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MM: 503 Deformation Behavior and Failure Analysis of Materials

Engr. Muhammad Ali Siddiqui Assistant Professor

Course Teachers

NED University of Engineering and TechnologyDepartment of Materials Engineering

Lecture 2 Series

Failure Analysis Methodology

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Failure Analysis Methodology ANALYZING FAILURES is a critical process in determining the physical root

causes of problems. The approach to failure analysis and the methodology adopted must

required expert analyst(s) - because a single analyst may not be equipped with knowledge in various disciplines such as metallurgy, materials science, structural mechanics, corrosion engineering, propulsion engineering, aerodynamics, and so on,

(So it is multidisciplinary activity). A team effort is always required to arrive at the correct solution to the

problem. The process is complex, draws upon many different technical disciplines,

and uses a variety of observation, inspection, and laboratory techniques. One of the key factors in properly performing a failure analysis is keeping

an open mind while examining and analyzing the evidence to foster a clear, unbiased perspective of the failure.

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1. Background Information:

Whenever a failure analysis is carried out, at the

beginning it is essential to collect all the relevant

background information, because it helps in developing

a complete case history about the failure.

The collection of information can fall into two groups:

i. Information about the failed component.

ii. Information about the failure itself.

Collect all available data concerned with specification and drawing, component design, fabrication, repairs, maintenance and services use.

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i) Information about the failed component includes Name of the component,

identification number, manufacturer, and user

Location Intended function Service life since new Service life since last overhaul

(repair) Design loads, actual service

loads, and load orientation Frequency of loading Service parameters such as

temperature, pressure, rotation, etc.

Environment Materials of construction Specifications and codes Concessions given in the

components Strength parameters Fabrication processes Thermomechanical treatments Surface treatments Inspection techniques and

records Maintenance records Ad hoc modifications

Cont.…

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ii) Information about the failure itself includes:Date and time of

failure/malfunctionExtent of damage to the

surrounding structure, personnel, etc.

Operating conditions just prior to failure

Service abnormalities Sketches and

photographs of the failed component and surrounding areas.

Wreckage distribution map (separated parts of a badly damaged object )

Eyewitness and earwitness accounts

Cont.…

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Wreckage distribution map

http://fourthelement.com/adventures/wreck-tours/thistlegorm/

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• The investigator should visit the failure site and make a firsthand examination and estimation of the damages. These should be documented immediately. (it is difficult task)

• Color Photography is the best method of recording the damages for further detailed investigation.

• Any delay can alter the condition of the failed component because of handling by unauthorized people, environmental influences, and attempts to put the pieces together.

Cont.…

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2. Location of the Failed Component:• Determining and documenting the location of the failed

component in a larger structure is an important task in the introductory examination of a failure.

• The part may sometimes get thrown a considerable distance as in the case of a rotating component or in the case of a moving or flying vehicle or a big object.

• It is important to photograph the entire wreckage. Also, a wreckage distribution map in Cartesian or polar coordinates must be prepared.

• Secondary failures are not uncommon and may destroy the primary failure.

• Hence, before selecting pieces for further laboratory examination, photographic documentation of the wreckage in the “as-is, where-is” condition should be completed.

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• Investigation should be properly planned.• All the tests to be carried out must be sensibly

selected and the sequence of tests should be carefully planned.

• Decisions on any test that would involve destruction of part of the component should be made very carefully.

• A wrong sequence of tests could destroy some of the important evidence or introduce features that could be confusing.

Cont.…

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3. Specimen Collection :• Collection of specimens for further laboratory

examination is a vital step in any failure analysis.• Wrong choice can, apart from wasteful work, lead to

confusion and wrong direction of the investigation.• Primary failure must be distinguished from numerous

secondary failures.• Samples from the suspected primary failure region must

be collected carefully, and their location in the wreckage and in the original structure must be recorded.

• Any other samples that can provide secondary or additional evidence must also be collected.

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• Fracture surfaces must be handled very carefully can provide a useful information about the mode and mechanism of fracture during detailed laboratory examination.

• In the field, it is better to keep the samples in plastic covers with suitable desiccants and seal them and secure the bags with suitable identifying tags.

• The fracture surfaces may be sprayed with a transparent lacquer before sealing.

Cont.…

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Don'ts

• Touching the fracture surfaces must be avoided because human sweat is a corrodant.

• The mating surfaces of a fracture should never be made to touch each other (though this is a normal, tempting tendency) because this would cause abrasion and thereby lead to loss of vital microfractographic evidences.

Cont.…

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3.1 Preservation Techniques of Fractured Specimen:

• FRACTURE SURFACES are fragile and subject to mechanical and environmental damage that can destroy microstructural features. Therefore, fracture specimens must be carefully handled during all stages of analysis.

• Unless a fracture is evaluated, immediately after it is produced, it should be preserved as soon as possible to prevent attack from the environment.

• The best way to preserve a fracture is to dry it with a gentle stream of dry compressed air, then store it in a desiccator, a vacuum storage vessel, or a sealed plastic bag containing a desiccant.

Cont.…

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• However, such isolation of the fracture is often not practical.

• Therefore, corrosion-preventive surface coatings must be used to inhibit oxidation and corrosion of the fracture surface.

• The main requirements for a surface coating are as follows:

1. It should not react chemically with the base metal2. It should prevent chemical attack of the fracture

from the environment3. It must be completely and easily removable without

damaging the fracture features.

Cont.…

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Fractures may be coated with fresh oil or grease.

Clear acrylic lacquers or plastic coatings are sometimes

sprayed on the fracture surfaces. These clear sprays are

transparent to the fracture surface and can be removed with

organic solvents.

However, on rough fracture surfaces, it can be difficult to

achieve complete coverage and to remove the coating

completely.

Cont.…

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Another type of plastic coating that has been successfully

used to protect most fracture surfaces is cellulose acetate replicating tape. The tape is softened in acetone and

applied to the fracture surface with finger pressure. As the

tape dries, it adheres tightly to the fracture surface.

The main advantage of using replicating tape is that it is

available in various thicknesses.

Rough fracture surfaces can be coated with relatively thick

replicating tape to ensure complete coverage.

The principal limitation of using replicating tape is that on

rough fracture surfaces it is difficult to remove the tape

completely.

Cont.…

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4. Laboratory Studies

a) cleaning of fracture surfaceb) Preliminary Examination (Macro-fartrography)c) Microscopic Examination (Micro-fractrography)d) Chemical Analysis. e) Mechanical Propertiesf) Nondestructive Evaluationg) Any other special technique

Discuss in detail in next couple of lectures

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5. Analysis of Data

• The most important task in any failure analysis is the consolidation and systematic connection of all the data obtained from the laboratory test/analysis.

• The results of tests must be compared against the specifications and deviations, if any, and should be carefully considered as possible contributing factors.

• At this stage, expertise from other related disciplines would be quite useful in interpreting the data.

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• Account of information has available through a proper analysis. These includes;

• The failure initiation site or sites, crack length, its propagation path and speed, and the nature and direction of load acting on the component.

• The role of other factors such as temperature, corrosion, wear, component manufacturing history, assembly and alignment, repair and maintenance history, service abnormalities, and abuses, if any, would also become clear during the analysis.

• Ultimately, the sequence of the failure would get established, differentiating between the primary cause of the failure event and the significant secondary failures and damages.

Cont.…

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6. Preparation of the Report:

• For effective communication of the results of failure analysis, documentation is extremely important. The report should be clear and contain the logic behind the conclusions.

• The following components form the essential sections of the report:

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Description of the failed component

Circumstances leading to the failure

Operational parameters and conditions at the time of failure

Background history Visual examination of general

physical features Laboratory investigations

including metallurgical, mechanical, chemical, and other tests and their results

Anomalies/irregularity , if any

Discussion of mechanism or possible mechanism of failure and sequence of events

Conclusions and recommendations for the prevention of recurrence

of such failures References to relevant

literature Summary

sections of the report

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• In addition to these elements, the report should contain an executive summary to enable the management to take appropriate action at various levels for avoiding future failures.

• It is extremely important for the investigator to keep track of the follow-up action based on his or her recommendations. Continued interaction with the designer, manufacturer, and operator is highly desirable.

• Failure analysis is a challenging task. It is exciting, and the analyst learns something new in every failure. If the analysis is carried out with utmost care and caution, the effort will be amply rewarding.

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Reference

1. Failure Analysis of Engineering Structures Methodology and Case Histories

V. Ramachandran, A.C. Raghuram, R.V. Krishnan, and S.K. Bhaumik

2. Failure Analysis and Prevention, Volume 11