ASM Metals HandBook Volume 12 - Fractography
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ASM Metals HandBook Volume 12 - Fractography

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nuclei for fatigue fractures or stress-corrosion fractures because they increase both local 
stresses and reactions to detrimental environments. Large discontinuities may reduce the strength of a part to such an 
extent that it will fracture under a single application of load. However, a discontinuity should not be singled out as the 
sole cause of fracture without considering other possible causes or contributing factors. Thorough failure analysis may 
show that the fracture would have occurred even if the discontinuity had not been present. 
Fractures that originate at, or pass through, significant metallurgical discontinuities usually show a change in texture, 
surface contour, or coloration near the discontinuity. Examination of a suspect area at several different magnifications and 
under several different lighting conditions will often help to determine whether a significant discontinuity is present and 
may provide information about its size and type. Varying the angle of incident light during examination with a low-power 
stereomicroscope may be especially helpful. Segregation or unfavorable grain flow sometimes contributes to fracture 
without showing evidence that can be detected by direct visual examination. Even when visual indications of a 
metallurgical discontinuity are present, corroborating evidence should be obtained from other sources, such as 
examination of metallographic sections through the suspect area or study of local variations in chemical composition by 
electron microprobe analysis or Auger electron spectroscopy (AES). 
Even though cracks usually originate at discontinuities, the type of discontinuity does not necessarily determine fracture 
mechanism. For example, fracture from a gross discontinuity, such as a rolling lap, can occur by any of the common 
fracture mechanisms. In general, discontinuities act as fracture initiation sites and cause fracture initiation to occur earlier, 
or at lower loads, than it would in material free from discontinuities. Additional information on material defects that 
contribute to fracture/failure is available in the "Atlas of Fractographs" in this Volume and in Volume 11 of ASM 
Handbook, formerly 9th Edition Metals Handbook. 
Laps, Seams, and Cold Shuts. An observer familiar with the characteristics of various types of fractures in the 
material under examination can usually find indications of a discontinuity if one was present at the fracture origin. A flat 
area that, when viewed without magnification, appears black or dull gray and does not exhibit the normal characteristics 
of fracture indicates the presence of a lap, a seam, or a cold shut. Such an area may appear to have resulted from the 
peeling apart of two metal surfaces that were in intimate contact but not strongly bonded together. A lap, a seam, or a cold 
shut is fairly easy to identify under a low-power stereomicroscope because the area of any of these discontinuities is 
distinctly different in texture and color from the rest of the fracture surface. 
Failures in valve springs that originated at a seam are shown in Fig. 99. The failure shown in Fig. 99(a) began at the seam 
that extended more than 0.05 mm (0.002 in.) below the spring wire surface. The fatigue fracture front progressed 
downward from several origins. Each one of these fronts produces a crack that is triangular in outline and is without fine 
detail due to sliding of the opposing surfaces during the later stages of fracture. This occurs when the fracture plane 
changes to an angle with the wire axis in response to the torsional strain. These surfaces are visible in the lower part of 
Fig. 99(a). 
Fig. 99 Fractures in AISI 5160 wire springs that originated at seams. (a) Longitudinal fracture originating at a 
seam. (b) Fracture origin at a very shallow seam, the arrow indicates the base of the seam. (J.H. Maker, 
Associated Spring) 
Fig. 100 Laps formed during thread rolling of a 300M steel stud. (a) Light fractograph showing laps (arrows). 
(b) SEM fractograph giving detail of a lap. (c) SEM fractograph showing heavily oxidized surfaces of a lifted lap; 
the oxidation indicates that the lap was present before heat treatment of the stud. Arrow at right points to area 
shown in fractograph (d), and arrow at left points to area shown in fractograph (e). (d) and (e) SEM 
fractographs showing oxidized surfaces of the lifted lap in fractograph (c). See Fig. 101 for views of the stress-
corrosion crack initiated by the laps. 
The failure shown in Fig. 99(b) has many of the characteristics of that shown in Fig. 99(a), except that the seam is 
scarcely deeper than the folding of the surface that results from shot peening. Observation of it requires close examination 
of the central portion of the fractograph. This spring operated at a very high net stress and failed at less than 106 cycles. 
The fractographs in Fig. 100 show laps that had been rolled into the thread roots of a 300M high-strength steel stud during 
thread rolling. The laps served as origins of a stress-corrosion crack that partially severed the stud. Both surfaces of a 
lifted lap (Fig. 100c) were heavily oxidized (Fig. 100d and e), indicating that the lap was formed before the stud was heat 
treated (to produce a tensile strength of 1930 to 2070 MPa, or 280 to 300 ksi). The stress-corrosion crack near the origin 
is shown in Fig. 101. 
Fig. 101 SEM views of the corrosion products (a) and the intergranular fracture and secondary grain-boundary 
cracks (b) that were the result of the laps shown in Fig. 100 
Cracks. The cause and size of a pre-existing crack are of primary importance in fracture mechanics, as well as in failure 
analysis, because of their relationship to the critical crack length for unstable crack growth. Figure 102 shows a fracture in 
a highly stressed AISI 4340 steel part. A narrow zone of corroded intergranular fracture at the surface of the part is 
adjoined by a zone of uncorroded intergranular fracture, which is in turn adjoined by a dimpled region. The part had been 
reworked to remove general corrosion products shortly before fracture. It was concluded that the rework failed to remove 
about 0.1 mm (0.004 in.) of a pre-existing stress-corrosion crack, which continued to grow after the part was returned to 
Fig. 102 Fracture caused by a portion of a pre-existing intergranular stress-corrosion crack that was not 
removed in reworking. The part was made of AISI 4340 steel that was heat treated to a tensile strength of 
1790 to 1930 MPa (260 to 280 ksi). (a) and (b) Remains of an old crack along the edge of the surface of the 
part (arrows); note dark zone in (a) and extensively corroded separated grain facets in (b). (c) Clean 
intergranular portion of crack surface that formed at the time of final fracture 
The heat treat cracks that most commonly contribute to service fractures are the transformation stress cracks and 
quenching cracks that occur in steel. When a heat treat crack is broken open, the surface of the crack usually has an 
intercrystalline or intergranular texture. If a crack has been open to an external surface of the part (so that air or other 
gases could penetrate the crack), it usually has been blackened by oxidation during subsequent tempering treatments or 
otherwise discolored by exposure to processing or service environments. 
Heat treatment in the temperature range of 205 to 540 °C (400 to 1000 °F) may produce temper colors (various shades of 
straw, blue, or brown) on the surface of a crack that is open to an external surface. The appearance of temper colors is 
affected by the composition of the steel, the time and temperature of exposure, the furnace atmosphere, and the 
environment subsequent to the heat treatment that produced the temper color. Additional information on heat treat cracks 
and the appearance of temper colors can be found in the article