Generation and use of standardised load spectra and load-time histories

The reliability of test results from simple test samples in predicting the fatigue performance of automotive components

Lightweight-structural durability design by consideration of variable amplitude loading

Most components and structures are submitted to load-time histories with variable amplitudes during their service life. Their safe design, in the context of weight reduction requirements, d...

Effects on Lifetime under Spectrum Loading

Assembly of off-road bicycle handlebars with a stem that clamps the handlebar around its circumference would be expected to affect fatigue performance by introducing both assembly stresses and stress concentration. Because the effect of clamping on fatigue performance is unknown and because of the need to insure structural reliability in the stem-handlebar assembly to prevent serious injury, the objectives of the work reported by the present article were fourfold. One was to determine the stresses due to assembly and the stress concentration induced in a handlebar for two different clamp designs (i.e. 1-bolt and 2-bolt), a second was to determine experimentally the high cycle constant amplitude load fatigue lives of the two stem-handlebar assemblies, a third was to determine experimentally the variable amplitude load fatigue lives, and the fourth was to predict the variable amplitude load fatigue life with constant amplitude load fatigue test results. The handlebar was instrumented with strain gages and the assembly strains were measured when the stem clamps were tightened. The handlebar was also loaded as a cantilever beam while the applied strains were measured for each assembly. Stresses were computed and the maximum stresses induced by clamping exceeded 200 MPa for both assemblies. A method unique to this study was devised to determine the stress concentration at an arbitrary angular location around the circumference of the handlebar and for an arbitrary loading direction in the plane of the bicycle. For a load directed along an angle of -38° (clockwise rotation from horizontal viewed from the right), both stems created similar stress concentration; the location of maximum applied stress was shifted by 30° from the point that would be expected in the absence of assembly and the stress was increased by 40% at this location. The measured fatigue lifetimes for constant amplitude loading were similar for the two stem designs but the variable amplitude load fatigue lifetime for the 1-bolt stem assembly was shorter than that for the 2-bolt stem assembly by 19%. The fatigue lifetimes for variable amplitude loading based on constant amplitude load fatigue test results were predicted to within 3% and 33% for the 1-bolt and 2-bolt stems, respectively. Thus, constant amplitude load fatigue test results can be used to approximate the variable amplitude load fatigue life. However, the ranking of different assemblies may not be accurately indicated by constant amplitude load fatigue data.

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Methods for Fatigue Testing Off-Road Bicycle Handlebars Based on Assembly Effects Using Two Different Stem Designs

Experimental fatigue and durability assessment plays a prominent role in lightweight design. There is broad evidence, however, that the fatigue strength of materials and structures determined under constant amplitude loading may be not relevant-quantitatively or even qualitatively-for variable amplitude or spectrum loading. Various phenomena both on a microstructural and a macrostructural level contribute to this finding as discussed in the paper including consideration of the fatigue limit, the role of crack growth mechanisms and fracture surface topography, residual stresses, scatter, relative proportions of crack initiation and crack propagation phases, detection of fatigue-critical locations, and ranking of materials and design details. Some general aspects regarding the application of variable amplitude testing for design and clearance purposes such as the required experimental efforts are also considered.

Dieter SCHüTZ

Papers

The Significance of Variable Amplitude Fatigue Testing