A failure mechanism is a physical, chemical, logical, or other process or mechanism that may lead to failure. Examples of failure mechanisms include wear, corrosion, fatigue, hardening, swelling, pitting, and oxidation. Failure mechanisms are hence specific failure causes as shown in Figure 3.12.
Figure 3.12 Failure causes and mechanisms. A failure mechanism is a specific type of failure cause.
Each mechanism can have its root in different stages of the item's life cycle. Wear can, for instance, be a result of wrong material specification (design failure), usage outside specification limits (misuse failure), poor maintenance, inadequate lubrication (mishandling failure), and so on.
A failure mechanism may be seen as a process that leads to a failure cause.
3.6.4 Software Faults
An increasing number of item functions are being replaced by software‐based functions and a fair proportion of item failures are caused by software bugs. IEV defines a software fault/bug as:
Definition 3.7 (Software fault/bug)
State of a software item that prevents it from performing as required (IEV 192‐04‐02).
Combined with a particular demand or trigger, the software bug may lead to item failure. Such a failure is a systematic failure and is sometimes called a software failure (see Figure 3.10). If the trigger is a random event, the software failure is random. Software bugs are difficult to reveal and software development projects therefore include a detailed process for finding and correcting bugs. This process is called debugging.
Software does not deteriorate and software bugs do not occur at random in the operational phase. They have been programmed into the software and remain until the software is modified. New software bugs are often introduced when new patches or new versions of the software are installed to remove known bugs. The same software failure occurs each time the same activation condition or trigger occurs. If relevant activating conditions or triggers do not occur, the software bug remains undetected. Installations of the same software may show very different frequencies of software failures because the failure frequency is proportional to the frequency of the occurrence of activating conditions or triggers.
3.6.5 Failure Effects
Failure effect is an undesired consequence of a failure mode. Failure effects may be categorized as follows:
1 Injuries or damage to personnel or to the public.
2 Damage to the environment.
3 Damage to the system where the failure occurred.
4 Material or financial loss.
5 Interruptions of the system operation (e.g. loss of production, cancelled or delayed transport means, interruptions of electric or water supply, interruption of computer/telephone network service.)
A failure mode may lead to many different failure effects, on the item where the failure occurred, and on other items. Failure effects are classified as local effects, next higher effects, and end effects. These effects are illustrated in Example 3.13.
Example 3.13 (Failure effects of brake pad failure)
Consider a (total) wear‐out failure of a brake pad on the left front wheel of a car. The local effect is that the braking effect on the left front wheel is strongly reduced and that the brake disc may be damaged. The next higher effect is that the braking effect of the car is uneven and not adequate. The end effect is that the car cannot provide a safe drive and must be stopped.
A general picture of the relationship between cause and effect is that each failure mode can be caused by several different failure causes, leading to several different failure effects. To get a broader understanding of the relationship between these terms, the level of indenture being analyzed should be brought into account. This is shown in Figure 3.6.
Figure 3.6 shows that a failure mode on the lowest level of indenture is one of the failure causes on the next higher level of indenture, and the failure effect on the lowest level equals the failure mode on the next higher level. The failure mode “leakage from sealing” for the seal component is, for example, one of the possible failure causes for the failure mode “internal leakage” for the pump, and the failure effect (on the next higher level) “internal leakage” resulting from “leakage from sealing” is the same as the failure mode “internal leakage” of the pump.
Failure effects are often classified according to their criticality as discussed in Chapter 4.
3.7 Failure/Fault Analysis
A failure or fault analysis is a systematic investigation of a failure or a fault that has occurred, in order to identify the root causes of the failure/fault and to propose corrective actions needed to prevent future failures/faults of the same, or similar, types.
This section gives an introduction to two commonly used failure/fault analysis techniques (i) cause and effect analysis and (ii) root cause analysis. Both techniques are primarily used to analyze real failures/faults that have occurred, but may also be used to analyze potential failures or faults.
3.7.1 Cause and Effect Analysis
Cause and effect analyses are frequently used in quality engineering to identify and illustrate possible causes of quality problems. The same approach may also be used in reliability engineering to find the potential causes for system failures or faults. The cause and effect analysis is documented in a cause and effect diagram.
The cause and effect diagram, also called Ishikawa diagram (Ishikawa 1986), was developed in 1943 by the Japanese professor Kaoru Ishikawa (1915–1989). The diagram is used to identify and describe all the potential causes (or events) that may result in a specified failure. Causes are arranged in a tree structure that resembles the skeleton of a fish with the main causal categories drawn as bones attached to the spine of the fish. The cause and effect diagram is therefore also known as a fishbone diagram.
To construct a cause and effect diagram, we start with an item failure. The item failure is briefly described, enclosed in a box and placed at the right end of the diagram, as the “head of the fish.” The analysis is carried out by a team, using an idea‐generating technique, such as brainstorming. Failure causes are suggested by the team and organized under headings such as
1 Manpower
2 Methods
3 Materials
4 Machinery
5 Milieu (environment)
This is a common classification for failure/fault analysis and is referred to as the 5M approach, but other categories may also be used.