Chinese Steel links
Failure Analysis of Exercise Bike Made in China
The failure of an exercise bike purchased in June 2010 from Big Box type store was investigated by Chemaxx. A female used the bike approximately 2-3 times per week and within two months the right pedal failed and she incurred a serious gash on her right leg. Figure A presents a schematic of the exercise bike, which was Made in China.
Figure B presents a photograph of the broken right pedal arm and Figure C presents a close up photograph of the fracture
The original failure analysis plan was to use the left (non-failed) pedal arm to perform a tensile test to determine the ultimate tensile strength of the material and the mode of failure. However, upon removing the left pedal, it was clear that it had already cracked and was itself near failure, as shown in Figure D, which also shows that the location of the failures in both pedals is nearly the same
Metal from the left pedal arm was used to obtain a sample for metallography as shown in Figures E-1 and E-2.
The as-polished specimen (E-1) shows a significant number of inclusions. Such inclusions are typically detrimental to the strength of the material and are often found in steels from China. The etched metallography (E-2) indicates that the material is most likely cast steel.
Hardness measurements were taken at multiple locations along the left pedal arm using two different hardness scales. Based on the results of the hardness, the tensile strength of the material was on the order of 85,000 pounds per square inch (85,000 psi or 85 Ksi). This is higher than the values found in handbooks for cast steel, most likely because the pedal arm material has a very fine grain size. The value found in handbooks for cast steel is typically 65-70 Ksi.
Using the conservative estimate for tensile strength of 65 Ksi and the cross-sectional area of the failure (as seen in Figure C), the pedal should have withstood a load of over 5,200 lbs. Since woman weighed far less than 5,200 lbs, it is not possible that this failure was a simple over-load type failure. The only way the pedal could have failed was if there were pre-existing cracks that precipitated the failure. Such pre-existing cracks act as “stress-risers” and reduce the load needed for total, sudden failure. No doubt there was some degree of fatigue crack growth as the bike was used several times before it failed. Fatigue is a mechanism by which the pre-existing cracks grow under cyclic stress, as would be the case when the bike is used for exercise. The inclusions, mentioned above, most likely contributed to the failures. Additionally, the non-failed left pedal had incipient cracks and was about to fail.
The pre-existing cracks were likely formed via improper cooling in the casting process. It would be a surprise if these exercise bikes have not had a number of similar failures that have already been reported to the manufacturer and/or distributor. In Chemaxx’s view, the bikes should be recalled to have the pedal arms inspected and replaced so no one else gets injured.
In summary, the right pedal arm failed due to a manufacturing defect that led to the pre-existing cracks in the threaded hole of the pedal arm. The failure may also be related to a design defect, as no doubt the design specifies the use of cast steel for the pedal arms. The inclusions, which are manufacturing defects, most likely also played a role.
The case settled prior to trial.
Dr. Fox is a nationally recognized metallurgy expert, corrosion expert and failure analysis expert who has published numerous peer-reviewed, scientific papers in these fields. He worked for years as a bench scientist (General Electric) and a research manager (EPRI) in the fields of corrosion, metallurgy and failure analysis. While a research manager, Dr. Fox managed over $100 million worth of research in metallurgy, corrosion and failure analysis.
©2012 CHEMAXX, INC