326 Effects of Damage on HCF Properties
While the maximum serviceable limits for FOD in some jet engines in operation today
range from 0.36 to 0.76 mm (0.014"–0.030"), the damage depths found in service vary
significantly (see Appendix G). Of major concern is if FOD as small as sometimes
found in service provides a significant fatigue strength debit in simulated engine blade
specimens. Whether the damage should be blended out or the component be removed
from service and replaced is very difficult to ascertain in many cases. Another item of
concern is the determination of whether damage depth is a good indicator of the actual
remaining fatigue life of an FOD impacted blade.
7.4. BACKGROUND
There has not been a substantial amount of work done relating FOD to fatigue strength.
Peters et al. [2] researched the effects of FOD on HCF thresholds by shooting steel spheres
onto the flat surface of modified K
b
(edge cracked) specimens. They found that the overall
effect of FOD markedly reduced the fatigue life, compared to that obtained on undamaged
smooth-bar specimens, by providing preferred sites for the premature initiation of fatigue
cracks. Mall et al. [3] tested both diamond cross section and uniform rectangular cross
section samples to study the effects of FOD. The samples were either ballistically impacted
or damaged by quasi-static indentation or shearing. They found that the different damage
methods created distinctly different damage mechanisms. It was suggested that a total
damage depth parameter could be utilized to allow the use of inexpensive and easily
controlled methods of simulating FOD, such as the quasi-static chisel indentations, to
replace more difficult and expensive means, such as ballistic impacting.
Due to the continuing concern of FOD, studies characterizing the damage sites have
been conducted in order to understand the effects of FOD on the HCF strength of engine
blades. There are several ways of simulating FOD in a laboratory environment which
do provide an impact site and fatigue strength debit; however, each method produces
different types of damage which could lead to different crack initiation mechanisms.
Characteristic material, geometries representing the leading edge of engine blades, and
“realistic” impact conditions have been explored. In this section we attempt to quantify the

the airfoils,there isa good chancethat the damagecame fromthe firstdamaged stageor from
in front of that. If the damage is on the suction side, this usually means the damage came
from behind which would indicate some sort of stall in the airflow. If none of the engine
components actually failed, the witness marks have to tell the entire story.
•
Disassembly of the engine and examination of the blade. Once the primary damage
component is located, the witness marks will be subjected to microscopic inspection
using both optical and scanning electron microscopes. The shape of the damage can
be determined from these inspections and potential components (such as bolts, nuts,
washers) that match the damage geometry can be isolated. After the detection of the
most likely damage origination, the broken surfaces must be examined. Hopefully, at
least one of the fracture surfaces will have ‘beachmarks’, crack growth striations or the
general appearance of brittle fracture – as these are all indications of fatigue. If all of the
components have signs of tensile rupture, the cause of failure is difficult to determine.
Once the components with fatigue damage are identified, a more complete microscopic
examination can be performed. Using a combination of experimentation and numerical
analysis, both ‘beachmarks’ and striations can be counted to determine approximately
when the damage was initiated. Basically, the prediction is based on the identification of
the stresses, in conjunction with experiments in the lab using specimens to simulate the
loadings if the necessary data are not available from the bibliography or the engineering
data provided from the manufacturer. If there is the appearance of brittle fracture (a
relatively flat and featureless fracture surface) without crack growth striations, this can
indicate high R-value fatigue as a damage mechanism. This could be indicative of HCF
328 Effects of Damage on HCF Properties
or LCF, but the appropriate type of fatigue must fall within the realm of what is predicted
numerically.
•
In addition, a spectrographic inspection of the fracture surface can be used to determine
if any material has been deposited on the blades from the impactor. This is crucial
to determining which of the geometrically compatible components is most suspect. For

the engines (real FOD) and he still was able to bring it home.
The story behind this is that two F/A-18 Hornets made a head on pass, just a bit too
close. One got home with part of the left wing and left vertical fin and rudder missing,
while the other jet, shown in Figure 7.3, is missing everything forward of the cockpit
pressure bulkhead – and is a flying convertible because the canopy is shattered too. This
illustrates the extreme nature of FOD and the possible consequences. In this chapter, we
will deal primarily with small hard objects that are commonly ingested into engines.
7.7. TYPES OF DAMAGE
When an engine blade experiences FOD, the damage can be in many different forms.
Some of these, like curling, denting, and distortion, have little effect on the fatigue
properties of the component. In a recent report [5], in Annex C, the definitions of the
330 Effects of Damage on HCF Properties
various forms of engine blade damage from FOD have been summarized along with
photos showing examples. Figures 7.4–7.15 are taken directly from that report, defining
the different types of damage.
Burred: Rough edge or sharp projection on edge or surface of parent material. Most
commonly associated with surge damage.
Figure 7.4. Example of a burred blade.
Chipped: Breaking away of surface of parent material usually caused by heavy impact
(not flaking). Most commonly associated with snubber abutment face damage.
Figure 7.5. Example of a chipped blade.
Foreign Object Damage 331
Cracked: Partial separation of material which may progress to a complete break, either
visible or detected by NDT.
Figure 7.6. Example of a cracked blade.
Curled: Condition where edges of outer portion of blade have been rolled over. Often
associated with rubbing on casing or vanes, or sometimes soft body.
Figure 7.7. Example of curled blades.
Dented: Indentation with rounded bottom, usually on leading/trailing edge, sometimes on
surface. Parent material is displaced, seldom separated.

Torn: Separation by pulling/ripping apart.
Figure 7.15. Example of a torn blade.
As noted above, not all of these damage modes are related to HCF. The specific types
of damage treated in this book are, using the definitions above, burred, chipped, cracked,
nicked, pierce out, scored, scratched, and torn. These, in turn, can be analyzed using
some of the tools dealing with notches, particularly including the extension to cracks and
short crack/shallow notch behavior discussed in Chapter 5. In some cases, these tools are
inadequate because of the many complex mechanisms associated with FOD.