is higher than the 5deld stress in quasistatic tests owing... 

The relationship (9.24) implies, that the exerted in microhardness tests pressure under indentor is higher than yield stress in quasistatic tests owing to restriction, imposed by undeformed polymer, surroimding indentor. However, in works [12, 16, 22, 27, 28] it has been shown that the value c can differ essentially from 3 and varied in wide enough limits 1.5-30. In the work [28] it has been found out, that for the composites HOPE/CaCO depending on strain rate s and type of quasistatic tests, in which the value Oj, was determined (tensile or compression) c magnitude varies within the limits of 1.80-5.83. To c=3 the ratio HJOy approaches only at minimum value s and at using Oy values, received by compression tests. Therefore, in the work [28] the conclusion has been obtained, that the value c=3 can be received only at comparable strain rates in microhardness and quasistatic tests and at interfacial boundaries polymer-filler failure absence. 

Table 3.3. Comparison of results from torsion pendulum and from tensile tests (quasistatic) at T = 500 K...

Indentation hardness determinations were performed in dynamic mode ( 1500 mm/sec impact speed) using a pendulum impact device and in quasistatic mode ( 0.008 mm/sec impact speed) with a custom-built indentation tester. The spherical indenters were of 2.54 cm diameter and 65.6 g mass, and the pendulum length was 92.3 cm with a release angle of 30°. Quasistatic indentation forces were selected to produce indentations of a similar size to the dynamic indentation test (1.5 to 2.0 mm radius). The compact indentations were measured using a white light interferometer (Zygo Corporation, Middlefield, Connecticut, U.S.A.) and the dent depth, dent diameter, apparent radius of curvature, and pendulum initial and rebound heights were used to calculate the indentation hardness of the compacts. 

First, the role of rubber modification in high rate impact is to suppress spallation by inducing the material to yield in the presence of dynamic tensile stresses arising from impact. Second, this yield-spall transition occurs at different strain rates for different rubber contents and may be predictable using quasistatic, low temperature tests of this type. These tests can also provide information concerning the basic nature of the yield process in these materials through the activation parameters which are obtained. Third, the Bauwens-Crowet equation seems to be a good model for the rate and temperature sensitive behavior of the American Cyanamid materials and is therefore a likely candidate for a yield criterion to use in the analytical code work on these materials which we hope to perform as a continuation of this work. 

Figure 8.6 Double logarithmic plot of delamination rate 6a/dN versus Gin,ax and Gnnuix. respectively, from cyclic fatigue (fat) with an R-ratio of 0.1 of carbon fibre epoxy (IM7/977-2) under mode I and mode II from ESIS TC4 round robins (2009, 2012), and under fixed-ratio mixed mode I/II from preliminary single-specimen testing (black dots) at the author s laboratory expected values for fixed-ratio mixed mode VYL (mixed mode I/n fat expect) were calculated from cyclic fatigue mode I and mode H round-robin data (2009 and 2012, respectively), quasistatic (static) and cyclic fatigue mode I and mode H values from literature [39] (nsr = no shear reversal sr = with shear reversal), and earlier cyclic fatigue mode H [60] are shown for comparison.

The Fitch method [50] and its various modifications [31] are the most common quasistatic techniques used to measure the thermal conductivity. The main advantage of this method is that the test is simple and can be carried out in 10 min. For absolute measurements, however, the accuracy is rather low. Figure 27.4 shows a modified Fitch apparatus [51]. The sliced sample is placed between two copper plates. One plate acts as a heat source and the other plate as a heat sink. The thermal conductivity is calculated by Equation 27.12, which is the solution of the governing differential equation for the temperature field within the sample [51]. 

A wide variety of analogous experiments with different conditions of pressure, velocities, surface characteristics, and temperature are required to fully characterize the friction phenomenon at variable sliding speeds. Various research techniques are available for the study of dynamic friction such as pressure-shear plate impact frictional experiments, Split-Hopkinson bar method, pin-on-disk tests, and others, ranging from quasistatic conditions to very low sliding velocities. At low velocities (Vc < 1 m/s), for steel-on-steel non-lubricated contact of sliding surfaces, the friction coefficient demonstrates a strong... 

Part of the work performed on a sample will be converted irreversibly into random thermal motion by movement of the molecules or molecule segments. This loss passes through a maximum at the appropriate transition temperature or relaxation frequency in the associated alternating mechanical field (torsion pendulum test). A similar effect is obtained by the delayed response of the dipoles with dielectric measurements. Therefore, dielectric measurements can be made only on polar polymers. According to the frequency used, the glass-transition temperatures measured with dynamic methods lie higher than those obtained by quasistatic methods (see Section 10.5.2). 

We have already referred to various kinds of data on mechanical behavior of polymers. We are now going to consider methods of acquisition of such information. The most fi equently used are the so-called quasistatic methods which involve relatively slow loading. Tension, compression, and flexure belong here. The quasistatic methods have to be distinguished from so-called transient tests which include stress relaxation and creep. There are also impact tests and dynamic mechanical procedures which will be defined later. 

Uniaxial and triaxial compression tests of silicon-carbide ceramics under quasistatic loading conditions were performed by Brannon et al. [33]. Their SiC-N specimens were prepared in the form of a right circular cylinder, as is indicated schematically in Fig. 1.43a next to the experimental set-up. 

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