Time dependent failure

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

This behavior provides evidence of the fading memory property of the material. Therefore, the entire strain (and temperature) history must affect the constimtive behavior of filled rubber elastomers. While the strain-rate sensitivity and the failure time dependency are recognized and well-documented in the case of other materials such metals, the incorporation of history-dependent properties of elastomers requires further clarification. 

Figure 2. Failure time dependence of PI interconnect devices on absolute water vapor pressure.

Flexural stress SiC mpture curves are shown in Figure 3 (27). AU. the forms tend to be fairly resistant to time-dependent failure by elevated temperature creep. In addition, SiC shows outstanding resistance to oxidation even at 1200°C as a result of formation of a protective high purity siUca surface layer (28). 

Hiestand Tableting Indices Likelihood of failure during decompression depends on the abihty of the material to relieve elastic-stress by plastic deformation without undergoing brittle fracture, and this is time dependent. Those which relieve stress rapidly are less... 

Many attempts have been made to obtain mathematical expressions which describe the time dependence of the strength of plastics. Since for many plastics a plot of stress, a, against the logarithm of time to failure, //, is approximately a straight line, one of the most common expressions used is of the form... 

The Poisson distribution for observing M events in time r is given by equation 2.5-1, where / is the failure rate estimated as M/i. This model may be used if the failure rate is time dependent rather than demand... 

This FMEA/FMECA shows failure rates that are both demand and time dependent. Adding the demand failure rates gives a train failure rate of 5. 1 E-3/demand. The sum of the time dependent failure rates is 3Ei-10/hr. A standby system such as this, does not exhibit its operability until it is actuated for which the probability is needed that the train has failed since the last use Val " are considered to be part o ng envelope and... 

Fleming et al. (1985) define this as similar to the model of Marshall and Olkin (1967) except that BPM is only for time-dependent failure rates. Equations 3.5.8-la-d are for four parameters, but the method may be generalized to n components. These parameters may be related to the MGL parameters as shown in equations 3.5.8-2a-d. 

OD -T alpha-lies, control, fault-tree description, failure data AND OR NOT K-of- N Bottom-up modularization and decompo-dtion of fault tree into t>est modular nepresentation Top event probability, time dependence, c s, r rCLs, and Linceitainty Option of not generating minimal cut >ets forquali ing fault ree IBM 360/370 Fortran IV, Available j from Software < enter... 

A type of time dependence that is available in most codes evaluates the exponential distribution at specified times. This is the constant failure rate - constant repair rate approximation ( Section 2.5.2). This may not be realistic as indicated by Figure 2.5-2 in which the failure rate is not constant. Furthermore, Lapides (1976) shows that repair rates are not constant but in many casc. appear to be lognormally distributed. 

There are two reliability modeling codes used for Tech. Spec, modification to address time-dependent failure rate and repair FRANTIC developed by BNL/NRC and SOCRATES developed by BCl,/EPRI. 

Recalculates sequence values after event failure data antPor cutsets have been modified. After choosing Sequence from the menu, the Sequence dialog appears listing all sequences in the current family. The pop-up menu lists Solve, Quantify, Uncertainty, CutSets. Display, and Time Dependent. 

The pressure and temperature of a container s contents at the time of failure will depend on the cause of failure. In fire simations, direct flame impingement will weaken container walls. The pressure at which the container fails will usually be about the pressure at which the safety valve operates. This pressure may be as much as 20 percent above the valve s setting. The temperature of the container s contents will usually be considerably higher than the ambient temperature. 

Although failures are recorded as time related or demand related, the distinction between the two failure types is not always clean cut. The total failures on any piece of equipment are usually a combination of some that are time dependent and some that are demand related. In other words, a piece of equipment that fails on demand may already be in a failed state when the demand arrives, or the demand may actually cause the failure. 

In the analysis of pumps, IPRDS failure data for 60 selected pumps at four nuclear power plants were statistically analyzed using FRAC. The data cover 23 functionally different pumps, respectively, for two BWRs. Catastrophic, degraded, and incipient failure severity categories were considered for both demand-related and time-dependent failures. 

For catastrophic demand-related pump failures, the variability is explained by the following factors listed in their order of importance system application, pump driver, operating mode, reactor type, pump type, and unidentified plant-specific influences. Quantitative failure rate adjustments are provided for the effects of these factors. In the case of catastrophic time-dependent pump failures, the failure rate variability is explained by three factors reactor type, pump driver, and unidentified plant-specific Influences. Point and confidence interval failure rate estimates are provided for each selected pump by considering the influential factors. Both types of estimates represent an improvement over the estimates computed exclusively from the data on each pump. The coded IPRDS data used in the analysis is provided in an appendix. A similar treatment applies to the valve data. 

The second limitation is the life dispersion of machinery components. It is difficult to predict time-dependent failure modes because even they do not occur at the exact same operating intervals. Consider the life dispersion of mechanical gear couplings on process compressors. Both components are clearly subject to wear. If we conclude that their MTBF (mean-time between failure), or mean-time-between-reaching-of-detect-limit is 7.5 years, it is possible to have an early failure after 3 years and another... 

Since most time-dependent failures have larger life dispersions, we must consider the maximum and minimum ratios of 4 1 and 40 1. Generally, relative life dispersion increases with the absolute value of MTBF. That is, wear items with a relatively short life expectancy such as rider rings on reciprocating compressors will have a comparatively smaller dispersion than components such as gear tooth flanks, which can be expected to remain serviceable for long periods of time. 

Tests in a Clj + Oj mixture at 427°C have shown that the worst elements for promoting susceptibility are Al, Sn, Cu, V, Cr, Mn, Fe and Ni, while the least harmful are Zr, Ta and Mo. a-phase alloys are generally more susceptible than )3-phase alloys. Heat treatment has not been examined extensively, but some heat treatments render some a-alloys more susceptible or change the mode of fracture. The general effect will depend upon the alloy and the heat-treatment cycle. Subsequent cold work can sometimes considerably lower susceptibility. Failure times decrease as either the testing temperature or initial stress value is raised. 

Materials subjected to high temperatures during their service life are susceptible to another form of fracture which can occur at very low stress levels. This is known as creep failure and is a time dependent mode of fracture and can take many hours to become apparent (Fig. 8.88). 

The Fig. 2-30 shows the curves of a family of TPs describe as failure that is fairly typical of the behavior of certain TPs. The time-dependent strains resulting from several levels of sustained or creep stress are shown,... 

Avoiding product failures can depend, in part, on the ability to predict the performance of plastic materials and their shapes. With available time, the usual approach of product prototype and/or field-testing provides useful and reliable performance data when conducted properly. As an example designers continue to develop sophisticated computer methods for calculating stresses in complex structures. 

Promoting an Optimal Response to Therapy Superficial and deep fungal infections respond slowly to antifungal therapy. Many patients experience anxiety and depression over the fact that therapy must continue for a prolonged time Depending on the method of treatment, patients may be faced with many problems during therapy and therefore need time to talk about problems as they arise Examples of problems are the cost of treatment, hospitalization (when required), the failure of treatment to adequately control the infection, and loss of income. The nurse must help the patient and the family to understand that therapy must be continued until tlie infection is under control. In some cases, therapy may take weeks or months. 

Failure to properly account for the time dependence of inhibition can result in grossly misleading SAR and potentially cause the researcher to overlook promising inhibitor molecules. 

Fig. 64 Creep and creep failure can be modelled by the time-dependent shear deformation as described by the Eyring reduced time model...

Low temperature can lead to embrittlement of plastics. This is not seen as a time-dependent effect, but it can be the cause of rapid failure should environmental degradation be followed by a fall in temperature. 

The possible fatigue failure mechanisms of SWCNT in the composite were also reported (Ren et al., 2004). Possible failure modes mainly include three stages, that is, splitting of SWCNT bundles, kink formation, and subsequent failure in SWCNTs, and the fracture of SWCNT bundles. As shown in Fig. 9.12, for zigzag SWCNT, failure of defect-free tube and tubes with Stone-Wales defect of either A or B mode all resulted in brittle-like, flat fracture surface. A kinetic model for time-dependent fracture of CNTs is also reported (Satapathy et al., 2005). These simulation results are almost consistent with the observed fracture surfaces, which can be reproduced reasonably well, suggesting the possible mechanism should exist in CNT-polymer composites. 

The Landau-Zener expression is calculated in a time-dependent semiclassical manner from the diabatic surfaces (those depicted in Fig. 1) exactly because these surfaces, which describe the failure to react, are the appropriate zeroth order description for the long-range electron transfer case. As can be seen, in the very weak coupling limit (small A) the k l factor and hence the electron transfer rate constant become proportional to the absolute square of A ... 

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