To improve fatigue life of the landing gear the attachment should be placed close to the neutral layer of the main beam. A bolted connection will improve the fatigue life substantially (2).
Landing gear attachment welded to the reinforced lower flange of a conventional trailer main beam.
The nominal stress due to bending of a beam is given by:
Characteristic fatigue strength of welded joints subjected to transverse loading (a) FAT = 63 MPa, and longitudinal loading (b) FAT = 125 MPa.
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The height transition results in high stresses, which do not generally affect the trailer’s static load-carrying capacity. However, extra care is needed when designing secondary structures, such as the landing gear attachment, in this area.
If the landing gear attachment is designed to be welded to the flanges, the weld joint will be in the most highly stressed area of the beam cross-section. Redesigning the attachment bracket to be attached to the web instead moves the weld joint into an area with lower stresses (see illustration below). This will improve the fatigue life of the weld joint substantially (Example A).
It is common to design the landing gear in a conventional trailer chassis to be welded to a reinforcement plate that is attached to the lower flange in the neck region of the trailer, as in the illustration below. This weld is placed in a critical region from a fatigue perspective. When developing a lightweight trailer chassis and reducing the thicknesses, the working stress level will be higher. This will result in a reduced fatigue life for this weld if no redesign is performed. This example demonstrates how a redesign of the attachment bracket affects the fatigue life.
The calculations are performed on a conventional main beam manufactured from mild steel (a) and an upgraded alternative in HSS (b) according to the illustration. The fatigue life of this weld in the conventional trailer chassis is assumed to be 16 years. The upgraded design assumes that the attachment bracket is welded directly onto the lower flange, without a reinforcement plate.
Geometry and cross-sectional properties of the conventional (a) and upgraded main beams (b) included in the calculations.
1) Main beam. 2) Reinforcement plate. 3) Landing gear attachment.
Moment of inertia: Ia = 87E-06 m4, Ib = 52E-06 m4.
Section modulus: Wa = 685E-06 m3, Wb = 348E-06 m3.
This shows that the stress level at the critical weld is 100 MPa in the conventional chassis. It will be 100 × 2 = 200 MPa in the upgraded trailer. The fatigue life of a weld is related to the applied stress range by a power of 3; hence the fatigue life of the critical weld in the upgraded trailer will be reduced by:
That is, the fatigue life of the critical weld in the upgraded de-sign will be reduced from 16 years to 16 / 8 = 2 years!
If the welded joint is redesigned according to the illustration in the previous section about load history, and the critical weld joint is removed, the longitudinal weld between the flange and the web becomes the dimensioning factor from a fatigue perspective. The strength of a longitudinal weld is much higher than the transverse weld. If we compare the fatigue strength of these welds, we find that the critical weld at the attachment has a characteristic fatigue strength, FAT, of 63MPa but the longitudinal has a FAT of 125 MPa, as in the illustration below. This means that the longitudinal weld can tolerate twice the stress compared to the transverse weld.
So even though the working stresses have been increased by a factor of 2 in the upgraded I-beam, the fatigue strength of the critical weld joint has been improved by a factor of 2 through a simple redesign. Hence, we have maintained the fatigue life of the original design.
