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 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.

The features revealed during macroscopic examination are:

i. Type of fracture

ii. Origin of fracture

iii. Presence of secondary cracks

iv. Presence of external debris or corrosion products

v. Discoloration

vi. Presence of wear marks in the vicinity of fracture

vii. Plastic deformation preceding fracture

viii. Dimensional changes in the component

ix. Evidence of any overheating

x. Post-fracture damage such as rub marks

Example:

Macrofractography of ductile tensile sample:

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.

Fig. 166 "Cup and cone" tensile fracture of cylindrical test specimen is typical for ductile metals; annealed AISI 1035. Fracture originates near the center of the section with multiple cracks that join and spread outward. When cracks reaches a region near the surface, the stress state changes from tension to macro shear, forming a fracture at approximately 45° to the plane of the major fracture--the familiar "shear lip." (D.J. Wulpi, Consultant)

Example: Macrofractography of Brittle sample:

Figure: Fast, unstable crack propagation may sometimes be manifested as

a series of chevron marks on the fracture surface

Example:

Four fatigue-fracture surfaces that were produced in bending in the laboratory with the use of assorted notches to simulate service conditions; the material is AISI W1 tool steel. All at 2×

The specimen in Fig. 760, which was tested without a notch and fractured after 432,000 cycles at 365 MPa (53 ksi), shows two cracks origins at opposite corners where bending was maximum.

The specimen in Fig. 761 had a single notch at top and broke after 20,500 cycles at 434 MPa (63 ksi); note the small shear lip around the region of fast fracture at bottom.

The specimen in Fig. 762 was notched at two corners that were along an axis not in the plane of bending and broke after 50,000 cycles at 338 MPa (49 ksi); the two resulting fatigue cracks therefore had somewhat distorted shapes.

The specimen in Fig. 763 was notched at four sites--two in the bending plane and two in a plane 60° from it--and broke after 10,260 cycles at 434 MPa (63 ksi). The four notches produced fatigue zones, which formed two major crack fronts that nearly penetrated the section.

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.

The additional information one can obtain through microscopy includes:

Microstructure of the material through metallography

Path of fracture

Mode of fracture

Length of the crack that pre-existed and propagated by fatigue

Length of the fatigue crack before it became critical

Presence of inclusions, pits, or other flaws at the origin

Striation spacing of the fatigue crack

Presence of corrosion products

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)

Possible combinations of mating dimple shapes resulting from different stress states, which cause the crack tip to deform by various

modes

Example-1 SEM Fractograph Elongated and equexed dimples:

Example-2 TEM Fractograph Elongated and equexed dimples:

Example 3:

Fig. 17 Surface of tensile-test fracture in specimen of low-carbon, high-oxygen iron that was broken at room temperature. Many of the equiaxed dimples contain spheroidal particles of FeO. The rectangle marks the area shown at higher magnification in Fig. 18 and 19. SEM, 500×Fig. 18 Enlargement of the area within the rectangle in Fig. 17, showing the surface contours of the dimple cavities of the very ductile fracture. Dimly visible in the central dimple is a globular particle of FeO; the particle is shown more clearly in Fig. 19. SEM, 2400×

Example-4: Stereo pair of scanning electron microscope fractographs of the

surface of a tensile-test fracture obtained at room temperature

The alloy was a low-carbon iron to which an appreciable amount of Fe2O3

had been added to form an aggregate of FeO inclusions. The dimples that are characteristics of ductile rupture are evident here, and many of these dimples contain one or more globular oxide inclusions that are readily apparent. There appear to be two sizes of these oxide inclusions--some being about 6 μm in diameter and other about 3 μm in diameter. Unlike many inclusions displayed in other fractographs, which have relatively smooth, unbroken contours several of the particles shown here possess sizable surface defects. Some of these defects may be exposed internal shrinkage cavities. The surfaces of the dimples show contours that vaguely resemble fatigue-striation marks. The differences in topographic contours of the dimples displayed in this stereo pair of fractographs can be appreciated by viewing the fractographs stereographically, which provides a three dimensional effect. It then becomes apparent that the dimples are chimney like cavities with nearly vertical walls in many instances and with bottoms at great depth that appear black and without detail. The FeO inclusions appear to cling to the cavity walls, many at a point part way to the bottom of the "chimney." Most of the separating walls between adjacent chimneys are extremely thin, which makes it surprising that these walls did not rupture at a point closer to the bottom of the chimney. 1200×

Example- 5: Sequence of SEM fractographs:

Sequence of SEM fractographs, at increasing magnifications (80×, 950×, and 5000×, respectively), that show a 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 rectangle in Fig. 6 indicates the area that is shown at higher magnification in Fig. 7, and the rectangle in Fig. 7 indicates the area that is shown at still higher magnification in Fig. 8. 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. It is quite evident that, during the process of microvoid coalescence, the iron matrix has become detached from the globular inclusions at the metal-to-oxide and metal-to-sulfide interfaces, leaving these inclusions unaffected by the applied stresses and severe deformation taking place around them.

Example- 6

Example- 7: Sequence of SEM fractographs:

Fig. 14 Intergranular fracture that was generated in a specimen of oxygen-embrittled Armco iron by a Charpy impact test at room temperature. The grain features appear sharp and clean. Note the secondary cracks, which follow grain boundaries. See also Fig. 15 and 16 for views of other regions of this fracture. SEM, 55×

Fig. 15 View of another region of the surface of the impact fracture shown in Fig. 14, showing facets that resulted from a combination of intergranular rupture and transcrystalline cleavage. Note the array of small river patterns at the bottom edge of the large facet at center. See also Fig. 16. SEM, 655×

Fig. 16 View of a third region of the surface of the impact fracture shown in Fig. 14 and 15. Note the almost perfect grain-boundary surfaces and the sharp edges and points at which the separated-grain facets meet. The secondary cracks are equally clean separations. SEM, 670×

Example-8:

Fracture of SiC initiated at corrosion pits. The boron- and carbon-doped SiC was injection molded and pressure less sintered. The sample was then coated with 2 mg/cm2 (0.07 oz/ft2) Na2SO4 and exposed for 48 hat 1000 °C (1830 °F) in 0.1% SO2/O2 gas. Corrosion products were removed with a solution of 10% HF inwater and the sample was broken in four-point bending at a strain rate of 0.5 mm/min (0.02 in./min).

Fig. 1278: Fracture surface shows radial crack lines emanating from the origin, which was a corrosion pit (top).SEM (30° tilt), 115×.Fig. 1279: Close-up of pit-ceramic interface reveals preferential grain-boundary attack in advance of the pit. SEM (30° tilt), 4300× (J.L. Smialek and N.S. Jacobson, NASA Lewis Research Center)

Example-9

Metallography of Fracture Specimen: 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.

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.

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.

Figure shows the intergranular and transgranular modes of crack

propagation, revealed by metallography.

Figure: Optical microstructure showing transgranular crack propagation

Figure: Optical microstructure showing intergranular crack propagation.