Failure Effects Overview

Feature Description

What are Failure Effects?

- At any time in the development of a design, it is possible to complement the functional characteristics with failure-based characteristics. While failure modes represent the lowest element of failure behavior, failure effects provide the ability to extend the failure modes to create data consistent with FMECA analysis. Each effect that is caused by the existence of a failure mode is captured using failure effects. Failure effects come in 4 types as described below:

What are Object Failure Effects?

- Object failure effects (sometimes referred to simply as "object effects") are failure effects that describe the immediate consequences of a failure that occurs on an object. Lower-level object failure effects (for which failure modes are listed as the causes) can themselves be listed as causes of design failure effects. In higher levels of the design hierarchy, any design failure effects within a lower-level design will be automatically inherited as object failure effects on the associated assembly. Object failure effects, by describing the way in which a failure manifests itself throughout the design hierarchy, create the information needed for populating a FMECA.

What are Design Failure Effects? -

Design failure effects describe how object failure effects manifest themselves at the design level. Furthermore, since the design becomes the definition of an object at the next higher level in a hierarchical design, design failure effects are used to create the next higher level's object effects (on the linked assembly). Design failure effects, also called simply "design effects," are failure effects that describe the design-level consequences of the failures that have been described on individual objects (i.e., object effects). Furthermore, design effects automatically roll up to create object effects on assemblies in the next higher-level design.This propagation of object effects to design effects to higher-level object effects (and so on) provides for a full hierarchical FMECA capability.

What Are Intermediate Effects? -

In a FMECA The number of Causes can sometimes be lengthy (Too many causes to make the FMECA read easily). Intermediate Effects can be used for those situations. These are a group of design effects lumped into one effect to reduce Bulky FMECAs (example - Decoupling capacitor failures on voltages)

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The use of intermediate effect greatly reduces the content of bulky FMECAs and is recommended where applicable.

What are External Failure Effects? -

External Failure Effects Causes are outside influences on a design that cause it to fail. Example: A road Hazard that causes a tire to blow out. (Not a function of the design - ex: Wet or frozen Pavement). The probability of External Effects Causes are hard to define. For example the occurance or degree of a Hurricane or earthquate cannot be defined but its statistics probability can be estimated in the environment that the system will experience (Earthquake probability varies with regions of the earth). eXpress allows External Failure Effects Causes to be defined for Design failures FTA utilization but does not provide statistical data for these failures. An event that is normally expected to occur. In general, these events can be set to occur or not occur (i.e., they have a fixed probability of 0 or 1).

How do I Determine Failure Effects?

- The determination of what failure effects to add is normally made by examining the severity of the failure. Since failure effects are based on failure modes, it is possible to create effects that are comprised of different combinations of those failure modes. This is desirable when more than one failure mode exhibits the same overall failure behavior. As an example, a resistor is usually described as failing in one of three ways: open, short and change in value. While three failure modes would be defined for the resistor, there might only be two effects identified. Open and short might result in a severity "Loss of Operation" while change in value might result in a severity of "Degraded Performance". For this example, two effects would be identified with open and short as the cause of one effect and change in value as the cause of the second effect. The reduction to two failure effects also has the benefit of simplifying the development the FMECA, since design effects will become simpler to define.