Structural Engineering & Statics
where the load goes
Concept

Energy absorption — where it appears

The work a member or material takes in by deforming permanently or by damping, which an impact must spend before it is stopped. How much is absorbed, and how much is handed on, decides the force a blow puts into what lies beyond.

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

The force climbs back to the elastic one. The peak force of each of 40 drops of a 500 kg weight dropped 1.0 m onto a structure whose stiffness gives it 2.0 mm of static deflection under the weight, onto a structure yielding at 80 kN — half the 160 kN an elastic structure takes (dotted). Perfectly plastic, every drop peaks at 80 kN and the saving holds, at the price of the ratchet. Hardening at 2 per cent, the peak climbs drop by drop: 82 kN at the first, 91 at the fifth, 131 at the 40th; hardening at 5 per cent, the peak climbs drop by drop: 86 kN at the first, 104 at the fifth, 152 at the 40th. A hardening structure ends up carrying almost the elastic force it was made weaker to avoid.

The same weight, dropped again

A structure allowed to yield under a falling weight needs only a fraction of the strength an elastic one needs — half of it for a ductility of two and a half — because the energy goes into bending the steel rather than into force. That is the whole saving, and it is spent on the first drop. Drop the same weight again and the structure yields again: a perfectly plastic one adds the same permanent set every time and never stops, and a hardening one stops only when it has stiffened itself back up to nearly the elastic force it was made weaker to avoid, after moving more than half a metre.

dynamics · Impact factor The forging is a pad nobody designed. For a 2.0 t tup at 7.0 m/s, 49 kJ, on the 150 t foundation with its 30 t anvil on a pad: the largest force through the anvil's pad (solid) against the forging's flow force (logarithmic), with the dies meeting at the right; dashed, the pad force if the same impulse arrived all at once, as a fixed-impulse calculation takes it. Through a forging flowing at 0.1 MN the pad carries 0.15 MN, against 3.70 for the same impulse delivered at once; at 1.5 MN, 2.32 against 3.93; with the dies meeting, 5.86 against 5.93.

The forging that cushions the blow

A forge hammer's foundation is usually designed for a blow treated as a fixed impulse: the tup's momentum, multiplied by a factor for its rebound. The blow is not fixed. Between the tup and the anvil lies a hot forging, a plastic body that flows under the tup, stretches the blow to 48 milliseconds and absorbs nearly all of its 49 kJ, so that the anvil's pad feels 0.46 MN. As the part cools it flows less, the blow shortens, the tup rebounds, and the pad force climbs to 5.86 MN when the dies meet with nothing between them. The foundation is designed by the blow that does no work.

dynamics · Vibration isolation

Named alongside it

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

DuctilityDynamic loadFoundationImpact factorImpulsePermanent setPlasticityRatchetingRestitutionShakedownStrain hardeningVibration isolation

All concepts