Advanced high strength steels in trailers

When looking into the financial benefits for the trailer manufacturer it is important to take all aspects affecting the overall production economy into account. Simply comparing price level per ton for different steel grades does not give a true picture of the manufacturing cost level. Reducing the sheet thickness will in most cases give a substantial cost reduction in both processing of the material and material cost. Even if the price per ton is higher for AHSS, less steel is consumed due to the weight reduction. Using thinner gauges in the work- shop allows the cost for cutting, bending and welding to be reduced. Laser cutting in high strength steel is no different from cutting in mild steel and the producer will decrease the cutting time due to the thinner gauge. Welding in thinner material gives, in most cases, the largest cost reduction due to the reduction of consumables and the opportunity to increase the welding speed. Introducing AHSS with good bendability may also allow the number of welds to be reduced. Bending of profiles in AHSS generally does not require greater force than a profile in a thicker gauge made from conventional steel. However, the spring-back of AHSS is greater compared to conventional steel and needs to be compensated for in the process.

To give a better insight into these issues a comparison of the production costs of a traditional, flatbed trailer chassis and a lightweight solution manufactured from AHSS is per- formed. The traditional trailer chassis included in this study is manufactured from hot-rolled standardized I-beams, which are cut and welded back together in the goose-neck region in order to produce the transition in height of the main beams. The cross-members consist of bent profiles welded to the longitudinal beams and there are also some side-wing profiles to support the floor. The side rail profiles are manufactured from standardized U-beams. All parts are produced from a S355-steel grade. In the lightweight solution the hot-rolled I-beam has been replaced by a laser-cut and welded longitudinal I-beam manufactured from AHSS. Upgrading the traditional chassis by introducing AHSS allows the thickness of all major structural parts to be reduced and the weight of the chassis is thereby reduced by about 1,500 kg.

In addition to the weight reduction a 30 % production cost reduction can be observed. Cost has been decreased for both cutting and welding operations, but also for the material. In this case a slight increase of the bending cost can be observed. New profile cross-sections have been introduced, due to design issues, which require more bending operations. A 30 % cost reduction give obvious benefits for the producer and combined with a more attractive lightweight trailer great market advantages can be expected. It is also interesting to notice that this study was performed on an existing chassis where most structural parts consisted of hot-rolled profiles.

If the existing traditional chassis would have been produced from welded beams even greater cost savings could have been achieved.

Comparison of production cost of conventional and lightweight trailer chassis including material cost. Using thinner material gives cost reductions in both welding and cutting operations and since less steel is consumed the material cost will also be reduced.

A lighter and stronger trailer also has a direct and obvious benefit for the logistics operator, since the maximum weight of the vehicle is limited by law. A lighter configuration allows the payload to be increased on every trip and in many cases less fuel consumption can also be observed. The amount of fuel savings can be estimated in a conservative way to about

0.55 l/100 km per 1,000 kg of weight reduction. This directly affects the operating figures of any logistics company. Depending on the type of vehicle and the upgrading philosophy, the total weight can generally be reduced by 500 to 2,000 kg by introducing AHSS. By selecting the most suitable AHSS grades for the application, the costs associated with maintenance can also be reduced. Combining high strength with abrasion or weather resistance can improve the resistance to the tough demands on the performance of these vehicles.

In addition to the financial benefits, a lighter vehicle will reduce the environmental impact by saving primary energy resources and reduce greenhouse gas emissions.

In a life cycle assessment of a vehicle, different phases are often analyzed. When an upgraded trailer is compared to an original design in conventional steel, the influence of steel production and the service life is dominating. The latter often counts for 90 % of the total environmental savings for vehicles.

In analyzing the service life in the case of volume limited cargo the energy balance of the vehicle is considered. The basic energy consumption of road vehicles depends on several resistance factors that the vehicle has to overcome during its operation.

With the exception of aerodynamic resistance, all resistance factors are linearly dependent of the mass. The aerodynamic resistance however, depends on the dimensions of the vehicle and the speed. Therefore, besides the mass, speed, acceleration and the gradient (hilly or flat) also affect the energy consumption. These factors are highly dependent on the driving situation and driving behavior. With the same driving situation assumed, the correlation between energy consumption and vehicle weight is linear, and the energy saving corresponding to a specific weight saving is independent of the absolute weight of the vehicle.

Fast vehicles with a steady speed will therefore have a high aerodynamic resistance and low acceleration resistance and thus moderate specific energy savings by weight reduction. Consequently slow vehicles with frequent stops and accelerations will have high energy savings by weight reduction.

< />Overview of the iresistance factors affecting the fuel consumption of any road vehicle.

Large semi-trailers are the dominant modes of road transport in Europe as well as in the U.S. and accounts for a significant proportion of the fuel used in the transport sector. Either direct or indirect savings can be achieved by weight reduction. If the cargo is limited by volume, a lighter vehicle uses less energy for hauling, and if the cargo is limited by weight, additional cargo can be transported.

In this case the gain in fuel consumption following a weight reduction is, as mentioned, heavily dependent of the driving situation which can be seen from the figures presented in the table, which shows typical values of fuel consumption for semi-trailers in different driving situations.

If we consider a 40 tonnes semi-trailer with a tipper body we can exemplify the influence of weight reduction on the environmental emissions and primary energy resources. We assume volume limited cargo and that these types of vehicles mainly drive on motorways and rural main roads. A specific fuel saving of 0.055 liter/100 km for 100 kg weight reduction is a conservative estimate. For the life cycle assessment (LCA), the total life for the trailer is set to 1.2 million km.

Fuel consumption of typical semi-trailers in different driving situations and specific gain in fuel consumption achieved by weight reduction of the vehicles.
1) Gabi LCA software and data base PE International, Leifelden-Echterdingen, Germany.
2) NTM Environmental data for international Cargo Road Transport Europe, Preliminary, Working document subject to updates, version 2009-12-22 Network for transport and the Environment, NTM, Sweden.

For weight limited cargo, the weight reduction of the vehicle permits higher payload to be carried and thus less vehicle-km are needed to transport a certain amount of goods. Energy savings for weight limited cargo are therefore higher than energy savings for volume limited cargo. As an example the savings in fuel consumption and CO₂-emissions for a light- weight tipper-trailer is calculated. The weight of the vehicle has been reduced by the use of AHSS, which has increased the payload by 1,839 kg, see table. With a life of 1.2 million km, 2,210,000 ton km transport performance can be saved during the life-time of the tipper-trailer; which is equivalent to 76,550 vehicle-km of fully loaded vehicles. If a fuel consumption of 40 l/100 km is assumed, this means a life cycle saving of 30,620 liter of diesel fuel, 81.4 ton saving of CO₂-emissions and 316 MWh primary energy savings per vehicle.

Example of upgrading of a semi-trailer with a tipper body.
WR= weight reduction, R-CO₂=reduction in CO₂, R-FC= reduction in fuel consumption, R-Energy= reduction in primary energy consumption.
* Environmental figures refer to volume limited cargo.
** Environmental figures refer to weight limited cargo with a load factor of 100 %.