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Multi-axial loaded parts like lower control arms are routinely tested in the laboratory for durability verification. But the full anticipated complex road load data is not normally applied because it would be too expensive and complex to test all parts this way. Instead, a simplified loading condition is used. Ideally, this will be as simple as a single sine wave loading applied to one loading point and in one direction. The specification of which hard point to use, which loading direction, and which frequency and load magnitude, requires very good engineering judgement and a high degree of experience. Even then, it is unlikely that the optimum solution will be obtained and the risk of creating a non-representative test is high. Recently, a new FEA technique has been developed which simultaneously derives both an optimum loading profile (surrogate load) and loading direction (vector direction) from full Proving Ground [PG] real loading conditions.

The fatigue analysis of a full car body requires the sheet metal (sheet fatigue), spot welds (spot weld fatigue) and seam welds (seam weld fatigue) to be thoroughly evaluated for durability. Traditionally this has always been done in the time domain, but recently new frequency domain techniques are able to perform these tasks with numerous advantages. This paper will summarize the frequency domain process and then compare the results and performance against the more usual time domain process.

The time domain is currently the most widely chosen option in fatigue testing to fully represent random events occurring in multiple simultaneous input channels. In vehicles for example, time domain tests can represent the same conditions of the road, by applying the same loads at the hard points of the vehicle along a time history. The main drawback of this methodology is the extensive testing duration and hardware cost. Time domain based fatigue tests are composed of a complex hardware, which requires servo motors to work, in order to induce the specific amount of load at a specific time window. These tests are time consuming, since they require the same length duration of the event they are reproducing, times the required repetitions. The frequency domain method for fatigue testing, on the other hand, requires simpler hardware, since there are no need for servomotors and the test length is reduced, since there is no need to run the full event times the required repetitions.

It has been recognised since the 1960's that the frequency domain method for structural analysis offers superior qualitative information about structural response (refs 1, 2, 3, 4); But computational and technological issues have held back the implementation for fatigue calculation until now. Recent technological developments (see refs 5, 6, 7, 8, 9) have now enabled the practical implementation of the frequency domain approach and this paper will focus on the accuracy of the approach when compared with the traditional time based (transient dynamic) approach.

Techniques for calculating fatigue life from random structural responses were first proposed in the 60's but these early methods were limited to narrow band responses (ref 1). When used for wide band responses these same techniques could become very conservative. In order to reduce this conservatism much effort was devoted from the 1980's onwards to develop methods that worked more accurately for the wide band situation. Several methods now exist for the wide band case and these typically exist alongside Finite Element (FE) based random analysis tools like Nastran, Ansys or Abaqus to take the PSD's of stress response and return the Rainflow cycle count and fatigue damage (ref 2). Several problems still exist with todays design methods. Firstly, for large models, these stress transfer functions have to be generated and stored for subsequent use in the fatigue life calculation and these files can be very large.