Failure modes degradation

Another example is a safety valve in standby service. If demands occur very infrequently, time-related stresses such as external corrosion may have a significant influence. Repeated demands in very dirty service could easily lead to faster degradation and failure, whereas repeated demands in lubricated service might actually enhance performance if the failure mode of interest is failure to open. Failure data based on time or demands can also be skewed if the relief valve is initially damaged or installed incorrectly. 

If a plant had periodic inspections, the impeller corrosion in No. 5 might be detected before it became a significant problem, thereby altering the failure mode from catastrophic to a degraded or an incipient failure. In a plant with routine maintenance, it is possible that Nos. 1 and 5 may be eliminated completely by routine seal and impeller changes. 

The IEEE Std 500 document is based on a hierarchical structure of component types set down in the manual s table of contents. The preface for each subsection (defined by a component type) provides a tree diagram that clearly shows the way the component classes have been subdivided to determine "data cells". The failure modes for each component class are also hierarchically organized according to failure severity catastrophic, degraded, or incipient. Rates per hour and demand rates (per cycle) are both included, as well as upper and lower bounds. 

This report provides an aging assessment of electric motors and was conducted under the auspices of the USNRC NPAR. Pertinent failure-related information was derived from LERs, IPRDS, NPRDS, and NPE including failure modes, mechanisms, and causes for motor problems. In addition, motor design and materials of construction were reviewed to identify age-sensitive components. The study included consideration of the seismic susceptibility of age-degraded motor components to externally-induced vibrational effects. 

All of the analysis techniques discussed to this point have been methods to determine if a potential problem exists within the machine-train or its associated systems. Failuremode analysis is the next step required to specifically pinpoint the failure mode and identify which machine-train component is degrading. 

Plastics. Part of the trend to substitute plastic and composite substrates for metals can be attributed to a desire to avoid the process of metallic corrosion and subsequent failure. Relatively little attention has been called to the possible failure modes of plastics under environments considered corrosive to metals. More extensive work should be conducted on the durability and life expectancy of plastic and composite materials under end-use environments. A further consideration is the potential for polymer degradation by the products of metal corrosion in hybrid structures comprising metal and polymer components. Since it is expected that coatings will continue to be used to protect plastic and composite substrates, ancillary programs need to be conducted on the mechanisms by which coatings can protect such substrates. 

Much recent ongoing work has focused on the chemical degradation mechanisms and revealed that radical attack is the root cause of the membrane decomposition. However, there is still a lack of fundamental understanding of the mechanisms for the degradation of mechanical strength, which is related to the membrane failure mode responsible for the sudden death behavior of fuel cells. 

For machinery lubricating oils, the failure modes are well known and understood. They are generally characterized as either contamination or degradation related. Contamination faults occur when the oil becomes contaminated with liquid or solid materials such as water, fuel, antifreeze, insolubles (soot), ingress dirt or process materials. 

In a different battery test with a simulated EV load pattern, a SWP-7 cell with an assembly pressure of 60 kPa achieved 450 cycles versus 270 cycles for an AGM cell with 73 kPa. The failure mode was found not to be the expansion of positive plate but, rather, sulfation of the negative plate. This led to the conclusion that the favourable mechanical properties of SWP-type separators suppress degradation of the positive active-material. 

Although cyclic environmental chamber test procedures may suffice for failure processes Involving, for example, mechanical stress, kinetic controlled processes dependent upon time and temperature such as oxidation and diffusion do not lend themselves to adequate Identification and analysis based solely on number of cycles. Thus Sandia National Laboratories developed an accelerated aging protocol for environmental testing which (1) identifies material incompatibilities and subsequent failure modes in Phase I and (2) proceeds with kinetic analysis of the Arrhenius type of failure mode processes which allow extrapolation necessary for lifetime prediction of components in Phase II. Thus two phases are necessary in a complete analysis to accurately predict system lifetimes. The accelerated aging protocol requires the Identification of the stresses that are most likely to damage the performance of the component under test. However, data is frequently not available on the performance of a system under a particular stress. When this is the case, it becomes necessary to make predictions of those stresses most likely to cause degradation and then test to see if the stresses selected are dominant. 

In addition to facilitating the acceptance of CMCs by designers, field tests will also determine CMC component degradation mechanisms and life-failure modes in operational environments. On the basis of these tests, promising composite systems can be refined, and less viable CMCs can be vetted. 

Thomas, S., Gupta, B. R., De, S. K., Mechanical properties, surface morphology and failure mode of gamma-ray irradiated blends of polypropylene and ethylene-vinyl acetate rubber. Polymer Degradation and Stability 1987,18(3), 189-212. 

Markov models are generally considered more flexible than other methods. On a single drawing, a Markov model can show the entire operation of a fault tolerant control system including multiple failure modes. Different repair rates can be modeled for different failure situations. If the model is created completely, it will show full system success states. It will also show degraded states where the system is still operating successfully but is vulnerable to further failures. The modeling technique provides clear ways to express failure sequences and can be used to model time dependent probabilities. 

A safety instrumented function has a valve energized and open. The valve must close when a demand is detected. The valve is fitted to a piston type pneumatic actuator that has an O-ring seal around the piston. This seal degrades with time and gets sticky. If it is left in position a long period of time, it will cause the actuator to stick in place. How would this failure mode be classified ... 

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