Concept

Brittle fracture — where it appears

Fracture with almost no plastic deformation before it, which happens without warning and at a load a strength calculation calls safe. Low temperature, thick material and a sharp detail all push a steel toward it.

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

Two materials pulled until they stop. Two stress-strain curves — mild steel, cast iron — plotted to a strain of 2.0%. One of them has a plateau, so the stress at which yielding starts is something the specimen does rather than something anyone chooses. No offset construction is drawn.

The property that appears in none of the equations

Ductility is in no design formula on this site. Every method on this site depends on it — and a brittle structure does not merely fail early, it makes the analysis wrong.

materials · Ductility $The crack length at which the strength stops mattering. Failure stress against crack length for a toughness of 100 MPa√m, with two steel grades drawn. The falling curve is fracture — Kc divided by Y times the root of pi a — and it does not know what the yield stress is. The horizontal lines are the grades. At 275 N/mm² the two cross at a crack 33.5 mm long; At 460 N/mm² the two cross at a crack 12.0 mm long. The stronger grade's transition is the shorter one — raising the yield stress does not raise the strength of a cracked member, it only shortens the crack that takes it away.$

The flaw that sets the strength

A member with a crack twenty millimetres long fails at its yield stress. Make the steel stronger and the crack that does it gets shorter, so the same flaw that was harmless in the weaker grade decides the stronger one.

materials · Fracture

Named alongside it

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

Ductility Critical crack Fracture Fracture toughness Griffith Load path Lower-bound theorem Moment redistribution Plastic hinge Robustness Rotation capacity Stress concentration

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