Shear cyclic strain tests

Most of the rubbers are deformed dynamically and specified dynamic properties are required. Therefore the effect of strain amplitude on the dynamic modulus was observed very intensively. The modulus of filled mbbers decreases with increasing applied dynamic strain up to intermediate amplitudes. A detailed smdy of the low frequency dynamic properties of filled natural rubber was carried out by Fletcher and Gent [71] and was later extended by Payne [72, 73]. In cyclic strain tests the shear modulus can be simply expressed as a complex modulus, G = G + iG" where G is the in-phase modulus and G" the out-of-phase modulus. The phase angle 8 is given by, tan 5 = G"/G. ... 

Laboratory testing methods are presently used on almost an exclusive basis to establish the requisite information for seafloor soils. These testing methods typically include the cyclic triaxial test (ASTM D5311), the cyclic simple shear, and the resonant column (ASTM D4015) techniques. The range of strain levels produced by these test apparatus are... 

A series of cyclic simple shear strain tests were performed on partially saturated sand specimens prepared at degrees of saturatirm ranging from S = 40 % to S = 90 %. The effects of the main parameters degree of saturation (S), relative density (D,.), shear strain (y) , on the maximum excess pore water pressure generation were evaluated. Consequently, based on the test results, an empirical model was developed, which predicts maximum excess pore water pressure ratios in partially saturated sand specimens. 

Krieger has argued that the fatigue life of structural adhesives cannot be usefully predicted by thick adherent lap shear specimens.He has described a test sample configuration more appropriate for determining the durability of skin-doubler assemblies. The skin-doubler specimens proposed were found to survive peak cyclic strains that produced failures with conventional thick adherent specimens. 

Finally, laboratory tests able to measure the dynamic soil properties at high strain levels have been derived from conventional tests by adding cyclic loading capabilities to the testing apparatus. Examples are represented by cyclic triaxial tests with local strain measurements and cyclic simple shear tests. 

Tyres are very definitely fatigued during use and, as mentioned for fabric/rubber adhesion above, it is very important to carry out dynamic tests to assess bond efficiency. Methods have not apparently been standardised but a variety of procedures have been reported71 79 Some workers have used the same or a similar test piece as in static tests and applied a cyclic tensile stress or strain, whilst others have used some form of fatigue tester operating in compression/shear to repeatedly stress or strain cord/rubber composite, or even to flex samples in the form of a belt. Khromov and Lazareva80 describe a method using test pieces cut from tyres. 

Experiments have also shown that the threshold shear strain increases with plasticity index (PI). The volumetric threshold shear strain of a clay with PI = 50 is approximately an order of magnitude greater than that of a sand with PI = 0 (Vucetric, 1994). Experimental evidence from resonant column (RC) tests also shows that soils exhibit linear elastic behavior below a linear cyclic threshold shear strain, yd, that is approximately 30 times smaller. 

Controlled strain (or, more properly, controlled displacement) oscillatory shear instruments, exemphfied by the Weissenberg Rheogoniometer [Macsporran and Spiers, 1982] readily facilitate tests in which independent measurements of both changing length scales and time scales of applied deformation can be performed. In this way it is, in principle, possible to separate effects due to strain and strain rates as the frequency of oscillation may be held constant while the maximiun (cyclic) shear strain amplitude is varied. Alternatively, the frequency of deformation can be varied at constant maximum shear strain amplitude. 

A special liquefaction box was designed and built in which fully and partially saturated sand specimens can be prepared and tested under uniform cyclic simple shear strains induced through a shaking table. The liquefaction box, named Cyclic Simple Shear Liquefaction Box (CSSLB), can accommodate an integrated set of instrumentation, minimize the sidewall boundary effects and induce uniform shear... 

Fig. 21.4 (a) Partially saturated sand specimen tested in Cyclic Simple Shear Liquefaction Box (CSSLB) (b) A micro picture of air induced sand specimen (S = 80 %) (Gokyer 2009) (c) Simple shear strain record applied using the shaking table (for shear strain amplitude is 0.1 %)... 

A parametric study was performed on experimental results to estimate individual, as well as cooperate effects of S, and y on rmnax- Test results showed that r nax depends significantly on the degree of saturation (S) and to a lesser extent on relative density (Dr) and amplitude of the cyclic shear strain (y). Ultimately, a mathematical model was developed to predict maximum excess pore water pressure ratios (r nax) in partially saturated sands. The r max model function (Eqs. 21.1,21.2,21.3,21.4 and 21.5) was obtained by the product of a base function fb and scaling factor functions Fp and F. Details on the formulation of these functions and estimation of model parameters and their statistics are presented in (EseUer-Bayat 2009). 

The challenges inherent in the measurement of stress-strain response of thin film materials by means of direct tensile testing are commonly more than offset by the distinct advantage that properties characterizing deformation resistance of the material in the plastic range can be determined under isothermal conditions for a relatively simple state of stress on the specimen. However, these techniques are not readily amenable to modifications that can accommodate uniaxial compression, simple shear stress, equi-biaxial stress or states stress on the specimen. As a result, it is difficult to draw conclusions concerning the dependence of plastic response on stress path history. It is noted that results for some cyclic tension-compression experiments were reported by Hommel et al. (1999). 

Modulus, dynamic Ratio of stress to strain under cyclic conditions that is calculated from either free or forced vibration tests in shear, compression, or tension. 

Oscillation Test. During an oscillation test, a cyclic shear strain is applied to the sample and resulting shear rate response and time lag of response are measured. From these measurements, viscons and elastic properties of the material can be separated. The elastic modulus G measures the amount of energy stored, whereas the loss modulns G" measures the energy loss dne to viscous dissipation. Figure 8.83 is an illustration of G, G", and r] as a function of freqnency for a Ag paste. The viscoelastic properties of materials are further... 

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