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Technical Paper

Time vs Frequency Domain Analysis for Large Automotive Systems

2015-04-14
2015-01-0535
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.
Technical Paper

Solver Embedded Fatigue

2014-04-01
2014-01-0904
This paper presents a fundamental conceptual change to the traditional CAE based fatigue analysis process. Traditional approaches take the responses from a stress solver and these are then transferred into a secondary fatigue analysis step. In this way fatigue is, and always has been, treated as a post processing step. The new conceptual change described in this paper involves combining the two separate tasks into one (stress and fatigue together). This results in a simple, elegant and more powerful Durability Management concept. This new process requires no large data files to be transferred, no complicated file management and it is likely that whole fatigue calculation process can be done in memory. This makes it possible to perform optimization with fatigue life as the constraint. It also facilitates full body fatigue life calculations, including dynamic behavior, for much larger models than was previously possible.
Technical Paper

Simultaneous Durability Assessment and Relative Random Analysis Under Base Shake Loading Conditions

2017-03-28
2017-01-0339
For many automotive systems it is required to calculate both the durability performance of the part and to rule out the possibility of collision of individual components during severe base shake vibration conditions. Advanced frequency domain methods now exist to enable the durability assessment to be undertaken fully in the frequency domain and utilizing the most advanced and efficient analysis tools (refs 1, 2, 3, 4, 5). In recent years new capabilities have been developed which allow hyper-sized models with multiple correlated loadcases to be processed. The most advanced stress processing (eg, complex von-Mises) and fatigue algorithms (eg, Strain-Life) are now included. Furthermore, the previously required assumptions that the loading be stationary, Gaussian and random have been somewhat relaxed. For example, mixed loading like sine on random can now be applied.
Technical Paper

Modern Methods for Random Fatigue of Automotive Parts

2016-04-05
2016-01-0372
Conventional approaches for the fatigue life evaluation of automotive parts like headlamps involves the evaluation of random stress conditions in either the time or frequency domain. If one is working in the frequency domain the fatigue life can be evaluated using one of the available methods like the Rayleigh (Narrow Band) approach or the more recent Dirlik method. Historically, the random stresses needed as input to these methods have been evaluated by the FEA solver (eg Abaqus, or Nastran) and these “in built” stress evaluations have limitations which relate to the fact that the stress conditions are complex and so the common “equivalents” for stress like von-Mises or Principal have not been available. There have also been limitations in the location and method of averaging for such stresses. In addition, the fatigue calculation approach for doing the evaluation has been constrained to the linear stress based (S-N) method.
Technical Paper

Loads Cascading in the Frequency Domain

2018-04-03
2018-01-0138
A previous SAE paper (ref. 1) did a comparative study of automotive system fatigue models processed in the time and frequency domain. A subsequent paper (ref. 2) looked at relative random analysis under base shake loading conditions. This paper proposes to merge these two analysis procedures to implement a new “Loads Cascading” procedure. The objective of this paper will be to show how loads (accelerations, displacements, forces) can be cascaded (transferred) from input load position such as road load data (RLD) body loads to some internal location, for example a battery pack location. Also note that the response from one “module” could form the input to another, therefore, once the loadings are in the frequency domain, the possibility exists to “cascade” the loads through a system. For example, from the chassis, to the subframe to attached components.
Technical Paper

Frequency FE-Based Weld Fatigue Life Prediction of Dynamic Systems

2017-03-28
2017-01-0355
In most aspects of mechanical design related to a motor vehicle there are two ways to treat dynamic fatigue problems. These are the time domain and the frequency domain approaches. Time domain approaches are the most common and most widely used especially in the automotive industries and accordingly it is the method of choice for the fatigue calculation of welded structures. In previous papers the frequency approach has been successful applied showing a good correlation with the life and damage estimated using a time based approach; in this paper the same comparative process has been applied but now extended specifically to welded structures. Both the frequency domain approach and time domain approach are used for numerically predicting the fatigue life of the seam welds of a thin sheet powertrain installation bracketry of a commercial truck submitted to variable amplitude loading. Predicted results are then compared with bench tests results, and their accuracy are rated.
Technical Paper

Frequency Domain Fatigue Analysis of Exhaust Systems

2018-04-03
2018-01-1396
Today in the automotive industry, there is a continual reduction in available development time. There is also an urgent need to reduce cost and weight, to adapt to customer and legislation which drives to an increase in design complexity. These challenges are sometimes made harder by the late availability of hardware and this creates the need to extend and continually improve the established CAE methods which are used to develop automotive parts. This holds especially true in the field of exhaust systems and their components, which experience loads from various sources like temperature, engine or road. In the field of road excitation the use of dynamic transient simulation and subsequent damage calculation is state of the art in terms of simulations methodology.
Technical Paper

Advances Relating to Fatigue Calculations for Combined Random and Deterministic Loads

2014-04-01
2014-01-0725
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.
Technical Paper

A Comparative Study of Automotive System Fatigue Models Processed in the Time and Frequency Domain

2016-04-05
2016-01-0377
The objective of this paper is to demonstrate that frequency domain methods for calculating structural response and fatigue damage can be more widely applicable than previously thought. This will be demonstrated by comparing results of time domain vs. frequency domain approaches for a series of fatigue/durability problems with increasing complexity. These problems involve both static and dynamic behavior. Also, both single input and multiple correlated inputs are considered. And most important of all, a variety of non-stationary loading types have been used. All of the example problems investigated are typically found in the automotive industry, with measured loads from the field or from the proving ground.
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