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Introduction

Chapter Overview and Learning Objectives

• To understand the importance of the field of reliability in modern society.
• To learn the history and motivation of the field of reliability.
• To understand the definitions of reliability and failure.
• To explore several causes of failures.
• To understand different types of failure time.
• To learn the concept of the reliability bathtub curve.

1.1 Reliability Concepts

1.1.1 Reliability in Our Lives

In modern society, we rely on various complex and advanced devices and systems to enjoy convenience and enhance our lives. For example, autonomous vehicles allow drivers to engage in other activities while driving, and advances in space technology have ushered in an era in which civilians can also go to space.

However, advances in technology have some side effects. With the development of technology, the area in which humans can intervene is reduced, and when there is a problem in a system or device, humans can be adversely affected. For example, cars that are being produced these days are often electronic systems that systematically operate the overall functions of the car. If the central computer unit that controls the car breaks down, the driver will be limited in what they can do with it. Autonomous vehicle accidents due to system errors are also occasionally encountered, which can be viewed in a similar context.

With the development of these technologies, the necessity of the reliability field can be felt more acutely. A complex system may have thousands or tens of thousands of large and small parts interlocked. If the failure characteristics of such a system and the time it takes to failure can be predicted, very serious accidents can be prevented, and customer satisfaction can be increased.

1.1.2 History of Reliability

When did the field of reliability begin to gain attention? The United States is one of the major countries that pioneered the field of reliability. The need for reliability was highlighted during World War II. Electronic military equipment was shipped immediately after development, but when it arrived at the destination (far east), the equipment underwent many failures. At the time of development, there was no defect when the quality inspection was conducted. After investigation, it was found that the place where the equipment was actually used was a high-temperature and high-humidity environment, which caused equipment (vacuum tube) failure. In other words, the environment in which the equipment was developed and the environment in which it was used were very different, causing failure. At this time, a professional reliability analysis team was founded, and efforts to reduce the occurrence of failures in the actual equipment-use environment began.

At the same time, Germany also began to actively consider using the reliability field to increase the mission success rate of missiles. In modern society, reliability-related fields are widely used in addition to their use in the military industry. Reliability is considered to be an extended concept of quality control, and it is not an exaggeration to say that reliability is considered in designing and manufacturing almost all equipment and products, from industrial equipment to household appliances.

1.1.3 Definition of Reliability

According to the dictionary definition of the word reliability, it is as follows:

The quality of being trustworthy or of performing consistently well.

This definition can also be considered in connection with product or system failure. It can be said that minimizing failure is one of the important factors of reliability.

In addition, time is another very important factor in defining reliability, because reliability is dependent on time. For example, the failure rate within 1 year of buying a car and after 10 years will be very different. For this reason, the concept of time is an essential element in reliability.

Finally, reliability can be expressed as a probability. That is, it can be evaluated through a quantified method. For example, the probability that the purchased car will fail within 1 year could be 5%, and the probability that it will fail within 10 years could be 90%.

In summary, reliability can be defined as:

The probability that a system or product will perform the expected function in a specific environment over a specific period of time.

In other words, the defect in military equipment (vacuum tubes) in World War II previously mentioned can be seen as a result of the lack of consideration for the specific environment in which the equipment was used. In order to understand and predict reliability in such a case, the concept of probability and distribution of statistics can be applied, and it will be mainly covered in this book.

Case

In the US auto market, the warranty is usually 3 years or 36,000 miles. Automaker A is planning a 5-year or 60,000-mile warranty to give it an edge over its competitors. The automaker wants to know whether the parts they receive from their subcontractors are 99% or more reliable over 5 years. How can subcontractors prove the reliability of parts?

1.1.4 Quality and Reliability

Quality and reliability are often considered together, but the difference between these two concepts needs to be explored, as shown in Table 1.1.

Table 1.1 Description of quality and reliability.

Quality Reliability Before a product is shipped, check it to be sure there are no defects and the product is satisfactory. Evaluate how satisfactorily the product or system operates throughout the entire lifespan. Assess whether the characteristics of the product or component are within the specification and whether the process is within control. Focus on how long a product or system can maintain satisfactory functioning with minimal failures.

The American rating company J.D. Power and Associates considers the quality and reliability of automobiles as important metrics, and automotive customers are also interested in these metrics. The new car Initial Quality Index (IQS) is an index that quantifies the number of problems per 100 new cars within 90 days. The Vehicle Dependability Study Index (VDS) quantifies the number of problems per 100 vehicles that occurred in the previous year among vehicles older than 3 years. We can see by the stated time spans that the concept of reliability is implied.

1.1.5 The Importance of Reliability

As products and systems become more sophisticated and complex, reliability becomes more and more critical. In the case of a smartphone, more than 500 parts are normally included. Failure of certain parts can lead to serious failures that make the entire smartphone unusable. In addition, the complexity of products and systems can expose them to more human error in the design and the development stage.

Failure of products and systems has a substantial impact on safety and cost. According to the factor of 10 rule, the more delayed the response to the reliability problem is, the more the cost of tenfold or greater increases as it moves to the next stage, as seen in Figure 1.1.

Figure 1.1 The factor of 10 rule.

The development cycle of products is gradually shortening due to advances in technology, heated competition for products, and increased customer expectations. In the case of smartphones, development cycles often range from six months to less than a year. Such a tight schedule increases the risk of human errors and failures. The importance of reliability concepts throughout the entire product cycle could grow in these circumstances.

1.2 Failure Concepts

1.2.1 Definition of Failure

The dictionary definition of failure is as follows:

Lack of success.

The omission of expected or required action.

If we approach the definition of failure from an engineering perspective, it is as follows:

The inability of a system or component to perform its required functions within specified performance requirements.

In other words, the concept of failure is closely related to the definition of reliability mentioned previously. Failure can also be classified into hard and soft failures, as seen in Table 1.2.

Table 1.2 Description of hard and soft failures.

Hard Failure Soft Failure Failure in which all system functions are lost due to sudden stop of function. Failure that reaches the endurance limit due to gradual deterioration of operating characteristics and deterioration of performance.

Adverse effects caused by failure can be divided into three stages according to the scale shown in Figure 1.2. A small failure can cause inconvenience in our daily life, such as an air conditioning malfunction. Intermediate failures can take a toll on our bodies, property, safety, and finances, such as with a vehicle's brake failure. In addition, large failures cause disasters in society. Examples include breakdowns or crashes of airplanes and spacecraft.

Figure 1.2 The adverse effects of...