Root Cause Failure Analysis. Trinath SahooЧитать онлайн книгу.
are located at the surface or below the surface. It is also important to note the fracture direction in relation to the normal or expected loading. Markings on the fracture surface formed during the initiation and propagation of the crack can be used to evaluate the fracture and determine origin of cracking. Examination of the origin by the experienced failure analyst may reveal the cause and determine any contributing factors toward failure.
Microscopic Examination
Microscopic examination is usually performed in a Scanning Electron Microscope or SEM. A Scanning Electron Microscope is a high‐power magnifying and imaging instrument that uses an accelerated electron beam as a light source. The SEM allows magnifications up to 50 000× and improves depth‐of‐field resolution. A Metallurgical Expert can examine the fracture surface in the SEM and determine fracture topography while inspecting the origin area for anomalies. It is in the SEM that the Metallurgist can classify the fracture and determine the fracture type. Semi‐quantitative chemical information can also be gathered while in the SEM by Energy Dispersive X‐Ray Analysis.
Metallographic Examination
Metallography is defined as the science of the constitution and structure (or microstructure) of a metal. During metallographic inspection, the failure analyst or technician, sections the area of interest usually through abrasive‐wheel cutting and mounts the specimen without deformation. The metallographic cross section is then polished to a mirror‐like finish. The sample is subsequently examined by the Metallurgist in a metallurgical microscope.
Many important macroscopic properties of metallic materials are highly sensitive to the microstructure. Critical mechanical properties, like tensile strength or elongation, as well as other thermal or electrical properties, are directly related to the microstructure.
Metallographic examination involves the use of high magnification microscopy. These examinations may include optical and scanning electron microscopy. Optical microscopic examination is used to determine grain size, microstructure, and inclusion type and content. On the other hand, scanning electron microscopy is used to determine abnormalities, such as inclusions, segregation, and surface layers, as well as fracture features.
Chemical Analysis
A chemical analysis is sometimes performed by the Metallurgical Expert to determine bulk chemistry, local elemental concentration, surface corrosion products, and coating chemistry. A failed component chemically analyzed to determine whether the grade is indeed as claimed, because mixes occasionally occur at the mill, in the warehouse, or at the fabrication or manufacturing shop. A small percentage of all failures are caused by grade mixes. Chemical analysis can verify conformance to a standard or specification, detect impurities, identify alloys, and analyze trace elements. Analytical chemistry can be performed by a variety of techniques including optical emission spectroscopy, atomic emission spectroscopy, and inductively coupled plasma analysis, to name a few.
Simulations
Occasionally, the investigator must simulate the environmental conditions encountered during service to ascertain suitability of the material to environmental conditions and to determine the effect of prior heat treatment or other processing on the service performance of the material. For example, improper heat treatment may render the material susceptible to certain types of attack. Simulation of the heat treatment as reported in the case history is valuable for both confirmation purposes and for further testing, particularly on a comparison basis (e.g., comparison of the toughness of the failed material as received and after various experimental heat treatments). Certain types of simulation tests require accelerated testing to obtain the desired information in a reasonable length of time. Interpretation of accelerated test data must be done with care.
Data Analysis, Conclusions and Report
After the completion of the outlined steps, the investigator should be ready to interpret and summarize the data that has been collected. Some of the work performed may not contribute in determining the root cause of the failure, yet it may be helpful in eliminating some possible causes. In combination, the steps that have been outlined will, in most cases, enable the investigator to conclude the root cause of the failure. The report should provide the following:
Description of the failed component
Conditions at the time of failure
Background service history
Mechanical and metallurgical data about the failed part
Evaluation of the material quality
Discussion of mechanisms that explain the root cause of the failure
Recommendations for prevention of future failures or for action to be taken with similar parts.
Impact of working condition on metallurgical failure
The failure of an engineering component in actual working conditions can occur due to very large of factors related with design, materials, manufacturing, service conditions, etc. To have systematic understanding on various factors which can lead to metallurgical failure of engineering components, these can be groups under following headings.
Improper 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