Failure modes environments

Photo 51 The current on this river is too fast to allow containment by a boom placed directly across the flow. Loss of containment in this case is by several failure modes. (Environment Canada)... 

The rehabihty level of a product also depends on the operating or environmental conditions, which may produce a variety of failure modes. Rehabihty can only be assessed relative to a defined environment. Unless these points are estabhshed clearly, confusion surrounds any quoted rehabihty number for a product. 

Erosion-corrosion is a fairly complex failure mode influenced by both environmental factors and metal characteristics. Perhaps the most important environmental factor is velocity. A threshold velocity is often observed below which metal loss is negligible and above which metal loss increases as velocity increases. The threshold velocity varies with metal and environment combinations and other factors. 

All of these factors determine the stress experienced by the workers and the extent to which operational errors will be recovered before disastrous consequences have ensued. In this context, hazard identification techniques, such as hazard and operability studies (HAZOP), failure modes and effects and criticality analysis (FMECA), fault trees, and others are useful in making the process environment more forgiving. 

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. 

Nonelectronic Parts Reliability Data 1991 (NPRD-91) and Failure Mode/Mechanism Distributions 199V (FMD-91) provide failure rate data for a wide variety of component (part) types, including mechanical, electromechanical, and discrete electronic parts and assemblies. They provide summary failure rates for numerous part categories by quality level and environment. 

From studies of service behavior and from extensive laboratory investigations, the well-established terms stress-corrosion cracking (SCC) and corrosion fatigue have been shown to relate to a continuum of failure modes classified as environment-sensitive fracture. In many environments, the addition of stress, with associated strains, introduces a variable that can result in brittle failure in the sense of very limited plastic flow in otherwise ductile materials such as the stainless steels. Environment-sensitive fractures propagate at an advancing crack tip at which, simultaneously, the local stresses can influence the corrosion processes, and the corrosion can influence the crack-opening processes. Since these processes proceed by kinetic mechanisms, they are time and stress dependent with the result that the crack propagation rate can become very sensitive to the stress application rates. Conventional SCC usually has been associated with static stress, but this is seldom realized... 

One of these failure modes is time-dependent crack propagation under static stress. Structures exposed to hydrogen gas can experience crack propagation at static stresses that are less than the maximum allowable stress for structures in inert environments. This subcritical crack propagation... 

Failure Modes and Effects Analysis (FMEA) This will document a review of the effects of faiiure of the component parts of the system. This review is mainiy aimed at assessing the system hardware, interfaces and environment. This review shouid be performed on two lev-eis The first ievei reviews the possibie failure modes of each individual system and the second level assesses the possible failure modes of the combined system. The overall objective of the FMEA is to identify the potentiai weak points and then to identify how these weak points may be designed out of the system. This may be achieved by instaiiing redundancy, redesigning parts of the system, recommending procedural controls and so on. The results of this review should be documented as a FMEA Review Report or as part of a DQ Report. 

The reliability of an adhesive and its impact on the performance of an electronic assembly should be considered in the initial selection of the adhesive and the design of the system. The function that the adhesive must perform for a specific application, the environment it is expected to encounter, and its duration are all important. Various approaches may be used to predict and assure reliability. Key among these approaches is a basic understanding of possible failure modes and mechanisms. Most failure modes attributed to adhesives are now well understood and documented so that they can be avoided in the initial selection and qualification of the adhesive and in its processing. 

A particular problem, which is explored in detail in Section 7.5.1, is that the storage environment itself is able to influence the properties of these materials, since they are susceptible to moisture uptake. In extreme cases, storage in water has been found to cause the mechanical failure mode to change from brittle to tough [25]. 

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. 

Three distinct failure modes have been observed [33-35] which involve a synergism between the applied (and/or internal) stress and environment. They have been characterized as Type A, B or C. 

Protection System Failure Modes. The protection system sha11 be des i gned to fa iI i nto a safe state or i nto a state demonstrated to be acceptable on some other defined basis if conditions such as faults, disconnection of the system, loss of energy, or postulated adverse environments are experienced. 

Quality, environment, occupational hazards, ethics or R+D+i management Failure mode significative if CPN>30... 

Failure mode A condition having an effect on the aircraft and its occupants, either direct or consequential, which is caused or contributed to by one ore more failures or error, considering flight phases and relevant adverse operating or environment conditions or external events [AMC25.1309]. 

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