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Project Overview
Determined root cause of failure in bicycle crank puller, using visual inspection, SEM imaging, EDS, hardness testing, and stress analysis.
Crank pullers are mechanical hand tools used to remove the crank arms from the bottom bracket spindle of a bike. They provide controlled, threaded extraction to avoid damage during maintenance.
Project Duration
2 months
Skills
Spectroscopy, fractography, materials analysis, fracture mechanics, stress analysis, Rockwell hardness and Vickers microhardness testing
Initial visual inspection exhibits a partial cup-cone fracture surface, with asymmetry in the surface profile and a shear lip along one edge. This indicates more than simply axial loading and contribution of bending stress in a ductile material.
Materials Analysis
The manufacturer shared limited information about the product, simply that it was case hardened. As such, tests were conducted to glean additional information.
EDS Spectra
Steel Grade-Hardness Matches
A Rockwell B hardness test resulted in a value of HRB = 92 and the elements as shown in the EDS spectra. According to Volume 1 of the ASM Handbook, this suggests the material being a medium-carbon steel, such as those described in the table, likely AISI 1030-1050.
Map of Microhardness Test Points
Map of Microhardness Test Points
Edge vs. Center Microhardness
Edge vs. Center Microhardness
A Vickers microhardness test confirmed the presence of case hardening, as shown by the much larger average hardness at the edge of the part.
According to Volumes 11 and 12 of the ASM Handbooks, the images shown indicate brittle edge crack formation, leading to ductile voiding and eventually transgranular fracture as a plausible cause of failure.
Stress Analysis and Human Factors Analysis
Assumed part geometry for stress calculations
Formulas for stress analysis
The crank puller, under intended use, is only loaded in the axial direction. The manufacturer specifies torque for square-taper crank bolts to be 30-44 Nm, which (with the most conservative estimate) creates a force of 260 N at the end of the handle, and assuming this full force is transferred through the smaller cylinder, the axial stress would still only be 9.3 MPa, significantly less than the yield strength of 1040 steel. Under this case, this part would not have failed.
In fact, even with over-torqueing, assuming the maximum exertable human force (link) of 400 N and using the same formulas as above, the axial stress would still only be 14.1 MPa. Therefore, this part could not have failed by axial loading alone.
Fracture Mechanics and Incipient Flaw
However, it is known that this tool is often misused and subjected to bending loads. With a bending stress of 480 MPa and fracture toughness as seen, the incipient flaw would only need to be 1 mm long to induce fracture. This flaw is realistic for case hardened surfaces, where small inclusions and machining marks are common, and surface brittleness increases crack susceptibility.
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