This article is a chapter originally planned for inclusion in the professional technical training textbook "Resistance Strain Gauge Load Cells" compiled by Professor Liu Jiuqing in 2006 by the China Weighing Instrument Association. If it infringes on any rights, please contact the editor of this public account for deletion. The following is the main text: All load cells and electronic weighing systems composed of load cells will experience performance fluctuations over time or with use, failing to perform their intended functions and eventually malfunctioning (unstable). The time from the start of use to failure of a load cell is uncertain. Reliability studies the mean time between failures (MTBF), failure rate, and reliable life of a load cell under specified conditions and within a given time. This is crucial for improving the reliability of electronic weighing instruments and electronic weighing systems assembled from load cells. Therefore, reliability has become an important quality indicator for load cells and a major indicator of market competitiveness. The International Organization for Standardization (ISO 8402) defines reliability as: "The ability of a unit to perform its intended function under given environmental and operating conditions and within a given time." Here, the term "unit" refers to an object considered as a whole; it can be a component, a subsystem, or a system. A load cell considered alone is a single component; considered within a weighing system, it becomes a subsystem or a crucial link in the system. "Time" should be understood broadly; for a load cell or electronic scale, it refers to the number of stress cycles. The definition of quality is: "The quality of a product reflects the sum of its characteristics that enable it to meet explicit and implicit needs." "Needs," by definition, can include performance, availability, reliability, and dependability. Here, "performance" is one aspect of the need, the central consideration, while reliability is the product's ability to perform its intended function under specified conditions and within a specified time. Based on this definition, the inability to perform the intended function is called a fault or failure. The core of reliability is fault; that is, reliability is caused by product failures. Failure—fault—reliability—consistency are closely related, forming a causal chain. From the above reliability theory, we can conclude that the reliability of a load cell is its continuous, fault-free operating time under specified environmental conditions. Reliability is caused by load cell failures, and reliability is a numerical value used to measure the reliability level of a load cell; it is the result of data processing based on statistical analysis of the load cell's operating time and the number of failures. To achieve a high level of reliability, it is essential to ensure the reliable design, control, and management of load cells. Section 1: Reliability of Strain Gauge Load Cells Since the 1990s, load cells and electronic scales have been widely used in industrial and commercial weighing in my country. The stability and reliability of load cells have increasingly attracted widespread attention from users. Under current conditions, developing and producing load cells with high accuracy is not difficult, but developing and producing load cells with high stability and reliability is not easy. In numerous international academic conferences and exchanges, experts from various countries have unanimously agreed that, in terms of working principle, structural characteristics, manufacturing process, and application conditions, load cells should be considered semi-permanent devices. Load cells with IP67 and IP68 protection ratings should be able to operate stably and reliably for more than 10 years. Based on the definition of product reliability and the actual application of load cells, reliability is defined as: the ability or probability of a load cell to perform its specified function under specified operating conditions and within a certain time. Specifically, load cells are the ability of a weighing sensor to maintain its technical performance and operate stably under specified operating conditions and within a certain time period. This is often measured by mean time between failures (MTBF) or reliable lifespan. As is well known, load cells are mainly used in various electronic scales and electronic weighing systems, requiring good and stable overall performance. This means that the sum of all deviations, including nonlinearity, hysteresis error, and sensitivity temperature effects, should be within a certain allowable error band. The stability of the zero point and sensitivity directly affects the long-term stability and operational reliability of the load cell. Currently, domestic and international research on sensor reliability focuses primarily on systems with high reliability requirements, such as launch vehicles and missiles, satellites and spacecraft, military and civilian aircraft, and large-scale industrial control systems. For the various sensors used in these systems, various tests are conducted based on the four basic reliability functions, resulting in extensive data acquisition, analysis, and processing. The U.S. Reliability Analysis Center and my country's military research departments have accumulated a large amount of sensor reliability data. Research on the reliability of weighing sensors, both domestically and internationally, is still limited to reliability analysis and follow-up testing. Reliability analysis is integrated throughout the entire process of load cell design, manufacturing, and use; it is a crucial aspect of reliability research. The main focus is on fault (failure) mechanism analysis. This involves macroscopic to microscopic analysis of the faults to identify their causes, understand their inherent patterns, and then implement corresponding countermeasures to improve the inherent reliability of the load cell. Follow-up testing is a simple and economical method for studying the reliability of load cells. Some domestic and international companies have conducted two types of research in this area. One involves tracking the storage life of the load cell under laboratory conditions; the other involves tracking the fault-free operating time (service life) of the load cell under operating conditions. Life testing is an important part of reliability testing, including not only storage life testing but also wear life testing and accelerated life testing. Reliability testing of load cells requires scheme design and mathematical modeling, and it demands significant funding and time, which is generally difficult for load cell manufacturers. It is recommended that manufacturers and users cooperate closely, exchange information, and accumulate reliability data. According to traditional reliability analysis methods, the product failure rate follows a bathtub curve, as shown in Figure 10.1. The image shows the following: The left side represents the early failure period, which is decreasing. In the early stages of product use, due to internal defects left over from manufacturing, the failure rate is often high. Early failures are generally eliminated through aging tests before the product leaves the factory. The middle part represents the random failure period, which is close to a constant level. This is because after the early failure period, the failure rate stabilizes at a low level. Failures during this period are often caused by random factors, hence the term "random failure period." The right side represents the wear and tear failure period, which is increasing. Due to component aging and wear, the failure rate begins to increase. The stability and reliability of load cells perfectly conform to this pattern, divided into an initial unstable period, a stable period, and a fatigue unstable period. The initial unstable period is due to residual stress generated in the elastic element after forging, machining, heat treatment, surface polishing, sandblasting, bonding of resistance strain gauges, curing, and post-curing processes. This residual stress continuously relaxes and releases, causing changes in the zero point and sensitivity, resulting in fluctuations in the load cell's performance. It can screen components under environmental stress, age and stabilize the load cell, and release and eliminate residual stress as much as possible during the early failure period (R(t)t), the accidental failure period, and the wear failure period. This allows it to overcome the initial instability period during production and enter a stable and reliable operating period after leaving the factory. After a long period of trouble-free operation, due to factors such as aging of protective sealing materials, wear of circuit compensation and adjustment components, and fatigue of resistance strain gauges, the load cell experiences performance fluctuations, instability, and eventually failure; this is called the fatigue instability period. The reliability theory of load cells studies and analyzes various systematic and random factors affecting their reliability, and scientifically and rationally proposes qualitative and quantitative requirements for reliability, such as failure mode and effects analysis, trouble-free operating time, and reliable lifespan. Section 2 Reliability Design of Strain Gauge Load Cells Introducing the reliability design methods of more complex systems into the reliability design of load cells, using systematic analysis procedures for failure mode and severity analysis, identifying potential hidden dangers, and taking measures to improve the design, still presents significant challenges. This is because whether the reliability of load cells follows an exponential or Weibull distribution requires further understanding. However, based on the working principle, manufacturing process, and application of load cells, and considering the effects of residual stress, aging of components and sealing materials, and fatigue accumulation in strain gauges, a logical approach is to independently study the reliability of each interacting and interdependent component of the load cell according to its function. This includes studying the elastic element material and heat treatment, the strain gauge and strain adhesive, the circuit compensation components and compensation process, and the protective sealing materials and sealing process. Once the technical specifications of the load cell are determined, they cannot be assigned to individual subsystems, units, or components as in designing a more complex system, providing quantitative targets for reliability design. Because the components of a load cell interact and depend on each other, they are fundamental to the load cell; a high failure rate in any one component significantly impacts the inherent reliability of the load cell. Therefore, a sound fundamental design for the reliability of the load cell is essential. I. Reliability Design of Elastic Elements The structure of the elastic element is fundamental to the load cell and is a crucial factor in its stability and reliability. Therefore, the design of the elastic element and its protective shell, welded sealing diaphragm, upper load-increasing head, and lower load-bearing pad must ensure that the load cell does not experience performance fluctuations under load, or minimizes such fluctuations. To achieve high reliability, the elastic element structure design should strive to achieve the following: single-zone stress distribution and uniform stress distribution within the strain zone; strong resistance to eccentricity and lateral loads through its own structure or the protective shell, overcoming the influence of non-measuring loads; installation force away from the strain zone; reasonable stress distribution on the load-increasing head and load-bearing pad, avoiding displacement at the load introduction point and support point; and ensuring the surface of the bonded resistance strain gauge is as flat as possible with sufficient working area to facilitate bonding and protective sealing. To ensure a reasonable elastic element structure and stress distribution, a relatively complex mechanical model should be established during design and calculation. Interactive or highly interactive finite element programs should be used, with CAD software to display the stress and displacement fields, and dynamic simulations performed. Finally, dimensional parameters are determined to obtain optimized design results. Due to advancements in science and technology and improvements in structural design in recent years, three-dimensional digital design is preferred to enhance the rationality of the elastic element structure and boundary design. Although load cells are assembled components, to achieve optimal performance and ensure stable and reliable operation, the structural design must minimize the number of parts, increase integration, and strive for a monolithic structure. II. Selection of Elastic Element Materials and Heat Treatment Processes The materials and heat treatment processes of elastic elements significantly impact the comprehensive performance indicators, long-term stability, and operational reliability of load cells, making them key issues in load cell reliability design. In particular, the solution heat treatment process and aging method for stainless steel elastic element materials directly determine the performance and reliability of the load cell. Considering the elastic modulus, hysteresis effect, residual stress influence, and fatigue resistance of elastic element materials, the following metal materials should be selected: high elastic limit and yield limit; good time and temperature stability of elastic modulus; small elastic hysteresis and elastic aftereffect; strong resistance to vibration and impact; strong fatigue resistance; good forging, machining, and heat treatment process performance; and low residual stress generation. It is difficult to make a single metallic material possess all the aforementioned properties simultaneously. However, through certain control methods and treatments, it can be made to meet or approach these requirements as closely as possible. An effective way to improve the reliability of load cells using elastic element materials is to strictly control the purity of the components and the uniformity of the material, avoiding an emphasis on any single indicator and pursuing good overall performance. In particular, it is crucial to experimentally determine the optimal heat treatment process and aging regime. For example, for 0Cr17Ni4Cu4Nb precipitation-hardening stainless steel, extensive testing of solution heat treatment and double aging methods or cooling treatment followed by aging is necessary to determine the heat treatment and aging process regime that meets the overall performance and reliability requirements of the load cell. III. Selection of Resistance Strain Gauges Resistance strain gauges are the core component of load cells. Their stability and reliability largely determine the stability and reliability of the load cell itself, making them a critical aspect of reliability control. Since resistance strain gauges cannot be reused after testing and can only measure fatigue life under laboratory conditions, this presents significant challenges for research. Currently, domestic and international load cell manufacturers typically focus on controlling the reliability of resistance strain gauges by addressing issues such as the structure of the sensitive grid, factors affecting reliability, and the selection of operating characteristics. 1. Structure of Resistance Strain Gauges The structural design of resistance strain gauges for load cells cannot remain at the level of traditional empirical design, merely focusing on grid length, grid width, and resistance value. Instead, the focus should be on researching the strain transfer theory, operational stability, and fatigue life of resistance strain gauges. Production practice has shown that, even with load cells of the same structure, bonding resistance strain gauges with different structures results in significant differences in creep parameters, the pass rate for achieving the same accuracy level, and fatigue life. This is due to insufficient research into the strain transfer theory and creep mechanism of resistance strain gauges. Resistance strain gauges for load cells often employ planar and three-dimensional finite element analysis methods to analyze and select appropriate structures based on strain transfer, the mechanical effects and optimal thickness of the substrate and overlay, the end effect of the sensitive grid and its relationship to creep, and the influence of substrate thickness on hysteresis. This allows for the design of resistance strain gauge structures that meet the comprehensive performance and reliability requirements of the load cell. 2. Factors Affecting the Reliability of Resistance Strain Gauges The structure, manufacturing process, and strain transfer principle of resistance strain gauges inevitably lead to some process defects in mass-produced products, directly affecting the long-term stability and operational reliability of the resistance strain gauges. The main factors affecting stability and reliability can be summarized as follows: (1) Influence of Sensitive Grid Foil Material and Heat Treatment Process The sensitive grid foil materials for resistance strain gauges used in weighing sensors are mostly constantan (copper-nickel alloy) and Karma/Evan alloys (nickel-chromium modified alloys). Besides the purity of the alloy composition and the uniformity of the microstructure, the main factor affecting stability and reliability is the heat treatment process of the alloy foil material. During repeated rolling and pressing, defects such as dislocations, slip, and vacancy breakage occur in the crystal lattice, and the atoms near these defects are in a thermodynamically unstable state, which is an important reason for unstable electrical performance. Therefore, stability treatment, i.e., annealing, must be performed. When the annealing temperature is reached, these atoms absorb heat energy and diffuse, causing the lattice defects to migrate and disappear, and the resistivity and temperature coefficient of resistance tend to stabilize. Annealing temperature, holding time, and number of cycles are the three key elements of stability treatment. If the heat treatment and stabilization processes for the foil are not properly selected, the electrical and temperature characteristics of the foil will become unstable, which is the main factor contributing to the instability of the resistance strain gauge. (2) Influence of Substrate, Cover Layer, and Strain Adhesive The substrate, cover layer, and strain adhesive of the resistance strain gauge are all organic polymer materials. All polymer materials are affected by moisture and oxygen in the air. Water can penetrate almost all polymers, causing plasticization, and will also deteriorate over time, i.e., its performance decreases due to physical or chemical changes. This leads to instability between the substrate, cover layer, and the sensing grid.