Reliability engineering: Reliability is the probability that a system or component will perform without failure for a specified period of time under specified operating conditions. Reliability engineering is the study of the causes, distribution and prediction of failure. Assessing the reliability of a design (product or process) is an essential step in modern quality-engineering products.
Causes of Unreliability These causes fall into five categories: Design mistakes: failure to include important operating factors, poor materials selection, incomplete information on loads and environmental conditions. Manufacturing defects Maintenance Exceeding design limits Environmental factors: subjecting equipment or products to environmental conditions for which it was not designed.
Failure Failure is the inability of a process or product to function as desired. That is the performance will drop to a level below predetermined acceptance standards. Minor failures are not necessarily bad, they give us the method to control the process or improve the product. If the minor failures occur very frequently, however, there is a chance (risk) that some of them may escalate to a higher level.
Event escalation model: Before this can occur, three barriers, PEOPLE, PLANT and PROCEDURES must be overcome. PEOPLE: Whose competence, training and monitoring enable them to put and correct the conditions that cause minor failure and therefore reduce their impact. Severe failures Procedure Plant PLANT: Consisting of the hardware (or software). Various protective systems are provided to prevent the escalation of minor failures. First aid boxes, fire extinguishers, furnace protection systems, emergency shut down or release systems. People Minor failures
PROCEDURES: Are the means of transferring other people s knowledge and experience to those operating the process. Manufacturers will tell you how to operate their equipment and software vendors will give you navigation guides and help screen. Some of the knowledge may have been gained as a result of earlier failures. The three barriers act in combination with one another. Well-trained and motivated people can compensate for the lack of procedures. A well documented and organised plant can be managed with relatively few trained people.
Failure Rate Curve Infant mortality Few (random) failures Time Failures due to wear-out
Reliability Engineering Measurement: Many techniques have been established for measuring and improving the reliability of a product design. Generally, reliability engineering is identified based on risk assessment methods. Hazards Analysis Failure Mode and Effect Analysis (FMEA) Failure Mode Effect and Criticality Analysis (FMECA) Fault Tree Analysis
1. Failure Mode and Effect Analysis (FMEA) FMEA (and FMECA) is a group of activities designed to: Recognize and evaluate the potential failure of a product/process and its effects Identify actions which could eliminate or reduce the chance of potential failure occurring Document the process Product/ Design FMEA is a tool used to assure that potential product failure modes and Their associated causes have been considered and addressed in the design or manufacturing process
The FMEA approach is to: 1. Identify known or potential failure modes, which may affect a product 2. Identify those design or process elements which may cause a product to fail 3. Assess and prioritise potential failure modes for corrective action 4. Assess the effectiveness of correction action and provide follow up Key resources necessary to conduct successful FMEA programs: Commitment of top management Knowledge in: design, manufacturing, assembly, service, quality, reliability
Advantages of FMEA: Enhance design and manufacturing efficiencies Reduce late change crises Minimise exposure to product failures Increase business records Add to customer satisfaction Limitations of FMEA Training of employees Initial impact on product & manufacturing schedules Financial impact required to upgrade design, manufacturing, process equipment and tools
FMEA Procedure There are two phases in FMEA To identify the potential failure modes and their effects. To perform criticality analysis to determine the severity of the failure modes. We need to construct a table with columns for: Component Failure mode Effect of failure Cause of failure Add columns for estimating value
Determine value (scale of 1 10) for: Occurrence 1 low, 10 - high Severity 1 minor, 10 - serious Problem of detection 1 certainty, 10 nearly impossible Calculate Risk Priority Number Risk priority number (RPN) = occurrence x severity x problem of detection For each, give corrective action
FAILURE MODE AND EFFECTS ANALYSIS (FMEA) Subsystem/Name: DC motor P = Probabilities (chance) of Occurrences Final Design: d/m/y Model Year/Vehicle(s): 2000/DC motor S = Seriousness of Failure to the Vehicle Prepared by: D = Likelihood that the Defect will Reach the customer R = Risk Priority Measure (P x S x D) Reviewed by: Name FMEA Date (Org.): d/m/y (Rev.) d/m/y 1 = very low or none 2 = low or minor 3 = moderate or significant 4 = high 5 = very high or catastrophic No. Part Name Part No. Functi on Failure Mode Mechanism (s) & Causes(s) of Failure Effect(s) Of Failure Curre nt Contr ol RPN P S D R Recommen ded Corrective Action(s) Action (s) Taken 1 Position Control ler Receiv e a deman d positio n Loose cable connecti on Incorrec t demand signal Wear and tear Operator error Motor fails to move Position controlle r breakdo wn in a long-run 2 4 4 4 1 3 8 48 Replace faulty wire. Q.C checked. Intensive training for operators.
Failure Mode Effect and Criticality Analysis (FMECA) The term criticality is important because it develops priorities where the design team should be spending it resources. Criticality refers to how often a failure will occur, how easy it is to diagnose and whether it can be fixed. The purpose of an FMECA is to : analyse the probable causes of product failure,
determine how the problem affects the customer Identify which process-control variable to focus on for prevention and detection. Quantify the effect on the customer. One of the key results of FMEA or FMECA process is the document it produces, which is structured to: facilitate the thinking process, focus the mind on what is important and document the thinking process in a standardised easy-to-follow manner.
Techniques of Failure Analysis When the problem of determining the cause of failure and proposing corrective action must be faced, there is a definite procedure for conducting a failure analysis. There are a number of reasons why a problem of failure should be investigated. for scientific purposes In order to apportion blame (product liability) To identify and eliminate the cause of failure To improve performance
1. Visual Examination: Without doubt visual examination carried out by a skilled and competent investigator is by far the most important aspect of mechanical failure analysis. It identifies the mechanism of failure. Whenever possible, visual examination should be carried out on site. Visual examination should be documented with photographs The following critical pieces of information should be obtained during the on site inspection: location of all broken pieces relative to each other Identification of the origin of failure Orientation and magnitude of stresses
Direction of crack propagation and sequence of failure presence of obvious material defects, stress concentrations.. Presence of oxidation, or corrosion products 2. Background History and Information A complete case history on the component that failed should be developed as soon as possible. Name of item, identifying numbers, owner, manufacturer Function of item Data on service history including inspection of operating logs and records
Discussion with operating personnel Documentation on materials used in the item Information on manufacturing and fabrication methods used including any codes or standards Documentation on inspection standards and techniques used Date and time of failure, temperature and environmental conditions Date and time of failure, temperature and environmental conditions Documentation on design standards
3. Macroscopic Examination This is part of the visual examination. Macro-examination used x5 to x25 magnifications where the object can remain in one piece. Often it is possible to identify the type of fracture from macroscopic examination 4. Microscopic Examination After visual examination, micro-examination is the most important aspect of failure analysis. This is made at magnifications greater than x100. The investigator covers the use of instruments as the optical microscope, SEM, TEM and EDAX. Correct micro-examination requires firstly the professional skill to identify the areas where specimens should be cut.
Secondly, there is the technical skill to correctly cut, mount, polish and etch the selected specimens. Finally, there must be the metallurgical ability to recognise the structure and effects produced. 5. Additional Tests Chemical Analysis Chemical analysis of metals or alloys is frequently included in failure analysis reports. It is part of the process of identifying the material and finding whether it meets the specifications.
Mechanical Testing: It is important to measure the mechanical properties (hardness) of an unused specimen of the material that failed. Non-Destructive Testing (NDT) The aim is to search for defects. The various NDT techniques include: 1. Liquid Penetrant Inspection (LPI) 2. Magnetic particle Inspection (MPI) 3. Radiography 4. Eddy Current 5. Ultrasonic Inspection
6. Report of Failure The technical report is an end product of the failure analysis and should be written in clear language which can be understood by the client. Reports should have a format and should contain the following: Introduction and background Conclusion Recommendations Visual examinations Technical investigation (NDT, macro and micro-examination, mechanical tests, chemical analysis etc.. Discussion