Fatigue Fracture of a Fan Blade

Robert A. McCoy, Materials Engineering Department, Youngstown State University


From: Handbook of Case Histories in Failure Analysis, Vol 2, K.A. Esakul, Ed., ASM International, 1992

Abstract: 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.

Keywords: Automotive components; Engine components; Trucks

Material: 301 (Austenitic wrought stainless steel), UNS S30100

Failure type: Fatigue fracture


Background

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.

Pertinent specifications

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 notch sensitivity.
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 relieved.

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 plastic deformation.
Figure 1

Fig. 1  Engine cooling fan, showing 0.15× location of fractured blade

Testing Procedure and Results

Surface examination

Macrofractography. 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. Fig. 2 shows the fractured blade still attached to the backer plate by three clips.
Figure 2

Fig. 2  Fractured blade still attached to backer plate (lower section) by three clips. 0.43×

Figure 3 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, left) appeared to follow an irregular path and was sheared at 45° to the plane of the blade.
Figure 3

Fig. 3  Smaller portion of fractured blade, showing fatigue crack origin near a rivet hole. 0.7×

Scanning Electron Microscopy/Fractography. Examination by scanning electron microscopy (SEM) revealed fatigue striations on the flat portion of the fracture surface (Fig. 4 and 5). 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), indicating a ductile fracture.
Figure 4

Fig. 4  Fatigue striations on fractured blade surface to left of rivet hole shown in Fig. 3. 800×

Figure 5

Fig. 5  Fatigue striations on fractured blade surface to the right of rivet hole shown in Fig. 3. 320×

Figure 6

Fig. 6  Dimpled fracture surface on 45° shear crack shown in Fig. 3. 0.7×

Metallography

Microstructural analysis. was difficult because of the highly cold worked condition of the blade. Basically, the microstructure appeared similar to that shown in Fig. 7, with highly elongated and distorted grains and dark streaks, indicating strain-induced martensite
Figure 7

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: Ref 1

Mechanical properties

Hardness. 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.

Discussion

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.

Remedial action

Because this blade failure appeared to be an isolated case, no specific remedial action was taken.

Reference

  1. Metals Handbook, 9th ed., Vol. 9, Metallography and Microstructures, American Society for Metals, 1985, p 287.

Related Information

Fatigue Failures, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 700–727