Thursday, November 23, 2017

Failed parts

There is a good chance this strut tower was stamped out of aluminum based on the bright-white of the fractured area.  A trained person can often tell where the crack initiated from based on "beach marks" of the fractured surface.  In this case, the failure may have been so quick and catastrophic that no beach marks were generated.
There is a saying in the automotive world:  "There are two kinds of aluminum parts.  Broken ones and ones that are about to break."

Airplane pilots will disagree.  They point to the DC-3 and ask, as a percentage, how many 1936 model, steel, automobiles are still in service.

An important difference between the two types of vehicles are the rates at which they are produced.  Automotive parts are typically die-cast or stamped while aeronautical parts are forged or machined out of billet.

Die-casting alloys are rich in silicon to make the liquid metal less viscous so it easily flows into the mold's details.  Silicon also makes the solidified alloy brittle.  Another attribute of die-cast parts is that the the skin "freezes" first.  As deeper layers of metal solidify and shrink it places the skin into compression.  Cracks that want to initiate at the surface must first overcome the compressive stresses in the skin.  However, all bets are off if the part is machined, which removes the skin, or if the part is nicked or scratched, or if their are internal voids or porosity.

Stamped parts are banged out at rates that depend on the size of the part but hoods can be banged out at one every couple of seconds.  Smaller parts are produced even more rapidly.

Why so fast?  It is because that same press is scheduled to punch out scores of different parts so every part it makes must be made many multiples more quickly that the production rate of the vehicles they will be installed on.

There are some "tricks" in stamping.  One is to punch multiple parts out of a single blank and to separate them in the final operation.  Then each separated part can slide down a dedicated chute into a waiting bin.

A well designed stamping operation allows the punch-and-die to suck in metal from outside the region that is currently being formed.  That portion of the part is called binder-wrap.  Part of the magic is to provide enough tension with the pressure pads to pull out the wrinkles and yet loose enough to let the metal sheet suck in.

The circled region shows the transition between localized thinning (called necking) and cracking.
Necked down regions and cracks form when the metal cannot flow to the most highly stressed regions.  This can be a failure of the metal (too low a work-hardening rate), a failure of the pressure pads or, most likely, a lubrication failure.  Remember, the sheet has to slide from between the pressure pads and across the face of the punch to feed the most highly stressed areas.

The reason a lubrication failure is the most likely root-cause is because lubrication requires attention.  Most stamping shops use an emulsifiable oil that is mixed with water.  It is sprayed onto the blanks as they are peeled off the stack.  Tanks run dry.  Job setters can make mistakes in the ratio of oil:water.  Nozzles get plugged.  Nozzles get bumped and spray misdirected.

So if I had to guess about the root cause of the failure shown in the first picture, I would guess that the stamping plant had a lubrication failure at the blank feeding operation at the start of their process.  I would inspect both left-and-right strut caps because they were either made from the same blanks or were stamped on the same press line.  I would inspect several thousand vehicles made before and after the failed vehicle and I would look for both cracks and necked down regions in the deepest parts of the draw.

1 comment:

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