Appendix B

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FAILURE ANALYSIS ASSIGNMENT

The purpose of this assignment is to introduce you to the procedures generally followed when conducting a failure analysis. Each student will have the opportunity to present a 6 minute failure analysis report to the class during the practical sessions on a case study of their choice. The following section will give an outline of how to lead a failure investigating and introduce you to some case studies.

Learning Outcomes:

Engineering problem solving Demonstrate competence to identify, assess, formulate and solve convergent and divergent engineering problems creatively and innovatively.

Application of fundamental and specialist knowledge Demonstrate competence to apply knowledge of mathematics, basic science and engineering sciences from first principles to solve engineering problems

Engineering methods, skills, tools and information technology Demonstrate competence to use appropriate engineering methods, skills and tools, including those based on information technology

Professional and general communication Demonstrate competence to communicate effectively, both orally and in writing, with engineering audiences and the community at large.

It is up to you to find a case study and put together the relevant information in a systematic report covering the topics listed below as is applicable to your case study. There is a significant amount of examples to choose from, be creative when looking for a topic. In other words: you are not expected to conduct your own failure analysis, but rather to find examples of such disasters in history and get behind the metallurgical aspect of the what went wrong? and present it in a report and as a short PowerPoint presentation.

Bear in mind that the weight of this assignment (including the report and presentation) is 10% of your participation mark for the semester and that entrance to the exam will be denied if you fail to complete any part of it.

Reports are to be submitted electronically via Edulink and will be scanned for plagiarism.

B.1. Introduction

Failure analysis and prevention are important functions to all of the engineering disciplines. The materials engineer often plays a lead role in the analysis of failures, whether a component or product fails in service or if failure occurs in manufacturing or during production processing. In any case, one must determine the cause of failure to prevent future occurrence, and/or to improve the performance of the device, component or structure.

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To increase the odds of completing a conclusive failure analysis while at the same time saving time and money, investigations should be carried out using a systemic approach similar to that outlined in Figure 1 to determine what went wrong.

Figure 1. Chart outlining the major steps that are usually taken when conducting a failure analysis

B.2. Procedure

The failure analysis sequence generally follows an order of increasing "destructiveness" of the test and/or sample removal. In conducting a failure analysis it is therefore crucial to collect as much information at each stage before continuing to the next. Significant deviation from this recommended hierarchy may prevent critical evaluation because of damage caused by previous tests.

B.2.1. Background information

The first step in conducting any failure analysis is to gain a good understanding of the conditions under which the part was operating. The investigator must ask questions from those who work with, as well as those who maintain the equipment and visit the site whenever possible. Contacting the manufacturer may also be necessary.

Part information: Detailed information about a failed component often facilitates selection of analytical methods and can provide insight into some of the factors that may have contributed to the failure. Certain test methods may be suggested by knowledge of the component manufacturing history, and this could lead to a quicker solution. This information should include as a minimum: specifications, manufacturing information, part number and serial number, and drawings with a bill of materials.

Service history: The history of a failed part is also of great importance to the analyst. All information concerning the actual record of a part can serve to illuminate the causes of a failure. Even "typical" service, which may be ostensibly identical to similar units in similar conditions, may initiate failure due to apparently innocuous or mundane differences that may not initially seem worthy of mention.

Investigation planning and sample selection: The planning portion of an investigation is crucial to determining the proximate cause of failure. Proper planning can ensure that an investigation is efficient and cost effective. Particularly in the case of a high visibility failure or if an assembly line shutdown is imminent, careful planning is necessary to hasten problem resolution. Haphazard or unsystematic investigations are unprofessional and wasteful of time, effort, and manpower.

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Unfortunately, in many instances the investigator will receive a failed part with little information about its history and operating conditions. In such cases the physical evidence will have to be more heavily relied upon.

B.2.2. Visual examination & cataloguing

The second step is to conduct a visual examination, cataloguing and recording the physical evidence at the same time. This serves the functions of:

Familiarising the investigators with the evidence.

Creating a permanent record that can be referred to in light of new information.

Samples should be examined, photographed and sketched taking particular care to identify and record any area of particular importance, such as fracture surfaces and surface defects. Visual examination can be aided by the use of a stereomicroscope with lights that can be easily directed. Shadows can give depth to a surface making it easier to analysis and photograph.

Pieces should always be examined and recorded before any surface cleaning is undertaken. In some cases substances such as dirt, paint and Oil on the surface can themselves be important clues, indicating such things as how old the fracture surface is and in what kind of environment the piece was operating. A good general rule is to be conservative when destroying evidence of any kind.

The visual examination is a good time for the investigator to examine the fracture surfaces in detail and try to identify the mode of fracture (brittle, ductile, fatigue, etc.), points of initiation, and direction of propagation. Each mode of fracture has distinct characteristics that can be easily seen with the naked eye or the use of a stereomicroscope; however, sometimes a scanning electron microscope (SEM) will have to be used.

B.2.3. Outline plan of action

The third step is to decide on a course of action. Based on the visual examinations and the background information the investigator must outline a plan of action, which is the series of steps that will be needed to successfully complete the case. There are several resources that an investigator can draw on to determine the cause of failure, which can be classified into one of the following categories:

Macroscopic examination

Non-destructive testing (NDT)

Chemical analysis

Metallographic examination

Mechanical Testing

Many of these categories will require steps that use the same equipment and therefore much time can be saved with a little forethought. The macroscopic examination is best performed when cataloguing the samples; however the investigator will often want to return to examine the part in more detail once other evidence is gathered. Use of a scanning electron

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microscope (SEM) is often useful at this stage because of its large range of magnifications and its large depth of field.

Since undamaged fracture surfaces are not always available, it is often a good idea to open other cracks that may be present in the piece. This often reveals good quality fracture surfaces similar to those that caused failure.

B.2.4. Non-destructive tests (NDT)

Non-destructive tests (NDT) are a good way to examine parts without causing permanent damage. Often, results obtained from examining failed parts in the lab using NDT's can be used to examine parts in the field and remove them from service before failure occurs. There are several NDT's that are available to the investigator and it would be a good idea to read up on each ones abilities.

NDT Method Capabilities

Radiography Measures differences in radiation absorption.

Inclusions, Porosity, Cracks

Ultrasonic

Uses high frequency sonar to find surface and subsurface defects.

Inclusions, porosity, thickness of material, position of defects.

Dye Penetrate Uses a die to penetrate open defects.

Surface cracks and porosity

Magnetic Particle

Uses a magnetic field and iron powder to locate surface and near surface defects.

Surface cracks and defects

Eddy Current

Based on magnetic induction.

Measures conductivity, magnetic permeability, physical dimensions, cracks, porosity, and inclusions.

Table 1. Commonly used non-destructive tests and there capabilities in detecting defects.

NDT is part of MTK3A practical and you will have some exposure to a few of these techniques. Familiarise yourself with the theory behind these techniques.

B.2.5. Chemical analysis

Chemical analysis is done on the bulk of the material to confirm the material composition. Depending on the investigation, chemical analysis should also be done on any overlay materials or surface residues. There arc several techniques that can be used to check composition, and the choice of which to use often depends on accessibility and sample type. In many cases, the SEM can be a powerful tool for fast identification of surface materials. Care should be taken not to contaminate samples taken for chemical analysis by surface residue or cutting instruments.

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B.2.6. Metallographic examination

Metallographic examination involves the sectioning of samples to examine the microstructure. The sections that are selected for examination are dependent on the type of piece and the mode of fracture. Sections from the sample should be taken in different planes so that any differences in the microstructure can be seen. Sometimes it is useful to take a cross section through the fracture surface so that the microstructure below the fracture and the surface profile can be examined. A section running parallel to the fracture surface is also often taken for examination. Samples should be mounted, ground, and polished using metallographic techniques. They should be examined before etching for porosity, inclusions, and other defects. Microstructures should be identified and their properties researched. There are several referenced that the investigator can refer to for identification of uncertain structures.

B.2.7. Mechanical testing

Mechanical testing is done to verify that the mechanical properties of the material conform to the standards. There are many types of mechanical testing that can be performed and their procedures can be found in the ASTM mechanical testing standards. The most common method used is hardness testing because of its relative simplicity, low cost, and the fact that for many materials tables exist to relate hardness with yield strength. Macro hardness is usually sufficient to determine material properties; however microhardness measurements are helpful in determining property variations within the material. Use the microhardness measurement to compare the surface hardness to that of the body or to verify the microstructure. Other mechanical testing such as tensile tests and impact tests can be used, however their use is usually limited by insufficient material and high costs.

We will be looking at some of these methods in the MTK3A laboratory sessions. Familiarise yourself with the theory on hardness tests, as well as impact testing.

B.2.8. Conclusion & recommendations

Once all the data is gathered, the investigator must come to a conclusion based on the evidence present. This requires that the investigator draw heavily on background experience and research performed. This step can be difficult because when conducting the investigation clues will lead the investigator down paths that seem to be the cause but which are merely consequences.

The final and most difficult step in any investigation is coming up with recommendations. Some cases will be simple, however many cases are not obvious even though the cause and theory are known. Recommendations are not to be taken lightly. Serious failures can occur if recommendations are in error. The system may have to be redesigned or a new material put in place. Sometimes all you will be able to recommend is that inspections be carried out more often.

B.3. Final remarks

Experience has shown that in spite of intelligent material selection and design, failures still occur. Being knowledgeable on failure analysis will extend the life of your design and prevent catastrophic failure. An example of a case study follows in the next section.

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B.4. FAA Schedule

The FAA Schedule will be made available via Edulink.

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CASE STUDY: CRANE BOLT FAILURE

This case study is an actual report submitted in response to industrial failure. The purpose of this report is to demonstrate by example. It is written at a basic level and further reading is recommended to better understand the failure mechanism.

Introduction

One of two bolts supporting a load of 16 200 lbs failed while in service causing eight hours of downtime on an essential machine to production. The bolts were in operation on a crane used to transfer anodes into the machine. Figure 1.1 shows a drawing of the set-up and the location of fraction above the nut. The crane cycled 600 time a day 7 days a week.

The broken bolt (Figure 1.2) and a new unused bolt, recommended by the supplier for the application, were supplied to conduct the investigation. The original designers of the crane specified a bolt that conforms to SAE standards grade 5. The supplier of the new bolt confirmed that it was made to conform with ASTM standard A 193 grade B7.

Figure 1.1. Drawing of the bolt and crane set-up.

Figure 1.2. Photograph of broken bolt

Figure 1.3. Photograph of fracture surface.

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Results:

Observations

Examination of the fracture surface revealed characteristics such as a beachmarks associated with fatigue (Figure 1.3). The zone of final fracture was located between two areas of fatigue propagation suggesting the presence of bending forces. The surface area of final fracture was approximately 12% of the total fracture surface suggesting that the bolt was not overloaded. Cracks where also found between threads near the fracture surface indicating that the bolt was highly susceptible to fatigue initiation.

Results from chemical analyses (Table 1.1) show that the original broken bolt had a carbon content slightly below those required by the SAE standards for a grade 5 bolt. This lower carbon content would have acted to decrease the material properties. The chemical composition of the new sample bolt conformed to the ASTM standard A193/A grade B7 that requires an AISI-SAE 4140 composition.

Table 1.1--Chemical analysis results on both bolts.

Element

Original broken bolt

(%) SAE Standard

Grade 5 (%) New Sample

Bolt (%)

ASTM Standard B7 AISI 4140 (%)

Carbon 0.20 0.28-0.55 0.42 0.37-0.49

Manganese 0.65 -- 0.85 0.65-1.10

Silicon 0.22 -- 0.22 0.15-0.35

Phosphor 0.013 0.048 max. 0.015 0.035

Sulphur 0.011 0.058 max. 0.030 0.040

Chrome 0.08 -- 0.79 0.75-1.20

Nickel 0.06 -- 0.07 --

Molybdenum 0.01 -- 0.15 0.15-0.25

Microscopic examination of the bolts where done using longitudinal and latitudinal mounts for each. The sections taken from the fractured bolt were taken close to the fracture surface. Examination before etching of the two bolts showed no cracking or unusually large inclusions. The original broken bolt did show some flaking at the base of the threads (Figure 1.4) but this is expected for a bolt that has been in service. Etching the sections revealed a microstructure of coarse pearlite in a matrix of ferrite (Figure 1.5). The SAE grade 5 standard requires that the bolt be quenched and tempered to conform and therefore should have a tempered martensite structure.

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Martensite has higher material properties such as yield strength and hardness, which increases its resistance to fatigue initiation. The ferrite matrix of the original bolt has low yield strength, which in turn reduces its resistance to fatigue initiation. The new bolt was found to be quenched and tempered as required by the ASTM standard (Figure 1.6). However rolling seems where found at the tips of the treads (Figure 1.7). This is not a serious defect because of the defects location in a low stress area however, if the bolt was placed in a corrosive atmosphere these seams would corrode and then act as fatigue initiation sites.

Figure 1.4. Micrograph of flaking found at the base of a thread in the fractured bolt. 2% nital 100X

Figure 1.5. Micrograph of fractured bolt. Ferrite matrix with pearlite. 2% nital 200X

Figure 1.6. Micrograph of new bolt. Tempered martensite. 2% nital 500X

Figure 1.7. Micrograph of the new bolt thread showing a rolling seam. 2% nital 200X

Tensile tests were done on the bolts to test their material properties in comparison with the standards. The results (Table 1.2) show that the yield strength and ultimate tensile strength of the original bolt are only two thirds that required by the standards. This conforms to the microstructural observations. The properties of the new bolt conformed to the standard even though they were slightly elevated.

Table 1.2--Results and standard requirements of tensile tests.

Original Broken Bolt New Sample Bolt

Standard Grade 5 SAE

Standard Grade

By AISI

Sample # 1 2 1 2

Ultimate Tensile Strength (KSI)

69.5 69.5 148 146 100 125

Yield Strength

42.7 44.4 134 133 80 105

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(KSI)

Elongation (%)

26 24 20 20 16 min. 16 min.

Surface Reduction (%)

67 67 59 59 50 min. 50 min.

Conclusions and Recommendations

Examination revealed that the bolt failed as a result of high cycle low load fatigue. Chemical analysis and tensile tests confirmed that the bolt did not meet the SAE grade 5 standards required by the original design of the crane. The major cause for this lack of conformity is because the bolt was not quenched and tempered. Since the resistance of steel to fatigue initiation in proportional to its yield strength, the low properties of the steel in this case left it open to fatigue initiation.

Examination of the new bolt revealed that it conformed with the ASTM standards A 193 for a grade B7 bolt, as the supplier specified. However, rolling seams were found in the thread tips. Due to the relatively low loads this area is subjected to this is not a major problem but if the bolt is subjected to a corrosive environments these seams could grow and become fatigue initiation sites.

The SAE grade 5 bolt specified by the original designers should continue to be used in future and the upgrade to the ASTM B7 is unnecessary

*For more case studies: http://www.tms.org/Students/Winners/Davidson/Davidson.html

http://www.tms.org/Students/Winners/Davidson/Davidson.html

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Failure Analysis Report Assessment Form

Student: Student Number:

Title:

Edulink:

Introduction Scientific and logical approach to introduce the failure. It creates an expectation, motivation for study clearly set out.

15

Background information Information presented in a logical and

structured manner 15

Results Information presented in a logical and structured manner, including appropriate information regarding:

Visual examination & cataloguing

Non-destructive tests (NDT)

Chemical analysis

Metallographic examination

Mechanical testing

20

Conclusion and recommendations Appropriate conclusions drawn from results

and logical arguments presented. 15

Documentation Spelling, punctuation and grammar

Academic register, vocabulary and sentence construction

Cohesion and overall structure of the report

25

References Appropriate references cited in text

Reference style

List of references

Figures and tables integrated into the report

Descriptive captions

10

Total 100

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Failure Analysis Presentation Assessment Form

Criteria Exemplary Basic Sub-standard

Presentation Clarity/audibility: Body Language: Organisation: Register/ Tone/Language Visuals/multimedia

[17 25] * Speaks in a clear voice and shows a flair for communicating with the audience *Keeps audience engaged *Makes eye contact with audience *Holds attention of audience *Well organized with beginning, middle & end *Ideas flow smoothly & logically *Delivers ideas in clear and concise manner *Appropriate reliance on notes *Uses appropriate academic register *Uses appropriate tone *Uses wide, appropriate vocabulary and sophisticated language structure Aids/supports the presentation

[10 - 16.5] *Speech clear most of the time *Keeps audience engaged most of the time *Some eye contact with audience *Is able to hold attention of audience most of the time *Some organization *At times ideas do not flow smoothly & logically *Many of the ideas presented are clear & concise *Tends to rely on notes at times. *Uses appropriate academic register most of the time/some slips *Tone mostly appropriate/some slips *Language mainly correct/some slips in vocabulary Appropriate

[0 9.5] *Speaker difficult to hear. Speaks too fast or too slow *Does not keep audience engaged *No eye contact with audience *Fails to hold attention of audience *Not well organized *No sequence of information *Is not clear & concise *Complete reliance on notes. *Uses mostly inappropriate register/colloquialisms * Speech littered with fillers * Inappropriate tone * Mainly basic vocabulary; serious language errors affecting understanding Distracts from the presentation

Content Evidence/support/ Understanding

[11 - 15] *Content relevant, well researched *Provides support for main ideas *Understands the topic and is confident *Conclusions and recommendations made are relevant and correct *Listeners are likely to gain new insights about the topic

[5 10.5] *Some content not relevant, some evidence of research *Some support for main ideas *Some understanding of the topic *Conclusions and recommendations made *Listeners may learn some isolated facts, but they are unlikely to gain new insights into the topic

[0 4.5] *Content irrelevant, no evidence of research *No support for main ideas *Lacks understanding of the topic *Conclusions and recommendations not presented *Listeners are unlikely to learn anything or may be misled

Time [7 10] Within 15 seconds of allowable time

[4 6.5] Uses most of the time/runs a little over

[0 3.5] Too short or too long Asked to conclude