Current US debt stands at 20+ trillion. Is there there a clause or rules/procedures on defaulting in the US constitution? If so, how are the distribution of assets and liabilities handled?
The only relevant parts in the constitution itself would the section 4 of the 14th amendment.
The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any state shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.
What it actually means hasn't been litigated since the US has never really "defaulted" despite political posturing over the debt ceiling before.
Due to the mechanics of sovereign debt, there is no real danger of "default" for the US (outside of a politically generated debt crisis). The danger from extremely high debt would be unacceptably high levels of inflation. Hearing about "default" in a case like Greece is because Greece does not issue their own currency and control their own monetary policy. The USD on the other hand is the reserve currency of the world and gives the US lots of advantages because of how much trust is placed in it.