Analysis of the fatigue failure of a mountain bike front shock
We present an analysis of a mountain bike front shock failure. The failure of the 1-year-old shock occurred catastrophically as the bike was ridden off of a 1-m drop. The failure was the result of fast fracture through both shock tubes at the location where the tubes were press fit into the shock upper crown. Examination of the fracture surfaces of the tubes revealed regions of fatigue crack growth that nearly penetrated the entire thickness of both tubes. An estimate of the forces during use, coupled with stress analysis, revealed three stresses near the fracture site-axial compression, bending, and hoop stresses. During operation, the axial compressive stress is negligible while the hoop and bending stresses are significant. Based on fracture mechanics, and an estimate of the bending stress from a 1-m drop, it is confirmed that the fatigue cracks present on the fracture surface were large enough to induce fast fracture. Prior to the existence of the fatigue cracks, the stresses were magnified locally near the fracture site by a significant stress concentration caused by the sharp transition from the shock tube to the crown. The fatigue cracks initiated at a circumferential location in the tube commensurate with high tensile bending stress and the stiffest region of the crown (highest stress concentration). Based on the evidence, the most probable cause of the bike shock fatigue failure was the shock design, which facilitated high local stresses during use. © 2003 Elsevier Ltd. All rights reserved.
Shelton, H; Sullivan, JO; Gall, K
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