Cyclic straining, frequency

Young s moduli were determined in tensile tests using samples of 4 mm thickness. Slow cyclic loading (frequency 0.01 Hz) with small strain amplitudes (s < 3%) was used for the tests in order to maintain the thermal equilibrium as much as possible. The temperature range was limited to 260 °C as thermal decomposition became noticeable above this temperature [11],... 

Dynamic Mechanical (Low Strain Deformation). When a cyclic strain of small ampUtude is applied to a strip of material, a cyclic stress will be generated in response by the sample. If the material is ideal (Hookian) and stores all the input energy, the cyclic stress is in phase with the applied cyclic strain. Viscous components cause a finite phase lag or phase angle, 8, between the stress and strain. represents the elastic, real, or storage modulus while E" is the viscous, imaginary, or loss modulus. Tan 8 is equal to the ratio E /E" and is related to the molecular relaxations that occur in the sample. Transition temperatures and associated activation energy can be determined (72) by varying the frequency of oscillation at a fixed temperature or the temperature at a fixed frequency. 

In addition, PLCL scaffolds can be easily twisted and bent. In contrast, PLGA scaffolds largely deform and are broken even at strains as low as 20%. These data indicate that PLCL scaffolds are flexible and highly elastic, whereas PLGA scaffolds are stiff and brittle. To further examine the elastic properties of PLCL scaffolds, scaffolds with varying porosity were subjected to cyclic strain at 10% amplitude and 1 Hz frequency for 27 days in culture medium (Jeong, 2004b). PLCL scaffolds of all tested porosities maintained excellent elasticity even in the hydrolytic medium over a 27-day experimental period. 

Nfp = number of cycles to failure in the field NfL = number of cycles to failure in the lab V = plastic portion of the cychc strain in the field SpL=plastic portion of the cyclic strain in the lab fp = cyclic frequency fp = cychc frequency... 

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. ... 

The case of mechanical vibration is less well defined in terms of the predominance of creep versus time-independent (plastic) deformation. Although the applied stresses are often quite low, the strain rates can be sufficiently high, so that a mixture of creep and plastic deformation modes define the fatigue response of the material. This sensitivity of fatigue behavior to cyclic loading frequency has been widely studied for tin-lead (Sn-Pb) solders and is recognized as an important variable in the fatigue response of lead-free alloys (Ref 1-3). 

The mechanical sohcitation is described by an imposed cyclic strain that maintain nniform vertical strains in the whole system. The imposed strain is given by 33 = 0 e>q)(2/ ot) where q and /g are the amplitude and frequency of the strain. 

Rheological measurements under oscillating conditions yield the dynamic mechanical properties of polymers, i.e. the storage modulus, G, the loss modulus, G , and a mechanical damping or internal friction, tan 5. In studies of the response of a material to vibrational forces, stress, strain, frequency and temperature are the key variables. When a material is subjected to cyclical stress under conditions analogous to those encountered in the intended applications, the data reflect both... 

The experimental activity was carried out on a cylindrical pressure vessel whose capacity is 50 litres and made from steel 3 mm thick. Fig. 2 shows the layout of the pressure vessel considered. The pressure vessel was connected to an oil hydraulics apparatus providing a cyclical pressure change of arbitrary amplitude and frequency (fig.3). Furthermore the vessel was equipped with a pressure transducer and some rosetta strain gauges to measure the stresses on the shell and heads. A layout of the rosetta strain gauges locations is shown in fig.4. 

It may also be mentioned at this point that the carbonyl absorption of the cations of 2,2-dimethylquinuclidone-6, which is certainly N-protonated, is found at the very high frequency of 1799 cm (Pracejus, 1959). This bicyclic system is strained, however, and strain is known to increase the carbonyl frequency in cyclic ketones (Cross, 1964, p. 59). 

Box 3.3 Effect of Ring Strain on the Carbonyl Stretching Frequencies of Lactones (Cyclic Esters) and Lactams (Cyclic Amides)... 

---