Monday, March 2, 2009

Effect Of BVID On Carbon Fiber Bike Frames

Barely Visible Impact Damage (BVID) is something we all fear in our nice looking carbon fiber bikes. It looks good on the outside but the material within is not structurally sound. This can happen after an impact such as that caused in an accident and depends upon the extent and severity of it.

Structurally, under these circumstances, there is a gradual reduction in residual strength with load cycles and further growth of the damage without detection (delamination growth, See What Is Delamination) which opens up the imminent danger of catastrophic failure. Its just a question of when. It would do us all good if we recognized that the bike was involved in an incident, and promptly send the bike back to manufacturer if its possible for inspection. I take it that frame inspection from within is not an easy procedure (See Radiographic Frame Inspection) and most manufacturers will replace the frame if its under warranty and if certain other conditions are met.

The effect of BVID and fatigue on compressive residual strength of carbon/epoxy composites is shown in the following clipping from the book Composite Materials For Aircraft Structures by Baker, Dutton and Kelly. The two graphs show the reduction in residual strength as a function of damage size, and the reduction in the static failure strain plateau after typical load cycles. The sample is a 56-ply-thick laminate typical for a fighter aircraft upper wing skin. For composite bicycles, the tubes usually have around 5 to 6 main plies but a large amount of carbon fiber pieces which are all oriented and laid by hand into a prepreg assembly, ready to be molded and cured.

Click to Zoom

Additional Resources :

Radiographic Frame Inspection
Inspecting Carbon Bike Frames
Carbon Fiber Inspection Procedures
Design Considerations For Carbon Fiber Bicycles
Taking The Witchcraft Out Of Advanced Composite Bicycle Frames - Specialized Engineering

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