Abstract
Additive manufacturing (AM) offers design freedom and ability to fabricate parts of complex shapes which are not often possible with the conventional methods of manufacturing. In an AM part, even with optimum build parameters, a complete elimination of defects is not possible and this makes it hard to fully deploy the AM technology to build load bearing parts operating under cyclic loading conditions. Many of these defects are < 1 mm in size and are categorised as ‘small cracks’. Local interaction of cracks with microstructural features and closure effects at the wake of the crack tip are some of the factors which make the growth behaviour of small and long cracks different. A crack growth life prediction method, which effectively considers the small crack growth behaviour, has been discussed in this paper. This proposed method includes a detailed finite element-based crack growth simulation using the ANSYS SMART fracture technology. The lifing calculations utilise the modified NASGRO equation and small crack growth data which was obtained from the published long crack growth data, corrected for closure effects. The predicted stress versus number of cycles curves were compared against the fatigue test results for the AM specimens in Ti–6Al–4V material. A good correlation between the predictions and test results suggests that the proposed method can be used to assess the small crack growth life of AM parts where the fatigue effects of cyclic loading can be quite significant.
Highlights
In comparison with subtractive manufacturing methodologies, the additive manufacturing (AM) is defined as ‘a process of joining materials to make objects from 3D model data, usually layer upon layer’ [1]
Though the predicted lifing curve for 0.1 mm initial defect size appears to be following the three failure points observed in experiments [20], it can be seen that all the experimentally observed failure points fall between the predicted curves for the initial defect of 0.1 mm and 0.170 mm in size
The small crack growth life prediction method proposed in this paper includes a detailed finite element (FE)based crack growth simulation and lifing calculations using the modified NASGRO equation and small crack growth data
Summary
In comparison with subtractive manufacturing methodologies, the additive manufacturing (AM) is defined as ‘a process of joining materials to make objects from 3D model data, usually layer upon layer’ [1]. The physical meaning of effective lower fatigue threshold is not clear, and the values of both these parameters could be different from those obtained from the experiments In both the studies [16,21] the effect of size, shape and distribution of defects in AM parts have not explicitly been discussed, and the use of these two theoretical methods for practical applications is judged to be limited. Fomin et al [22] in their research have proposed a fatigue life assessment model which considers the effects of small cracks for laser-welded Ti–6Al–4V butt joints in HCF regime Their proposed small crack growth model considered the ‘cyclic R-curve’ as a mean to reflect the reduced threshold value and the crack length dependent Kth(a) for small cracks. Three different test cases were studied and details of analysis together with results, discussion and conclusions, and some recommendations are presented in this paper
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