CONTENTS CHAPTER-1 Introduction 1.1 General consideration for good design CHAPTER-2 Design Reliability 2.1 Probabilistic Design Methodology 2.2 Stress strain Distribution 2.3 Relaibility Activities during design cycle CHAPTER-3 Designing for Higher Reliability 3.1 Methods for improving design for reliability CHAPTER-4 Reliability Testing 4.1 Evaluating design by test 4.2 Reliability Tests CHAPTER-5 Failure Analysis 5.1 Modes of Failure 5.2 Causes of Failure and Unreliabilty CHAPTER-6 Case study CONCLUSION BIBLIOGRAPHY

CHAPTER NO. 1

INTRODUCTION The development of science and technology and the needs of modern society are racing against each other. Industries are trying to introduce more and more automation in their industrial processes inorder to meet the ever increasing demands of society. The complexity of industrial systems as well as their products are increasing day-by-day. The improvement in effectiveness of such complex systems has therefore acquired special importance in recent years. The effectiveness of a system is understood to mean the suitability of the system for the fulfilment of the intended tasks and the efficiency of utilizing the means put into it. The suitability of performing definite tasks is primarily determined by the reliability and quality of the system. From time immemorial engineers recognised that a product should have a long service life and that during this life it should give service with few failures. As products became more complex, there were increasing problems with failures over time. What has evolved is a collection of tools called reliability engineering. It is evident that this definition relates to the ability of a system to continue functioning without failure. This interpretation of the term ‘reliability’ makes it totally unsuitable as a measure for those continuously operated systems which can tolerate failure. Modern reliability evaluation techniques are used in a much wider range of applications including domestic appliances, automobiles and other products which individually have little socio-economic effect when they fail. All engineers should have some awareness of the basic concepts associated with the application of reliability evaluation techniques. Two main categories into which systems can be divided are mission-oriented systems and continuously operated systems. Mission- oriented systems continue to function with failures for the duration of the mission. Continuously operated systems are those in which a number of system down-states are considered tolerable provided they do not happen too frequently or last too long. Clearly, reliability is an important element of achieving high performance since it directly and significantly influences the item’s performance and ultimately its life-cycle cost and economics. Poor reliability in design, manufacturing, construction, and operation would directly cause increased warranty costs, liabilities, recalls, and repair costs. Therefore, a high quality design, production, manufacturing and operation programme leads to low failures, effective maintenance and repair, and ultimately high performance. General consideration for good design :- 1) Appearance 2) Functional efficiency 3) Safety 4) Reliability 5) Maintainability 6) Ease a production 7) Standerdisation 8) Review of design

CHAPTER – II DESIGN RELIABILITY [4]

Reliability is an inherent attribute of a system just as is the system’s capacity or power rating. The reliability level is established at the design phase, and subsequent testing and production will not raise the reliability without a basic design change. Because reliability is an abstract concept that is difficult to grasp, many organizations find themselves unable to implement a comprehensive reliability program. This is not to say that system designers or managers in the firm are not interested in a reliable product. Rather, the pressures on the designer and, very often the organizational structure of the design department impede the development of such a program. A system desing usually requires the efforts of several design groups, each of which is well versed in such things as the determination of power ratings, material selection, or tolerances. However, designers frequently are not well versed in the philosophy and principles of reliability.

2.1 PROBABILISTIC DESIGN METHODOLOGY [4] The conventional design approach is not adequate from a reliability standpoint. Hence, another design methodology that does consider the probabilistic nature of the design is needed so that component reliability can be calculated at the design stage. Such a design methodology is called probabilistic design. Recently, engineers and designers have become increasingly concerned about problems of design adequacy in various disciplines. This has led to the widespread interest in the use of the probabilistic design approach. This approach, which originated in aerospace engineering, has now spread to the consumer products industry. Figure shows a block diagram for the methodology. For the component under consideration, the first step is to perform environmental computations, which affect the stress and strength computations. For the strength computation consideration must be given to the properties of the material used and the probability distributions of factors affecting the strength, such as the surface finish and surface treatment. For the stress computations, the load satisfies history and the probability distributions of factors affecting the stress such as stress concentration and temperature must be considered. Based on this computation the stress strength distribution and their satisfies can be obtained. These distributions are then used to compute the reliability of the component, which is defined to be the probability that the strength of the probability that the strength of the component is greater than the stress acting on the component. Imp. If we consider the total design reliability program, the steps will be as summarized below : 1) Define the design problem. 2) Identify the design variables and parameters involved. 3) Conduct a “failure modes, effects, and criticality analysis” 4) Verify the significant design parameter selection. 5) Formulate the relationship between the critical parameters and the failure governing criteria involved. 6) Determine the failure governing stress function. 7) Determine the failure governing stress distribution 8) Determine the failure governing strength function 9) Determine the failure governing strength distribution 10) Calculate the reliability associated with these failure governing distributions for each critical failure mode. 11) Iterate the design to obtain the design reliability goal. 12) Optimize the design in terms of performance, cost, weight, etc. 13) Repeat optimization for each critical component. 14) Calculate system reliability 15) Iterate to optimize system reliability.

2.2 STRENGTH AND STRESS DISTRIBUTION [4] The probability distributions for strength can be approached in two ways. If it is assumed that the strength of the components is determined by the weakest point, then the distribution will be determined by the lowest value of samples taken from a distribution that describes the strength of all points in the material. This assumption leads to an extreme value distribution. An alternative assumption is that the weaker points receive support from the stronger points surrounding them; that is, an averaging process occurs. Accordingly, the distribution of the strength is related to the mean value of samples from the distribution of strength of all points, and this leads to a normal distribution. It should be noted that the normal approximation has some limitations. Negative values for strength are in admissible, hence the strength cannot be truly normally distributed since the normal distribution has the limits from minus infinity to plus infinity. However, when the coefficient of variation is less than 0.3 the probability of negative values is negligible for the strength data.

2.3 RELIABILITY ACTIVITIES DURING THE PRODUCT DESIGN CYCLE.

DESIGN REVIEW A design review is a formalize, documented, and systematic review of the design by senior company personnel. The review from a reliability, maintainability, and service ability standpoint must be a technical review aimed at ferreting out weak points in the design and concerned with design improvements that will ensure early product maturity and improve reliability.

FAILURE MODE, EFFECTS, AND CRITICALITY ANALYSIS. For failure prevention during system design, two techniques are presented; failure mode, effects, and criticality analysis and fault-tree analysis. Failure mode, effects and criticality analysis (FMECA) represents a “bottom-up” approach while fault-tree analysis (FTA) is a “top-down” approach. Both represent qualitative as well as quantitative approaches for assessing the consequences of failure and determining means for prevention or mitigation of the failure effect. Failure mode and effects analysis (FMEA) is a procedure by which potential failure modes in a system are identified and analyzed. Since design is usually approached from an optimistic viewpoint, the FMEA procedure assumes a pessimistic viewpoint to identify potential produce weaknesses. The terms FMEA; design FMA (DFMA); and failure mode, effects, and criticality analysis (FMECA) are also used. The main purpose of the technique is to identify and eliminate failure modes early in the design cycle where they are most economically dealt with. A documented FMECA procedure can be found in MIL-STD-1629A (1980).

FAULT-TREE ANALYSIS Fault-tree analysis is a technique used for systems safety and reliability analysis. The analysis proceeds from a designated “top event” to basic failure causes called “primary events.” A fault tree is a model that graphically portrays the combination of events leading up to the undesirable top event.

RELIABILITY DESIGN GUIDELINES We will now discuss some basic principles of reliability in design that are useful for the designer. Each concept is briefly discussed in terms of its role in the design of reliable system. Simplicity - Simplification of system configuration contributes to reliability improvement by reducing the number of failure modes. A common approach is called component integration, which is the use of a single part to perform multiple function. Use of Proven Components and Preferred Designs - (1) If working within time and cost constraints, use proven components because this minimizes analysis and testing to verity reliability. (2) Mechanical and fluid system design concepts can be categorized and proven configuration given first preference.

Stress-Strength Design - The designer should use various sources of data on strength of materials and strength degradation with time related to fatigue. The traditional and common uses of safety factors do not address reliability, and new techniques such as the probabilistic design approach should be used. The designer would use derating factors, proper reliability or safety margins, and develop stress-strength testing to determine stress and strength distributions. The probabilistic design approach is explained in the following section.

Redundancy - Redundancy sometimes may be the only cost-effective way to design a reliable system from less reliable components.

Local Environmental Control - A severe locate environment sometimes prevents the achievement of required component reliability. In that case, the environment should be modified to achieve high reliability. Some typical environmental problems are 1) shock and vibration, 2) heat, and 3) corrosion.

Identification and Elimination of Critical Failure Models – This is accomplished through FMECA and also by falut-tree analysis.

Self-Healing - A design approach which has possibilities for future development is these of self-healing devices. Automatic sensing and switching devices represent a form of self-healing

Detection of Impending Failure - Achieved reliability in the field can be improved by the introduction of methods and/or devices for detecting impending failure. Some of the examples are 1) screening of parts and components 2) periodic maintenance schedules, and 3) monitoring of operations. Preventive Maintenance – Preventive maintenance procedures can enhance the achieved reliability, but the procedures are sometimes difficult to implement. Hence effective preventive-maintenance procedures must be considered at the design stage.

Tolerance Evaluation – In a complex system, it is necessary to consider the expected range of manufacturing process tolerances, operational environmental, and all stresses, as well as the effect of time. Some of the tolerance evaluation methods are worst-case tolerance analysis, statistical tolerance analysis and marginal checking.

Human Engineering - Human activities and limitations may be very important to system reliability. The design engineer must consider factor which directly refer to human aspects, such as 1) human factors, 2) human-machine interface, 3) evaluation of the person in the system, and 4) human reliability.

CHAPTER III DESIGNING FOR HIGHER RELIABILITY

A number of techniques are available to enhance the system reliability. Some of the important methods are : 1. Parts improvement method. 2. Effective and creative design 3. System simplification 4. Use of over rated components 5. Structural redundancy. 6. Maintenance and repair In the parts improvement method either the reliability of all the constituent components are improved or at least the most critical components are identified and their reliability improved. This involves application of improved production techniques and automation and therefore, is a costly and difficult means of achieving the reliability. Nevertheless, it is quite effective up to a certain point. Since the production of a perfect component is almost impossible and the cost of part improvement is very high, the approach becomes unwidely when one deals with large and complex system. The effective and creative design approach needs some thinking on the part of the design engineer to create a new or improved circuit or system with better reliability. In cases where the systems are poorly designed and overly complex, the proper use of the components and reducing the complexity can prove to be an important technique for improving system reliability. However, over simplification can lead to poor quality and efficiency of the system. Failure rates of almost all components change with their operating stresses, and therefore can be reduced significantly by the application of over-rated components. The degree of improvement depends upon the type of component. Although it is often possible to trade money an other resources to buy over rated equipment’s, its application is obviously limited by the availability of the equipment with the required ratings. Maintenance and repairs, where ever possible, undoubtedly boost the system reliability. A maintained system when combined with redundancy may have a reliability of almost one.

METHODS FOR IMPROVING RELIABILITY DURING DESIGN Action to improve reliability during the design phase is best taken by the designer. The designer understands best the engineering principles involved in the design. The reliability engineer can help by defining areas needing improvement and by assisting in the development of alternatives. The following actions indicate some approaches to improving a design : 1. Review the users’ needs to see if the function of the unreliable parts is really necessary to the user. If not, eliminate those parts from the design. Alternatively, look to see if the reliability index correctly reflects the real needs of the user. For example, availability (see below) is sometimes more meaningful than reliability. If, so a good maintenance program might improve availability and hence ease the reliability problem. 2. Consider trade-offs of reliability for other parameters, e.g., functional performance, weight. Here again it may be found that the customer’s real needs may be better served by such a trade-off. 3. Use redundancy to provide more than one means for accomplishing a given task in such a way that all the means must tail before the system fails

CHAPTER – 5 FAILURE ANALYSIS 5.1 MODES OF FAILURE The reliability of an item which can only fial once, such as a transistor or a ligh-bulb is usually defined as the probability of no failure over a specified time period, which might be the expected life of the item. During this period the instantaneous probability of failure is called the hazard rate. Other reliability characteristics, such as the mean time to failure (MTTF) of the expected life by which a specified proportion say 10% might have failed, are also used. Many years of experience of failure data of various devices has shown that the failures, in general, can be grouped into different modes depending upon the nature of failure. When we put a large collection of units into operation. It is likely that there are a large number of failures initially. The early failures are called initial failures or infant mortality. These failures are primarily due to manufacturing defects, such as weak parts, poor insulation, bad, poor fits etc.