Yield Strength of 6061-T6 Aluminium

Aluminium has no sharp yield point, so its "yield" is defined by convention: the stress at 0.2 % permanent offset strain. For 6061 in the T6 temper, solution heat treated and artificially aged, that is 40 ksi (276 MPa), with 45 ksi ultimate and about 10 % elongation. Temper is everything: the same alloy in the O (annealed) condition yields at only 8 ksi, so a 6061 part is only as strong as its heat treatment history.

The trap is welding. Fusion welding 6061-T6 destroys the aging in the heat-affected zone, and the material there reverts toward annealed properties: codes require the welded zone to be designed at roughly 15–20 ksi yield unless the assembly is re-heat-treated after welding. Bolted and adhesive joints avoid the penalty entirely, which is why aluminium truck bodies and ladders are riveted or bolted rather than welded. Aluminium also lacks a fatigue endurance limit, so cyclic applications need a finite-life design rather than a stress below some threshold.