Failure Analysis of a Solid ™ BMX Crank Arm

1. Failure Analysis of a Solid™ BMX Crank Arm Chad Smith and James Walsh E45 Fall 2004 Instructor: Younes Ataiiyan

2. What are Cranks? • Cranks transmit rotary motion and consist of two arms attached to a shaft. • A crankset consists of bearings which are pressed into the bike frame, a spindle or shaft, and a crank arm attached to each side of the shaft. • Since BMX bike riding consists of doing tricks which in turn heavily abuses the bike, the cranks and shaft take an extreme amount of force in all directions (lateral and vertical) - and therefore must be stronger than on a normal road or mountain bike.

4. Our Objective… • We obtained a crank arm which was fractured under heavy stress, which is expected by manufacturer. • Our objective was to analyze the fracture in order to determine the cause for failure and to determine the metal’s Carbon content.

5. Visual Inspection • Our failure analysis consisted mainly of visual inspection. • We noted the fracture at the weld which propagated through the actual material.

6. Top image: Weld fracture • Bottom image: Propagation of fracture through arm

7. Next, we put our crank arm in the vice and separated the arm at the fracture point. • Note the size difference between the top of the fractured material compared to the right side of the material which is much thinner. The thicker part is from the original fracture, while the thinner part is from the forced separation of the crank arm. The first propagation occurred on the left hand side along the weld.

8. Weld Penetration • After separating the crank arm, we noted that the weld penetration was almost non-existent. The weld bead was essentially sitting on top of the material, and therefore we concluded that the strength of the weld was the ultimate cause of fracture. • Due to geometry of the crank arm and analysis of regular force application (at pedal boss), a high stress point would be at the point where the spindle interacts with the crank arm. Therefore, our fracture location would be the expected site for failure.

9. Material Composition • The standard material used in high quality BMX crank arms is chromoly steel. • After cutting the crank arm and mounting a sample, we examined the microstructure under a microscope.

10. Material Composition Hypothesis • After comparing our sample to the graphs in the laboratory, we concluded that our sample was somewhere between .3% and .4% Carbon, meaning the crank arm was between 4130 and 4140 chromoly steel.

11. After making our conclusions, we contacted Solid Bikes through e-mail, and informed them of our analysis. • Ryan Puckett, at Solid Bikes, informed us that the tubing of the crank arm is 4130 chromoly. He also went on to say that “Despite the heat treating, crank fractures tend to be caused by the HAZ of the weld, and the weld penetration restrictions of the crank geometry. They will always break at the spindle on most cranks, regardless of material, treatment, etc.”

12. Microstructure • The microstructure of SAE 4130 steel after cyclic stress. • 100x Magnification • (source: http://www.ndt.net/article/wcndt00/papers/idn520/idn520.htm)

13. 4130 Properties • Carbon 0.28 - 0.33 % • Chromium 0.8 - 1.1 % • Manganese 0.7 - 0.9 % • Molybdenum 0.15 - 0.25 % • Phosphorus 0.035 % max • Silicon 0.15 - 0.35 % • Sulfur 0.04 % max • E = 29 ksi • Source: http://www.suppliersonline.com/propertypages/4130.asp

14. Why Chromoly? • Applications Typical applications for 4130 low alloy steel include. structural use such as aircraft engine mounts and welded tubing applications. • Welding 4130 alloy is noted for its weldability by all of the commercial methods. Source: http://www.suppliersonline.com/propertypages/4130.asp

15. Conclusions • Crank failed at weld due to shallow weld penetration • 4130 Chromoly Steel • Although the crank arm fractured, 4130 Chromoly was still the best choice based on strength and elasticity.