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Topography Analysis (FRASTA)

In conventional fractography using optical and SEMs, fracture surfaces of broken components are examined to determine the crack initiation site, crack propagation direction, and fracture mode. When distinct features such as fatigue striations are present on the fracture surface and when there is a one-to-one correspon- dence between such features and the load cycle, the crack growth rate can be readily determined from striation spacing. In stress- corrosion cracking (SCC), repeated formation and fracture of the corrosion product layer at the crack tip may produce distinct fea- tures that can be measured and related to crack growth rate. There are many situations where well-defined features are not readily discernible on the fracture surfaces and the load/environment his- tories of the failed component are not known. For such cases, an advanced technique is now available for reconstructing the fracture event in microscopic detail. In this technique, the three-dimen- sional topographies of the conjugate fracture surfaces are simul- taneously analyzed to obtain quantitative information on fracture toughness, crack nucleation times, and crack growth rates without the need for a fracture mechanics test or instrumented impact test. This technique, called fracture surface topography analysis (FRASTA), was developed by SRI International, Menlo Park, Ca- lif. (Ref 11, 12). The technique enables reconstruction of the frac- ture process and allows identification of the changes in load and environment with the position of the crack front.

The Concept. When a component is loaded, events occur in

the following sequence: elastic deformation, localized inelastic de- formation at sites of defects or stress concentration, and develop- ment of a microcrack in or near the inelastic deformation zone.

Fig. 5.3 Perspective view of conjugate fracture surfaces. Source: Ref 11 Fig. 5.4 Fractured-area-projection plot. Source: Ref 11

ograph is inverted and positioned with respect to the other so that the pair of topographs represent the fracture surfaces. The two conjugate topographs are first brought together until they overlap everywhere. Then they are gradually separated. As the displace- ment between them increases, small nonoverlapping areas appear. These nonoverlapping areas represent areas on the fracture sur- faces that formed first; i.e., they are the sites where microcracks nucleated. As the relative displacement between the topographs is increased, new nonoverlapping areas appear, representing nucle- ation of new microcracks; existing nonoverlapping areas become larger, representing growth of microcracks; and adjacent nonov- erlapped areas merge, signifying microfracture coalescence. Thus, the fracture process is reconstructed. When the two conjugate to- pographs are aligned and juxtaposed so that the most protruding features just touch, the configuration of the space between the two topographs is a representation of the crack configuration at the stage of final separation. From a knowledge of total fractured area as a function of topograph displacement, crack growth rate can be computed.

The information is used to create a series of fractured-area-pro- jection plots (FAPP) and a series of cross-sectional plots (XSP). A FAPP is equivalent to a radiograph taken perpendicular to the fracture plane. It provides information on microcrack initiation sites and projected areas of microcracks and macrocracks (Fig. 5.4). A XSP is a section perpendicular to the fracture surface. It shows the profile of the microcracks, the macrocrack tip opening The stresses drop to zero on the newly formed microfracture sur-

faces. The applied load is redistributed to sound material ahead of the crack tip, which continues to deform inelastically to micro- fracture, and the process repeats to other areas. The deformation that existed at the moment of microcracking is thus “frozen” in the material immediately below the fracture surface. This material undergoes no further inelastic deformation.

The previously mentioned sequence of nucleation, growth, and coalescence of microfractures produces different amounts of in- elastic deformation in the component as a function of distance from the microfracture nucleation site. The varied amount of local inelastic deformation results in an irregular fracture surface (Fig. 5.3). The topography of the fracture surface is a permanent record of the evolution of all the microfracture events that resulted in the macrofracture.

Surface irregularity is also produced as the crack tip interacts with the microstructure of the component. When there is no in- elastic deformation, crack tip interaction with the local microstruc- ture would produce conjugate fracture surfaces with their profiles matching precisely. Any mismatch between the profiles of the con- jugate fracture surfaces indicates inelastic deformation, the extent of which can be determined with a computer. This information is used to reconstruct the fracture sequence at the microscopic level.

The Methodology. Using a scanning laser microscope (SLM),

the fracture surface photograph and the topographic map of each of the two conjugate fracture surfaces are first recorded. The top- ograph data are digitized and stored in a computer. The sequence of the fracture event is reconstructed using the computer. One top-

Chapter 5: Advanced Techniques of Failure Analysis / 35

Fig. 5.5 Cross-sectional plot. Source: Ref 11

angle and displacement, and the amount of inelastic deformation necessary before fracture (Fig. 5.5).

Fracture surface topography analysis is a powerful tool and its applications include identification of fracture mechanisms, deduc- tion of crack history in components in service, and determination of fracture parameters. The following examples illustrate the use of the FRASTA technique.

Stress-Corrosion Cracking (SCC) in a Stainless Steel. Stress-

corrosion cracking is difficult to detect prior to component failure, and unexpected SCC can lead to disasters. The crack initiation and propagation may respond in different ways in different environ- ments. Crack initiation in a given material generally depends on the bulk environment, whereas crack propagation depends on the crack-tip environment, which could be different from the bulk en- vironment. Figure 5.6 shows the SEM fractographs and the cor- responding topographs of the two conjugate fracture surfaces of a notched tensile test specimen of sensitized 304 type stainless steel tested for SCC in clean water for one week at a strain rate of 7⳯ 10ⳮ8sⳮ1and then pulled to fracture at room temperature (Ref 13). Intergranular SCC can be seen at the ten o’clock and two o’clock positions in the fractographs. Cracks have initiated on the surface and propagated inward.

A series of FAPPs were taken at various displacements of the topographs (Fig. 5.7). Cracked regions are seen as white patches, corresponding to nonoverlapping areas of the topographs.

The unfractured area (dark area in the FAPP) is plotted against the topographic map displacement and the result is shown in Fig. 5.8. The displacement is correlated to specimen elongation and hence, time. Thus, knowing the time for the duration of the test (total elongation) and the time for crack growth (elongation as- sociated with the crack growth period), the time for crack initiation (first appearance of microcrack) could be computed. In the present case of SCC in clean water, it is done as follows. Test duration (171.8 hours) multiplied by the specimen gage length (25 mm) and the strain rate (7⳯ 10ⳮ8sⳮ1) gives the total elongation as 1082 lm. From the ratio of the elongation associated with the crack propagation period to the total elongation, the time of crack propagation can be found (577lmⳮ 64 lm) ⳯ 171.8 hours/1082 lm⳱ 81.4 hours. Thus, the time taken to initiate the first micro- crack is 171.8ⳮ 81.4 ⳱ 90.4 hours. It was also found that this time corresponded to a discontinuity in the load/time record in the testing machine. Knowing the crack penetration length and the time for crack propagation, the crack-growth rate is readily com- puted. In the present case, it is about 2⳯ 10ⳮ6mm/s.

In a test for SCC of the same material in water deliberately contaminated with 1 ppm sulfuric acid, FRASTA technique re- vealed that the crack initiation and growth started in the interior and only after considerable growth, the internal cracks merged with the surface. The crack initiation time in this case was 46 hours and the crack propagation rate about 4⳯ 10ⳮ6mm/s.

Fracture Parameters. The cross-sectional plot XSP of a frac-

ture surface can be used to compute the fracture toughness of the material. A compact specimen of a thermally embrittled stainless steel was fatigue-precracked, then loaded monotonically to extend the crack. The cracked area was heat-tinted for easy identification and then the fracture surfaces opened (Ref 14, 15). Figure 5.9 shows the XSP obtained by displacing the topographs along a

center plane normal to the fracture surface. The crack tip opening displacement,d, is measured at the tip of the fatigue precrack, i.e., at the onset of crack extension.

The fracture toughness J-integral or stress intensity factor K of the material for crack initiation can be computed from the crack tip opening displacement, using the relations:

J⳱ r d0

and

2 1/2 2 1/2

K ⳱ [JE/(1 ⳮ m )] ⳱ [Er d/(1 ⳮ m )]0

where E is the elastic modulus;d is the crack opening displace- ment;r0is the flow stress, i.e., the average of yield strength and

ultimate tensile strength; andm is the Poisson’s ratio. The values computed through FRASTA agreed well with the values obtained by conventional fracture toughness testing (Ref 14, 15).

Other Practical Applications. The FRASTA technique has

been successfully applied to determine when a crack initiated in a boiler tube during its 22 years of service and at what rate the crack grew. The technique revealed that the tube had periods of accel- erated and decelerated crack growth (Ref 16).

In a nuclear reactor vessel of stainless steel subjected to repeated thermal shock, the crack initiation time was computed through the application of FRASTA technique (Ref 15).

Fig. 5.6 Scanning electron fractographs and corresponding topographs of conjugate fracture surfaces of sensitized 304 stainless steel tested in clean water. Source: Ref 11, 13

There are many instances where corrosion pits in structures have led to fatigue cracks, ultimately resulting in failure. This is par- ticularly true in structures such as aircraft flying in marine envi- ronment. The accident of the Boeing 737 aircraft of Aloha Airlines near Maui, Hawaii on April 28, 1988 is one such incident in which a section of the fuselage tore away in flight. Tiny cracks at the rivet holes had joined to form a larger crack of critical length. An insight into the evolution of corrosion pit to fatigue crack and the time for such a transition was obtained by the application of FRASTA technique to specimens of aircraft skin material, i.e., 2024-T3 aluminum alloy tested in fatigue after exposure to salt solution (Ref 17).

Other applications of FRASTA include understanding the frac- ture process in ceramic fiber (silicon carbide) reinforced metal ma- trix (titanium aluminide) composite, and in polyethylene gas pipe- line material (Ref 12).