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Metal Casting Technologies : March 2005
INTRODUCTION hen a component or system ceases to function in a safe, satisfactory manner (i.e. it is no longer "fit for purpose") before the end of its expected life then it is considered to have failed in service. Reliability can be quantified in terms of the probability that a given component or system will perform in a safe and satisfactory manner under specified conditions of use for a pre-determined period of time. With respect to reliability failure in service can be considered as: Early failures which arise due to faulty material or manufacturing defects which have not been prevented and detected by quality systems. Random failures which arise due to chance overload or other occurrences such as the accidental loss of lubricant. These situations can be minimized by design, fool-proofing and condition monitoring. Wear Out failures which arise when products reach the end of their intended service lives. "Worn" parts which still operate may cause vibration and/or extra loading on other components leading to failure elsewhere in the system. Overloading of a part beyond its safe capacity may cause failure or damage leading to eventual failure under subsequent normal loading. The majority of service failures involve gradual degradation and loss of performance and do not take place in a catastrophic manner. However it must be recognized that deterioration due to fatigue damage, wear, corrosion, etc. can often lead to sudden failure and disaster if the damage is not monitored and the affected material or components are not replaced when necessary. Fatigue damage is generally considered to be responsible for about 90% of service failures in metallic materials. Clearly the most effective form of Failure Analysis is carried out before production at the design and production planning stages. This is achieved by consideration of : a "service conditions" profile that examines the expected conditions in service e.g. loading, temperatures, environment, possible corrosion, contamination, etc. a "production profile" that examines the processing & manufacturing stages via flow diagrams that provide a technical description of all treatments applied to the materials before entering service. a "failure profile" that considers all possible ways in which failure of an item in question can occur and the conditions that may lead to failure. The "Before" approach during design reviews increases reliability and minimizes the probability of failure during the intended design life. Effective design must not only prevent failure in service but must also optimize the economic use of materials and their processing. It follows that correct use must be made of available property data on materials e.g. fatigue data, fracture toughness, creep data, corrosion rates and behaviour in a given environment, etc. Nowadays the failure model profile is performed routinely as part of the Quality Assurance procedures in the design process. This is achieved using Failure Modes and Effects Analysis (FMEA) studies to provide a Design FMEA. In the automobile industry this analysis is also applied to manufacturing processes as a Process FMEA in order to improve efficiency, reduce variation and to ensure process capability and effective process control . A discussion of "analysis before failure" techniques via design reviews and FMEA studies is outside the scope of this short article which focuses on the case of "analysis after failure" which needs to be done when a service failure has unfortunately already occurred. In such cases failure analysis may well be required as evidence in legal actions under Product Liability law and in insurance claims as well as for the obvious engineering reasons. It is interesting to note that much of the early work on Failure Analysis was performed by the specialist engineering insurance companies that were founded during the period 1850-1900 at the height of the Industrial Revolution. Many of their initial investigations tackled the then very frequent and dangerous problem of exploding steam boilers . Since that period these companies have regularly published case studies of failure analysis which together with specialist handbooks [3-5] on failure provide extremely useful sources of information for engineers. The major objective of a service failure investigation is to determine the factors responsible for the failure so that further failures of similar type can be prevented. The experience gained will be invaluable in assessing the service behaviour of other similar components or plant and in the evaluation of new designs. Failure analysis together with service reports, demand for spares, customer complaints, etc. provides essential feedback information that is needed to update Design FMEA studies on the path to continual improvement. REASONS FOR FAILURE IN SERVICE Failure during service is often associated with faults but correctly produced parts may also fail due to abuse or lack of maintenance or may fail after being in use for periods longer than their design 14 METAL Casting Technologies March 2005 An Introduction to Failure Analysis Dr. John Pearce W TECHNICAL FEATURE