design
Improper selection of materials
Defects and discontinuities in metal itself
Improper processing of materials
Poor service conditions
Poor assembling
Poor maintenance
Improper Design
The deficiency in design of a component can be in various forms such as presence of stress raisers owing to sharp change in cross section, changing the design without proper consideration of its influence on stress distribution especially in high stress areas of the component. Many a time duplicating a successful design for more severe loading conditions or the design is developed owing to lack of knowledge to use proper criteria for designing the engineering components may lead to failure. Designers frequently also come across the situation when accurate calculations and clear analysis of stress (under prevailing technological understanding and capabilities) is not practicable due to complexity in geometry of the component.
Improper Selection of the Materials
For each type of expected failure mechanism, a combination of the mechanical, physical, and chemical properties should be possessed by the material to be selected for developing a design. For example, if failure of a component is expected to occur by excessive plastic deformation at room temperature and high temperature conditions, then yield strength and creep respectively will be important criterion for design. Similarly, if failure of a component is expected to occur by fracture under overloads, fluctuating loads, and impact loads then ultimate strength, endurance strength, and impact strength respectively should be considered for design purpose. Deficient material selection can occur due to reliance on tensile data for selection of materials, and inability to select of metal in light of the expected failure mechanism and so as to develop suitable criteria for the design purpose. The problem of the materials selection is further complicated when the performance of materials varies as function of time, e.g. creep, corrosion, embrittlement, etc. The criteria for the selection of metal for designing a component for a particular service conditions must be based on the expected failure mechanism.
UnFavorable Manufacturing Processing Conditions
A wide range of manufacturing processes are used for obtaining the desired size, shape, and properties in stock material which includes primary and secondary shaping processes such as castings, forming, machining, and welding apart from the processes like heat treatment. The selection of inappropriate combination of the process parameters for each of above mentioned manufacturing processes can lead to development of discontinuities, defects, unfavorable transformation, and metallurgical changes and so deterioration in the performance of final product during the service. These imperfections and discontinuities are mostly process specific and can exist in variety of forms due to improper selection of manufacturing process and their parameters. Therefore, due care must be taken by failure analyst to investigate the presence of any defect, discontinuity or unfavorable features in end produced by manufacturing processes and failed prematurely during the service.
Poor Assembling
Error in assembly can result due to various reasons such as ambiguous, insufficient or inappropriate assembly procedure, misalignment, poor workmanship. Sometimes, failures are also caused by the inadvertent error performed by the workers during the assembly. For example, failure of nut and stud assembly (used for holding the car wheel) by fatigue can occur owing to lack of information regarding sequence of tightening the nuts and torque to be used for tightening purpose; under such conditions any sort of loosening of nut which is subjected to external load will lead to fatigue failure.
Poor Service Conditions
Failure of an engineering component can occur due to abnormal service condition experienced by them for which they are not designed. These abnormal service conditions may appear in the form of exposure of component to excessively high rate of loading, unfavorable oxidative, corrosive, erosive environment at high or low temperature conditions for which it has not been designed. The contribution of any abnormality in service conditions on the failure can only be established after thorough investigation regarding compatibility of the design, manufacturing (such as heat treatment) and material of the failed components with condition experienced by them during the service.
Stress overloads, excessive temperatures(hot or cold), inadequate abnormal corrosive environments, excessive speed, shock, thermal and mechanical, and excessive wear are all common misuse or misoperations that can promote premature and material failures. Loss of metal from corrosion including chemical attack, liquid metal corrosion, stress accelerated corrosion, and corrosion fatigue are common. Inadequate or maintenance such as poor lubrication, weld repairs, contamination, and cold straightening without thermal stress relieving and wear compensation can lead to accelerated wear resulting in deformation that promote fatigue, of rotating or static components.
Poor Maintenance Strategy
The failure of many moving mechanical components takes place due to poor maintenance plan. A well‐developed maintenance plan indicating each and every important step to be used for maintenance such as what, when, where, who, and how for maintenance, is specified explicitly. Lack of information on proper schedule of maintenance, procedure of the maintenance, frequently causes premature failure of moving components.
Conclusion
In failure analysis, investigation takes place to determine the cause of failure, usually with the aim of taking corrective action to fix the problem and mitigate against further failures. A metallurgical failure analysis takes in to account as much of the information as possible during analysis. The end goal of failure analysis is to provide a determination of the root cause. Analysis of a failed part can be done using destructive testing or non‐destructive testing (NDT). Failure analysis is undertaken across all branches of manufacturing industry to prevent future asset and product fails as well as protecting against potentially dangerous risks to people and the environment.
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