16 Prakt. Met. Sonderband 52 (2018) 
stack. This definitely comes in handy for macrofractography, since this technique retains 
valuable colour information from the fracture surface. Macrofractographic fracture features 
can also be studied using SEM images at low magnification. Apart from the much higher 
resolution an SEM offers over LOM, arguably the most valuable advantage of the electron 
column is the much greater depth of field of up to 2 mm, compared with only some nm in a 
an LOM. With an SEM, large fracture surfaces can be brought entirely into focus. 
Among the macrofractographic fracture features, the tell-tale, half moon-shaped beach 
marks are the most prominent and well-known. They offer evidence, often detectable with 
the naked eye, that the failed component did indeed fracture by fatigue. In between the 
beach marks and parallel to them, the lucky failure analyst might find microfractographic 
fracture features that are called fatigue striations. They are the exclusive realm of the 
SEM. Fatigue striations are very small ridges on the fracture surface, which are produced 
in ductile materials by the propagating crack. There number and spacing might be 
indicative of the fracture mode that eventually "killed" the part. Sometimes they even 
provide some means to estimate the loading situation and the level of alternating stresses 
that caused the failure. Fatigue striations could be as finely spaced as only some tens of 
nm and as coarsely as some pm. Sometimes the number of beach marks as 
macrofractographic fracture features can be related to individual loading change events, 
Figs. 1-3. 
mn) l#2][#] =) ws] ov Lins! 
Root C 
As ever 
factogre 
fo deck 
ie 
wel. Fe 
or 
Fig. 1: High Cycle Fatigue (HCF) Fracture of Combustion Chamber Casing ne 
in a Heavy-duty Gas Turbine Engine. Material is GX23CrMoV12-1, 1.4931 ei 
approximate location of muitipt 
‘crack initiation sites 
beach mark~ 
final forced ~ —— IE side) 
rupture shear lip of final ~~ 
forced rupture 
Fig. 2: HCF Fracture of Compressor Vane in a Heavy-duty Gas Turbine Engine. 
Material is X20Cr13, 1.4021 
20 mm