Fatigue

The life of a trailer chassis is determined by the load history, which consists of collected loads of varying number and magnitude. The appearance of the load history will vary depending on the type of trailer, road conditions and loading situation. When upgrading a trailer chassis using HSS, the sheet thickness of the structural parts is usually reduced. This reduction in thickness will result in an increased working stress level in the complete chassis. However, a stronger material will result in 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. Therefore, the fatigue life of welded joints is more dependent on design and manufacturing rather than choice of material. If the same weld joint design and weld quality are deployed, this will result in reduced fatigue resistance for the chassis.

The fatigue resistance of a material is demonstrated in S/N curves, which are created by testing specimens using a load history of constant amplitude. This means that a specimen is subjected to the same load cycle repeatedly until it fails. After testing several specimens at different load levels, an S/N curve can be plotted. In the graph below, the upper curves show that the fatigue resistance is determined by the static properties of the material. In the lower right part of the graph, the fatigue resistance is determined by discontinuities in the specimen. Discontinuities can include surface texture from rolling of the sheet, cut edges, holes, notches and welds. These are listed in order of decreased fatigue resistance.

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

Welded joints have a much lower fatigue resistance compared to the base material due to the sharp geometry of the weld and residual stresses introduced from the heat input during welding. While the fatigue resistance of welds is often discussed in relation to microstructures, heat affected zones and hardness, the major cause of the weakening of the weld is local stress concentration and defects. All post-treatment methods of welds aim to reduce residual stresses and improve the weld geometry. To achieve good fatigue resistance, it is important to have a smooth transition radius and angle at the weld toe, as shown in the illustration below.

Sharp and smooth weld toe geometry.

Discontinuities in a weld are oriented in the welding direction and follow the root and weld toes. If the discontinuities are parallel to the principal stress direction, they have a 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. For example, 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.

The start and stop positions of a weld are the most critical to its fatigue resistance. Since the welding process is not in a steady state, defects and inclusions are more likely to occur in these positions. Therefore, due to their limited length, tack welds have lower fatigue resistance than continuous welds. Tack welding of longitudinal beams should be minimized, and tack welds should be positioned in low-stress areas. The weld between the upper flange and the web is less sensitive to fatigue, since this area 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 can be used to move the start and stop positions away from the most highly stressed area, such as at the end of a reinforcement plate (see illustration below).

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

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 load cycles have a very small magnitude, they can still be potentially critical for fatigue when combined with larger loads, which can be perceived as crack initiators. On the other hand, small loads can be viewed 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, it is important that welds in high-stress areas have good fatigue resistance, such as welds loaded in the lengthwise direction. Welds with less fatigue resistance should be placed in low-stress areas, such as near the neutral layer of the web of the main beams.

As an example, we can make a comparison of an alternative design for an attachment bracket welded to a beam that is subjected to bending in the vertical direction. When loaded in global bending, the maximum stresses occur at the flanges of the beam and vary in compression and tension over the neutral layer. The design at the top (1) in the illustration below has the attachment bracket 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. The configuration at the bottom (2) has the attachment bracket redesigned to be plug welded closer to the neutral layer. This results in the stress level at the welded joint being reduced by 50%, which 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.