The crack occurs because of shrinkage.
Not sure why you think moisture from a washing machine would be significant, and would cause shrinkage.
Plenty of chipboard core tops, unsealed, above washing machines that are fine.
It's the heat I suspect. If that's what has caused it of course.
There's something here about installing foil underneath.
http://worktops.org/wood-worktops/technical-specifications/[/QUOTE]
A thin sheet of metal will not prevent heat gain, common misunderstanding. Foil can reflect 90% of radiant heat, but radiant heat will be a tiny portion of heat gain.
And washing machines are found in kitchens, same as ovens, not like the manufacturer can say 'well I didn't expect the wood to be exposed to this'.