Lecture No. 3

Under reliability is understood as the property of an object to maintain over time, within established limits, the values ​​of parameters that characterize the ability to perform the required functions in given modes and conditions of use of maintenance, repairs, storage and transportation. Reliability is a complex property, which, depending on the purpose of the object and the conditions of its use, consists of a combination of safety, maintainability and storability (Figure 1).

Figure 1 – Equipment reliability

For the vast majority of year-round use technical devices When assessing their reliability, three properties are the most important: reliability, durability and maintainability.

Reliability- the property of an object to continuously maintain an operational state for some time.

Durability- the ability to maintain operability until the limit state occurs when installed system maintenance and repair.

Maintainability- a property of a product that consists in its adaptability to maintaining and restoring its working condition through maintenance and repair.

At the same time, equipment for seasonal use (agricultural harvesting machines, some municipal vehicles, river vessels on frozen rivers, etc.), as well as machines and equipment for eliminating critical situations (fire-fighting and rescue equipment), which have a long service life in standby mode, should be assessed taking into account persistence, i.e. indicators of all four properties.

Storability- the property of a product to maintain within specified limits the values ​​of parameters characterizing the ability of the product to perform the required functions during and after storage or transportation.

Resource(technical) - operating time of a product until it reaches the limit state specified in the technical documentation. The resource can be expressed in years, hours, kilometers, hectares, or number of inclusions. The resource is distinguished: full - for the entire service life until the end of operation; pre-repair - from the start of operation to the overhaul of the restored product; used - from the start of operation or from the previous major overhaul of the product to the considered point in time; residual - from the moment in time under consideration until the failure of a non-repairable product or its overhaul, between-repairs.

Operating time- the duration of operation of a product or the amount of work it performs over a certain period of time. It is measured in cycles, units of time, volume, run length, etc. There are daily operating time, monthly operating time, and time to first failure.

MTBF- reliability criterion, which is a static value, the average operating time of the repaired product between failures. If operating time is measured in units of time, then MTBF refers to the average time between failures.

Finally, there is a whole range of products (for example, rubber products) that are assessed mainly by their storage and durability.

The listed reliability properties (reliability, durability, maintainability and storage) have their own quantitative indicators.

Thus, reliability is characterized by six indicators, including such important ones as probability of failure-free operation. This indicator is widely used in the national economy to assess the most various types technical means: electronic equipment, aircraft, parts, components and assemblies, vehicles, heating elements. These indicators are calculated on the basis of state standards.

Refusal- one of the basic concepts of reliability, which consists in a malfunction of the product (one or more parameters of the product go beyond the permissible limits).

Failure rate- the conditional probability density of the occurrence of a failure of a non-repairable object is determined provided that the failure did not occur before the considered point in time.

Probability of failure-free operation- the possibility that within a given operating time, an object failure does not occur.

Durability is also characterized by six indicators representing different types of resource and service life. From a security point of view, the greatest interest is gamma percentage resource- operating time during which the object will not reach the limit state with probability g, expressed as a percentage. Thus, for metallurgical equipment objects (machines for lifting and moving liquid metals, pumps and devices for pumping harmful liquids and gases) g = 95% are assigned.

Maintainability is characterized by two indicators: probability and average time to restore a working condition.

A number of authors divide reliability into ideal, basic and operational. Ideal reliability is the maximum possible reliability, achieved by creating a perfect design of an object with absolute consideration of all manufacturing and operating conditions. Basic reliability is the reliability actually achieved during the design, manufacture and installation of an object. Operational reliability is the actual reliability of an object during its operation, determined both by the quality of the design, construction, manufacturing and installation of the object, and by the conditions of its operation, maintenance and repair.

The basic principles of reliability will be unclear without defining such an important concept as redundancy. Reservation- this is the use of additional means or capabilities in order to maintain the operational state of an object in the event of failure of one or more of its elements.

One of the most common types of redundancy is duplication - redundancy with a reserve ratio of one to one. Due to the fact that redundancy requires significant material costs, it is used only for the most critical elements, components or assemblies, the failure of which threatens the safety of people or entails severe economic consequences. Thus, passenger and cargo-passenger elevators are suspended by several ropes, airplanes are equipped with several engines, have duplicate electrical wiring, and cars use a double and even triple brake system. Strength reservation, based on the concept of a safety factor, has also become widespread. It is believed that the concept of strength is directly related not only to reliability, but also to safety. Moreover, it is believed that engineering safety calculations of structures are almost exclusively based on the use of a safety factor. The values ​​of this coefficient depend on specific conditions. For pressure vessels it ranges from 1.5 to 3.25, and for elevator ropes it ranges from 8 to 25.

When considering the production process in the relationship of its basic elements, it is necessary to use the concept of reliability in a broader sense. In this case, the reliability of the system as a whole will differ from the total reliability of its elements due to the influence of various connections.

In reliability theory, it has been proven that the reliability of a device consisting of individual elements connected (in the reliability sense) in series is equal to the product of the probabilities of failure-free operation of each element.

The connection between reliability and safety is quite obvious: the more reliable the system, the safer it is. Moreover, the probability of an accident can be interpreted as “system reliability”.

At the same time, safety and reliability are related, but not identical concepts. They complement one another. So, from the consumer’s point of view, equipment can be reliable or unreliable, and in terms of safety precautions, it can be safe or dangerous. In this case, equipment can be safe and reliable (acceptable in all respects), dangerous and unreliable (unconditionally rejected), safe and unreliable (most often rejected by the consumer), dangerous and reliable (rejected due to safety regulations, but may be acceptable to the consumer, if the degree of danger is not too great).

Safety requirements often act as restrictions on the resource and service life of equipment or devices. This occurs when the required level of safety is compromised before a limit state is reached due to physical or mental aging. Limitations due to safety requirements play a particularly important role when assessing the individual residual life, which is understood as the duration of operation from a given point in time until the limit state is reached. Any parameter characterized by the duration of operation of the object can be selected as a resource measure. For aircraft, the measure of resource is the flight time in hours, for vehicles - mileage in kilometers, for rolling mills - the mass of rolled metal in tons, etc.

The most universal unit from the point of view of general methodology and reliability theory is the unit of time. This is due to the following circumstances. Firstly, the operating time of a technical object also includes breaks, during which the total operating time does not increase, and the properties of the materials may change. Secondly, the use of economic and mathematical models to justify the assigned resource is possible only using the assigned service life (service life is defined as the calendar duration from the start of operation of the object or its renewal after a certain type of repair until the transition to the limit state and is measured in calendar time units) . Thirdly, calculating the resource in time units allows us to pose forecasting problems in the most general form.

The initial impetus for the creation of numerical methods for assessing reliability was given in connection with the development of the aviation industry and the low level of flight safety at the initial stages. A significant number of aviation accidents with an ever-increasing intensity of air resources has necessitated the development of reliability criteria for aircraft and requirements for the level of safety. In particular, a comparative analysis of one of the many aircraft was carried out from the point of view of successful completion of flights.

Indicative from a safety point of view is the chronology of the development of reliability theory and technology. In the 1940s, the main efforts to improve reliability were concentrated on comprehensive quality improvements, with the economic factor being predominant. To increase the durability of components and assemblies of various types of equipment, improved designs, durable materials, and advanced measuring instruments were developed. In particular, the electrical engineering department of General Motors (USA) increased the active life of locomotive drive motors from 400 thousand to 1.6 million km through the use of improved insulation and the use of improved tapered and spherical roller bearings, as well as testing at high temperatures. temperature. Progress has been made in developing repairable designs and in providing facilities with the equipment, tools and documentation to carry out preventative and maintenance activities.

At the same time, the preparation and approval of standard schedules for periodic inspections and control cards for high-performance machine tools has become widespread.

In the 50s, great importance began to be attached to security issues, especially in such promising industries as astronautics and nuclear energy. This period marks the beginning of the use of many currently widespread concepts on the reliability of elements of technical devices, such as expected durability, compliance of the design with specified requirements, and prediction of reliability indicators.

In the 60s, the urgent need for new methods of ensuring reliability and their wider application became obvious. The focus has shifted from analyzing the behavior of individual elements various types(mechanical, electrical or hydraulic) to the consequences caused by the failure of these elements in the relevant system. During the early years of the spaceflight era, significant effort was expended on testing systems and individual components. To achieve a high degree of reliability, block diagram analysis has been developed as the main models. However, with the increasing complexity of block diagrams, the need for a different approach arose, and the principle of analyzing systems using a fault tree was proposed and then became widespread. It was first used as a program to evaluate the reliability of the MINITEMAN missile launch control system.

Subsequently, the methodology for constructing a fault tree was improved and extended to a wide range of different technical systems. After catastrophic accidents at underground intercontinental ballistic missile launch complexes in the United States, the study of system safety as a separate independent activity was officially introduced into practice. The US Department of Defense has introduced a requirement for reliability analysis at all stages of development of all types of weapons. At the same time, requirements for the reliability, performance and maintainability of industrial products were developed.

In the 1970s, the most notable work was to assess the risk associated with the operation of nuclear power plants, which was carried out based on the analysis of a wide range of accidents. Its main focus was to assess the potential consequences of such accidents on the population in search of ways to ensure safety.

Recently, the problem of risk has acquired very serious importance and still attracts increasing attention from specialists in various fields of knowledge. This concept is so inherent in both safety and reliability that the terms “reliability,” “hazard,” and “risk” are often confused.

Among the technical causes of industrial accidents, causes associated with insufficient reliability of production equipment, structures, devices or their elements occupy a special place, since most often they appear suddenly and are therefore characterized by high rates of injury severity.

A large number of types of metal-intensive equipment and structures used in industry, construction and transport are a source of hazardous production factors due to the existing possibility of emergency failure of individual parts and assemblies.

The main goal of analyzing the reliability and associated safety of production equipment and devices is to reduce failures (primarily traumatic ones) and associated human casualties, economic losses and violations in environment.

Currently, there are quite a few methods for analyzing reliability and safety. So, the simplest and most traditional method for reliability is the method of block diagrams. In this case, the object is presented as a system of individual elements for which it is possible and appropriate to determine reliability indicators. Structural diagrams are used to calculate the probability of failures, provided that only one failure is possible in each element at a time. Such limitations have led to the emergence of other methods of analysis.

The preliminary hazard analysis method identifies hazards to the system and identifies elements to determine failure modes in consequence analysis and to construct a fault tree. It is the first and necessary step in any research.

The analysis of consequences by failure mode is focused mainly on the equipment and considers all failure modes for each element. The disadvantages are that they are time consuming and that the combination of failures and human factors is often not taken into account.

Criticality analysis identifies and categorizes elements for system improvement, but often does not consider common cause failures between systems.

Event tree analysis is useful for identifying major sequences and alternative failure outcomes, but is not suitable for parallel sequences of events or for detailed study.

Hazard and performance analysis is an expanded form of consequence analysis by failure mode that includes the causes and consequences of changes in key production variables.

Cause-effect analysis demonstrates sequential chains of events well, is quite flexible and rich, but too cumbersome and time-consuming.

The most common method, widely used in various industries, is fault tree analysis. This analysis is clearly focused on finding failures and, in doing so, identifies those aspects of the system that are important to the failures in question. At the same time, graphic, visual material is provided. Visibility gives the specialist the opportunity to penetrate deeply into the operation of the system and at the same time allows you to focus on individual specific failures.

The main advantage of fault trees over other methods is that the analysis is limited to identifying only those system elements and events that lead to that particular system failure. At the same time, building a fault tree is a certain kind of art in science, since there are no analysts who could create two identical fault trees.

To find and visually represent a causal relationship using a fault tree, it is necessary to use elementary blocks that subdivide and connect a large number of events.

Thus, the currently used methods for analyzing the reliability and safety of equipment and devices, although they have certain disadvantages, still make it possible to quite effectively determine the causes of various types of failures, even in relatively complex systems. The latter is especially relevant due to the great significance of the problem of the emergence of hazards caused by insufficient reliability of technical objects.

Need to install windows, but don't know what to choose? On the one hand, the well-known wooden ones, and on the other, the now popular plastic ones. In both cases, the environmental friendliness, safety and reliability of the design corresponds to the price and honesty of the manufacturer. And yet, when it comes time to install new windows, you can find a significant difference between these two types.

Need to install windows - pros and cons of wooden and plastic structures

If you need to install wooden window, then you shouldn’t trust companies that promise to deliver the structure the day after tomorrow. This is basically impossible, because the minimum production time for a wooden structure is 30 days. The wood needs to be dried, painted or tinted, and varnished if you need to install wooden windows. But when you need to install plastic window, then the company can produce it within 24 hours. Especially if the manufacturer has its own production.

When to install windows, then wooden structures give way to the palm for two reasons. This is a painstaking installation and a high price. To actually install wooden European structures, you will need to pay about 3-4 times than for a structure with a PVC profile.

When need to install windows However, you should remember that even the most expensive plastic structures are made of polyvinyl chloride. And this means that when high temperatures, in extreme heat or during a fire the greatest amount of harmful substances will be released

Service life must also be taken into account when installing windows. After all, plastic structures will last on average about 40 years. They have already proven themselves well in the difficult Russian climate. Wooden structures will last about 10 years, and then the sun, wind and moisture will do their dirty work and gradually destroy the structure.

If you need to install a plastic window, then at least because it is easier and faster to do. When you need to install windows, you can install plastic structures yourself with minimal experience. This trick with a wooden structure will no longer work. Installing a wooden structure is quite a process that requires both experience and special tools.
Another reason why you need to install plastic windows is easy maintenance. You will only need to wipe the profile with a cloth, adjust and lubricate the fittings, and change the seal. A wooden profile that dries out or absorbs moisture requires more attention. But on the other hand, the wood must be restored, and the plastic will have to be completely changed.

A glass unit in a plastic structure is easier to replace. This can be done in a few days. But in a wooden structure this is much more difficult to do. In it, the glass unit is firmly glued into the sash with silicone sealant, and the bead is securely attached. Therefore, it is very difficult to remove a double-glazed window without damaging the glazing bead. This is also taken into account when installing windows. If it is a domestic design, then it will take a week or two to replace the double-glazed window. And if the manufacturer turns out to be foreign, then you will have to wait at least a month for a replacement.

Basic concepts of reliability. classification of failures. Components of reliability

The terms and definitions used in reliability theory are regulated by GOST 27.002-89 "Reliability in technology. Terms and definitions."

1. Basic concepts

Reliability– the property of an object to perform specified functions, maintaining the values ​​of established operational indicators over time and within specified limits.
An object– a technical product for a specific purpose, considered during the periods of design, production, testing and operation.
Objects can be various systems and their elements.
An element is the simplest component of a product; in reliability problems, it can consist of many parts.
A system is a set of jointly operating elements designed to independently perform specified functions.
The concepts of element and system are transformed depending on the task at hand. For example, a machine tool, when establishing its own reliability, is considered as a system consisting of individual elements - mechanisms, parts, etc., and when studying the reliability of a production line - as an element.
The reliability of an object is characterized by the following main states and events.
Serviceability– the state of the object in which it meets all the requirements established by the normative and technical documentation (NTD).
Performance– the state of an object in which it is capable of performing specified functions, maintaining the values ​​of the main parameters established by the normative and technical documentation.
The main parameters characterize the functioning of the object when performing assigned tasks.
Concept serviceability broader than the concept performance. An operational object must satisfy only those requirements of the technical documentation, the fulfillment of which ensures the normal use of the object for its intended purpose. Thus, if an object is inoperative, then this indicates its malfunction. On the other hand, if an object is faulty, this does not mean that it is inoperable.
Limit state– the state of an object in which its intended use is unacceptable or impractical.
The use (use) of the object for its intended purpose is terminated in the following cases:

in case of an unrecoverable security breach;

in case of irreparable deviation of the values ​​of the specified parameters;

with an unacceptable increase in operating costs.

For some objects, the limit state is the last in its operation, i.e. the facility is decommissioned; for others, it is a certain phase in the operational schedule that requires repair and restoration work.
In this regard, objects can be:

unrecoverable, for which operability in the event of a failure cannot be restored;

recoverable, the functionality of which can be restored, including by replacement.

Non-recoverable objects include, for example: rolling bearings, semiconductor products, gears, etc. Objects consisting of many elements, for example, a machine tool, a car, electronic equipment, are recoverable, since their failures are associated with damage to one or a few elements that can be replaced.
In some cases, the same object, depending on its characteristics, stages of operation or purpose, can be considered recoverable or non-recoverable.
Refusal– an event consisting in a violation of the operational state of an object.
Failure criterion is a distinctive feature or set of features according to which the fact of a failure is established.

2. Classification and characteristics of failures

By type, failures are divided into:

operational failures(the performance of the main functions of the object stops, for example, the breakdown of gear teeth);

parametric failures(some object parameters change within unacceptable limits, for example, loss of machine accuracy).

By their nature, failures can be:

random, caused by unforeseen overloads, material defects, personnel errors or control system failures, etc.;

systematic, caused by natural and inevitable phenomena that cause gradual accumulation of damage: fatigue, wear, aging, corrosion, etc.

Main characteristics of failure classification:

nature of occurrence;

cause of occurrence;

nature of elimination;

consequences of failures;

further use of the object;

ease of detection;

time of occurrence.

Let's take a closer look at each of the classification features:

Sudden failures usually manifest themselves in the form of mechanical damage to elements (cracks - brittle fracture, insulation breakdowns, breaks, etc.) and are not accompanied by preliminary visible signs of their approach. Sudden failure is characterized by the independence of the moment of occurrence from the time of previous operation.
Gradual failures are associated with wear of parts and aging of materials.

cause:	structural failure caused by deficiencies and poor design of the facility;
	production failure associated with errors in the manufacture of an object due to imperfections or violations of technology;
	operational failure caused by violation of operating rules.
character of elimination:	sustained failure;
	intermittent failure (appearing/disappearing).
	consequences of failure: easy failure (easily remedied);
	average failure (not causing failures of adjacent nodes - secondary failures);
severe failure (causing secondary failures or leading to a threat to human life and health).	further use of the object:
	complete failures that prevent the facility from operating until they are eliminated;
partial failures, in which the object can be partially used.	ease of detection:
	obvious (explicit) failures;
hidden (implicit) failures.	time of occurrence:
	running-in failures that occur during the initial period of operation;
	failures during normal operation;

wear failures caused by irreversible processes of wear of parts, aging of materials, etc.

3. Components of reliability

Reliability is a complex property that includes, depending on the purpose of the object or its operating conditions, a number of simple properties:

reliability;

durability;

maintainability;

Reliability preservation.
– the property of an object to continuously maintain operability for some operating time or for some time.
Durability Operating time is the duration or volume of work of an object, measured in any non-decreasing quantities (unit of time, number of loading cycles, kilometers, etc.).
Maintainability– the property of an object to maintain operability until a limit state occurs with an established system of maintenance and repairs.
Storability– a property of an object, which consists in its adaptability to preventing and detecting the causes of failures, maintaining and restoring operability through repairs and maintenance.
– the property of an object to continuously maintain the required performance indicators during (and after) storage and transportation.

Depending on the object, reliability can be determined by all of the listed properties or part of them. For example, the reliability of a gear wheel and bearings is determined by their durability, and the reliability of a machine tool is determined by its durability, reliability and maintainability.

4. Main reliability indicators Reliability indicator
quantitatively characterizes the extent to which a given object has certain properties that determine reliability. Some reliability indicators (for example, technical resource, service life) may have a dimension, a number of others (for example, the probability of failure-free operation, availability factor) are dimensionless.
Let's consider the indicators of the reliability component - durability.– operating time of an object from the start of its operation or resumption of operation after repair until the onset of the limit state. Strictly speaking, the technical resource can be regulated as follows: up to average, capital, from capital to the nearest average repair, etc. If there is no regulation, then we mean the resource from the start of operation until reaching the limit state after all types of repairs.
For non-repairable objects, the concepts of technical resource and time to failure coincide.
Assigned resource– the total operating time of an object, upon reaching which operation must be stopped, regardless of its condition.
Life time– calendar duration of operation (including storage, repair, etc.) from its beginning until the onset of the limit state.
In Fig. a graphical interpretation of the listed indicators is given, with:

t0 = 0 – start of operation;
t1, t5 – shutdown moments for technological reasons;
t2, t4, t6, t8 – moments of switching on the object;
t3, t7 – moments when the object is taken out for repairs, medium and major, respectively;
t9 – moment of termination of operation;
t10 – moment of object failure.

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Safety assessment of buildings and structures.

Technical inspection of structures makes it possible to establish their reliability at the time of inspection. However, to make a conclusion about further operation, establish the service life and repair of a structure, it is necessary to know the change in these properties over time. For example, if over time concrete structures retain their strength characteristics, then many new synthetic materials often lose their construction properties over a period of 10-20 years, which cannot be acceptable for permanent buildings and structures.

When operating structures for assessment technical condition Visual inspections are widely used in structures. For this purpose, there are methodological recommendations and tabular data for assessing the results of observations, which establish the reliability of the structures being examined based on external signs of their condition and damage assessment. More accurate data is obtained through instrumental measurements using various devices based on physical, radiological, electromagnetic and other influences.

As observations have shown, during the operation of structures there is a cyclic change in their reliability, which is associated with variability of loads and load-bearing capacity due to various damages.

Damage to a structure can be of two types depending on the reasons for its occurrence: from force effects and from environmental influences (temperature changes, corrosion processes, microbiological effects, etc.). The latter type of damage not only reduces the strength of the structure, but also reduces its durability.

Particular attention should be paid to the danger of terrorist influences, which have become relevant recently. The degree of protection from terrorist and other emergency impacts and the economic justification for protection measures should be determined depending on the significance of these objects for the life of the city (management facilities, etc.).

Forecasting emergency situations

An analysis of extreme situations in construction practice has shown that accidents are directly or indirectly related to violation of the requirements of the rules and regulations for the design and construction technology of buildings and structures.

Compliance with current norms and rules guarantees the reliability of construction projects under various natural influences and ensures human safety during their qualified operation. The probability of damage to these objects usually does not exceed 2.4 · 10-6, which is acceptable from the conditions of economic feasibility.

Risk assessment under emergency forecast conditions

The study of the causes of accidents served as the basis for assessing the possibility of the occurrence of conditions affecting the reliability of the structure. These conditions include the reliability of design solutions, quality of construction and operation.

Insufficient design reliability may arise due to:

• 1) inconsistency of the adopted calculation model with the actual operation of structures due to the absence or incomplete use of the requirements of design codes and standards, ambiguity of design schemes, incorrect definition of loads and operating conditions of the facility, as well as incorrect consideration of the resistance of load-bearing and enclosing structures to temporary and accidental influences;
• 2) insufficient verification and incorrect engineering assessment of the adopted design solution in real conditions (lack of experience in operating the designed buildings and structures, significant differences in the dimensions of the designed object and loads in comparison with previously constructed similar structures, etc.);
• 3) violations building codes and rules when performing design in terms of: completeness and reliability of engineering-geological studies, taking into account the aggressiveness of the external environment, errors in determining loads and impacts, incorrect tolerances for the manufacture of structures and products, low quality of materials, violations of construction methods and operating rules, etc.;
• 4) mistakes made due to the lack of sufficient experience and qualifications of designers, lack of time or funds for detailed design.

Poor quality construction of facilities can arise due to:

• - use of materials and structures that do not comply with the design;
• - low quality of construction and installation works;
• - use of unusual or untested construction methods;
• - poor control over the quality of construction, unsatisfactory interaction between designers and builders;
• - low qualification of production personnel or their frequent change;
• - unsatisfactory conditions at the construction site: lack of time, funds, poor staff relationships;
• - deviations from building codes and rules of construction practice during the construction of a structure, deviations from the original project;

Poor operation may occur due to:

• - loads exceeding the calculated design values;
• - lack of control over the condition of the structure and operation of the structure with unrepaired defects;
• - deviations from the rules of operation, use of the structure for other purposes.

The accident analysis showed that if any of the specified conditions are not met, an accident at the construction site is possible.

The probability of an accident is determined based on an analysis of space-planning and design solutions that affect the reliability of structures, the use of expert assessments, as well as calculated data or materials from field surveys.

The survey questionnaire, to which experts answer anonymously, contains a number of evaluation conditions, each of which has its own specific weight, with the total sum of all conditions equal to 1 (see Appendix 3). This appendix provides typical conditions for analyzing the reliability of a structure, taking into account design features and operating conditions.

In specific conditions, if necessary, a design reliability analysis can be carried out taking into account additional requirements, and the number of conditions can be increased or changed.

Each condition is assessed on a point scale and has five answer options: 1 (unacceptable), 2 (unsatisfactory), 3 (satisfactory), 4 (good), 5 (excellent).

The conditional reliability of a building or structure β is determined by the formula

Where R i - specific reliability assessment, obtained by multiplying the specific weight of the condition by the score.

The obtained values ​​for the structure are compared with the reliability rating scale (Table 6.1).

Table 6.1. Scale for assessing the reliability and probability of accidents of structures based on expert assessments

Although determining the susceptibility of structures to an accident using the above method can be done quite approximately, the advantage of this method is that it is less dependent on subjective assessments.

For a more reliable assessment of the reliability of the structure and identification of possible emergency situations, an inspection is carried out by several independent experts.

In case of an unfavorable forecast, additional measures are prescribed to check the reliability of the initial materials for design, the quality of design solutions, construction and operation processes in order to identify and eliminate the causes of a possible decrease in the degree of reliability of the facility.

In addition to expert assessments, the reliability of a structure design can be established from an analysis of the structure as a structural system consisting of individual structures connected to each other in a certain sequence and interacting with various events.

Construction experience has shown that different structural systems of structures with the same purpose may have different reliability, and accidents occur when one or more joint failures within the system lead to a dangerous situation.

The solution to the complex problem of identifying the failure of the entire system is carried out by simplifying it by constructing a so-called logical fault tree.

The fault tree is a graphical representation of the relationships between the initial failures of individual system elements and events leading to the occurrence of various emergency situations, connected by the logical signs “and”, “or”.

Initial failures are events for which there is data on the probability of their occurrence. Usually these are failures of system elements: destruction of structures and structural joints, various initiating events (personnel errors during operation, accidental damage, etc.).

Establishing the reliability of a structure begins with a preliminary analysis of hazards, which is then used when constructing a fault tree.

The analysis is carried out on the basis of studying the process of operation and operation of the structural system, a detailed consideration of environmental impacts, and existing data on failures of similar structures.

First, they determine what constitutes a system failure and impose the necessary constraints on the analysis. For example, they establish the need to take into account the intensity and frequency of earthquakes, equipment failures, consider only the initial failure of a structure (failure in start date operation) or failure during the entire service life, etc.

Then, system elements that can cause hazardous conditions are identified, for example, structures, joints, foundation soils and foundations of a structure, external triggering events, etc. At the same time, they raise the question of what will happen to the system if any of the elements fails.

In order to obtain a quantitative assessment of reliability using a fault tree, you need to have data on the original failures. This data can be obtained on the basis of operating experience of individual construction projects, experiments and expert assessments of specialists.

The construction of a fault tree is carried out in compliance with certain rules. The top of the tree represents the final event. Abstract events are replaced with less abstract ones. For example, the event “oil tank failure” is replaced by the less abstract event “tank destruction”.

Complex events are divided into more elementary ones. For example, “tank failure” (Fig. 6.1), which can occur during its service life, is divided into failure during the test stage and failures in the first and subsequent 10 years of operation. This separation is caused by various causes of failure: the initial reliability of the structure and the accumulation of damage as a result of long-term operation.

Rice. 6.1. Failure tree of a steel oil tank during operation

When constructing a fault tree, for the purpose of simplification, events with very low probability are usually not included.

A quantitative indicator of system failure is the probability (Q) of one failure occurring during the accepted service life. System reliability ( R ) is determined by the expression

If a system consists of i elements connected using the "or" sign, its failure will be defined as

Where q, - probability of failure of the i-th system element.

At small value q i formula (6.3) can be approximately expressed as

For a system or subsystem of i elements connected by the sign “and”, the failure will be

Thus, the study of the reliability of structural systems allows us to solve several problems that are important for practice: qualitatively assess the reliability of a designed construction project and, in case of increased danger, implement measures to increase it, determine the relative reliability of a structure during design for various variants of structural schemes, quantitatively assess the reliability of structures and safety environment.

Determination of expected damage and destabilizing factors

Expected damage from natural and man-made impacts depends on two main destabilizing factors:

• - intensity and frequency of natural and man-made impacts on buildings and structures;
• - engineering (quantitative) knowledge about the resistance or protection of construction sites and residential areas from the destructive effects of man-made and natural phenomena.

The algorithm for calculating and assessing the economic consequences of the expected impacts is as follows.

For natural influences:

• - determine the scientifically substantiated possibility of the occurrence of destructive natural phenomena in the territory under consideration that can cause damage to engineering structures (transport communications, hydraulic engineering and energy facilities), industrial and civil facilities;
• - assess the probability of occurrence of each type of natural impact, their intensity and frequency of recurrence;
• - determine the state of the soil environment and establish the strength characteristics of load-bearing and enclosing structures;
• - perform a complex of analytical work and engineering calculations to determine the reliability of foundations and the resistance of building structures to loads arising from natural and man-made impacts during the design period of operation;
• - carry out work to strengthen the structures of buildings and structures, if necessary, to change transport communications schemes (for example, in avalanche-prone areas or mudflow areas) and other necessary solutions.

For technogenic impacts:

• - determine the possibility of man-made accidents and the likelihood of their occurrence;
• - assess the impact of man-made accidents on the environment and the safety of the population;
• - consider the possibility of preventing or preventing man-made impacts;
• - carry out work on the reconstruction and modernization of the facility to increase the level of safety and reliability of potentially dangerous facilities;
• - develop measures to localize the impact of the accident on the environment and to protect the population and production personnel.

Based on the expected impacts and the determination of possible damage and destruction of construction projects and the harm caused to the environment, the estimated values ​​of damage and losses are calculated, both in the area of ​​economic losses and in matters of health and livelihoods of the population. In this case, recommendations and conclusions can be of a restorative nature or reconstruction and modernization, as well as a fundamental change in the structure of the region’s economy and even relocation of the population from areas with serious dangers and damages that are not economically feasible to develop (for example, in areas of strong earthquakes, constant floods and avalanches ). Qualified analysis and serious public debate must be carried out on a case-by-case basis.

Development of measures to improve the reliability of construction projects and the life of the population

To ensure the reliability of construction projects, the strength characteristics of buildings and structures must be determined and compared with all types of loads and impacts that may arise during the design period of operation.

If insufficient stability and load-bearing capacity of construction objects is detected in relation to existing loads and impacts, the following types of work must be performed:

• - examine, using instruments and tools, all objects whose reliability raises doubts or concerns;
• - determine the strength characteristics of load-bearing structures and assess the condition of foundation soils, taking into account their behavior under vibration and other loads that can reduce the stability of the soil environment or cause damage to foundations;
• - develop a strengthening or reconstruction project that excludes damage or destruction of the object or loss of its overall stability under possible and expected loads and impacts in emergency situations;
• - in accordance with the developed project, they carry out the necessary complex of strengthening or reconstruction of the construction site;
• - carry out strict quality control of construction and installation works, taking into account the increased requirements provided for by the norms and standards for areas with high loads and impacts;
• - when performing construction and installation work, it is necessary to require a quality certificate for the materials and structures used with guaranteed durability during the estimated period of operation of the facilities;
• - acceptance into operation of a strengthened or reconstructed facility is carried out in accordance with norms and standards in accordance with project materials and actual performance data;
• - develop recommendations for the operation of buildings and structures, taking into account ensuring their reliability and durability under maximum design loads and impacts during the standard period.

Lecture . RELIABILITY INDICATORS

The most important technical quality characteristic is reliability. Reliability is assessed by probabilistic characteristics based on statistical processing of experimental data.

Basic concepts, terms and their definitions characterizing the reliability of equipment and, in particular, mechanical engineering products are given in GOST 27.002-89.

Reliability- the property of a product to maintain, within a specified time limit, the values ​​of all parameters characterizing the ability to perform the required functions in given modes and conditions of use, maintenance, repairs, storage, transportation and other actions.

Product reliability is a complex property that may include: reliability, durability, maintainability, storability, etc.

Reliability- the property of a product to continuously maintain operability for a given time or operating time under certain operating conditions.

Operating state- the state of the product in which it is capable of performing specified functions, while maintaining the acceptable values ​​of all basic parameters established by regulatory and technical documentation (NTD) and (or) design documentation.

Durability- the ability of a product to maintain operability over time, with the necessary breaks for maintenance and repair, up to its limiting state specified in the technical documentation.

Durability is determined by the occurrence of events such as damage or failure.

Damage- an event consisting of a malfunction of the product.

Refusal- an event resulting in a complete or partial loss of functionality of the product.

Working condition- a state in which the product meets all the requirements of regulatory, technical and (or) design documentation.

Faulty condition- a condition in which the product does not satisfy at least one of the requirements of regulatory, technical and (or) design documentation.

A faulty product may still be functional. For example, a decrease in the density of the electrolyte in batteries or damage to the lining of the car means a faulty condition, but such a car is operational. An inoperative product is also faulty.

Operating time- duration (measured, for example, in hours or cycles) or volume of work of the product (measured, for example, in tons, kilometers, cubic meters, etc. units).

Resource- the total operating time of the product from the start of its operation or its resumption after repair until the transition to the limit state.

Limit state- the state of the product in which its further operation (use) is unacceptable due to safety requirements or is impractical for economic reasons. The limit state occurs as a result of resource exhaustion or in an emergency situation.

Life time- calendar duration of operation of products or its resumption after repair from the beginning of its use until the onset of the limit state

Inoperative state- a condition of a product in which it is not able to normally perform at least one of the specified functions.

The transfer of a product from a faulty or inoperable state to a serviceable or operational state occurs as a result of restoration.

Recovery- the process of detecting and eliminating failure (damage) of a product in order to restore its functionality (troubleshooting).

The main way to restore functionality is repair.

Maintainability- a property of a product, which consists in its adaptability to maintaining and restoring an operational state by detecting and eliminating defects and malfunctions through technical diagnostics, maintenance and repair.

Storability- the property of products to continuously maintain the values ​​of established indicators of its quality within specified limits during long-term storage and transportation

Shelf life- calendar duration of storage and (or) transportation of the product under specified conditions, during and after which serviceability is maintained, as well as the values ​​of indicators of reliability, durability and maintainability within the limits established by the regulatory and technical documentation for this object.

N

Rice. 1. Product state diagram

reliability constantly changes during the operation of a technical product and at the same time characterizes its condition. The diagram for changing the states of the operating product is shown below (Fig. 1).

To quantitatively characterize each of the product reliability properties, single indicators such as time to failure and time between failures, time between failures, service life, service life, shelf life, and recovery time are used. The values ​​of these quantities are obtained from test or operational data.

Complex reliability indicators, as well as the availability factor, technical utilization factor and operational readiness factor, are calculated based on the given single indicators. The range of reliability indicators is given in Table. 1.

Table 1. Approximate nomenclature of reliability indicators

Reliability property		Indicator name		Designation
Single indicators
Reliability		Probability of failure-free operation Average time to failure Mean time between failures Average time between failures Failure rate Failure flow of a restored product Average failure rate Probability of failures
Durability		Average resource Gamma Percentage Resource Assigned Resource Installed resource Average service life Gamma percentage life Assigned life Assigned life
Maintainability		Average recovery time Probability of recovery Repair complexity factor
Storability		Average shelf life Gamma percentage shelf life Assigned shelf life Established shelf life
Generalized indicators
Set of properties	Availability factor Technical utilization factor Operational readiness ratio

Indicators characterizing reliability

Probability of failure-free operation of an individual product is assessed as:

Where T - time from start of work to failure;

t - time for which the probability of failure-free operation is determined.

Magnitude T may be greater than, less than or equal to t. Therefore,

The probability of failure-free operation is a statistical and relative indicator of maintaining the operability of serially produced products of the same type, expressing the probability that, within a given operating time, product failure does not occur. To establish the probability of failure-free operation of serial products, use the formula for the average statistical value:

Where N- number of observed products (or elements);

N o- number of failed products over time t;

N R- number of functional products at the end of time t testing or operation.

The probability of failure-free operation is one of the most significant characteristics of product reliability, since it covers all factors affecting reliability. To calculate the probability of failure-free operation, data accumulated through observations of operation during operation or during special tests is used. The more products are observed or tested for reliability, the more accurately the probability of failure-free operation of other similar products is determined.

Since trouble-free operation and failure are mutually opposite events, then the assessment probability of failure(Q(t)) determined by the formula:

Calculation average time to failure (or average time between failures) based on the results of observations is determined by the formula:

Where N o - number of elements or products subjected to observations or tests;

T i - uptime i th element (product).

Statistical assessment of the mean time between failures calculated as the ratio of the total operating time for the period of testing or operation of products under consideration to the total number of failures of these products for the same period of time:

Statistical assessment of the average time between failures calculated as the ratio of the total operating time of a product between failures for the period of testing or operation under consideration to the number of failures of this (their) object(s) for the same period:

Where T - number of failures over time t.

Durability indicators

The statistical estimate of the average resource is:

Where T R i - resource i-th object;

N- number of products delivered for testing or commissioning.

Gamma percentage resource expresses the operating time during which a product with a given probability γ percent does not reach the limit state. Gamma percentage life is the main calculation indicator, for example, for bearings and other products. A significant advantage of this indicator is the possibility of its determination before the completion of testing of all samples. In most cases, the 90% resource criterion is used for various products.

Assigned resource - the total operating time, upon reaching which the use of the product for its intended purpose must be stopped, regardless of its technical condition.

P odestablished resource is understood as a technically justified or specified value of resource provided by the design, technology and operating conditions, within which the product should not reach the limit state.

Statistical assessment average service life determined by the formula:

I

Where T sl i - life time i-th product.

Gamma percentage life represents the calendar duration of operation during which the product does not reach the limit state with probability  , expressed as a percentage. To calculate it, use the relation

Appointed date services- the total calendar duration of operation, upon reaching which the use of the product for its intended purpose must be stopped, regardless of its technical condition.

Underspecified service life understand the technically and economically justified service life provided by design, technology and operation, within which the product should not reach its limit state.

The main reason for the decrease in the durability of a product is the wear of its parts.