DA42 instructor pilot reported left main gear collapsed during landing roll.

Narrative

I was conducting a training flight for a commercial multi-engine add-on student. We both completed a thorough preflight and walk around before getting in the airplane. We ran all before engine start; engine start; and pre taxi checklists. We left the ramp around XA:50 and taxied to the runway XX run up area. We completed all pre takeoff checklists and then departed runway XX. In the upwind; we observed in flight weather conditions favored runway XY (compared to reported wind conditions). We brought the gear up as normal and noted no abnormal indications. Upon reaching a safe altitude (~1600' MSL); we turned to the SW and entered ~3 mile base to final for runway XY. We deployed the landing gear on final and noted no abnormal indications or annunciations. The approach was stable; and the landing was both smooth and on runway centerline. The landing was to a full stop; we cleared the runway on 1; completed after landing checklists; and then taxied back to runway XY. The pre takeoff checklist was completed and we departed runway XY; then made left traffic. We deployed the landing gear midfield downwind. The gear was down and locked (three green lights on the gear light indicator) - no unsafe light or annunciations were observed. The approach to land was stable. Upon getting into the round out and flare; the power of both engines was smoothly brought to idle. Touchdown was smooth; not side loaded; and on runway centerline. As the airplane continued to roll out; a beeping sound was observed and we felt the left wing start to dip. Initially; we thought it was a flat left main tire but the dipping motion continued (as well as the beeping). I took controls and maintained directional control; using full right rudder and right aileron to keep the airplane from veering/dipping to the left. At this point; the gear unsafe light became illuminated and the 'check gear' annunciation came on. I secured the left engine by turning off the left engine master and left fuel pump as we felt the left wing scrape. I then secured the right engine by turning off the right engine master and right fuel pump as we came to a complete stop. I alerted airport operations of a disabled aircraft on runway XY on airport CTAF. I then turned off the electric master; removed the ignition key; and we both exited the aircraft. At this point; I called our director of maintenance. Upon looking at the aircraft; they determined it was a structural failure. There was a composite bonding failure on the left main landing gear. This was around approximately XB:15.