How to Conduct

FAILURE ANAYLSIS

Proper failure analysis requires
following the proper sequence of tests,
collecting appropriate data,
and evaluating results.


This bolt failed because of ductile overload


Typical nondestructive examination methods
Typical chemical analysis methods
Typical mechanical testing methods

Failure analysis consists of investigations to find out how and why something failed. Understanding the actual reason for failures is absolutely required to avoid recurrence and prevent failure in similar equipment. The analysis should also help with the understanding and improvement of design, materials selection, fabrication techniques, and inspection methods.

This article reviews the steps to follow when conducting a full failure analysis investigation.

Acquiring background information

The failure analysis sequence generally follows an order of increasing "destructiveness" of the test and/or sample removal. Significant deviation from this recommended hierarchy may prevent crucial evaluation techniques through damage caused by previous tests. Naturally, methods may be added or deleted from this sequence provided consideration is given to the best order of analyses.

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 can be wasteful of time, effort, and manpower.

Equation for successful failure analysis

Proper background information

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correct analyses and tests

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Thorough knowledge of materials behavior and processing

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Pertinent experience

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Correct Analysis

Preliminary examination

All pertinent features should be examined visually, and thorough notes should be taken for each component associated with a failure analysis. Many macroscopic features and characteristics can suggest certain failure modes and circumstances. Fracture surfaces can exhibit an identifiable origin, progression marks, direction marks, and postfracture damage, all of which can be significant to the failure.

All of the pertinent features noted in the visual examination should be well documented by photographs. The component condition as well as any stamped information /part numbers should be illustrated. This should be done as soon as possible after failure, especially if the fracture surfaces continue to corrode. If smaller sections are to be excised for analysis, this step can document their locations and orientations. Photography is necessary because subsequent dissection of the sample may preclude further visual examination.

Failure analysis rules

  • The failure investigator has only one objective. to determine the failure mechanism that caused the failure and to use that knowledge to prevent another occurrence:
  • Start with and maintain an open mind. Emotion obscures objectivity and must be edged from the investigation.
  • The theory, however elegant, must agree with the observed evidence, however humble.
  • The simplest solution is the best solution.
  • Having the wrong solution is frequently much worse than having no solution.
  • Major incidents are often triggered by very minor or apparently innocuous details.
  • Only one thing is worse than knowing you have a crack growing in a component, and that is not knowing you have a crack growing in a component.
  • Cracks never get smaller, nor do they ever disappear. Either they stay the same size (invariably in a minor, insignificant, or easily repaired component), or they get bigger (usually in a critical component).

Material evaluation

Thoroughness and diligence mandate that prudent material evaluation be an integral part of any failure analysis. Many individuals (with and without qualifying experience) feel sufficiently confident to render sophisticated failure analysis judgements without the requisite testing. Absurd as this practice may seem, it is done every day. Naturally, a wise analyst will always select test methods that are appropriate to the physical evidence of an investigation. A general rule in this regard is that failure analysis minus the analysis is just a failure.

Equally deleterious is to disregard some of the necessary tests. Sometimes analysts take a "target of opportunity" approach, stopping an investigation at a point where a discrepancy is found (improper chemical composition, low mechanical strength, etc.). The conclusions would then be composed around a hypothesis based on an incomplete investigation. Remember that most material failures are complex and may contain several significant contributory causes. Although it may be economically attractive to conduct a minimal evaluation, the proximate failure cause may be missed, and improper recommendations may be made as a result of this ill-advised practice. The following tests should all be considered:

Environmental testing: Environmental tests may simulate the corrosivity and temperature extremes experienced by a component in service. Either the failed part or an exemplar part that has been processed similarly could be subjected to these procedures, which may reveal behavior that could cause failure.

Mechanical testing: Mechanical properties have been historically measured by material suppliers and are often reported to the end user. Properties measured on a failed material can often be directly compared to pertinent specifications and prior property data. Discrepant strength levels can be determined, and in many cases degradation of these properties during service can be identified.

Chemical analysis: Chemical analysis is an integral part of an investigation, because it indicates whether a component is made of the specified material. Subtle variations in composition can often dictate the strength and property values that processing can develop. In addition, relatively low amounts of impurity elements can cause significant changes in these same properties.

Metallography: Examination of cross sections of materials involved in a failure can provide important insights into the probable cause of the incident. One of the basic expressions of materials science can be expressed as follows: Properties = structure=processing.

Determination of material structure can indicate the likely mechanical and physical properties of a component. Similarly, this examination can reveal crucial processing information, which may indicate incorrect or incomplete heat treatment or other required structural alteration. Metallography can reveal macrostructure, such as depth of surface hardening or banding, and it can also reveal microstructure, such as grain size and the phases present.

Carefully prepared cross excellent depth of field, which is a boon to morphological identification.

In addition to the fracture mode, electron microscopy can help identify types of mechanical damage, such as adhesive or abrasive wear, corrosion, and other distinct features. To some extent, surface roughness or machining quality can also be evaluated. Corrosion products and inclusions can be examined via electron microscopy, and these minute features can be analyzed and identified by X-ray spectroscopy or other techniques.

In many cases, the skill of the metallographer is key to revealing the critical evidence that leads the analyst to the correct solution. Poorly prepared Tension specimens can ruin the analysis by destroying critical inclusions, porosity, or other crucial evidence.

Fractography: Fractography generally involves a stereoscope along with an electron microscope. It is an indispensable tool for the analyst, because much fracture information cannot be revealed by any other technique. Magnifications as high as 10,000X can reveal many features that cannot be seen by standard light microscopy. The limit of resolution of a microscope is the wavelength of the imaging illumination, hence visible light would exhibit far less resolution than electron microscopy. Electron microscopes can also offer excellent depth of field, which is a boon to morphological identification.

In addition to the fracture mode, electron microscopy can help identify types of mechanical damage, such as adhesive or abrasive wear, corrosion, and other distinct features. To some extent, surface roughness or machining quality can also be evaluated. Corrosion products and inclusions can be examined via electron microscopy, and these minute features can be analyzed and identified by X-ray spectroscopy or other techniques.

Special testing

This group of tests includes all those that do not easily fit into the previous categories. The following are a few of the more important techniques.

Fracture mechanics: This tool can approximate the stresses surrounding a fracture to better explain its occurrence. By making.certain assumptions and isolating the fracture location, simulations of crack

growth scenarios can be calculated. By altering the determined model parameters, the analyst can estimate the magnitude of the mechanical stress applied to a component at the time of failure. This is often used in tandem with NDE-measured flaw sizes to determine the reduced loads necessary to produce crack growth with a flaw of certain dimensions. Mechanical properties measured during destructive physical analysis can be placed in this model to acquire more specific information.

Finite element analysis: Finite element analysis, or FEA, is an advanced modeling technique that can help to predict the magnitude of stresses on individual components within complex assemblies. This type of analysis also utilizes the computational capabilities of current technology. Changing the 1ocanon and magnitude of simulated loads on the three-dimensional model may indicate the reason for a failure. In some circumstances, this type of engineering analysis is of utmost importance when physical evidence is not available but mechanical loading is known.

Simulation: In some instances, sophisticated test beds and test apparatus can be developed for approximating the service conditions involved in a materials failure. Although this tool is more often used during product development, the ability to reproduce a failure is a very clearcut proof of a failure hypothesis. By altering the physical simulation through inserting the specific parameters associated with the failure, an exemplar part similar to the failed one can be tested. Naturally, the actual factors contributing to a failure may be too numerous and varied to adequately simulate; however, failures in which mechanical stresses are the only significant causes are not uncommon.

Typical nondestructive examination methods<

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Acronym

Technique

Information provided

EC

Eddy current

Detection of anomalies by differential electrical current response

MRI

Magnetic resonance imaging

Identification of features and structure via introduction of

alternating magnetic fields

MT

Magnetic particle examination

Detection of surface and near surface flaws in ferromagnetic materials by flux leakage

PT

Penetrant testing

Identification of flaws and cracks open to the surface in many materials via liquid retention

RT

Radiographic examination

Identification of material flaws and features via density differences measured by penetrating radiation

UT

Ultrasonic examination

Detection of anomalies by differential reflection of ultrasonic pulses

Typical chemical analysis methods

Acronym Technique Information provided
AA Atomic absorption Analysis of dissolved materials in a gaseous flame.
AES Auger electron spectroscopy Composite analysis of surface layer including depth profiling.
DSC Differential scanning calorimetry A measure of heat flow related to temperature.
EDS Energy dispersive X-ray spectroscopy Chemical analysis of small features and particles in the electron microstructure.
ESCA Electron spectrometry for chemical analysis
FTIR Fourier transform infrared spectroscopy
GC Gas chromatography Quantitative analysis of organic materials after volatilization
GDS Glow discharge spectroscopy Analysis of metallic materials vaporized in an arc
GPC Gel permeation chromatography Form of liquid chromatography for molecular weight distribution of polymers
ICP Inductively coupled plasma spectroscopy
LC Liquid chromatography Quantitative analysis of dissolved materials by separation techniques
MS Mass spectroscopy Detection of mass units of organic materials in a magnetic field
OES Optical emission spectroscopy Analysis of metallic materials vaporized in an arc
RS Raman laser spectroscopy Qualitative identification of organic chemicals using Raman absorbance
Sims Secondary ion mass spectroscopy Surface analysis technique detects all elements
TGA Therxnogravimetric analysis A measure of weight change related to temperature
TMA Thennomechanical analysis A measure of physical properties related to temperature.
XPS X-ray photoelectron spectroscopy Elemental analysis of surface layers.
XRD X-ray diffraction Analysis of materials and crystal structures by X-ray impingement.
XRF X-ray fluorescence spectroscopy Bulls analysis of solids or liquids by X-ray excitation.

Typical mechanical testing methods

Typical mechanical testing methods Test Information provided
Bend Measures ductility of base materials and weldments
Brinell hardness Indentation hardness test has a high load and measures a relatively large area.
Compression Test measures inherent resistance to fracture under compressive loading. Compression modulus can be calculated.
Creep / stress rupture High temperature tension test measures resistance to rupture by a creep mechanism.
Drawability Measures the formability of a material to determine its suitability for severe forming during manufacturing.
Durometer hardness Surface hardness testing method for plastics and elastomers.
Fatigue Cyclic loading test to determine fatigue resistance.
Flexure impact Three or four point bending test measures ductility. Rapid point loading test determines impact resistance or toughness, often at simulated service temperature. (Charpy V-Notch, Izod, etc.)
Knoop microhardness Microindentation hardness testing method for very small areas or features.
Rockwell hardness Indentation hardness tests measure an intermediate amount of surface area.
Scleroscope hardness Surface hardness measurement of softer materials.
Shear Measures resistance to fracture when shearing loads are applied. Most commonly performed as single or double shear methods.
Superficial hardness Lower load varieties of the standard Rockwell tests for thin samples or thin surface layers.
Tension Mechanical test pulls a material apart and can determine the ultimate tensile strength, yield strength, elongation, reduction in area, modulus of elasticity, and other properties.
Torsion Mechanical test loads a material in a twisting manner to measure its strength in torsion.
Vickers microhardness Microindentation hardness testing method for very small areas or features.

Analyze data and prepare a report

After all of the systematic examination and data collection has been completed, the information must be organized and interpreted for its significance to the failure. The assembled results from the different tests must be considered collectively, because the final hypothesis needs to be in substantial agreement with all physical evidence and test results. As the investigation reaches its fruition, most of the possible failure causes can be conclusively discounted and a single, coherent explanation is indicated. At this stage of the investigation, it pays to have kept an open mind throughout, because the results may not conform to preconceived notions or preferred outcomes.

Presentation of the results in a technical report is a very essential portion of a failure analysis. The data should be arranged and organized in such a manner that the information proceeds from general to specific, similar to any professional technical report. The training and experience of the investigator are required to assemble the often large volume of information into a contiguous report that logically leads to the conclusions.

In many failure analyses, the resolution of a single incident is subordinate to the goal of avoiding similar failures in the future. A good investigation can provide specific recommendations concerning design, materials, and processing changes that will avert identical failures in other products. One of the possible exceptions to this is in litigation, where determination of the specific failure cause is paramount and may supersede other interests.

Naturally, changing the properties of any component in a complex assemblage of parts can alter the forces on adjacent components, and these changes may not always be readily predicted. All recommended changes must be evaluated prior to implementation, as other, potentially more severe failures may result.