Arma Supercharger Incident Report
Background
The incident occurred on 19 Oct 2011 at around 2300hrs in Singapore. The charger kit had been installed for about 3 months prior and the car was always driven around local roads and never raced nor tacked. At the point of failure, the ambient temperature was 28 Degree Celsius. The engine was working at normal operating temperatures and the approximate rpm when the compressor failed was at 3400rpm. The failure was so drastic that the metal shrapnel cut the transmission fluid cooling lines and the radiator heater hoses. The car was towed back to shop.
Facts about the case
Pieces of the broken charger and associated plumbing was recovered and collected. The following series of pictures are of the plumbing and of the compressor blade and housing.
Looking at the compressor wheel closely, you can see the grainy crystaline structure of the turbine blades that indicate a casting process was used to create the blade. The blade pieces in question that the ARMA representative brought up was these 2 photos.
Our conclusion
The primary cause of the failure was material fatigue that started from a hairline of fissure crack on the turbine blade. The crack slowly developed until the spinning blade could not hold itself together and thus broke off. This caused the turbine housing to be destroyed. The shrapnel had 2 kinds would of energy levels. The spinning blade pieces would posess the (1) high velocity but low/medium mass projectile while the housing itself would be the (2) low velocity high mass type. These 2 kinds of projectiles would give different kinds of damage as they would be either the “cutting” or “smashing” effects. The evidence of this is seen here. The ATF pipe was severed with a relatively thin 1.4mm cut which indicates that a piece of the turbine blade had stuck the pipe.
Meanwhile at the intake charge pipe, the point of impact suffered a smashing blow as you can see from the cross sectional photo. The outer surface is heavily indented and sunken while the side facing the engine block has torn the welds holding the pieces together.
As for the damage to the 2 pieces of turbine blades that the ARMA representative mentioned, There could not be a foreign object being injested into the intake as the air filter was whole and did not have any holes in the media. The screw indentation in the blade was a result of an impact to the charger housing clamping bolts. As the turbine blades broke off, it shattered the housing into pieces and characteristically, the fault lines occur at the bolt holes.
This breakage tore off some bolts and also left a couple of bolts in place. These bolts are the M4 variety and it perfectly matches the indentation. Thus as the bolts stood out from the flange, they were subsequently struck by the turbine blades and this left the imprint.
To dispute the ARMA hypothesis of a foreign object entering the compressor wheel, the bearing support was very conspicously free of any strike marks as any injested object would be clamped between the spinning blade and one of the 3 outer bearing supporting struts, and there would be visible damage, but there was not.
Thus the point of failure still revolves around material fatigue and that the testing protocols for each piece of turbine blades that leaves the foundry or manufacturing plant is less than satisfactory. The other possibility is a drop in material quality that was changed by the supplier that yielded less than satisfactory strength.
To cope with the immense inertia forces, it is suggested that the material itself be altered and that the 47000rpm design parameter be increased to about 60000rpm to provide about 80% more design headroom to allow for material fatigue and manufacturing tolerances.