Fatigue Fracture of a Fan Blade
Handbook of Case Histories in Failure Analysis,
Vol 2, K.A. Esakul, Ed., ASM International, 1992
A blade from the engine cooling fan of a pickup truck fractured unexpectedly. The blade was made from type 301 stainless steel in the extra full hard tempered condition with a hardness of 47 HRC. Failure analysis indicated that the blade fractured in three modes: crack initiation, fatigue crack propagation, and final rapid fracture in a ductile manner The fatigue crack originated near a rivet hole.
Automotive components; Engine components; Trucks
(Austenitic wrought stainless steel), UNS S30100
A blade from the engine cooling fan of a pickup truck fractured unexpectedly.
Circumstances leading to failure
The fan was original. The vehicle had been in use several years before
the fracture occurred.
The blade material was specified as type 301 stainless steel, extra
full hard temper. This temper is achieved by cold rolling to about a 60% reduction
of thickness, resulting in a tensile strength that exceeds 1310 MPa (190 ksi).
Because the austenitic structure of type 301 stainless steel is metastable,
this severe cold working results in the formation of a significant amount
of strain-induced martensite. The presence of this martensite is conformed
by the fact that the blades are moderately magnetic. The combination of the
very high degree of cold working and the formation of significant amounts
of martensite greatly reduces the ductility of the steel and increases its
After the stainless steel strip is roller to extra full hard temper,
the blades are blanked and the rivet holes are pierced. The blades are then
lightly rolled to impart a slight curvature. Finally, the blades are stress
Visual Examination of General Physical Features
Examination of the fan (Fig. 1
) revealed that it was dirty from use. However, aside from the fractured
blade, it was in good condition, with no signs of corrosion, dents, or other
Fig. 1 Engine cooling fan, showing 0.15× location of fractured blade
Testing Procedure and Results
In order to examine the fracture surfaces
at low magnification, the four rivets attaching the blade and the backer plate
to the spider arm were removed.
shows the fractured blade still attached to the backer plate
by three clips.
Fig. 2 Fractured blade still attached to backer plate (lower section) by
three clips. 0.43×
shows the smaller
portion of the fractured blade after separation from the backer plate. This
fractured piece was ultrasonically cleaned using acetone and then examined
at low magnifications using a stereomicroscope. The fatigue crack surface,
which ran a length of 64 mm (2.5 in.) (Fig. 3
, right), appeared very flat and normal to the plane of the blade.
No beach marks or other indications of fatigue were visible. The final rapid
fracture region (Fig. 3
appeared to follow an irregular path and was sheared at 45° to the plane
of the blade.
Fig. 3 Smaller portion of fractured blade, showing fatigue crack origin
near a rivet hole. 0.7×
Scanning Electron Microscopy/Fractography.
by scanning electron microscopy (SEM) revealed fatigue striations on the flat
portion of the fracture surface (Fig. 4
). From the
relative curvature of these striations, it was determined that the fatigue
crack originated next to one of the rivet holes (Fig. 3
). The distance from the rivet hole to the fatigue
crack origin was about 2 mm (0.08 in.). The 45° sheared fracture surface
was dimpled (Fig. 6
a ductile fracture.
Fig. 4 Fatigue striations on fractured blade surface to left of rivet hole
Fig. 3. 800×
Fig. 5 Fatigue striations on fractured blade surface to the right of rivet
hole shown in
Fig. 3. 320×
Fig. 6 Dimpled fracture surface on 45° shear crack shown in
Fig. 3. 0.7×
was difficult because of the
highly cold worked condition of the blade. Basically, the microstructure appeared
similar to that shown in
with highly elongated and distorted grains and dark streaks, indicating strain-induced
Fig. 7 Type 301 stainless steel sheet, cold rolled to 40% reduction (full
hard), showing almost complete transformation to martensite in severely deformed
austenite grains. 250×. Source:
The approximate hardness of the bladed was measured
as 47 HRC. This relatively high hardness for a stainless steel suggests that
its ductility was relatively low and that it was relatively notch sensitive
in fatigue loading.
SEM fractography clearly showed that a fatigue crack initiated near
a rivet hole and propagated in both directions for a length of 64 mm (2.5
in.). The exact crack origin area could not be identified. No surface defects
were observed that could have triggered the initiation of the fatigue crack.
Because this steel was so heavily cold worked, microstructural evaluation
proved worthless. The development of the slow crack growth portion of the
failure may have required thousands of stress cycles. Finally, the remaining
cross section of the blade was no longer able to support the applied load,
and rapid ductile fracture resulted. Because the blade was only about 0.5
mm (0.02 in.) thick, ductile fracture occurred as a shear crack 45° to
the blade surface.
Conclusion and Recommendations
Most probable cause
The blade fractured in three modes: crack initiation, fatigue crack
propagation, and final rapid fracture. The crack initiated near a rivet hole,
but it is not clear whether the hole, acting as a stress raiser, had a role
in initiating the crack. The relatively high hardness of the steel may have
contributed to its notch sensitivity. Once initiated, the fatigue crack propagated
in both directions, producing a flat fracture normal to the plane of the blade.
Finally, the fatigue crack so weakened the blade that its remaining cross
section was overloaded and failed rapidly in a ductile manner, producing a
45° shear fracture.
Because this blade failure appeared to be an isolated case, no specific
remedial action was taken.
, 9th ed., Vol. 9,
, American Society for Metals, 1985, p 287.
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