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Lister Racecar Kingpin Failure Analysis

RSIME performed a failure investigation of a failed kingpin of a vintage Lister racecar, similar to the one shown in Figure 1.  The kingpin is a critical suspension component that must endure high loads. 
  Lister Racecar
Figure 1. Example of a Lister Racecar
An overview of the failed kingpin can be seen in Figure 2.  The fracture surface was sectioned off the kingpin for closer examination.  Figure 3 depicts the fracture surface with the major characteristics of fatigue pointed out. These characteristics are:

  • Crack Initiation Site
  • Crack Growth Area
  • Overload
kingpin overview
Figure 2. Overview of failed kingpin. Note the fracture surface that was sectioned for better inspection
 fatigue crack overview
Figure 3.  Overview of fatigue fracture.  Note the different stages of the fatigue fracture.

Note the two crack initiation sites opposite of each other.  As the fatigue cracks grew toward the center of the kingpin, they reduced the cross-sectional area of the kingpin until its strength was diminished to the point of an abrupt failure.  This is known as reverse bending fatigue.  Near the crack initiation site, significant machining marks from lathe cut threads were discovered and are shown in Figure 4.  These machining marks acted as stress concentrations which help to initiate the fatigue fracture. 

machining marks
Figure 4.  Machining marks found on lathe cut threads of the kingpin.
Hardness and chemistry composition analysis was performed.  The material had a 52 HRC hardness and the chemistry results indicated a 4340 alloy steel, see Table 1.
chemical composition results
Table 1.  Kingpin chemical composition results.

Conclusions

  • The kingpin failed from reversed bending fatigue. This failure was caused by loading incurred during normal left and right hand cornering of the racecar.
  • The fatigue cracks initiated due to machining marks on the lathe cut threads.
  • The kingpin material was a high hardness, medium carbon alloy steel, which can be notch sensitive and prone to fatigue cracks initiating from stress concentrations.

Recommendations

Steel has a unique ability to avoid fatigue failures.  If the applied cyclic loading is below the material endurance limit, it will not fail from fatigue, no matter how many cycles.  Typically this endurance limit is proportional to the material's strength.  However, increasing strength does not always insure protection against fatigue.  With a high hardness, high carbon steel, the endurance limit is high, but the material is more notch sensitive and its fracture toughness is reduced, allowing fatigue cracks to start more easily.   When designing components against fatigue, a balance of strength and toughness is required.

With this particular component, the fatigue fracture initiated at lathe cut threads.  A better option is to use rolled treads which induce residual compressive stresses, that help to prevent fatigue fractures from occurring.