News

Failure Analysis

• satyendra
• December 16, 2014
• 0 Comments
• cause of failure, crack, Data, failure analysis, inspection, mode of failure, testing,

Failure Analysis

Failures of equipment components and assemblies, or structures in industry can cause loss of life, unscheduled shutdowns, increased maintenance and repair costs, and damaging litigation disputes.

To prevent future recurrence of the problem caused by the failures ,it is essential to carry out an investigation in each failure. The conducting of an  investigation for a failure is known as failure analysis.

 Failure analysis is a process of collecting and analyzing data and it is carried out to determine the causes or factors that have led to the undesired loss of functionality or failures of equipment components and assemblies, or structures. It is a multilevel process that includes physical investigation. The normal scope of a failure analysis is to find the failure mechanism and the most probable cause of the failure. The term failure mechanism is normally described as the metallurgical, chemical, mechanical, or tribological process leading to a particular failure mode.

Failures of the equipment components and assemblies, or structures occur as a result of some sort of a mistake causing a weak link in the chain of the continuous process of engineering, design, manufacturing, and operation. The cause of a failure can be any one or more of the following.

• Fault in design
• Defects in material
• Deficiencies during processing and manufacturing
• Defects in assembly or installation
• Off-design or unintended service conditions
• Deficiencies in maintenance (neglect and procedures etc.)
• Improper operation

Major steps while conducting a failure analysis are given in Fig 1.

Fig 1 Majors steps in failure analysis

The main principle of a failure analysis is to preserve evidence and the necessary information from the subject part or assembly in the as-received condition and the same is captured before anything is done to alter its condition. Further during failure analysis it is necessary to perform tests in order of less destructive to more destructive in nature.

Failure analysis starts with collecting the information concerning the history of the component involved in the failure, such as (i) process involved, (ii) the application, (iii) type of material, its specifications, shape, dimensions, and processing technique, (iv) design parameters, (v) service conditions, (vi) maintenance history, (vii) sequence of events preceding the failure, (viii) frequency of failure and whether it is of localized nature, etc.

Inspection of the site of the failure is a very important step that aids in the failure analysis. A thorough examination of the condition of the failed component at the failure site can provide a great deal of information. During site inspection, it  is necessary to give attention to such features as location of cracks, fracture characteristics, perforations, deposits, wall thinning, sagging, and detached pieces etc. Also, wherever possible, it is necessary to obtain as much information as possible from eyewitnesses about any abnormal conditions preceding the failure. In some cases, visual inspection of the failed component can reveal information which cannot be otherwise obtained.

During the visual inspection it is important to photograph the machine and surroundings, the failed part and the mating part. Photographic documentation of the failed component is always necessary for future reference and further inspection after leaving the site. It is important to photograph all features of the failed component.

Clear defining of the problem is an essential part of a failure analysis. The depth of the information obtained, site inspection, as well as the skill and judgment of the person carrying out the failure analysis helps in clearly defining the problem. For example, the problem may be defined as metallurgical, mechanical or environmental or a combination of two or more of them. Further through discussions with the concerned personnel at the site and through visual inspection the person carrying out the failure analysis can narrow down the various possibilities. Also, the failure may resemble a previous case of failure.

Based on the likely cause of failure as per the defined problem during the site inspection, an experimental program is developed to determine the cause of failure. The nature and number of selected experiments must be such that they should lead to identifying the cause of failure. Samples necessary to carry out the selected experiments are to be carefully drawn in such a way  that they represent as much as possible all features of the failed component. Deposits or pieces detached from the component can provide valuable information. Whenever possible, samples removed from sound sections, as well as samples never used in service, must also be collected for comparative purposes.

Investigative tools used in failure analysis studies are usually of two types namely (i) tools which can be used on site and (ii) laboratory tools.

Material verification with respect to the specification is an important component of the failure analysis. Chemical analysis and analysis of macrostructure as well as microstructure is carried out for this purpose. Also mechanical testing is carried out in case sufficient material is available to find out tensile strength, impact toughness etc., for evaluating the properties of the failed component. In many cases both surface hardness and micro hardness tests are carried out for evaluating the mechanical strength. These tests are performed on the failed component for finding out the compliance of the results with the specifications

Stress analysis, fracture mechanics, and corrosion failure analysis are very valuable activities in failure analysis investigations. These analyses provide information to the person investigating the failure whether the material used has been the right choice to meet the requirements of its application.

From knowledge of service conditions such as applied loads or pressure, and geometry and size of the component, it is possible to calculate the principal stresses developed in the part. Comparison of the maximum principal stress with the strength of the material used in the application provides important information to the person investigating the failure about the suitability of the material for the  application and whether the design parameters were strictly followed during service.

Analysis using the principles of fracture mechanics provides valuable information to the person investigating the failure about the stress developed in the failed component at the time of fracture in relation to the design stress, particularly in the case of crack propagation by a brittle mechanism.

In case of failure due to fatigue, it is important to determine the number of cycles preceding fracture by a fatigue mechanism. The method based upon measuring the spacing of fatigue striations is usually  convenient provided the striations are clearly visible on fracture surfaces.

A careful and thorough examination of the macrostructure (usual magnification 10) and microstructure of the material involved in the failure can provide important clues. If possible, the structural features of samples removed from the failed component with those representative of samples removed from sound sections, as well as of samples never used in service are to be compared. Also It is necessary to refer to any specific recommendations by the equipment manufacturer regarding the use of the material.

Macro structural examination can provide information about the fracture path and the location from which it was originated, e.g., internal surface or external surface.

Selection of the proper technique for micro structural examination is dependent upon the type of information required. However, optical microscopy is usually employed to  start with for knowing the overall micro structural features such as grain size and shape, secondary precipitates and their distribution, and micro cracks and their location. An unusually large grain size is an indication that the material is exposed to an excessively high temperature. Grain shape and the shape of annealing twins provide information about the amount of cold working. Elongation of grains indicates that the material is cold worked heavily.  Bent twin boundaries indicate that the material is deformed plastically. Fine slip lines within the grains also indicate the plastic deformation of the material. If secondary precipitates are seen in the micro structure then their nature and morphology is need to be examined.

During the examination of the micro structural features it is necessary to pay attention to the presence of the micro cracks. For example, grain boundary cracks can result from either creep deformation or embrittlement of the grain boundaries by precipitates of a secondary phase.

Though it is possible to get a lot of information about the characteristics of fracture by visual examination, yet micro structure examination of the fracture surface using a scanning electron  microscope is needed to know the mechanism which was responsible for fracture. A fracture can happen by sudden overloading or by propagation of a crack to the extent that it causes fracture. The propagation of crack can take place by different mechanisms. Hence it is necessary to know the morphology of the fracture surface for finding the mechanism which has caused the fracture.

During the visual examination, if any corrosion deposits are noticed, then it is necessary to determine the nature of the deposits. The deposit can be oxide, sulphide, chloride or  carbide etc. In case of corrosion deposits, it is necessary to know the nature of the operating environment and the corrosion resistance of material to that environment.

It is very important for the person investigating the failure to distinguish between the mode of failure and the cause of failure. Sometimes the two terms are intermixed, leading to a great deal of confusion. To prevent future failures, it is essential to identify the cause of failure. As an example, mode of failure is inadequate surface hardness of the failed component, but the cause of failure can be either related to the wrong selection of the material or improper operating conditions.

Accurate and detailed interpretation of entire data obtained during the various stages of the failure analysis is important for a failure analysis. Without it, proper inferences which are necessary for finding the exact cause of failure cannot be drawn. Improper determination of the cause during the failure analysis leads to wrong corrective actions which does not help in the prevention of future failures.

Failure analysis investigation are not completed without a detailed report. The report is to include a set of recommendations for preventing future failures. Recommendation are to be consistent with the results of the investigation. Further the recommendations may provide short term solutions as well as a long-term solutions.