Why use ultrasound to inspect carbon bikes?
Ultrasound scans are very useful for carbon composite inspection and is the primary method used in aerospace to determine the structural integrity of these parts.
Carbon composite structures are made up with a number of layers called plies, stacked on top of each other. Each ply needs to be bonded to the adjacent ply so it can transfer load. If this bond is compromised the structural integrity is significantly reduced.
Typical defects include:
Delaminations - where the plies are separated and can no longer transfer load. Very common after an impact or other failure.
Unbond - an unbond is when the plies or other fittings were not bonded properly during manufacture.
Porosity - dispersed air trapped in the resin during cure causing a reduction on mechanical properties. Aerospace standards are typically 2% max. porosity.
Void - a large air bubble trapped in the part.
Cracks - broken fibres and/or matrix.
Ultrasonic inspection is very sensitive to all these defects, it works by sending a high frequency sound wave into the part and then measuring what sound comes back. It takes very specialised equipment and knowledge.
Often highly loaded carbon composite parts such as aircraft components get 100% inspected by ultrasound methods during manufacture. This is usually conducted by automated inspection machines specific to the part and is very expensive. For aircraft repairs portable ultrasound systems are used to determine the damage prior to repair and most importantly to verify the structural integrity after repair.
Back to bikes!
So as carbon bikes become lighter and closer to the design limit it becomes very important to know that the structural material is made properly and capable of meeting the loads it was designed for. And just like aerospace, to repair high performance bikes you need to know what the damage is and that it has been repaired successfully.