Design

Trailer designs are often a result of experience and knowledge achieved over the years by the producing companies, and the know-how of the end users. Good solutions are in general also applicable to lightweight vehicles produced in high strength steels. However, advanced high strength steel, (AHSS) enables new solutions, but may also call for design changes in order to utilize the higher strength.

A common trailer chassis consists of two longitudinal main beams manufactured from either standardized hot rolled profiles or welded I-beams and a number of cross-member profiles. For the cross-members, solutions with open profiles, tubes or box-section profiles can be found. Depending on the type of trailer, floor members and different support profiles can also be attached to the chassis. The king-pin region of the trailer usually consists of a king-pin plate and some reinforcement profiles.

The upgrading potential of a trailer chassis is generally not only limited by static load carrying capacity, but even more so by fatigue and stability issues. Therefore, finding a solution with matching load carrying capacity to the existing design serves as a good starting point, but in order to have a vehicle with matching or improved performance it is essential to address the other technical aspects as well. It is important to notice that poor design or production quality rapidly reduces the upgrading potential.

For a trailer chassis manufactured from mild steel, the dimensioning load case is generally load carrying capacity with regard to permanent deformations. In a lightweight trailer chassis, where the thicknesses have been reduced and the working stress level is increased, the load carrying capacity and service life is limited by fatigue, elastic deflections and stability.

Trailers subjected to different loading situations during service.

To achieve a successful upgrading it is important to take all loading situations into account,
a) Fatigue at frequent low stress loading cycles,
b) Elastic deformations when operating,
c) Load carrying capacity; no permanent deformations at maximum loading,
d) Stability when operating.

Strenx^® 700 MC is commonly used in lightweight solutions for trailer chassis. Upgrading a trailer chassis from a 350-grade using Strenx^® 700 MC typically generates a weight reduction of about 30 % for the chassis structural parts, but depending on the chassis design, the weight reduction potential may be higher; even up to 50 %. As an example the potential weight reduction of an existing 13.75 meter long trailer main beam produced from a 350-steel grade by introducing Strenx^® 700 MC has been calculated. The load carrying capacity of the existing design has been determined and a matching alternative in Strenx^® 700 MC is suggested.

The total weight of the original main members manufactured from conventional steel is 1,085 kg and the total weight of the upgraded alternative in Strenx^® 700 MC is 704 kg. This gives a weight reduction of 381 kg or 35.2 %. These results should be considered as an example. Depending on the type of vehicle, specific requirements and design details, the upgrading potential may be less or greater compared to the example above. The calculations only consider static load carrying capacity, but serves as a good starting point in developing a lightweight chassis design.

For the main beams of a conventionally designed trailer chassis, there are some limitations in order to utilize a higher strength than Strenx^® 700 MC for the purpose of structural strength. In order to truly benefit from higher strength other concepts for the chassis need to be explored. However, for certain special trailers higher grades such as Strenx^® 960 may be an appropriate choice. For flanges or profiles subjected to wear or denting, such as the rear bumper and the bulkhead, a higher strength steel such as Strenx^® 1100 or a wear resistant steel such as Hardox^® 450 can be utilized. For other parts of the chassis, such as floor members, cold-rolled steels such as Docol^® 1000 DP and Docol^® 1200 M offer great opportunities for significant weight reductions. These parts can be produced by bending and, for larger series, roll-forming or stamping.

Overview of the cross-sectional properties and the weight reduction potential of a conventional main beam a) and upgraded, lightweight alternatives b) and c) in Strenx^® 700 MC.

Bending stiffness in the vertical direction is often pointed out as a critical aspect for lighter and stronger trailers in high strength steel. In certain markets the elastic deflections of the vehicles are regulated with regard to ground clearance, but in the majority of cases the limitations on deflections are a matter of functionality. That is, the deflections of the trailer chassis should not introduce problems in opening and closing doors, and of course there should be enough space between the truck and the trailer in the area of the fifth wheel when loaded. For certain special trailers, like low-bed trailers, the requirements on the elastic deflections may limit the choice of material.

Since all steel grades have the same Young’s modulus, the bending stiffness is governed by geometry. That is, simply reducing the sheet thickness of the consisting profiles will give a reduced bending stiffness if the outer geometry is the same. For a trailer chassis the bending stiffness in the vertical direction is governed by the geometry of the longitudinal beams. If the stiffness reduction should pose a problem the bending stiffness may be improved by increasing the height of the cross-sections.

Increasing the height of the beam is the most efficient way to increase the bending stiffness. However, in areas where the height of the beam is restricted, the bending stiffness can be improved by increasing the flange width. This measure can also be taken in critical areas to reduce the working stress level and improve the stiffness in the lateral direction of the beams. Utilizing modern manufacturing techniques the flange width can be tailored over the length of the beam according to the load distribution. Although, since the thick- ness of the flange is reduced, the width can only be increased to a certain degree, otherwise the flange on the compressed side will be too slender and local buckling may occur, which will limit the material utilization of the flange. If the existing trailer beam is already very high, shear buckling of the slender web may limit the possibility to increase the height and reduce the thickness of the web. Instability and calculation methods are described further in the SSAB Sheet Steel Handbook.

Variable flange width can be introduced to improve the bending resistance and the bending stiffness in critical areas.

The stability of the complete vehicle riding on the roads or, for tipper trailers, in an unloading situation depends on a number of different factors, where the torsional stiffness of the chassis is one component. For tipper trailers and other trailers where significant twisting loads are present this must be taken into account when upgrading the trailer chassis. The torsional stiffness of a chassis is governed by the design and the position of the cross-members and by the presence of cross-ties. Reducing the thickness of the web of the chassis main beams will have a very limited effect, while reducing the thickness of the cross-members will affect the chassis torsional stiffness significantly. In order to avoid stability problems, design changes can be introduced to reach a matching or even improved torsional stiffness, compared to the original design.

By introducing profiles with closed cross-sections for the cross-members the stiffness in torsion is significantly improved. But in order to get the optimum material utilization the position of the cross-members is equally important. Redistribution or introduction of one or two additional cross members influences the overall torsional stiffness. In general the cross members should be focused towards the rear part of the chassis. However, this shift has been exaggerated in practice many times, and by moving cross members forward or introducing an additional cross member in a strategic region of the front part, will improve the overall behavior significantly. Since a small rotation in the front part results in big displacements of the rear, an increased torsional stiffness of the front could improve the overall performance.

Another effective measure is introducing cross-ties. To get the optimum material utilization of the cross-tie it is important to design it to only carry tensile loading in one bar and allow the other bar to buckle. Hence, the bars should be slender and not welded to each other at the center. To illustrate the effect of these measures a comparison of the twisting angle corresponding to a torsional moment applied at the rear of a common tipper chassis is performed. The results from the calculations represent a unique case, but illustrate clearly the impact of these measures on the chassis stiffness in torsion. In all the calculated cases the total mass of the cross-members has been constant. That is, for the case where closed cross-sections have been used the thickness of the profiles has been reduced. The results show that reducing the thickness of the web gives a slightly reduced torsional stiffness compared to the original design. While introducing closed cross-members or a double cross-tie gives a significantly improved stiffness.

Comparison of the torsional resistance of a trailer chassis with open section cross-members to solutions with closed cross-member profiles and cross-bars.

All trailers are subjected to fatigue loads during driving and loading. The load history that determines the life of a trailer chassis consist of the collected loads of varying number and magnitude. Depending on the type of trailer, road conditions and loading situation the appearance of the load history will vary accordingly. When upgrading a trailer chassis using AHSS the sheet thickness of the structural parts is usually reduced. Reducing the sheet thickness will result in an increased working stress level in the complete chassis. With a stronger material comes higher fatigue strength for the base material. For welded joints, however, this influence is limited due to the stress concentration and the initial imperfections introduced at the welds. Hence, fatigue life of welded joints is more a question of design and manufacturing than choice of material. If the same weld joint design and weld quality is deployed this will result in a reduced fatigue resistance for the chassis.

Fatigue resistance

The fatigue resistance of a material is illustrated in S/N curves, which are created from fatigue testing of specimens using a load history of constant amplitude. That is, a specimen is subjected to the same load cycle repeatedly until failure. After testing of several specimens at different load levels a S/N curve can be plotted. At the upper left part of the curves in the graph the fatigue resistance is governed by the static properties of the material. At the lower right part of the graph the fatigue resistance is governed by discontinuities in the specimen. Discontinuities are for instance the surface texture from rolling of the sheet, cut edges, holes, notches and welds. They are mentioned in the order of decreased fatigue resistance.

S/N Curves of specimens of rolled sheet, with punched hole and welded joint.

Why is the welded joint a critical area?

Welded joints have a significantly reduced fatigue resistance compared to the base material due to sharp geometry of the weld and the residual stresses introduced from the heat input during welding. It is common that the fatigue resistance of welds is discussed in relation to micro-structures, heat affected zones and hardness, but the major cause of the weakening of the weld is due to local stress concentration and defects. All post-treatment methods of welds are focused on reducing residual stresses and improving the weld geometry. To retrieve good fatigue resistance, it is important to have a smooth transition radius and angle at the weld toe.

Sharp and smooth weld toe geometry.

Start and stop positions

Start and stop-positions are the most critical part of a weld with respect to fatigue. Since the welding process is not in a steady state, the risk of generating defects and inclusions in these positions is higher. Therefore, due to their limited length, tack welds have lower fatigue resistance than continuous welds. Tack welding of longitudinal beams should be kept to a minimum and tack welds should be positioned in low stress areas. The weld between the upper flange and the web is less sensitive to tack welding, since this part is mainly subjected to compressive stresses. It is important to design welded joints in general to allow the start and stop of the weld to be placed in low stress areas. In some cases fish-tail design could be used to move the start and stop-positions away from the most highly stressed area as at the end of a reinforcement plate.

Fish-tail design can be introduced to move the start and stop-positions of a weld away from a high stress area.

Transverse versus longitudinal fatigue loading of a weld

Discontinuities in a weld are orientated in the welding direction and follow the root and weld toes. If the discontinuities are parallel to the principal stress direction they have small impact on the fatigue resistance of the weld. On the other hand, if the stresses are transverse to the weld direction the fatigue resistance of the weld will be very low. E.g. the fatigue life of an attachment bracket welded to the lower flange has less than 5 % of the fatigue life of the weld between the web and the flange.

Load history

The load history of trailers is irregular and random by nature, and the total number of load cycles during its life is in the region of 108–109 cycles. Even if the majority of the load cycles have a very small magnitude, combining them with larger loads still make them potentially critical for fatigue. One can perceive large loads as crack initiators and small loads as crack propagators. Due to these combined effects the fatigue limit found in constant amplitude loading vanishes in trailer applications. The only exception is when all loads in the complete history are lower than the fatigue limit. Therefore, in high stress areas, it is important that the welds have good fatigue resistance, such as welds loaded in the lengthwise direction. Welds with less fatigue resistance should be placed in low stress areas, e.g. near the neutral layer of the web of the main beams.

As an example, a comparison of an alternative design of an attachment bracket welded to a beam subjected to bending in the vertical direction can be made. When loaded in global bending the maximum stresses occur at the flanges of the beam and varies in compression and tension over the neutral layer. In the design at the top (a) the attachment bracket is welded near the flanges with the start and stop positions of the weld located in the most highly stressed area of the beam cross-section. In the configuration at the bottom (b) the attachment bracket has been redesigned to be plug welded closer to the neutral layer. This results in the stress level at the welded joint to be reduced by 50 %. Such a reduction in the stress level increases the fatigue life 8 times compared to the previous design.

By redesigning the welded joints to be placed in low stress areas the fatigue life will be improved.

For trailers where the chassis is subjected to torsional loading, for example tipper trailers and timber carriers, it is strongly recommended to use profiles with a closed cross-section for the cross-members. In most cases such a solution allows the cross-members to be welded straight into the web without any additional reinforcements. For heavy-duty vehicles a web stiffener can be integrated in the cross-member attachment to increase the stiffness and reduce the stress level in this area.

Profiles with open cross-sections can be used in trailers where the cross-members are mainly subjected to bending, e.g. curtain-siders, container carriers and vans. Openings for every profile can be cut into the web and the profiles may be welded to the web plate of the longitudinal beam. However, it should be noted, though repeated, that profiles with open cross-sections are not recommended for chassis subjected to twisting loads.

Yet another solution is to use an attachment bracket to distribute the stresses over a larger area. The attachment bracket can be welded, riveted or bolted to the web of the longitudinal beam.

Different types of cross-member attachments. The type of cross-member to be used and the design of the attachment to the main beams depend on the type of trailer. For trailers subjected to substantial twisting loads closed cross- member profiles are recommended. For heavy- duty vehicles it is beneficial to combine such a profile with a u-shaped web stiffener welded to both the flanges and the web (a). Welding of protruding C-profile cross-members can be limited to the web of the profile (b). Cross-members can also be bolted or riveted to the main beams.