Stress-relaxation technique

For insoluble monolayers of cholesterol and dipalmitoyl choline the relaxation at pressures below the collapse point were studied by Joos et al. ), using oscillatory and stress relaxation techniques. They found experimental evidence (and presented theory) for a double-exponential decay, representing two consecutive processes. The longer r s are 0(10 s) and 0(10 s) for cholesterol and the lipid, respectively, so these relaxations are relatively slow and may therefore be overlooked, especicJly in automated apparatus. No molecular mechanism was proposed the two r s did not exhibit a clear relationship with the surface pressure at which the experiments were carried out. 

Although creep-compliance (Kawabata, 1977 Dahme, 1985) and stress-relaxation techniques (Comby et al., 1986) have been used to study the viscoelestic properties of pectin solutions and gels, the most common technique is small-deformation dynamic measurement, in which the sample is subjected to a low-amplitude, sinusoidal shear deformation. The resultant stress response may be resolved into an in-phase and 90° out-of-phase components the ratio of these stress components to applied strain gives the storage and loss moduli (G and G"), which can be related by the following expression ... 

The elastic nature of a fluid is characterized by dynamic mechanical or stress relaxation techniques. Dynamic mechanical (oscillatory) testing is a procedure in which a sample is sinusoidally strained and the resultant stress is measured. The shear stress T varies with the same frequency as the shear rate... 

In the first experiments over an extended frequency range, the biaxial viscoelastic as well as uniaxial viscoelastic properties of wet cortical human and bovine femoral bone were measured using both dynamic and stress relaxation techniques over eight decades of frequency (time) [Lakes et al, 1979]. The results of these experiments showed that bone was both nonlinear and thermorheologically complex, that is, time-temperature superposition could not be used to extend the range of viscoelastic measurements. A nonlinear constitutive equation was developed based on these measurements [Lakes and Katz, 1979a]. 

The stress-relaxation technique is an extension of the use of equilibrium modulus measurements to measure the concentration of stress-bearing chains. A sample of cross-linked rubber is stressed in tension to a constant extension and the stress required to maintain this constant extension is monitored, often continuously. If chain scission occurs so the stress required to maintain the extension will drop. 

In spite of the objections to the various methods of measuring scission events, good 1 1 correlations are found between the scission efficiency values obtained by the various methods in the cases of natural rubber latex, uncross-linked natural rubber, uncross-linked gutta percha and peroxide-cured vulcanizates. On the other hand divergent results are obtained using the stress-relaxation technique with sulphur-cured vulcanizates. 

These techniques have very important applications to some of the micro-structural effects discussed previously in this chapter. For example, time-resolved measurements of the actual lattice strain at the impact surface will give direct information on rate of departure from ideal elastic impact conditions. Recall that the stress tensor depends on the elastic (lattice) strains (7.4). Measurements of the type described above give stress relaxation directly, without all of the interpretational assumptions required of elastic-precursor-decay studies. 

Cosgrove, D.J., Van Volkenburgh, E. Cleland, R.E. (1984). Stress relaxation of cell walls and the yield threshold for growth Demonstration and measurement by micropressure probe and psychrometer techniques. Planta, 162, 46-52. 

Gillen, K.T., M. Celina, and R. Bernstein, Validation of improved methods for predicting long-term elastomeric seal lifetimes from compression stress-relaxation and oxygen consumption techniques. Polymer Degradation and Stability, 82(1), 25-35, 2003. 

Stress reduction techniques. Psychotherapy including stress management, guided imagery, and relaxation techniques are being used more frequently as adjunctive therapies for patients with psoriasis. Stress reduction has been shown to improve both the extent and severity of psoriasis. 

Psychotherapy (e.g., individual, group, and family), interpersonal therapy, and/or cognitive behavioral therapy / Stress reduction techniques, relaxation therapy, massage, yoga, etc. 

Scission yields are determined by stress relaxation measurements during irradiation of specimens. Permanent and temporary chain scissions are measured by this technique in contrast to gel measurements, which provide data only on permanent scissions. 

It turns out that stress relaxation following a simple shear deformation is seldom employed experimentally. A more common technique is to measure the steady state response to small sinusoidal deformations as a function of angular frequency to. The dynamic storage modulus G (to) and loss modulus G"(to) in small sinusoidal deformations are related to G(t) ... 

Stress relaxation has been mentioned in the context of several plasticity instruments. ASTM D6048140 gives background information about techniques and theory of stress relaxation testing and interpretation of results. Mention is made of the Mooney and capillary rheometers. 

Modern life is loaded with stress. While retirement takes you away from job-related stress, there will be other things like family, health, or money worries to upset you. Your heart becomes at risk when hostility is repressed or circumstances seem to overwhelm you. Participating in relaxation techniques such as resting quietly, breathing exercises, meditation, or yoga help keep stress under control. 

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