Motorcycle Fork Separation

Metallurgical failure analysis was requested to determine the cause of failure after a separation of the front fork, handlebars, and front wheel of a custom manufactured motorcycle occurred.

An overall view of the separated front fork, handlebars, and front wheel is shown in Photograph A. A close-up view of the failed front fork collar is shown in Photograph B.

                          Photograph A: Overall view of the                                     separated front fork, handlebars, and front wheel.

                            Photograph B: Close-up view of the                             failed front fork collar.

A close-up view of the failed upper tube weld is shown in Photograph C. The thickness of the weld, in the area denoted by white arrows, was 1/32".

                         Photograph C: Close-up view of the failed                          upper tube weld.

Visual and stereomicroscopic examination of the failed fork collar weld revealed welding defects in the form of lack of root penetration (LORP). An area of LORP is shown in Photograph D. The LORP welding defect is denoted by white arrows in Photograph D.

                            Photograph D: An area of LORP is shown.                          The LORP welding defect is denoted by white arrows.

Visual and stereomicroscopic examination of the failed fork collar weld also revealed lack of fusion (LOF) defects. A very pronounced LOF defect is shown in Photograph E. White arrows in Photograph E denote the LOF welding defect.

                      Photograph E: Very pronounced LOF defect.                  White arrows denote the LOF welding defect.

Scanning electron microscopy (SEM) was conducted around the failed fork collar weld to determine the mode (metal fatigue vs. fast fracture) of crack propagation and the direction of crack growth. A map showing SEM examination locations and weld crack propagation direction is shown in Photograph F.

                           Photograph F: Map showing SEM                       examination locations and weld crack propagation direction.

No evidence of metal fatigue (long term crack growth) was observed during the SEM examination. Instead, step-wise intermittent crack growth was observed. Two areas of step-wise intermittent crack growth (step-wise cyclic overload) are shown in Photographs G and H. The step-wise fracture pattern is denoted by vertical groups of red arrows in Photograph G and H.

Photograph G

Photograph H

As a result of this investigation, the following was concluded:

1. The weld joining the motorcycle frame tube to the front fork collar was defectively manufactured in that the weld was of insufficient size.

2. The weld joining the motorcycle frame tube to the front fork collar was defectively manufactured in that the weld lacked fusion (welding) in the root (corner) of the weld, between the frame tubes and front fork collar i.e. lack of root penetration.

3. The weld joining the motorcycle frame to the front fork collar was defectively manufactured in that the weld contained significant amounts of lack of fusion (LOF) defects.

4. The weld joining the motorcycle frame tube was probably defective in that after welding, very hard (Rc 43) areas were created in the weld heat affected zone. Such hard zones would render the weldment brittle and subject to unexpected, premature catastrophic cracking.

5. The stresses resulting from "normal" operation of the motorcycle were sufficient to initiate a crack in the frame tube to front fork collar weldment.

6. The insufficiency in weld bead size and other welding defects resulted in crack initiation and propagation of a crack through the frame tube to front fork collar weld.

7. The separation of the subject motorcycle frame from the front fork collar resulted from a progressive, step-wise (cyclic tearing) failure of the frame tube to front fork collar weld.

Back to Top