Complex sentence: Concorde selected Mach 2 as its maximum speed deliberately; at higher speeds the skin friction heats the airframe so much that conventional aircraft duralumin (an aluminum alloy) would lose its temper and strength.
Split into simpler sentences: Concorde selected Mach 2 as its maximum speed deliberately. At higher speeds the skin friction heating of an aircraft at Mach 2 was the maximum speed at which duralumin (an aluminum alloy) could be used before it would lose its temper and strength.