Fighting the terrorist threat
Concentrating in the case of bridges, as in many other structures, the general principles of deter, detect, delay, defend, and respond provide a layered response to the threatRead more
19-12-2018 | Posted by Joaquín Martí
Impacts occur all the time. Sometimes they are intended, as when hammering a nail, using a battering ram to smash a castle’s entrance, processing rocks with an impact crusher, or trying to pierce armour plate with kinetic energy projectiles. Other times, impacts occur as a result of some kind of accident. But they all tend to have one thing in common:
we want to understand the process and predict its outcome, which is where simulation lends a helping hand.
Impacts are effective because the momentum gathered by the moving element(s), possibly over a long time, is transferred very rapidly, over a time governed by the speed of wave propagation, not by the speed(s) of the moving element(s). This leads to the generation of very high interaction forces, albeit with only a short duration. Think that a steel bar will start to yield plastically if it hits a rigid target at more than a few metres per second.
Until relatively few years ago, most impacts resisted effective simulation: their dynamic character, the strong non-linearities activated in the process, and the almost inevitable 3-D modelling requirements, all combined to make the problem very hard to analyse. And it only gets tougher when perforation takes place, with its implications on the handling of the mesh employed to discretise the problem.
When studying an impact, sometimes we are interested in the survival of the target. This is the case when we analyse the impacts on aircraft produced by birds, tire fragments, hail, debris, etc. The same occurs in nuclear power plants, when impacted by aircraft, tornado missiles, flying valves and other accidental missiles. Or in storage tanks for liquefied natural gas, ammonia, etc., we have also studied similar postulated accidents in several tens of them. Or in bridges, where their piers or abutments must undergo impacts from ships. Or when we study ceramic armour, meant to withstand hits by high-speed projectiles. We have analysed even pedestrian walkways that could be affected by
Other times it is the moving element, i.e. the missile, which is of interest; those investigations are generally classified as crashworthiness studies. We have studied various barriers that keep stray vehicles on the road without damaging them too much, or structures that protect railway lines from the accidental drop of high-voltage lines. We have also analysed planes in forced landings on land and water, dropped containers for spent nuclear fuel or radioactive waste, as well as a variety of pieces of equipment that may suffer accidental falls.
In all the studies mentioned, the designer tries to maintain a safety margin to avoid catastrophe; this simplifies the studies, because the materials involved operate in an easier range of behaviour.
Forensic studies tend to be more complicated because the accident already occurred and one needs to investigate exactly what happened, however complex the response.
The impact on the sea bottom of the two fragments of the floundered Prestige tanker, the collision of a lorry against a 50-m road sign onto the A-6 motorway in Madrid or that of another lorry against an elevated pedestrian crossing in Torrevieja, or the failure of a large crawler crane onto the roof of the Grand Mosque in Mecca, they are all examples of accidents in which it was necessary to model what had really happened, proving that things remained on the safe side was not sufficient.
There are quite a few industries that need this type of investigations. In our experience the main ones are the nuclear industry, oil and gas, aerospace and defence, and the automotive industry, although the mining and civil sectors are also frequent generators of impact problems. The advances in simulation of the last few decades allow them to sleep better than they used to.