Crash barriers: matching choice to purpose

Barriers to protect drivers and passengers of road vehicles are of two types: those that are static (the central divider of a freeway, for instance) and those that move (the bumper of the vehicle itself), as shown in Figure 9.3 and Table 9.3. The static type lines tens of thousands of miles of road. Once in place they consume no energy, create no CO2, and last a long time. The dominant phases of their life in the sense of the life cycle of Figure 3.1 are those of material production and manufacture. The bumper, by contrast, is

Two crash barriers, one static, the other—the bumper—attached to something that

Table 9.3

Design requirements for crash barriers

Function

Crash barrier: transmit impact load to absorbing elements

Constraint

High strength

Adequate fracture toughness Recyclable

Objectives

Minimize embodied energy for given bending strength (static barrier)

Minimize mass for a given bending strength (mobile barrier)

Free variables

Choice of material Shape of cross-section

moves. Different ecocriteria are needed for each. The barrier is loaded in bending, as in the plan view in the

figure.

part of the vehicle; it adds to its weight and thus to its fuel consumption. The audit of Chapter 7 established that the dominant phase here is that of use. If ecodesign is the objective, the criteria for selecting materials for the two sorts of barrier will differ: minimizing embodied energy for the first, minimizing mass for the second.

In an impact, the barrier is loaded in bending (Figure 9.3). Its function is to transfer load from the point of impact to the support structure, where reac­tion from the foundation or from crush elements in the vehicle supports or absorbs it. To do this the material of the barrier must have high strength, ay, be adequately tough, and able to be recycled. That for the static barrier must meet these constraints with minimum embodied energy as the objective, since this will reduce the overall life energy most effectively. We know from Chapter 8 that this means materials with low values of the index

HmP

1 2/3

°У

where ay is the yield strength, p the density, and Hm the embodied energy per kg of material. For the car bumper it is mass, not embodied energy, that is the problem. If we change the objective to that of minimum mass, we require materials with low values of the index

M2 = -2L (9-2)

<7,

These indices can be plotted onto the charts of Figures 8.13 and 8.15; we leave that as one of the exercises at the end of this chapter to show here an alternative: simply plotting the index itself as a bar chart. Figures 9.4 and

105

103

9.5 show the result for metals, polymers, and polymer-matrix composites. The first guides the selection for static barriers. It shows that embodied energy (for a given load-bearing capacity) is minimized by making the barrier from carbon steel or cast iron or wood; nothing else comes close. The second figure guides selection for the mobile barrier. Here carbon fiber reinforced polymer (CFRP), for instance, excels in its strength per unit weight, but it is not recyclable. Heavier, but recyclable, are alloys of magnesium, titanium, and aluminum. Polymers, which rank poorly on the first figure, now become candidates;

Postscript: Metal crash barriers have a profile like that shown on the left of Figure 9.3 . The curvature increases the second moment of area of the cross-section, and through this, the bending stiffness and strength. This is an example of combining material choice and section shape (Section 8.10 and Table 8.9) to optimize a design. A full explanation of the coselection of material and shape can be found in the first text listed in Further Reading.