Lifetime under stress

How far the fracture parameters of a specimen, such as lifetime under stress and fracture strength, depend on Regime I or Regime II processes will depend on the material s physical properties, the specimen geometry (ie. are there pre-existing cracks) and the conditions of test (rate, temperature, environment). 

Experiments for accelerated aging have to be conducted under well defined conditions. The age-acceleration factor is defined as the lifetime in use (tu) divided by the lifetime under stress (ts) (Fig. 5.9.12). 

The devices must retain their performance characteristics over their claimed lifetime under the normal stresses associated with their use. 

Mechanism of Radiation-Accelerated Creep. Apparently the radiation-accelerated creep under stress and the radiation expansion under no stress are interrelated and may, in fact, result from the same cause. Thus, the mechanism of accelerated creep may be elucidated by better understanding the radiation expansion under no stress. Any hypothesis advanced to explain the mechanism of increasing creep rate during irradiation must also explain the reversible nature of the phenomenon— i.e., the creep rate returns to a low value when the beam is turned off. If the mechanism is based on chemical changes, a chemical species must exist during irradiation which does not exist before or long afterward. The only reasonable species of this type with adequate lifetimes are the free radicals, ions, or electrons, and gases formed when polymers are irradiated. 

The test arrangement is shown diagramatically in Fig. 3. The upper end of the specimen is screwed into a strain-gauged PMMA Hopkinson bar of 19 mm diameter and 1.5 m length, calibrated for short-time response. Pilot tests had shown that the craze lifetime under impact lay within the expected uniaxial stress wave return time of 1.2 ms. 

Figure 30 shows how the number of microvoids increased with time under load in a PVC film, though a simflar behaviour was found in oriented polycaproamide (nylon 6). The initial rate of formation of voids, dNjdt was logarithmic with tensile stress. A further important observation was that the critical concentration of cavities just before macroscopic failure appeared to be constant for specimens whose lifetimes under load varied over four decades of time (accordii to the applied load). This is shown in Fig. 31. The same critical concentration of microvoids was found close to the tip of a propagating crack.

Suitable models have to be developed so as to extrapolate from the experimental results to nominal real-life conditions by calculating the age-acceleration factor that relates the lifetime under experimental stress to the in-use lifetime. As with reliability functions, various models that apply to the different age-acceleration methods are described in the literature [1, 9]... 

In addition to the separate or combined effects of heat, oxygen, and radiation, polymers may deteriorate due to exposure to water (hydrolysis) or different types of chemical agents. Condensation polymers like nylons, polyesters, and polycarbonates are susceptible to hydrolysis. Structural alteration of some polymers may occur as a result of exposure to different chemical environments. Most thermoplastics in contact with organic liquids and vapors, which ordinarily may not be considered solvents for the polymers, can undergo environmental stress cracking and crazing. This may result in a loss of lifetime performance or mechanical stability and ultimately contribute to premature mechanical failure of the polymer under stress. 

The best tools for weighing the various aspects are quantitative expressions of properties and performance data valid under various conditions, such as corrosion rate and distribution, lifetime in eorrosion fatigue, mechanical or electrochemical threshold values (Kiscc, Kthi Ep etc.), compared with corresponding quantified requirements or service conditions, i.e. specified lifetime, actual stress intensity factors and functions, and corrosion potential. 

By substituting Eq. (9.14) into Eq. (9.8), it is possible to construct stress-probability-time (SPT) diagrams. For example, SPT diagrams showing lifetime under constant stress for different levels of failure probability can be formulated, as shown schematically in Fig. 9.6. 

One further topic merits discussion in this section in view of its success in dealing with the mechanical properties of oriented fibres, which are after all anisotropic polymers. That is the theory of kinetic fracture, developed mainly by Zhurkov and co-workers. Evidence has been presented from electron spin resonance (e.s.r.X " mass spectrometry, and infra-red spectroscopy that when highly oriented fibres or heavily cross-linked rubbers experience a tensile stress (along the axis for fibres) an appreciable fraction of main-chain bonds are broken by the applied stress. These scission events are observed to occur more or less homogeneously throughout the fibre and are not localised in the fracture plane. Many sets of data show that the lifetime tb of a fibre under stress is described approximately by the following equation... 

A detailed study of the strength and lifetime under constant stress of single PpPTA filaments using Weibull statistics and an exponentional kinetic breakdown model was carried out by Wu et al. [207], They found that filaments failed due to transverse crack propagation after very short creep times, but that after long creep times the failure mechanism was splitting and fibrillation. Activation energies of the failure process amounted to 340 kJ/mol, which seems to indicate rupture of the C — N bond in the chain backbone. 

One of the key limitations of the S-N curve is its inabUity to predict lifetimes at stress ratios different from those under which the curve was developed. To predict the lifetime of a certain component, a more useful presentation of fatigue life test data is the modified Goodman diagram. 

Acceleration factors are material dependent and can be significantly different for each material and for different formulations of the same material. Therefore, it is erroneous to attempt to establish a single acceleration factor for a laboratory accelerated test to be used to predict lifetimes under natural weather conditions for a variety of materials and formulations. Because of the complex nature of the interaction of the combined weather stresses with a material, there is presently no simple way to estimate the acceleration factor for a material. Increase in irradiance cannot be equated with acceleration of degradation. For most polymeric materials, the rate of degradation is not simply a linear function of the level of irradiance. Also, it does not take into account the effect of temperature, moisture, and other weather factors. Thus, there is no substitute for determining the acceleration factor for a given material experimentally. 

The effect of the (a) dry phase and (b) wet phase % RH for membrane lifetime under acceierated stress conditions of repeated wet/ dry cycles in inert gas conditions combined with an open-circuit voltage phase. (Source Lauritzen eta ., 2009.)... 

Neither can the lifetime of a polymeric material be predicted nor can an optimum method of stabilization be determined, unless the effect of stress on the kinetics and mechanism of ageing under stress has been assesed and taken into account [1818, 1819]. 

Creep tests can be conducted in eitho- tensile or flexural modes. The time-dependent viscoelastic deformation of polymers and composites is compared and the differences in material compliance is analyzed. The constitutive relationship for creep compliance that takes into account the effect of di-latational stresses is determined. Estimation of lifetime under non-isothermal conditions is also ixe-sented. Not only are the thermal and mechanical loading of great importance to estimation of life expectancy, but also the influence of the chemical medium and immersion time. Two possible methods of obtaining this information are discussed (1) time-temperature extrapolation of the measured aging process, and (2) a functional estimation of time-temperature collectives, the latter being more precise. 

Fatigue. Engineering components often experience repeated cycles of load or deflection during their service fives. Under repetitive loading most metallic materials fracture at stresses well below their ultimate tensile strengths, by a process known as fatigue. The actual lifetime of the part depends on service conditions, eg, magnitude of stress or strain, temperature, environment, surface condition of the part, as well as on the microstmcture. 

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