Fatigue Life Prediction of an EV Frame from Road Loads

The problem

Frames rarely fail from one big load; they crack after many smaller road loads, usually at welds and brackets. EVs add heavy battery mass that loads brackets on every bump. Static factors of safety do not show fatigue life, and weld fatigue needs methods beyond plain material S-N curves.

Objectives

• Build road load histories from measured or published acceleration data.
• Run unit-load static studies and fatigue studies with variable-amplitude events.
• Apply rainflow counting and mean-stress correction and compare S-N curve choices.
• Cross-check critical welds with a nominal-stress or hot-spot weld fatigue method.

What you need

Key concepts

S-N curve

Stress amplitude against cycles to failure for a material or detail. Welded joints have much lower curves than polished parent material.

Rainflow counting

A method of turning an irregular load history into a set of cycles with ranges and means, used with Miner's rule.

Miner's rule

Damage from each cycle is added as cycles applied over cycles to failure; failure is predicted when the total reaches 1.

Mean-stress correction

Goodman, Gerber or similar methods reduce allowable amplitude when the mean stress is tensile.

Step-by-step guide

1. 1

   Define the frame and duty

   Take an example e-scooter frame and define a target life, such as a stated distance on a mix of city and rough roads. Gather acceleration data: log vertical and longitudinal acceleration at the battery and steering head with a data logger if you can, or use published road profiles. Write down the duty assumptions: road mix, rider mass and how many repeats of the logged route make up the target life.

   Checkpoint: A duty definition and load data files with sampling rate, units and source.

2. 2

   Build load histories

   Filter the acceleration data, remove offsets and convert to force histories at each load point by multiplying by the supported mass. Keep separate channels for vertical, longitudinal and lateral loads. Plot histories and their statistics. Run a quick rainflow count in MATLAB or Python to see the range distribution and check that the data look sensible. Keep the sampling rate high enough to capture the peaks, and note the filter cut-off you used.

   Checkpoint: Force histories for each channel with plotted statistics and a rainflow histogram.

3. 3

   Run unit-load static studies

   Set up the frame static study with realistic fixtures and apply a unit load, such as 1000 N, in each direction at each load point in separate studies. Use beam mesh for members and a solid sub-model for critical welds and brackets. Check convergence of stress at critical locations. These unit-load results scale with the load history in the fatigue study.

   Checkpoint: Unit-load studies that solve and converge, with reaction forces balancing applied loads.

4. 4

   Set up fatigue events

   Create a fatigue study and add a variable-amplitude event for each load channel, linking each to its unit-load static study and importing the load history as a curve with a scale factor. Choose rainflow counting with a stated number of bins and a mean-stress correction such as Goodman. Assign S-N curves to materials, either from the library, derived from elastic modulus, or from your data.

   Checkpoint: A fatigue study with events, curves and settings that runs without errors.

5. 5

   Run and read results

   Run the fatigue study and plot damage, life and load factor. Identify the locations with the highest damage. Compare results with Goodman and with no mean-stress correction, and with two S-N curve choices, to see how sensitive life is. Check that the hot spots match where you expect from static results. Report life in the same units as the duty, such as kilometres, by dividing by the number of route repeats per target life. Check whether the critical location changes between settings.

   Checkpoint: Damage and life plots with hot spots identified and a sensitivity table for correction and S-N choices.

6. 6

   Check welds with a design-code method

   Plain material S-N curves overestimate weld life. At each critical weld, extract nominal stress or hot-spot stress from the solid sub-model, following the extrapolation rules of a recognised weld fatigue code such as the IIW recommendations. Use the matching weld detail fatigue class and Miner's rule with the rainflow-counted stress ranges. Compare this life with the SOLIDWORKS result. State the hot-spot extrapolation points and mesh size used, since these strongly affect the result.

   Checkpoint: A weld fatigue life calculated by the code method and compared with the software result, with the more conservative value used.

7. 7

   Study the effect of battery mass

   Vary the supported battery mass and its mounting position, since EV battery mass is a large share of the load at brackets. Scale the load histories and rerun. Plot life at the critical weld against battery mass. Note how mounting the battery on rubber isolators could reduce peaks, as a possible design change to check. Use the same rainflow results from the load history so the comparison is like for like. Keep battery position changes within the available packaging space.

   Checkpoint: A plot of predicted life against battery mass at the critical location.

8. 8

   Improve the design

   Change the critical detail: add a gusset, move the weld away from the highest stress, change weld type or improve the weld toe with grinding. Rerun the unit-load and fatigue studies and the weld code check. Record mass change and life gain. Stop when the life meets the target with a stated safety factor on life. Check that each change does not move the hot spot to a new weld that was not critical before.

   Checkpoint: A design change that meets the target life with the stated safety factor in both software and code methods.

9. 9

   Plan physical validation

   Fatigue predictions carry large scatter, so plan a test: a rig that cycles the battery bracket or frame joint at a stated load amplitude, with strain gauges at the predicted hot spot. If time allows, run it and compare measured strain with the model and cycles to crack initiation with the prediction. Otherwise write the test plan in detail. Place a reference gauge in a low-stress area to check the rig load.

   Checkpoint: A measured strain comparison or a detailed test plan with loads, cycles and gauge positions.

10. 10

    Report

    Report duty, load histories, unit-load studies, fatigue settings, life results, sensitivity to assumptions, weld code check, design changes and validation plan. State clearly that life predictions are estimates with scatter and that welds need code methods or testing. Give the predicted life as a range rather than a single number. Include the load data processing script and all fatigue study settings in an appendix so the work can be repeated. Recommend measuring loads on more roads to reduce uncertainty in the duty.

    Checkpoint: A report with traceable inputs, sensitivity, weld checks and a life range.

Testing & validation

• Unit-load studies converge and balance.
• Rainflow count from software agrees with an independent script.
• Weld life checked by a recognised code method.
• Strain gauge comparison or test plan for physical validation.

Expected results

A fatigue life prediction for an EV frame from road loads, with sensitivity to S-N curves and mean-stress correction, a weld code cross-check, a design change that meets target life and a validation plan.

Take it further

• Use a SOLIDWORKS Motion study to derive loads from a road profile instead of measured accelerations.
• Compare a steel and an aluminium frame for fatigue life.
• Study the effect of weld toe improvement methods.

Viva and review questions

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