Etydhv

The 1-1/2" copper waste pipe than runs from our kitchen just up and cracked two days along its length, and for no apparent reason.

The crack is in a section of pipe that runs vertically through the kitchen floor down into the basement, where it connects to an angled length of pipe (two 45-degree elbows) and then a section of pipe that connects to cast iron sewer pipe (lead and oakum seal).

I would venture to say that this cracked pipe is from the 1970s (when former owners remodeled the kitchen) and that the section of copper pipe (which didn't crack) going into the cast iron hub dates from around 1948-49, when the house was built.

Could the crack have been the result of twisting forces when the pipe connection was made to the garbage disposal beneath the sink? Or does copper waste pipe just eventually give out, perhaps because of corrosion over a period of 50 years?

Copper does erode over time through mechanical wear and chemical corrosion. My guess is that it simply got thin enough that thermal stresses popped the weld.

I'd put in plastic and be happy.

Back in the 1970's the company that I worked for ran into a slew of copper tubing sizes through 3" that was manufactured with a defect that weakened the whole length of the copper. Since the copper was new the supplier replaced all the tubing (pipe) that was installed. This could be a similar problem. As "Isherwood" said, just replace it with plastic.

Having worked with brass chrome, my personal bet is it just got old. I wanted to teach my son how to use a wrench and when he went to crank off the old nut, the pipe shattered like an eggshell. Yours looks like a a small crack just gave way, which doesn't surprise me at all. These older pipes are not terribly thick either (the cast iron drains look to be just fine).

Cut off the old pipe and use a compression coupler (like a Fernco) to make the transition to PVC.

It looks like it was thinned by corrosion until any stress opened it. Ammonia and it's compounds are notorious for attacking copper alloys ; Ammonia cracks brass not copper, but corrodes copper quickly. Maybe a previous owner liked to use ammonia for cleaning in the sink. That is why brass flex-tubes are not permitted for gas service, ammonia cleaners are common in kitchens and utility rooms and they will stress corrosion crack brasses rapidly. The answer is still replace with plastic.