### Failure mode, impact and hazard analysis (FMECA)

The failure mode, impact and criticality analysis (FMECA) is aimed at all possible failures of the product, and based on the analysis of the failure mode, determines the impact of each failure mode on the product’s work, finds out the single point of failure, and presses the severity of the failure mode The degree and its probability of occurrence determine its hazard. The so-called single-point failure refers to a partial failure that causes a product failure and does not have redundant or alternative working procedures as a remedy. FMECA includes failure mode and impact analysis (FMEA) and criticality analysis (CA).

Failure mode refers to a manifestation of component or product failure. Generally, it is a failure phenomenon that can be observed. Such as material bending, fracture, deformation of parts, poor electrical contact, short circuit, improper installation of equipment, corrosion, etc.

Fault impact refers to the impact of the failure mode on safety and product functions. The impact of the fault can generally be divided into three levels: partial, one level higher and final impact. For example, when analyzing a hydraulic pump in the aircraft hydraulic system, it has a failure mode of slight oil leakage. The partial impact on the pump itself may be reduced efficiency, and the higher level, that is, the impact on the hydraulic system may be reduced pressure. , The final impact may have no impact on the aircraft.

Combine the occurrence probability of the failure mode and the severity of the impact is called the hazard.

Failure mode and impact analysis (FMEA) is the process of product design, through the analysis of various potential failure modes of the product components and their impact on product functions, and propose possible preventive and improvement measures to improve product reliability A method of design analysis. It is a preventive technique, a pre-existing behavior, and a stage of talk on paper. It has now been extended from reliability analysis to product performance analysis. Its function is to check the correctness of the system design, determine the cause of the failure mode, and evaluate the reliability and safety of the system.

Criticality Analysis (CA) is to analyze each failure mode determined in the FMEA according to the comprehensive impact of its severity category and probability of occurrence, in order to comprehensively evaluate the impact of various possible failure modes. CA is the continuation of FMEA. According to the product structure and the availability of reliability data, CA can be qualitative analysis or quantitative analysis.

FMECA (potential failure mode and effect criticality analysis) analysis method can be used at any level from the entire system to the components, type I (catastrophic failure), which is a failure that can cause death or damage to the system (such as an aircraft).

Class II (fatal fault), this is a fault that causes serious injury to personnel, serious damage to equipment or systems, and thus fails the mission.

Class III (serious failure), this type of failure will cause minor injuries to personnel, minor damage to equipment and systems, resulting in mission delays, mission degradation, or system failure (such as aircraft flying by mistake).

Ⅳ (mild failure), this type of failure is not serious enough to cause personal injury, equipment or system damage, but requires unplanned maintenance or repair.

The implementation steps of FMECA are usually:

(1) Grasp relevant information about product structure and functions.

(2) Master product start-up, operation, operation and maintenance data.

(3) Grasp information on the environmental conditions where the product is located.

In the initial stage of design, these materials are often not available at the same time. In the beginning, only certain assumptions can be made to determine some obvious failure modes. Even the preliminary FMECA can point out many single-point failure locations, and some of them can be eliminated by rearranging the structure. As the design work progresses and the available information continues to increase, the FMECA work should be repeated, and the analysis should be extended to more specific levels as needed and possible.

(4) Define products and their functions and minimum work requirements. The complete definition of a system includes its primary and secondary functions, uses, expected performance, environmental requirements, system constraints and conditions that constitute failures. Since any given product has one or more working modes, and may be in different working stages, the definition of the system also includes each mode of the product’s work and its function description during the continuous working period. Each product should have its functional block diagram, which shows the work of the product and the relationship between the functional units of the product.

(5) Draw the reliability block diagram according to the product function block diagram.

(6) Determine the level of analysis based on the required structure and the amount of existing data, that is, specify the level of analysis.

(7) Find out the failure mode, analyze its cause and effect.

(8) Find out the fault detection method.

(9) Find out possible preventive measures during design to prevent particularly undesirable events.

(10) Determine the severity of the damage caused by various failure modes to the product.

(11) Determine the probability level of occurrence of various failure modes.

The probability of occurrence of failure modes can generally be divided into: Class A (occurring frequently). The probability of occurrence of the product during operation is very high, that is, the probability of occurrence of a failure mode is greater than 0.2 of the total failure probability. Class B (very likely Occurrence), the probability of product failure during operation is medium, that is, the probability of a failure mode occurring is 0.1-0.2 of the total failure probability. Level C (occurring by accident), product failure during operation is accidental, that is, one The probability of occurrence of this failure mode is 0.01-0.1 of the total failure probability. Level D (rarely occurs), the probability of a product failure during operation is very small, that is, the probability of occurrence of a failure mode is 0.001 of the total failure probability —0.01. E level (extremely unlikely), the probability of a product failure during operation is close to zero, that is, the probability of a failure mode occurring is less than 0.001 of the total failure probability. (12) Fill in the FMEA table and draw the hazard Matrix, if quantitative FMECA is required, CA form is required. If only FMEA is performed, step (11) and drawing the hazard matrix need not be performed.

Example 6: The severity of the impact of the failure mode on the product is called the severity, which is generally divided into four categories: I, II, III, and IV, where I represents ().

A. Fatal failure;

B. Catastrophic failure;

C. Serious failure;

D. Mild failure.

Answer: B

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