Induction time failure

If degradation is due to sequential or synergistic mechanisms, then a simple extrapolation may not be reasonable. For example, in accelerating the rate of oxidation of polyethylene it may be necessary to identify the induction time and subsequent degradation time separately and to produce Arrhenius diagrams for each. The total time to failure is the sum of the two times. Figure 9.3 shows an instance of where unknowing extrapolation of the short-term results in tests on polyaramid fibres could have led to overestimates of lifetime and premature failure. 

What has been shown in Figure 3 is that increasing the rates of all of the bimolecular reactions, proportionately and together, causes a net increase in the induction time (time to failure). This result probably reflects the importance of the bimolecular termination of polymeric... 

Radical trapping. To allow for stabilizaton by this mechanism, another reaction (number 49) was included to allow easy abstraction of a hydrogen atom from an additive (QH) by a peroxy radical to form a hydroperoxide and a harmless adduct. With the same value of the rate constant as for energy transfer and for concentrations as low as 10 M, the photooxidation process was efficiently slowed. Figure 9 shows the linear dependence of the time to failure (5% oxidation) as the concentration of QH is altered. Note that the trap is consumed in the process and the apparent induction time is associated with its removal. The stabilization is less effective for higher intensity (and probably higher temperature) because the faster photo (or thermal) decomposition of ROOH continues the degradation process. 

Finally, the researchers considered the PO s contact with copper wire over a cable s 50-year lifetime, and how this contact can contribute to "metal catalyzed thermal failures " Although the real-world tendency of a cable to oxidize over the long term is difflcult to measure in testing, these research ers evaluations showed that phenolic AOs, combined with a metal deactivator or chelator (see Table 3.1, for example), are effective in providing long oxidative Induction times at high temperatures. 

From Figs. 3.8, 3.9, and 3.10 it is evident that autocatalytic reactions are capable of strong temperature rises. The thermal induction time is all the shorter the higher the reaction temperature before cooling failure. These statements hold as well for autocatalytic reactions of other orders than the one of second order treated here. 

If thermal degradation is considered, it is reasonable to measure a time to failure at a series of temperatures and extrapolate to room temperature. Failure maybe defined in terms of an extent of discoloration, a degree of oxygen absorption, fracture imder a fixed load or other convenient test. Since oxidation processes show an induction time followed by accelerating reaction, most measurements will yield reasonably consistent data if they detect the end of the induction time. 

Stress crack formation - also known as crazing - according to Kausch is the formation of particular material areas (linearly confined) that open up perpendicular to the highest tensile stress after an induction time. In contrast to genuine cracks, the interior of a craze is not free of material, but contains many hollow spaces (10 to 20 nm diameter) between which molecular strands with diameters of 10 to 50 nm (fibrils) connect the opposite faces of the craze loads are still transferred between the craze faces. The presence of crazes does not represent total component failure. On the contrary, crazes are able to relief local load zones. Cracks initiating final fracture [22] do not occur until the crazes grow and break open, especially under impact load (s. Section 5.5.4 and 5.5.3.1). 

Figure 5.287 shows that the profiles for the viscosity index and the oxidation induction time i.e., degradation, of pipes in internal pressure creep tests is independent of internal pressure. One of two pipes exposed to 4 N/mm broke after 8,208 h, while the other one broke after 1,442 h, 3,720 h, and 13,690 h in the second case, the pipe was reinstalled and reloaded at 4 N/mm after elimination of the failure area [181]. 

Figure 5.288 shows the reduction in oxidation induction time (Oil) during a inter-nai pressure creep rupture test. The initiai OIT values are approximately proportional to the amount of antioxidant however, degradation onset can be influenced only in part by the amount of antioxidant, because stabilizer concentration on the pipe surface is decisive for failure [181]. 

Alia- testing, the following test methods were performed on the selected tested specimens Visual examination of the specimens was performed to identify the modes of failure. Oxidation induction time (OIT) was performed in general accordance with ISO 11357-6-2002 (E) [10] at 200 °C. Specimens wo-e taken liom the inner and outer surfaces as well as liom the bulk pipe wall and Micro-attenuated total reflection Fourier Transform Infrared Spectroscopy (micro-ATR) was performed. The inner surface and the fracture surface were examined. Scaiming Eleetron Microscopy (SEM) coupled with Energy Dispersive X-ray analysis (EDX) was performed on the iimer surface and the fiacture surface. 

GO Identification of hazards or dependencies Qualitative, inductive c.jnsidcrs only one failure at a time simple to apply, provides orderly examinaiion... 

Preanesthetic drugs must be administered on time to produce their intended effects Failure to give the preanesthetic drug on time may result in events such as increased respiratory secretions caused by the irritating effect of anesthetic gasesand the need for an increased dose of the induction drug because the preanesthetic drug has not had time to sedate the patient. 

If there is no hypothesis for the event, use an inductive method to find potential scenarios. Inductive methods speculate a given fault or failure, then look forward in time to determine the probable outcome, that is, What would happen if... Inductive methods include using a Checklist or a Hazard and Operability Analysis (HAZOP). 

It has only been discovered relatively recently that a natural remedy, St. John s Wort, used by millions for the treatment of depression, is a potent inducer of the major human drug-metabolizing enzyme, CYP3A4. This induction can lead to the therapeutic failure of a drug when the remedy is taken at the same time. 

Many nonsteroidal anti-inflammatory drugs of different chemical structures (Fig. 5) have been introduced for the treatment of inflammatory and painful conditions. Many years of clinical experience with these drugs have shown that there is no induction of tolerance or dependence and no respiratory depression as seen with opioids. The major side-effects of these compounds with COX-1 selectivity or balanced COX-1 and COX-2 inhibition are damage to the gastric mucosa, prolongation of bleeding time and renal failure. 

---