Claudia Cofano and Patrick Le Pense highlight the risks and dangers for road authorities to accept the reconditioning or reuse of safety barriers if damaged after impact
Steel material presents excellent mechanical characteristics. As well as strength it possesses high deformation capacity, which not only makes it suitable for cold (e.g. by roll forming and stamping) or hot forming, but also allows it to deform and absorb energy, notably in case of impact.
For this reason steel is one of the materials the use of which is most common for the manufacture of road barriers. In fact it can not only resist high loads without breaking but also absorb energy, which greatly increases survival chances for the vehicle occupants.
It can be tempting to reuse an old road barrier or a device damaged after a vehicle impact because, in theory, the mechanical characteristics of the steel permit it to be easily roll-formed again. To do this, after having been taken from the damaged/old system, barrier components are first flattened and then roll formed. This process leads to a modification of the mechanical properties of the steel, with a notable reduction in the elastic limit value.
In fact, because of its microstructure (crystal type and grain size) and the distribution of dislocations present (Orowan’s model), the elastic limit of a metal permanently deformed in one direction will be reduced if loaded again in the opposite direction (reversal loading). This phenomenon is known as the Bauschinger Effect.
In order to evaluate the variation of steel’s mechanical properties in a reconditioning process; testing was performed by ArcelorMittal R&D and its research centre CRM group-AC&CS. A batch of 30 specimens of S460MC steel (EN 10149-2:2013), 3mm thick was machined: 15 specimens in longitudinal direction (parallel to the rolling direction) and 15 in the transverse direction.
The test procedure consisted of the following steps:
- Tensile test on specimens to standard ISO20/80 (reference values);
- Bending of specimens to achieve a permanent residual deformation;
Two internal bending angles and two internal bending radii were chosen:
- Angle: 135° and 155°;
- Radius: 3 mm and 20 mm.
The internal bending angle values were chosen to develop in the specimen a permanent residual deformation.
- Unbending of specimens and verification of flatness (using a 3D machine);
- Tensile tests on flattened samples.
- Microstructure analysis of the samples using an optical microscope (Zeiss Axio Imager).
VARIATIONS ON A THEME
When comparing the results obtained during this test campaign, the difference in terms of elastic limit among specimens subject to reversal loading is obvious.
After post-damage reconditioning, the steel has lost part of its ability to deform: the reduction in the elastic limit can be very large and can go up to about -25 per cent. The value of Rp0.2 falls below the minimum elastic limit value specified by the reference standard. Therefore, it no longer corresponds to the original steel system that was tested and certified. These variations of the steel’s mechanical properties become more or less important depending on the type of steel considered.
The reduction in the elastic limit and in the elongation at rupture, even if the tensile strength value remains unchanged, thus lead to the loss of energy absorbing capacity. This can result in the case of impact of a vehicle on the road barrier in premature rupture of the device and to a more violent shock with major user risk. The acceleration measured inside the passenger compartment would be greater and, as a result, the ASI would increase.
The crash test for vehicle restraint systems’ certification (EN1317: 2010) takes into account the hardening of system components due to roll forming, while the structural performance is already evaluated taking work- hardened material into account. This test, however, does not provide any information on barrier performance in the case of component reconditioning through a new roll forming process: which could have extremely dangerous consequences for road users.
In addition the problems related to damage to the surface coatings must not be underestimated. Surface coatings have the function of protecting the system components from different corrosion agents. After the impact the surface of the crash barrier components in contact with the vehicle is damaged, even if the shock does not produce large deformations. The coating damage can be worsened due to the reconditioning process to restore the original profile. Before proceeding to reconditioning the damaged parts it is indispensable to treat the parts, for example by stripping or polishing, to prepare them for a new immersion in a galvanising bath.
The use of reconditioned rails must thus be considered as a potentially dangerous procedure because it increases the risk of vehicles crossing the barrier by increasing the probability that the reconditioned components (e.g. longitudinal rail) breaks during an accident.
Crash barrier performance depends on the energy absorbing capacity of each component. If this capacity is reduced due to vehicle impact, a road restraint system that originally had certain performance in terms of containment level and severity index can see its functionality reduced if its components are reconditioned to restore the original profile.
Although profile reconditioning and the risks related to this practice have been known for a long time by the European Road Authorities, the number of Countries that have made legislative provisions is unfortunately still very small. As already done by Belgium in its PTV 869, the practice of recondition must be prohibited by Road Authorities through the approval of stricter national standards. Without any monitoring this practice can represent a danger for road users.
Claudia Cofano (CRM group-AC&CS) is a member of the European Committee TC 226 in charge of the European Standardization for Road Restraint Systems – EN 1317 and a Research engineer at CRM group (ArcelorMittal Global R&D)
Patrick Le Pense (ArcelorMittal Europe- Flat Products) is member of the STA management committee. He is also chairman of the “Metal for Buildings” European Association.