Static stress level

Malfunction Under Static Load. Whenever pipes are subjected to excessive stress by static interior pressure to such a degree that they malfunction, the attained stress level is closely related to the quality of the pipe in question. The static stress level KS is defined as KS = ajcrs, whereby a stands for the rated stress in the direction of the pipe circumference, while ag constitutes the cracking resistance of the pipe material. The pipe quality— one measure for it is the defect magnitude in the pipe section—is described by the nondimension al crack characteristic... 

According to Equation (5-22), a static stress level of that magnitude requires a mean crack characteristic of... 

Dynamic mechanical analysis (DMA) was performed to determine the influence of the polymer constitution on tensile modulus and mechanical relaxation behavior. For this purpose, a Perkin Elmer DMA-7 was run in tensile mode at an oscillation frequency of 1 Hz with a static stress level of 5 x lO Pa and a superposed oscillatory stress of 4 x 10 Pa. With this stress controlled instrument, the strain and phase difference between stress and strain are the measured outputs. Typically, the resulting strain levels ranged from 0.05% to 0.2% when the sample dimensions were 8 mm x 2 mm x 0.1 mm. A gaseous helium purge and a heating rate of 3°C min" were employed. The temperature scale was calibrated with indium, and the force and compliance calibrations were performed according to conventional methods. 

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

Stress in crystalline solids produces small shifts, typically a few wavenumbers, in the Raman lines that sometimes are accompanied by a small amount of line broadening. Measurement of a series of Raman spectra in high-pressure equipment under static or uniaxial pressure allows the line shifts to be calibrated in terms of stress level. This information can be used to characterize built-in stress in thin films, along grain boundaries, and in thermally stressed materials. Microfocus spectra can be obtained from crack tips in ceramic material and by a careful spatial mapping along and across the crack estimates can be obtained of the stress fields around the crack. ... 

Test results provides the hypothesis that syntactic foam is rate insensitive and that the static uniaxial strain stress-strain curve actually represents the general constitutive relation. Disagreement between the experimental data and the predicted behavior is greatest at low stresses (1 kbar) where experimental stresses are about double those predicted analytically. The discrepancy decreases at the higher stress levels and virtually disappears at and beyond 7 kbar. This range... 

The phenomenon involving failure of a material subject to repeated loading is called fatigue. Failure occurs at stress levels below those observed in the "static" tests described above. Lee et al (22) examined the characteristics of some sulphur concretes subject to fatigue. Fatigue lives (the number of cycles to failure) considerably in excess of those for portland cement concretes were observed. Polymerization of the sulphur with di-cyclopentadiene was observed to reduce fatigue life. 

From Eq, (1) it is clear that a model of crystal polarization that is adequate for the description of the piezoelectric and pyroelectric properties of the P-phase of PVDF must include an accurate description of both the dipole moment of the repeat unit and the unit cell volume as functions of temperature and applied mechanical stress or strain. The dipole moment of the repeat unit includes contributions from the intrinsic polarity of chemical bonds (primarily carbon-fluorine) owing to differences in electron affinity, induced dipole moments owing to atomic and electronic polarizability, and attenuation owing to the thermal oscillations of the dipole. Previous modeling efforts have emphasized the importance of one more of these effects electronic polarizability based on continuum dielectric theory" or Lorentz field sums of dipole lattices" static, atomic level modeling of the intrinsic bond polarity" atomic level modeling of bond polarity and electronic and atomic polarizability in the absence of thermal motion. " The unit cell volume is responsive to the effects of temperature and stress and therefore requires a model based on an expression of the free energy of the crystal. 

For instance, appreciable tensile strength had occasionally been noted in water tunnels (ref. 16). (The tensile strength of a liquid is defined here as the minimum tensile stress in the liquid at which it ruptures or cavitates (ref. 57).) The inception of cavitation in these water tunnels has occurred at higher stress levels than ordinarily expected. Higher flow velocities or lower tunnel pressures than normal have been needed to produce cavitation about a test body. The tensile strength acts as if an additional static head were present in the system. In this case, appreciable tensile strength is undesirable in order to make for a uniformity of test results, and duplicate prototype conditions (ref. 16). 

From the safety point of view, cyclic loading means that fatigue can occur at stress levels far below the limits for static strength. 

---