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Technical Paper

An Evaluation of the Fatigue Performance of Automotive Steels

A rapid inexpensive evaluation and comparison of the cyclic properties of three steels used in the automotive industry is presented. This evaluation ranges from the endurance limit through the transition life and low cycle regions to the monotonic results. Smooth and notched specimens, tested in strain control and load control, respectively, provide data that are used to indicate notch sensitivity and size effects, cyclic strength and ductility, and cyclic deformation response. The effect of overloads on fatigue damage is given and prestrained smooth specimens demonstrate the possible effect of a few large plastic strain cycles on fatigue resistance. Overloaded notched specimens indicate reductions in life due to both large plastic strain cycles and the induced tensile residual stress. These data are suitable for direct insertion into the design process and also provide a broad base for continuing studies of cyclic behavior.
Technical Paper

Basics of Structural Fatigue

Various recent developments in mechanics theory are discussed with reference to the design of metal structures for fatigue. Particular attention is given to the fatigue notch factor and some test results on welded components are presented and discussed in this context. Emphasis is placed on relating design procedures to progress in fatigue research.
Technical Paper

Fatigue Life Prediction for Variable Amplitude Strain Histories

This paper presents a model for fatigue life prediction for metals subjected to variable amplitude service loading. The model, which is based on crack growth and crack closure mechanisms for short fatigue cracks, incorporates a strain-based damage parameter, EΔε*, determined from the effective or open part of a strain cycle along with a fatigue resistance curve that takes the form: EΔε* = A(Nf)b, where E is the elastic modulus, Nf is the number of cycles to failure, and A and b are experimentally determined material constants. The fatigue resistance curve is generated for a SAE 1045 steel and the model is used successfully to predict the fatigue lives of smooth axial specimens subjected to two variable amplitude strain histories. The model is also used to predict the magnitude of non-damaging cycles that can be omitted from the strain histories to accelerate fatigue testing.
Technical Paper

Fatigue Service Histories: Techniques for Data Collection and History Reconstruction

A number of service fatigue history summarization statistics are examined for their suitability in regenerating the history in the laboratory. Evaluation criteria applied include: fatigue damage-per-level equivalence, implementation simplicity, waveform similarity, and fatigue life equivalence. The results suggest that a three-dimensional “From-To” matrix that includes sequence information about the original variable amplitude histories, is optimal for service history reconstruction.
Technical Paper

Fatigue Testing of Sheet Metals Subject to Uniaxial Tension-Compression

The paper describes the fabrication and testing of thin sheet metal uniaxial fatigue specimens that have been laminated to prevent buckling. When hot or cold rolled metal thicknesses are below 5 mm, the usual fatigue specimens, having a uniform gauge length of 7.5 mm or more, buckle in the short life region (∼10000 cycles) of strain-life testing. For thinner materials, non-standard specimen designs or anti-buckling guides have been used, but each of these solutions requires additional instrumentation. The results presented in this paper show that laminating multiple sheets of material together to increase the specimen's effective thickness raises the strain level for the onset of buckling of the standard uniaxial specimen. Constant and variable amplitude fatigue tests extending into the high-strain short-life region were performed. Fatigue life data for multiple layer specimens were in good agreement with those obtained for single layer specimens.
Technical Paper

Techniques and Equipment for Axial Fatigue Testing of Sheet Steel

The techniques and equipment discussed in this paper were developed in order to characterize the fatigue behavior of sheet steels ranging in thickness from 0.10-0.20 in. The unique developments consist of a specimen gripping and alignment device, and an inexpensive, highly sensitive diametral extensometer. Test results indicate that the combination of specimen geometry and specially designed grips performed satisfactorily, and that longitudinal and diametral specimens are satisfactory for strain amplitudes up to 0.75 and 7%, respectively. The techniques could be used to obtain axial fatigue data on sheet steel as thin as 0.100 in.
Technical Paper

The Effects of Overloads in Service Load Histories on Crack Closure and Fatigue Damage

Fatigue life predictions for notched components are typically based on constant amplitude fully reversed strain-life data derived from smooth specimens. A mean stress parameter is applied to account for the effects of mean stress in the crack initiation location. Fatigue life predictions using this approach are usually accurate for constant amplitude fatigue but are almost always unconservative for variable amplitude service loading. A considerable amount of work has now related the unconservative predictions to reductions in crack closure during the large cycles in the variable amplitude load history which result in lower crack opening stresses for the small cycles than those in the constant amplitude reference tests used in design. This increases the effective strain range and the damage done by the small cycles and results in shorter than predicted fatigue lives.