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A B S T R A C T It is an accepted fact in fatigue community that compressive loads contribute to fatigue crack growth. Evidences range from fatigue crack growth under fully compressive loads to effects of compressive underloads to negative stress ratio loading. Because the crack closes under compression and the crack flanks transmit compressive stresses, the loading situation is completely different to those of tensile loading. The present paper addresses the comparability of crack growth testing procedures at negative stress ratios. It reveals that compressive loading at the crack tip differs in different specimens for an equal maximum stress intensity factor K max and negative stress ratio R. Furthermore, the crack length can significantly influence the loading conditions at the crack tip for tensioncompression loading. Depending on the specimen type and crack length, a negative force ratio may lead to a change of algebraic sign of the stresses at the crack tip or not. As a consequence, the comparability of available literature results for R ≤ 0 tests is not ensured. Proposals to improve the comparability of tension-compression crack growth testing will be given.

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The present study aims to carry out an experimental, analytical and numerical investigation of the monotonic and fatigue performance of electron beam melted Ti-6Al-4V structures. Therefore, tensile tests, multiple step tests and strain-life tests were performed on machined EBM Ti-6Al-4V solid samples. An elastic-plastic material model in combination with a numerical damage model was examined according to the experimental tensile tests. Analytical models proposed by Ramberg and Osgood, as well as Coffin and Manson were obtained to describe the cyclic stress-strain curves and strain-life curves, respectively. The fracture surfaces of the tested samples and the influence of different build directions were analyzed. A prediction of the static and fatigue material properties is of particular importance, e.g., for the safe application of additively manufactured load-bearing implant structures. Based on the determined analytical and numerical models, the material and product behavior of complex electron beam melted structures under cyclic loading and fatigue life determination can be investigated in the early stages of the product development process.

The present paper addresses two issues regarding the influence of compressive loadings for fatigue crack growth. The first issue is the crack tip loading in compression. It will be verified that Kmaxand R are not suitable to account for compressive loading conditions at the crack tip. The second issue is the investigation of some basic load interaction effects in tension-compression loading. Especially loading conditions that were reported leading to an acceleration of fatigue crack growth were revisited. Numerical simulations of the experiments are used to interpret the results.