Water jet cutters

[TheBigM]

Anyone have any idea what (if any) damage or degradation is caused by water jet cutting of monolithic composite plates? Water jet cutting is being suggested by some people for cutting of test specimens but I am concerned about the idea of blasting high pressure water at a composite.

Does it cause water to be forced in to the structure? Can the edge finish be good enough for testing?

 

[RPstress]

It doesn't seem to have any major disadvantages. You need to dry specimens out in an oven before any testing, but you probably need to do that anyway.

The surface finish can be good, but may cost a bit more to generate. If it's cut fast then it's typically poor. Just eyeballing some cut surfaces recently it looked worse than 128 microinches. Even at its best it is not good enough for a test specimen in my experience. (Typically specimen edges should be polished anyway.)

 

[SWComposites]

Water jet cutting works well for general composite cutting with the correct grits and speeds.

However, I do not recommend using water jet cutting for test specimens. The surface finish is typically a lot worse than desirable for test specimens, which should be 32-64 Ra to minimize variability. Straight sided test specimens should have their edges machined with a cooled diamond wheel saw. Period. Everything else that I have seen used results in (at some point) lousy edge finish and bad data. Water jet cutting can be used to cut large panels down to smaller panels to be then sliced with the diamond wheel saw. The edge finish of the ends of the specimen are typically not critical (unless an end loaded test) and therefore can have the rougher water jet finsi.

SW

 

[TheBigM]

Many thanks for your replies - I kind of figured that the edge finish would be no good for testing.

Thanks again.

 

[Demon3]

Just a note that the edge surface will be critical for some properties but not all. For example, if you only care about modulus, then that should be independent of the edge smoothness.

If you need strength or elongation to break, then the edge is critical and you will get very low values due to microcracks at the edge. Unnotched impact resistance will also be dramatically lower with a rough edge as you are effectively adding notches. In contrast, notched impact resistance should not depend on the smoothness of the edge because your intentionally introduced notch is a bigger flaw than those caused by the rough edge. Hope that helps.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry