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Fatigue Testing and Fatigue Calculation at TRW Lucas Varity

fatigue calculation LMS FALANCS brakeThe development of safety-related components for the automobile industry is subject to strict guidelines and time restrictions that should not be under-estimated. This is why Hermann Tumbrinck of TRW Lucas Varity Braking Systems in Germany has been investigating new technologies, such as LMS FALANCS, to maintain safety factors while dramatically reducing development times, and component weight.

For 18 years Hermann Tumbrink, Chief Fatigue & Simulation Engineer of TRW Lucas Varity in Koblenz, has been responsible for the development of new fatigue design methods. His first task has been to give the designer the necessary information as to whether the design is able to meet the target service life. And secondly, to develop theoretical and experimental guidelines using these methods to facilitate service life estimation prior to the prototype stage. More recently much work has focussed on the characterization of calipers made from a variety of aluminum alloys.

The engineering group has evaluated the fatigue lives of brake components using the operating strength method - i.e. determining Woehler (S-N) curves for every brake carrier and a number of brake housings. Tests take one week per caliper, and normally six calipers are tested for statistical reasons. Since 1982, almost 1000 S-N curves have been evaluated. Several years ago, TRW asked LMS to co-develop a software package for the statistical analysis of test results, with main focus on S-N curves. The program, which eventually became LMS FALANCS Lifestat, provides various tools and methods for computation and visualization in four highly related working areas: 1D distribution analysis; S-N Curve analysis using the stress-life approach; computation of pseudo damages; and the analysis of cyclic material data (strain life approach). All results can be easily compared, even output from different methods, and so traditionally used algorithms can be used along with more advanced routines. On the other hand, an interface to LMS FALANCS enables the usage of calculated S-N curves for any kind of numerical fatigue calculation.

In the past few years, stress analysis has been conducted on every new component using FEA, with LMS FALANCS being using for service life predictions. The input customer load history is a standard cumulative load history used by the Brake Working circle of the German Automobile Industry, and represents a worst-case customer usage for a life of 300,000km with a probability of failure of 10-5. Although the load history is currently uniaxial, the ease of using multi-axial loads within FALANCS has enabled further research into such loads. The cyclic fatigue properties for the aluminum materials were obtained through the statistical analysis of tests using LifeStat.

On first examination, TRW believes the comparison of test and prediction is quite reasonable, especially in the vicinity of the life for a scaled load of 1.2. At lower scaled loads, the strain-life concept tended to overestimate the service life. This discrepancy may have been due to the fact that the cyclic properties used for strain-life calculations were obtained from smooth laboratory specimens, and research was undertaken to determine the effects of surface finish, size effects, residual stresses and variance in the Miner factor. According to Hermann Tumbrink, “For components made of SG cast iron we know that the Miner factor is always >1, and we are already in a position to make satisfactory life predictions. However, the aluminum is very sensitive to environmental conditions, and we’ve determined that using a Miner’s factor of 0.24 gives reasonable estimates. Research is ongoing, but within a few years, we expect to make reliable predictions that our design team can use to dramatically shorten their development process”.

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