**1.8 Assessing the effects and importance (criticality) of failures**

Defects are usually assessed depending on two criteria: probability of occurrence (A) and probability of detection (D), which are expressed using the same notation scale.

The quantification of these probabilities depends on the type of product or process analyzed.

Severity (importance) means the value that characterizes how serious the established effect is for the failure mode and how it affects the customer, in terms of product/process failure/failure, their effects, and notation being presented in **Table 1**.

In assessing the significance (severity) of the faults, the following general rules must be observed:


The assessment of the significance of the defects is done using the notation scale. Based on probabilities A and D, and importance I, the RPN risk coefficient is determined using the relationship:

$$\mathbf{RPN} = \mathbf{A} \cdot \mathbf{D} \cdot \mathbf{I} \tag{1}$$

This coefficient has values between 0 and 1000. It is generally considered that measures are needed to prevent potential damage when the RPN risk coefficient is greater than 100.

In **Table 1**, the evaluation of the significance (severity, seriousness) of the defects ("I") when FMEA of the product or process is applied is presented.

The appearance index (A) estimates the probability of occurrence of the defect as a product of the probability of occurrence of the cause and the probability that this cause will provoke the considered defect [13].

**Table 2** shows how to score the occurrence index according to its probability of occurrence.

The detection index (D) estimates the probability of detecting noncompliance before it reaches the customer.

**Table 3** shows the scoring values of the detection index.

After evaluating and scoring each of the three indices, in the next stage, indications are given regarding the need for improvement measures, depending on the values of indices A, D, and I.

The assessment of the need for improvement measures (general guidance) is presented in **Table 4**.

The results of the analysis are written in tabular form, similar to that described in **Figure 14**

Advantages of applying FMEA:



#### **Table 1.**

*Ways to assess the importance index (I).*

#### *Failure Analysis – Structural Health Monitoring of Structure and Infrastructure Components*


#### **Table 2.**

*Evaluation of the Occurrence Index (A).*


#### **Table 3.**

*Scoring the detection index.*



#### **Table 4.**

*The need for improvement.*


#### **Figure 14.**

*FMEA spreadsheet example.*


The main advantages of applying the FMEA are shown in **Figure 15**.
