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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 3 Series

“Laboratory Studies in Failure Analysis”

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Laboratory Studies

a) cleaning of fracture surfaceb) Preliminary Examination (Macro- fartrography)c) Microscopic Examination (Micro -fractrography)d) Metallography Examinatione) Chemical Analysis. f) Mechanical Properties.g) Nondestructive Evaluationh) Any other special technique

Last Class

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Perform fractographic examination. Perform chemical analyses and compare

results with specification or standards. Analyze any important surface corrosion products, deposits, or coatings.

Determine mechanical properties and compare with specifications or standards.

Perform macroscopic examination to evaluate homogeneity, integrity, and quality.

Perform metallographic examination to evaluate microstructural features. Determine the deformation direction of wrought products and its relationship to the applied and residual stresses.

Perform microhardness testing to measure case depths, evaluate cold working, determine quality of weldments, and aid in identifying

phases. Perform high-magnification

metallographic examination using the electron microscope to study phases unresolvable with the light microscope.

Microprobe any critical abnormalities, such as inclusions and segregations, that are too small for bulk analysis.

Use x-ray techniques to determine: (a) level of residual stress; and (b) the relative amounts of phases, for example, austenite or retained austenite, ferrite, delta ferrite or martensite, sigma, and carbides in steels.

Perform simulation tests to evaluate critical characteristics of the material (such as the stress-corrosion cracking tendency in a particular environment), to determine the degree of embrittlement, or to confirm the method of heat treatment or hardenability.

Basic Approach to Failure Analysis (Important steps)

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Fracture-Cleaning Techniques

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Fracture-Cleaning Techniques• Fracture surfaces exposed to various environments generally contain

unwanted surface debris, corrosion or oxidation products, and accumulated artifacts that must be removed before meaningful fractography.

• Before starting cleaning procedures, the fracture surface should be surveyed with a low-power stereo binocular microscope, and the results should be documented with appropriate sketches or photographs.

• The most common techniques for cleaning fracture surfaces are:1) Dry air blast or soft organic-fiber brush cleaning2) Replica stripping3) Organic-solvent cleaning4) Water-base detergent cleaning5) Cathodic cleaning6) Chemical-etch cleaning

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1) Air Blast or Brush Cleaning:

• Loosely adhering particles and debris can be removed from the fracture surface with either a dry air blast or a soft organic-fiber brush, such as an artist's brush, should be used on the fracture surface because a hard-fiber brush or a metal wire brush will mechanically damage the fine details.

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2) The replica-stripping cleaning technique• It is very similar to that "Preservation Techniques". However,

instead of leaving the replica on the fracture surface to protect it from the environment, it is stripped off of the fracture surface, removing debris and deposits. Successive replicas are stripped until all the surface contaminants are removed.

• The given figure shows successive replicas stripped from a rusted steel fracture surface.

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• Note: The replicas can be retained, and the embedded contaminants can be chemically analyzed, if the nature of these deposits is deemed important.

• The one disadvantage of using plastic replicas to clean a fracture surface is that on rough surfaces; it is very difficult to remove the replicating material completely.

• However, if the fracture surface is ultrasonically cleaned in acetone or methyl acetate. after each successive replica is stripped from the fracture surface, removal of the residual replicating material is possible.

• Ultrasonic cleaning in acetone or the appropriate solvent should be mandatory when using the replica-stripping cleaning technique.

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3) Organic solvents

• Organic solvents such as xylene, naphtha, toluene, ketones, and alcohols, are primarily used to remove grease, oil, protective surface coatings, and crack-detecting fluids from the fracture surface.

• Sample can be cleaned by 3 methods:

1. soaked in the appropriate organic solvent for an extended period of time,

2. immersed in a solvent bath where jets from a pump introduce fresh solvent to the fracture surface,

3. placed in a beaker containing the solvent and ultrasonically cleaned for a few minutes

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• The ultrasonic cleaning method is probably the most popular of the three methods mentioned above, and the ultrasonic agitation will also remove any particles that adhere lightly to the fracture surface.

• However, if some of these particles are inclusions that are significant for fracture interpretation, the location of these inclusions relative to the fracture surface and the chemical composition of these inclusions should be investigated before their removal by ultrasonic cleaning.

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• It is important to avoid use of the chlorinated organic solvents, such as tri-chloro-ethylene and carbon-tetra-chloride, because most of them have carcinogenic (hazardous) properties.

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4) Water-base detergent cleaning:

• Water-base detergent cleaning assisted by ultrasonic agitation is effective in removing debris and deposits from the fracture surface and, if proper solution concentrations and times are used, does not damage the surface.

• A detergent Alconox, has proved effective in cleaning ferrous and aluminum materials.

Example:a) The cleaning solution is prepared by dissolving 15 g of Alconox powder in a

beaker containing 350 mL of water.

b) The beaker is placed in an ultrasonic cleaner preheated to about 95 °C (205 °F).

c) The fracture sample is then immersed in the solution for about 30 min, cleaned in water then alcohol, and air dried.

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• Fig: (a) shows the condition of a laboratory-tested fracture toughness sample (AISI 1085 heat-treated steel) after it was intentionally corroded in a 5% salt steam spray chamber for 6 h.

• Figure (b) shows the condition of this sample after cleaning in a

heated Alconox solution for 30 min.

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5) Cathodic cleaningGeneral Requirement:• Cathode = Fracture Sample• Anode = carbon/Platinum• Electrolyte = any current conducting solvent • vibrate the electrolyte ultrasonically or to rotate the specimen (cathode)

with a small motor

It is an electrolytic process in which the sample to be cleaned is made the cathode,

and hydrogen bubbles generated at the sample cause primarily mechanical removal of surface debris and deposits.

An inert anode, such as carbon or platinum, is normally used to avoid contamination by plating upon the cathode.

During cathodic cleaning, it is common practice to vibrate the electrolyte ultrasonically or to rotate the specimen (cathode) with a small motor.

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• The electrolytes commonly used to clean ferrous fractures are sodium cyanide, sodium carbonate, sodium hydroxide solutions, and inhibited sulfuric acid.

Example-1:

• A study in which AISI 1085 heat-treated steel and EX16 carburized steel fractures were exposed to a 100% humidity environment at 65 °C (150 °F) for 3 days.

• A commercially available sodium cyanide electrolyte, ultrasonically agitated, was used in conjunction with a platinum anode for cleaning.

• A 1-min cathodic cleaning cycle was applied to the rusted fractures, and the effectiveness of the cleaning technique without altering the fracture morphology was demonstrated.

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• The below given figure shows a comparison of an as fractured surface with a corroded and cathodically stable ductile cracking region in a quenched-and-tempered 1085 carbon steel.

• The relatively low magnification (1000 ×) shows that the dimpled topography characteristic of ductile tearing was unchanged as a result of the corrosion and cathodic cleaning.

• High magnification (5000 ×) shows that the perimeters of the small interconnecting dimples were corroded away

• The fractographs on the left show the as-fractured surface;

• those on the right show the fracture surface after corrosion exposure and cathodic cleaning

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6) Chemical Etching

• If the above techniques are attempted and prove ineffective, the chemical-etch cleaning technique, which involves treating the surface with mild acids or alkaline solutions, should be implemented.

• This technique should be used only as a last resort because it involves possible chemical attack of the fracture surface. In chemical-etch cleaning, the specimen is placed in a beaker containing the cleaning solution and is vibrated ultrasonically.

• It is sometimes necessary to heat the cleaning solution. Acetic acid, phosphoric acid, sodium hydroxide, ammonium citrate, ammonium oxalate solutions, and commercial solutions have been used to clean ferrous alloys.

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• Titanium alloys are best cleaned with nitric acid .

• Oxide coatings can be removed from aluminum alloys by using a warmed solution containing 70 mL of ortho-phosphoric acid (85%), 32 g of chromic acid, and 130 mL of water.

• However, it has also been recommended that fracture surfaces of aluminum alloys be cleaned only with organic solvents .

• Especially effective for chemical-etch cleaning are acids combined with organic corrosion inhibitors. These inhibited acid solutions limit the chemical attack to the surface contaminants while protecting the base metal.

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• Ferrous and nonferrous service fractures have been successfully cleaned by using the following inhibited acid solution:

a) 3 mL of hydrochloric acid (1.19 specific gravity), b) 4 mL of 2-butyne-1,4-diol (35% aqueous solution), and c) 50 mL of deionized water .

• This study demonstrated the effectiveness of the cleaning solution in removing contaminants from the fracture surfaces of a low-carbon steel pipe and a Monel Alloy 400 expansion joint without damaging the underlying metal.

• Various fracture morphologies were not affected by the inhibited acid treatment when the cleaning time was appropriate to remove contaminants from these service fractures.

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Fractrography

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Fractrography • The word fractography origin from the Latin word fractus, meaning

fracture, and graphy derives from the Greek term grapho, meaning descriptive treatment.

• “The science of studying the fracture surface is termed as fractography”.• Thus, depending on the level of examination, one can have

macrofractography and microfractography.

1. Macroscopic Examination /Macro-fractrography• Macroscopic examination is carried out with unaided eye or a simple

handheld magnifier, or a stereomicroscope with low magnification (up to 50X).

• In the stereo microscope, reasonably large specimens can be handled, and the microscope can easily be adopted for examination of components in the field or accident site.

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• The usual sequence for the examination of fractured components is as follows :1. Visually survey the entire component to obtain an overall understanding of

the component and the significance of the fractured area2. Classify the fracture from a macroscopic viewpoint as ductile, brittle, fatigue,

torsion, and so forth3. Determine the origin of failure by tracing the fracture back to its starting point

or points4. Based on the observed fracture features, estimate the manner of loading

(tension, compression, shear, bending, and so on), the relative stress level (high, medium, or low), and the stress orientation.

5. Examine areas selected by macroscopic examination at higher magnifications by light microscopy, SEM, or replica transmission electron microscopy (TEM) to determine the fracture mode, to confirm the fracture mechanism (observation of cleavage facets, ductile dimples, fatigue striations, and so on), and to detect features at the fracture origin

6. Examine metallographic cross sections containing the origin to detect any microstructural features that promoted or caused fracture initiation, and determine if crack propagation favors any microstructural constituent

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• The features revealed during macroscopic examination are:

1. Type of fracture 2. Origin of fracture 3. Presence of secondary cracks4. Presence of external debris or corrosion products5. Discoloration6. Presence of wear marks in the vicinity of fracture7. Plastic deformation preceding fracture8. Dimensional changes in the component9. Evidence of any overheating10. Post-fracture damage such as rub marks

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Examples of Macrofractrography

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A ductile tensile fracture in a component of circular cross section consists of three distinct zones as shown in the figure. 1. The inner flat/fibrous zone with a fibrous appearance is where the fracture starts and

grows slowly.2. The fracture propagates fast along the intermediate radial zone. The radial lines

extended backward point to the fracture origin. Sometimes the radial lines start from the origin itself.

3. The fracture finally terminates at the shear lip zone that is the annular region near the periphery of the fracture surface. The shear lip zone is at an angle of 45° to the tensile stress direction.

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Macroscopic appearance of ductile (a) and brittle (b) tensile fractures

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Bolt that failed in fatigue. The smooth dark areas are from initial fatigue cracking, now coated with dark adherent oxide. This initial cracking took place some years prior to the later fracture, the surface of which is covered by a less adherent rust coating

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Brittle Sample: series of chevron marks on the fracture surface

Compressor Blade in an Aircraft Engine: Beach Marks

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5 mm

The broken tie-rod of the developmental aircraft towing tractor:

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2. Microscopic Examination / Microfractography

• The information thus gathered at the macroscopic level has to be integrated with the observations on detailed examination at the microscopic level so that meaningful conclusions can be drawn regarding the cause of failure.

• Microscopic examination is carried out using optical microscopes and electron microscopes, the choice dictated by the magnification and resolution desired.

• Microscopic examination is carried out on the fracture surface to study the fracture features and also on a section transverse to the fracture surface to study the internal structure of the material.

• The latter is a destructive test and should be carried out only at the end after recording all the microfractographic features.

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• The additional information one can obtain through microscopy includes:

1) Microstructure of the material through metallography2) Path of fracture 3) Mode of fracture 4) Length of the crack that pre-existed and propagated by

fatigue5) Length of the fatigue crack before it became critical6) Presence of inclusions, pits, or other flaws at the origin7) Striation spacing of the fatigue crack8) Presence of corrosion products

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• The fracture surfaces are generally rough and cannot be easily studied entirely by an optical microscope because of its limited depth of focus and resolution.

• For microfractography, instruments with better depth of focus and resolution are necessary. These requirements are met by the electron microscope of which there are two types:

• The scanning electron microscope (SEM) • The transmission electron microscope (TEM)

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Examples of Microfractrography

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SEM Fractograph Elongated and equiexed dimples:

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TEM Fractograph Elongated and equiexed dimples:

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Sequence of SEM fractographs:

80 X 950 X 1000 X

Fracture in an iron alloy containing 0.14% S and 0.04% O. The fracture was obtained by bending at room temperature. Several spheroidal oxide inclusions are visible, most of them having diameters in the range of 1 to 3 μm.

The 6-μm-diam oxysulfide particle in Fig. 8 shows a shrinkage cavity plus a white spot from an electron beam impingement in fluorescent x-ray analysis.

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Metallography of Fracture Specimen:

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• The metallurgical microscope is yet another instrument very useful to the failure analyst.

• After collecting all the information through fractography of the failed component, a section of the component can be cut transverse to the fracture surface.

• This section is then polished and examined in the metallurgical microscope, both before and after etching. Inclusions present in the material are observed on the as-polished surface.

• The inclusion rating can be determined by standard quantitative microscopy techniques.

• By differences in color, reflectivity, and refractive index, they can also be identified with some prior experience.

• The polished specimen is then etched with suitable etchants to reveal the microstructure of the material.

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• Abnormalities in the microstructure that may have been responsible for the failure can be identified at this stage.

• The path of a crack, whether it is intergranular or transgranular, and branched or not branched, will be clear in the microstructure.

• Cracks due to stress corrosion, hydrogen embrittlement, and liquid metal embrittlement are generally intergranular with some exceptional situations.

• Fatigue cracks are transgranular. If a stress-corrosion crack propagates by fatigue, the transition from intergranular to transgranular mode can be seen in the microstructure.

• Stress-corrosion cracks in certain stainless steels are transgranular with extensive branching.

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• Plastic deformation of the component prior to fracture can be recognized in the microstructure by the elongated grains.

• Abnormal grain growth, segregation of brittle or weak phases at the grain boundaries, and recrystallization are some of the other features that can be identified by metallography.

Transgranular crack propagation

intergranular crack propagation

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Chemical Analysis of Fracture Specimen

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• Chemical analysis of fracture component provides information regarding any deviation from the standard specifications, compositional inhomogeneities, impurities, inclusions, segregations, also helpful in identifying the nature of corrosion products, coatings, external debris, and so on.

• Several cases of service failures are known to have been caused by the presence of deleterious inclusions from which cracks start in the component and propagate, leading to fracture.

• Certain impurities are known to cause embrittlement in metals. Segregation of constituent elements sometimes provides an easy path for crack propagation.

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• Hence, identification of these harmful constituents is very important in failure analysis. A variety of instruments are available for bulk chemical analysis and microchemical analysis as well…..

• A few features are briefly discussed here.

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A- Techniques for average bulk chemical analysis (accuracy, 2 to 5%)

● Spectrophotometry: Applicable to nearly all elements; accessible range, 0.001 to 50%.

● Atomic absorption spectrometry: Applicable to practically all elements; accessible range, 0.001 to 10%;

● Emission spectroscopy: Applicable to all elements; accessible range, 0.005 to 10%;

● X-ray fluorescence analysis: Normally applicable to elements heavier than sodium; accessible range, 0.005 to 10%;

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B- Techniques for local composition variations

● Laser probe microanalysis: Applicable to nearly all elements; accessible range, 0.01 to 100%; accuracy, semiquantitative; resolution, 20 to 200 µm

● Electron probe microanalysis: Applicable to elements heavier than boron; accessible range, 0.001 to 10%; accuracy, 5 to 10%; resolution, 0.2 to 1 µm

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C- Techniques for surface chemical analysis

● Auger electron spectroscopy: Applicable to all elements except hydrogen and helium; accessible range, >0.1%; accuracy, 5 to 10%; analysis depth, 10 to 20 A˚

● X-ray photoelectron spectroscopy: Applicable to all elements except hydrogen and helium; accessible range, >0.01%; accuracy, qualitative and semiquantitative; analysis depth, 5 to 25 Ao

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Analysis of Mechanical Properties of fracture component

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• One of the important step in any failure analysis.

• This process enables the investigator to judge whether the material with which the component is made meets the strength specifications and whether the component was capable of withstanding the service stresses.

• If the size of the failed component permits, samples can be taken from the component, and the conventional mechanical testing can be done by standard test procedures.

• Tensile test is generally the most useful one in many cases. Other properties such as impact strength, toughness, and creep rupture provide clues for the mechanism of failure.

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• Sometimes, even tests on miniature specimens would provide vital information.

• If the condition of the component does not permit tensile or other mechanical tests, even a hardness measurement would help in estimating the tensile strength.

• This method has been adopted in quite a few failure cases.

• Defects due to improper processing or inadequate heat treatment would result in poor mechanical properties.

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Next Class

• NDT Techniques. • Modes of Failure.

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Reference

1. ASM Hand book Fractrography Volume 12

2. ASM Hand book Failure Analysis and Prevention,

Volume 11

3. Failure Analysis of Engineering Structures

Methodology and Case Histories V. Ramachandran,

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