Fatigue evaluation of transportation trailer
Keeping a close eye on the condition of your assets are vital to ensure continuous productivity as well as profitability. MegChem’s Centurion office was asked to do a life assessment of a trailer fleet for the client to make a decision regarding the possible replacement of their trailer fleet.
As part of the life assessment of the trailers, a fatigue life evaluation of a transportation trailer frame was done for a typical transport cycle. This was done to evaluate the fatigue life of the frame for a typical loading cycle.
Axle displacement measurements were used as inputs to a dynamic analysis of the trailer frame. Strains and accelerations were also measured to be used as reference values. The force histories obtained from the dynamic analysis are used together with the stress transfer functions of the trailer frame to calculate the stress-life fatigue life of the trailer frame.
The outcome of the tests was that four failure locations were identified on the transportation frame. These locations can be inspected for initiation of cracks according to the inspection intervals based on the predicted fatigue life at these locations.
Also, decisions about the replacement of the trailer can be made based on the remaining life of the trailer taking the trailer service history into account. Another advantage of this approach is that the fatigue life of the trailer can be easily recalculated for an increase or reduction in payload should that be required. Finally, should modifications to the trailer frame be needed, the test results can easily be used to determine the effect on the required fatigue life.
For the strain gauge measurements, a typical half bridge configuration was used as shown in Figure 1. Figure 1 also shows the locations on the trailer frame of the installed strain gauges.
Four laser displacement sensors measured the vertical displacement of the trailer axles. The sensors measure the vertical displacement of the axle form the top of the axle to the inside of the main beams bottom flange. See Figure 2 for a typical view of the sensor position.
Dynamic and Structural analysis
The trailer and its load were modeled using shell elements. The figure below shows a bottom view of the model used in the dynamic analysis.
From the dynamic analysis of the trailer the following force histories were calculated. Figure 4 show the force histories at the two axles front and rear. Figure 4 also show the force histories at the front connection pin.
For the structural analysis several inertia relief analyses were used to determine the stress transfer functions of the frame for each load input. Figure 5 shows the Von Mises stress distribution in Pa for a 1000 N vertical force input at the front pin.
For the fatigue analysis of the trailer frame the following load cycles were included:
- Load and unload the full ladle
- Load and unload the empty ladle (no damage)
- Tramming with full ladle
- Tramming with empty ladle
To calculate the stress history over trailer frame, the transfer function for each input load is multiplied by the force history of the specified input. All the stress histories of the trailer frame, for the different inputs, are then combined using superposition to give the combined stress history of the trailer frame. The fatigue life is then calculated using the appropriate S-N curve, see Figure 6, for the specific location and the combine stress history at that location. This is done for all the nodes of the trailer frame to obtain the fatigue life contours of the frame.
Figure 7 shows the welds that was used in the fatigue life simulation. Figure 8 shows the fatigue life distribution in log of life plots. The critical locations are indicated by the red circles in these figures in sequence of failure.
Figure 9 shows a comparison between the measured strain (stress) and calculated value with the combined stress history model. The comparison is good, and the same trends are shown by both curves.