Module 4: Failure Investigation

Question 1

What are common pitfalls in failure investigations?

Answer 1

1) Jumping to conclusions
2) Not understanding the problem
3) Not understanding how the failed system is supposed to operate
4) Not considering all possible failure cases
5) Tearing system apart without a developed plan
6) Failure to follow through
7) Not asking for help
8) Thinking it is too easy to do
9) Destroying evidence due to lack of planning

Question 2

What is NOT failure analysis?

Answer 2

1) “Give me your best guess”
2) Not identifying the root cause(s)
3) Reworking or repairing
4) Swapping parts: Remove and replace mentality
5) Ignoring the problem
6) We are going to change it later
7) Band-aid fixes: return to the supplier;scrap;another syste, supplier or inventory
8) Never happens

Question 3

What are the principle stages of a failure analysis?

Answer 3

1) Get background information (documentation, eg procurement specs, materials list, processes lists, standards, parts list, drawings etc)

2) Catalog evidence

3) Non destructive testing, chemical analysis, mechanical testing, metallographic examination

4) Amass information to come to a conclusion

5) Make recommendations

Question 4

What is the first step in failure analysis?

Answer 4

Get a good understanding of the conditions under which the part was active. The investigator must ask questions from those who work with, as well as those who maintain the equipment and visit the site whenever possible.

It is essential that these questions are asked ASAP after the failure

There are three reasons one would collect data through interviews:
1) Firsthand data (witnesses, participants, etc)
2) Background and circumstantial data (historical experiences, related events, situational insights etc)
3) Expert information (to elicit technical knowledge)

Question 5

What is the second step in failure analysis?

Answer 5

Perform a visual exmaination, cataloguing and recording the physical evidence at the same time. This serves the functions of:

1) Familiarizing the investigators with the evidence
2) Creating a permanent record that can be referred to in light of new information

Question 6

What should investigators be cautious about evidence?

Answer 6

1) Pieces should always be examined and recorded before any surface cleaning is undertaken

2) A good general rule is to be conservative when destroying evidence of any kind

Question 7

What is the third step in failure analysis?

Answer 7

Devide on a course of action. On the basis of visual inspections and background information the investigator must outline a plan of action, which is the series of steps required to successfully complete the case. There are various means that an investigator can draw on to determine the cause of failure, which can be classified in the following categories:

• Macroscopic examination
• NDT
• Chemical analysis
• Microscopic examination
• Testing

Question 8

What are some general rules for colleting samples?

Answer 8

1) Collect failed parts, nearby fragments and lubricant and fluid samples
2) Collect evidence beyond what is apparent at the time of the initial assessment
3) Collect undamaged samples of similar components for comparison to the damaged ones
4) Draw diagrams to indicate the position of parts and sample collection locations
5) Do not be afraid to take many photographs while photo documenting the scene
6) Take shots from every angle and always have a scalable object in the photo, preferably ra rules scale
7) Protect samples, particularly delicate items and fracture surfaces, from each other and from other sources of damage
8) Label the samples in order to indicate when and why it was collected, how it was oriented, who removed it and what were the relevant in site observations

Question 9

What are steps undertaken in a laboratory analysis?

Answer 9

1) initial examination
2) Photodocumentation
3) NDE
4) Material verification
5) Fractographic examination
6) Microscopic analysis
7) Mechanical properties determination
8) Analysis of evidence

Question 10

What are some questions one should ask?

Answer 10

• Should the complex part be an assembly of several parts rather than one?
• How was the component loaded, and was anisotropy considered?
• Is the material capable of being produced with the required properties in the form used?
• Can any available material meet the specifications?
• Did the strength requirements preclude toughness or corrosion resistance needs?
• Was the wear resistance adequate for the materials in contact?
• Were the seried properties compromised by the use of low-cost materials or processes?
• Did the materials and processing comply with the applicable codes and standards?
• Was the product made with unique materials and processes?
• How did the scrap value contribue to repair and maintenance decisions in service?
• Does the material prossess adequate durability in the service envrionment?
• Was the CoC of the materials compliant to the specifications?
• Are there other samples available of the same material batch? (reproduce the failure)
• Were proprietary or obsolute materials and processes employed?
• Were the manufacturing procsesses used to create the desired shape appropriate?
• Did the individual processing methods make sense?
• Should it have been preheated prior to heat treatment or welding?
• Did the fabricality requirements compromise the desired mechanical or physical properties?
• Were the manufacturing methods appropriate for the quantity produced?
• Were the operating conditions and maintenance as intended?
• Were the service conditions easy to anticipate?
• Were the operators certified for the performed manufacturing process?
• Were anomalies documented during the manufacturing process?

Question 11

How to formulate conclusions?

Answer 11

• Has the failure sequence been established?
• If the failure involved cracking or fracture, have the initation sites been determined?
• Did cracks iniate at the surface or below the surface?
• Was cracking associated with a stress concentration
• How long was the crack present?
• What was the level of magnitude of the loads?
• What was the type of loading? static, cyclic or intermittent?
• Is the fracture surface consistent with the type of loading assumed in the design?
• What was the failure mechanism?
• Serive temperate at time of failure
• Did high/low temp. contribute? Were there temp excursions?
• Did wear contribute?
• Did corrosion contribue? what type of corrosion?
• Was proper material used?
• Was the cross section adequate? were the stresses high?
• Was the quality of the material acceptable in accordance with specifications?
• Was heat treatment properly done?

Question 12

What should a failure analysis report contain?

Answer 12

A failure analysis report is the summation of all the individual tests and alayses performed during the course of an invesrtigations. Reports should be written with the target audience in mind.

Failure analysis reports contain as minimum:
1) An executive summary (stating the main conclusions)
2) An introduction that included background information on the subject of the report and restates the work requested and may serve as a written record for verbal work requests
3) A section detaling investigative procedures and results
4) Conclusions

Question 13

What is the root cause method?

Answer 13

The principles of root cause analysis (RCA) may be applied to ensure that the root cause is understood and appropriate corrective actions may be identified.

These principles are rooted in the following method:
1) Identify the issue
- Describe the current situation
- Define the deficiency in terms of the symptoms (or indicators)
- Determine the impact of the dficiency on the component, product, system and customer
- Define a goal
- Collect data to provide a measurement of the deficiency

2) Determine root cause
- Design deficiencies
- Material defects
- Manufacturing/installation defects
- Service life anomalies
3) Reproduce the failure (not always possible but very useful)
4) Develop corrective actions
5) Validate and verify corrective actions
6) Standardize

Question 14

What is the fish bone analysis?

Answer 14

Every bone is a potential root cause to the problem. Each bone has a list of causes, which may contribute to the problem. Some examples could be:
- Machine
- Method
- Material
- Man/mind power
- Measurement/medium

Question 15

Discuss about developing, validating and standardizing corrective actions.

Answer 15

Develop corrective actions:
- List possible solutions to mitigate and prevent reccurence of the problem
- Generate alternatives
- Develop implementation plan

Validate and verify corrective actions
- Test corrective actions in pilot study. Measure effectiveness of change. Validate improvements. Verify that the problem is corrected and improves customer satisfaction.

Standardize
- Incorporate the corrective action into the standards documentation of the company, organization or industry to prevent recurrence in similar problems or systems
- Monitor changes to ensure effectiveness.

Question 16

What are the three levels of root-cause analysis?

Answer 16

1) Physical roots
Or the roots of quipment problems, are where many failure analyses stop. These roots may be what comes out of a laboratory investigation or engineering analysis and are often component-level or materials-level findings
(corrosion damage)

2) Human roots
(i.e people issues) involve human factors that causes the failure, an example being an error in human judgement
(inadequate inspection)

3) Latent roots
lead us to the caues of the human error and include roots that are organizational or procedural in nature, as well as environmental or other roots that are outside the realm of control
(inadequent inspector training)

Question 17

What are some causes and types of failures?

Answer 17

Causes of failures:
1. Misuse: conditions different from what designed.

2. Assembly error and improper maintenance. eg leaving off a bolt or using incorrect lubricants.
3. Design errors
   - Size and shape of part
   - Material
   - Properties

Types of failure:
- buckling
- corrosion
- creep
- fatigue
- fouling
- hydrogen embrittlement
- impact
- mechanical overload
- stress corrosion cracking
- thermal shock
- wear
- yielding

Question 18

Discuss about fracture and fracture surface.

Answer 18

Fracture in engineering alloys can occur by a transgranular or an intergranular fracture path. Howver, regardless of the fracutre path, there are essentialy only four principle fracture modes:
1) Dimple fracture
2) Transgranular cleavage
3) Fatigue
4) Decohesive rupture

• When overload is the principle cause of fracture, most commong structural alloys fail by a process known as microvoid coalesence
• In brittle crystalline materials, fracutre can occur by cleavage as the result of tensile stress acting normal to the crystallographic planes with low bonding
• Decohesive ruptur occurs along weak material surfaces. Surfaces such as grain-boundbary precipitation, low-strength phases, defect structures, stress-corrosion cracking and hydrogen embrittlement
• A fracture that is the result of repetitive or cyclic loading is known as fatigue fracture. Beyond the fatigue-zone limits, failure is by radial fibroud fast fracture.

Question 19

Discuss fraction dimples:

Answer 19

In tension, equitax dimples are formed on both surfaces

In shear, elongated dimnples point in opposite directions on mating surfaces

In tensile tearing, elongated dimples point toward the fracture origin on amting cracture surfacees

Question 20

How can the crack origin be found?

Answer 20

Usually the direction of crack growth can be detected from marks on a fracture surface, such as V-shaped chevron marks.

T-junction procedure is important: Crack A acts as a crack stopper of crack B, causing a T-shape.

Question 21

Dicuss about wear failures

Answer 21

Generally, tribologists can usually solve twothirds of their problems using a small magnet, a low-power optical microscope or hand-held lens, and surface roughness tracing. Few tribologists will need the more
sophisticated instruments, and even fewer can be expected to know how to operate them.

The very first and perhaps surprising suggestion is to avoid dismantling the device or cleaning the surfaces before performing the steps outlines below, either formally or informally:

1) Check Effect of Mechanical Test Sequence on Surface Chemistry.
In some instances the surface
chemistry will change with time after the machine is shut off, and surface chemistry will surely change
during cleaning. Dismantle the mechanical system in question in the presence of the personnel responsible for its performance

2) Conduct a Preliminary Investigation by Human Senses. Use eyes, fingers, and nose to make a first
judgment of the environment in which the surfaces are operating. A 10× eyepiece (magnifying glass) is
probably the best aid at this stage.

3) Observe the surfaces and debris in a binocular microscope that has a magnification range from about
2 to 40×.

4)Analyze Condition of Workpiece Surface. Surface materials may be worn away, rearranged, or built up
by transfer.

5)Proceed with patience. Interesting details of the debris and sliding surfaces are usually not obvious in the
first hour of study, but with practice the eye eventually “sees” differences.

Question 22

Discuss weld failures.

Answer 22

Cracks are perhaps the most serious defects that occur in the welds or weld joints in weldments. Cracks are considered dangerous because they create a serious reduction in strength. They can propagate and cause suddent failure. They are the most serious when impacting loading and cold temperature service are involved.

Types of cracks:
- Surface cracks
- Transverse cracks
- Longitudinal cracks
- Creater cracks
- Toe cracks
- Underbead cracks

Question 23

What are some failure analysis tools?

Answer 23

1) Visual examination
2) Dye-penetrant
3) Microscopic photography
4) Scanning electron microscopy (SEM)
5) Energy dispersive x-ray spectroscopy (EDS)
6) Electron backscatter detector (ebsd)
7)Wavelength dispersive x-ray analysis
8) Fourier transform infrared spectroscopy
9) Auger electron spectroscopy (AES)
10) X-ray photoelectron spectroscoph (XPS)
11) Dynamic Mechanical Analysis
12) Thermogravimetric Analysis
13) mechanical testing
14) X-ray computer tomography
15) Acoustic microscopy

Question 24

Discuss about failures in solder joints

Answer 24

The major cause of such fatigue failure is the cyclic deformation of the joint by the stresses that result from temperature excursions combined with CTE mismatch. Slow cycle fatigue has been observed with most solder joints. The progress of the fatigue damage of leadless components may be seen as:

1) Start of the crack, generally under the component at the edge of the metallisation (difficult to observe in inspection)

2) Progression of the crack to the outer surface of the fillet, generally first visible at the corners of the metallisation

3) Growth of the visible cracks from the corners of the components to the middle of the joint

4) Sometimes, depending on the configuration, the cracks follow the interface between component and solder

Question 25

Discuss failures in brazing.

Answer 25

Brazing is a metal -joining process whereby a filler metal is heated above melting point and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while proteced by a suitable atmosphere. It then flows over the base metal (known as wetting) and is then cooled to join the workpieces together. It is similar to soldering, except the temperatures used to melt the filler metal are higher.

As brazing work required high temperature, oxidation of the metal surface occurs in an oxygen-containing atmosphere. This may necessitate the use of an atmospheric environment other than air. The commonly used atmospheres are vacuum and noble gas.

Radiography or ultrasonic inspection shall be used fo rbaze joints Class A to detect subsurface or internal defects.

Question 26

Dicuss failures in platings/coatings

Answer 26

Blisters either originate from ionic contamination on the substrate prior to coating or are due to soluble material leaching out from the coating itself and migrating to the interface with the substrate.

A chemical analysis of the paint or coating, as well as the substrate and corrosion products is usually performed.

The majority of paint and coating-related failures can be attributed to six primary causes
1) Improper surface preparation - the substrate surface is not adequately prepared for the coating that is to be applied. THis may include cleaning, chemical pretreatment or surface roughening

2) Improper coating selection - either the paint or coating selected is not suitable for the intended service environment, or it is not compatible with the substrate surface

3) Improper application - this can be a problem with either shop-applied or field applied coatings, and occurs when the required specifications or parameters for the applications are not met

4) Improper drying, curing and over coating times - lack of comformance to the required specifications or parameters

5) Lack of protection against water and aqueous systems -this is a particularly serious problem with
aqueous systems containing corrosive compounds such as chlorides.

6) Mechanical damage -which results from improper handling of the painted or coated substrate,
resulting in a breach in the paint or coating.

Question 27

Discuss failures in ceramics.

Answer 27

Because of the brittle behavior of most ceramics, there may be many pieces resulting from a failure, and reassembly of the pieces can provide information about the form of loading and the point of fracture
initiation.

In practice, evaluation of a fracture in a brittle ceramic often means actually assembling the pieces together, although care must be taken not to damage or obscure features on potentially critical fracture surfaces. The situation is considerably different from that of metals, for which the failure analyst is normally very reluctant to conduct anyreassembly of parts because of the much greater potential for surface damage on metallic fracture faces.

Plastic components can fail via many different modes,
including catastrophic mechanisms, such as brittle
fracture, ductile overload, creep rupture, environmental
stress cracking, molecular degradation, and fatigue.
In the case of failure involving fracture, the determination
of the failure mode involves identifying how the crack
initiated and how it subsequently extended.

All of the factors that affect the performance of a plastic component can be classified into one of
four categories: material, design, processing, and service conditions. These factors do not act
independently on the component but instead act in concert to determine the performance
properties of a plastic component

Question 28

Discuss the visual examination.

Answer 28

THe visual examination should begin with unaided visual inspection

Eye has exceptional depth of focus and ability to examine large areas rapidly. Detect change in color/tecture.

Particular attention should be given to the surfaces of fractures and crack paths. The significance of any indications of abnormal conditions or abuse in service should be observed and assessed, and a general assessment of the basic design and workmanship of the part should also be made

Important features (eg dimensions) should be recorded

Question 29

Dicuss liquid-penetrant inspection

Answer 29

Liquid penetrant inspection is used to detect surface flaws in materials. Spreading of a liquid penetrant in the sample. The liquid is usually a very bright color or contains fluorescent particles that, under ultraviolet light, cause discontuinities in the materials to stand out.

• Discontuinities must be open to the surface
• Testpieces must be cleaned before and after testing because the liquid penetrant may corrode the material
• Surface films may prevent detection of discontuinities
• Penetrant may be a source of contamination that masks results in subsequent chemical analysis of fracutre surfaces
• Not suited to inspection of low-density powder metallurgy parts or other poroys materials

Question 30

Discuss about macroscopic examination.

Answer 30

Relating to observations made by unaided eye.

In macroscopy, the examination of the structural characteristics or chemical characteristics of a metal or an alloy is done by the unaided eye or with the aid of a low-power microscope or binocular, usually under 50x.

Question 31

Discuss grinding, polishing and etching

Answer 31

Grinding:
A small piece of specimen is cut by a metal-cutting saw. After cutting operation, burrs on the edges of the speciment should be carefully removed by a fine file or coarse grinding paper.

The silicon carbide grinding papers are held flat in a unit containing water facility for lubrication purpose. Each unit contains four grades of papers, starting with grade 400 and winishing with grade 1200. Grinding of the work piece is done by starting with the corase papers and then continuing with fine papers.

The specimen is washed thoroughly to remove coarse silicon carbide particles before proceeding to a fine paper, so that the removal of previous grinding marks is easility observed. At the end of the grinding sequence, the specimen is washed thoroughly and dried. Now the specimen is ready for polishing

Polishing:
The polishing is done on rotating wheels covered by a special cloth. Alumina, diamond or OPS is employed as a polishing agent.

The specimen should be held firmly in contact with the polishing wheel, but excessive pressure should be avoided. During polishing the speciment should be rotated or moved around the wheel so as to give an even polish. The speciment should be thoroughly cleaned and dried between each wheel.

If the final polishing has involved the use of magnesia or alumina then thorough washing followed by drying off with acetone or alcohol will give a suitable surface, although it must not be fingered afterwards.

Etching:
Etching is generally done by swabbin. Etching times will vary from speciment to specimen, however surface should be observed during etching. Microscopical examination will then reveal wheter the degree of etchign is sufficient. Further etching can then follow to strengthen up the details as required.

After etching, the speciment should be thoroughly washed in running water, followed by drying off with acetone or alcohol

Question 32

Discuss microscopic examination.

Answer 32

It can provide quantitative information about the following parameters:
1) The grain size of specimens
2) The amount of interfacial area per unit volume
3) The dimensions of constituent phased
4) The amount and distribution of phased
Magnifications up to 1000x can be obtained with a resolution of 0.5μm

A preparation is required before microscopic examination which includes:
1) Cutting
2) Embedding
3) Grinding
4) Polishing
5) Ettching

Question 33

Discuss about the scanning electron microscopy.

Answer 33

The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid speciments. The SEM is widely used to identify phased based on qualitative chemical analysis and/or crystalline structure. The magnification ranges from 6X to approximately 250,000X with a spatial resolution of 2nm.

Minimum preparation includes acquisition of a sample that will fit into the SEm chamber and some accomodation to prevent charge build-up on electrically insulating samples. Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. ALternatively, an electrically insulating sample can be examined withouta conduvtive coating in an instrument capable of “low vacuum” operation

The SEM is also capable of performing analyses of selected point locations on the sample; This approach is especially useful in qualitatively or semi-quantitatively determining:
- chemical compositions
- crystalline structure and crystal orientations
- backscattered electron images

SEMS cant detect very light elements (H, He, Li)