Structural Engineering & Statics
where the load goes
Concept

Work hardening — where it appears

The rise in a metal's flow stress as it is deformed plastically, which is why its stress-strain curve keeps climbing after yield. It lets a member spread strain along its length before necking, and a metal that hardens little concentrates its strain in whichever part yields first.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

The strength was bought in a furnace and the welder gives it back. Proof stress as delivered and beside a weld, for four aluminium alloys. The heat-treated alloys lose half of it: the strength of a 6xxx extrusion is in precipitates formed by an ageing treatment, and the arc dissolves them for 32 mm either side of the weld, permanently. The work-hardened tempers lose nearly as much, because the heat undoes exactly the work. The annealed ones lose nothing at all, because there is nothing left in them to anneal. The consequence is the crossover: 5083-H22 is 1.04 times 6061-T6 as delivered and 0.92 times it once welded, so the stronger alloy is the weaker member. The 240 mm member drawn, with two longitudinal welds, keeps 87% of its parent capacity — a weld along a member softens a strip and leaves a section, and the same weld across it softens the whole of one.

The strength the welder gives back

A 6082-T6 extrusion is twice as strong as a 5083-H111 plate and, welded across, the two are within a few per cent of each other. The heat of the arc anneals the metal for thirty millimetres either side, permanently, and the strength that was bought in a furnace is given back at the first joint.

materials · Heat-affected zone A few per cent between two welds is most of the second one's stretch. For a 2.0 m 6082-T6 member with two welds whose heat-affected zones are each 60 mm long, the stretch beyond the elastic at maximum load as a multiple of one weld's (solid), against how much stronger the second zone is than the first; dashed, the share of its own uniform plastic strain the stronger zone reaches when the weaker peaks, (1/(1 + gap))ⁿ with the zone's hardening exponent n = 10.4. Identical zones give 2.00 times one weld's stretch; 1.74 at a gap of 3 per cent, 1.23 at 15. The stronger zone gives half its stretch at a gap of 7.1 per cent. (uniform elongation of the zone assumed at 12%, and each zone's proof stress and ultimate scaled together)

Two welds, and the one that decides

A member welded twice has two soft zones in series. If they were identical, both would reach their ultimate strength at the same load and the member would stretch twice as far as with one weld before either necked. They are never identical, and the flat top of an aluminium zone's stress–strain curve means that a second zone five per cent stronger than the first gives only sixty per cent of its stretch. Every weld added makes the member weaker, by the statistics of its weakest link, and more ductile, by less than it would if the welds matched.

materials · Heat-affected zone

Named alongside it

The objects these essays reach for when they reach for this one.

AluminiumDuctilityHeat-affected zoneWeldAnnealingEffective sectionFabricationFatigueLocalisationMaterial selectionNeckingNet section

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