leometwu的部落格

Crankshaft nose.  Fatigue fracture.

也是枝晶界碳化物薄膜惹的禍. Again, interdendritic grainboundary carbide is the root cause of this fatigue fracture. These carbides are potential crack initiation sites. 微裂起源位置.

FCD 700  金相組織不良(枝晶界碳化物薄膜)為主因. With interdendritic carbide in its microstructure as shown above, fatigue cracking was waiting to happen for this crankshaft of ductile iron of FCD 700. And happen it did, at only 5000 km of mileage, as shown below.

下:主裂面primary crack 繞了360度deg.  次裂面secondary crack 才前進大約40度deg. 與單方向受扭力有關. Uni-directional torsion.

Primary fracture surface shows hilly feature of dendrite grain surfaces. These surfaces are weak in bonding force. Crack finds its way zig-zag thru these weak surfaces when  it advances.