Efficient Frequency-Domain Based Fatigue Life Estimation of Spot Welds in Vehicle Components

Project Description

Fatigue of vehicle components is caused by various external and internal excitations within a broad frequency range. As spot-welds are prevalent joining elements in the automotive industry – a typical modern body-in-white contains about 4000-6000 thereof –, reliable fatigue life estimation of spot-welds is very important.

Essentially, for spot-weld fatigue analyses in the time-domain two basic approaches have been followed:

  1. First, there are simple FE models with only a few elements that allow for a rather gross approximation of the stress state only.
  2. Second, there are detailed models with hundreds or thousands of nodes for a single spot-weld, which however are computationally very expensive.

Although the necessary CPU-time is a crucial issue, especially in time-domain calculations, it is relevant in frequency-domain methods, too. Indeed, whereas the general frequency-domain approach is well-established, there still is room for improvement concerning specific problems.

Main Goals

An advanced procedure in the frequency-domain is proposed. It is aimed to outline several measures to speed up frequency-domain fatigue analyses for vehicle components with huge numbers of spot-welds -- as implemented in the program FEMSITE of MAGNA STEYR. An efficient superposition of the stresses from a modal analysis, using a pre-processed database, with the PSD input may increase the performance and allow fast consideration of variants of the fatigue life estimations both for different PSD input curves and different spot weld and sheet geometries.

Moreover, the so-called cutting plane method used hitherto in the time-domain proves advantageous in the frequency-domain, too. Additionally, by appropriate a priori stress estimations the number of spot welds to be considered can be reduced essentially. To show the benefits of these measures, a fatigue life estimation of an actual vehicle component both with the suggested method in the frequency-domain and with the standard time-domain approach is presented and discussed. Figure 1 presents a sketch of the applied basic procedure.

Survey of the general frequency-domain approach

Figure 1: Survey of the general frequency-domain approach for fatigue life estimations

Results

An assessment of the proposed approach is applied to a bracket for an expansion tank of a cooling medium. This expansion tank is mounted at the front of a passenger car next to the radiator, see Figure 2. The figure shows also the comparison of frequency-domain and time-domain results for the bracket. As one observes, the general distribution of critical damage is almost indistinguishable. The (left) most vulnerable position found by the calculations is perfectly coinciding with the position of crack occurrence in tests shown thus validating the simulations. With regard to the spot welds, they are confirmed to be in a safe state (within the load history), and the difference between the D-values obtained for them by both methods is only about 11%. As compared to the time-domain approach, the frequency-domain method predicts slightly smaller damage. It is shown that the proposed algorithm may reduce the computation time as compared to time-domain calculations substantially.

Model of cracks in a bracket

Figure 2: Cracks in the bracket of the expansion tank and calculated damage for frequency and time domains

Figure 3 presents a comparison of CPU-times of time-domain and frequency-domain calculations for different numbers of spot-welds in a vehicle body-in-white.

Graphic comparison CPU time and frequency calculation

Figure 3: Comparison of CPU-times of time-domain and frequency-domain calculations

Publication

Zigo, Milan, Eray Arslan, Werner Mack, and Gerhard Kepplinger. "Efficient frequency-domain based fatigue life estimation of spot welds in vehicle components, opens an external URL in a new window." Forschung im Ingenieurwesen 83, no. 4 (2019): 921-931.

Zigo, Milan, Eray Arslan, Werner Mack, and Gerhard Kepplinger. "Computationally effective spot‐weld fatigue life estimation for vehicle components, opens an external URL in a new window." PAMM 19, no. 1 (2019): e201900137.

Cooperation Partner

Duration

  • September 2014 - April 2015

Contact

Privatdoz. Dr. Eray Arslan

Send email to Eray Arslan