I don't have any personal experience with any Terns. Your story is quite familiar with regard to Tern's willingness to accept responsibility for their frames poor safety record. Tern was formed by Josh Hon after a family rift drove him away from his father's company Dahon. Terns are close copies of Dahons with one big exception: As far as we know, Dahons have enjoyed a great frame reliability in general, as opposed to Terns, which have now had numerous cases of frame failure, all in exactly the same way. Tern has done damage control by claiming there was a manufacturing fault in a batch, and recalled that batch. Later, as more and more frames broke, they were forced to expand the recall.

In my opinion, the weld joint was poorly designed, as evidenced by the failures that span accross batches, accross models and even accross factories. Tern wants you to believe that it is either a batch problem or outrageously in your case, a customer error.

Don't accept those lies. Stand your ground. See if your lawyer would consider a class action. There is plenty of starting material in this forum under several topics, just do a search.

Tern needs to eat their frame guarantee. They need to be held accountable for a poor weld design. Their handling of this fiasco earned them only to go bankrupt.