Armored cable locks seem to be just cable locks with metal covering? It seems the metal is in pieces (so that the cable can bend). How secure is armored cable lock? Can they be broken by tough bolt cutters? Anyone heard of such locks being broken?

As we all know there is no such thing as a foolproof lock. At best all we can hope for it to slow the thief down enough to make stealing to much trouble.

Although we caution people that cable locks are not as secure as U-locks, it appears that big, thick cables are a reasonable deterrent if personal experience is any guide.
I'm in campus law enforcement, and we deal with bike theft all the time. The common cables that run anywhere from 1/4" to 3/8" in diameter are essentially useless; even a small bolt cutter will snip them instantly.
However, I can't recall a case where someone took a bike locked with one of those big 3/4" or 1" braided cables... Theyr'e too big for most bolt cutter jaws and take too long to saw through.
A good Kryptonite is better, though.

The tube sections are supposed to be tool-resistant steel, and there's not enough slack in the cable to spread the tube sections and expose the cable itself.
Yes, yes and yes.
But then again, any lock can be broken--given enough time and nobody around.
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I have been a locksmith for 10 years now, and get this question almost daily. The strands in the armoured cable make it extremely difficult to cut- they move and flatten and bend instead of holding still for bolt cutters. that doesn't make them impossible to cut, and the padlock you use to join the two ends needs to be of the highest quality.

in the end, if a thief REALLY wants YOUR bike, they're going to get it. a friend of mine used a pair of handcuffs (his dad was a cop) and a beat up old shwinn (or something) when riding downtown, never got stolen. finally rode his 'A' bike in to work one day, used 3 of the highest quality locks he could find and the guy that stole it used a reciprocating saw to cut the sing post he was locked to.

if your bike is that important, find a safe place for it. make friends with your local security departments and shipping-receiving offices, and have them stow your bike for you while you run errands or work.

I'm one of very few using one of them here, 8 ok years now. I don't think I have seen them in stores since that year. The outer coil is about 21 mm ( metal + plastic) , but the cable is 9 or 8 mm I think. They look like more trouble than they are. It's long enough to go around a light pole. They are a bulky nuisance to carry around. Probably lighter than a 1/2 " u-lock.

tough bolt cutters? Why not use portable battery powered cutting tools? The reality is the tools to cut a lock are fairly cheap and easily available. Really good bolt cutters are actually expense.

Just came back to see the replies--thanks to all! Think I can use such lock for short parking and low crime areas. The reason I consider using them is they are not as heavy as U-locks. They seem to be visually more deterrent than the regular cable locks. By the way I keep seeing bikes locked with thin cable locks here on NY streets--though they are obviously not so valuable bikes.