Characterization of internal fatigue cracks in aluminum alloys by simulation of phase contrast tomography.

Synchrotron Radiation Computed Tomography (SRCT) allows a better detection of fatigue cracks in metals than laboratory CT due to the existence of phase contrast. However the presence in reconstructed images of fringes at the edges of objects generated by Fresnel diffraction makes it difficult to identify and analyze the cracks quantitatively. Simulations of phase contrast synchrotron tomography images containing cracks with different sizes and shapes are obtained by using GATE software. Analyzing the simulation results, firstly, we confirmed that the bright parts with strong contrast in SRCT image are streak artifacts; secondly, we found that the gray scale values within the cracks in SRCT images are related to the crack size; these simulation results are used to analyse SRCT images of internal fatigue cracks in a cast Al alloy, providing a clearer visualisation of damage.

Damage tolerant design of additively manufactured metallic components subjected to cyclic loading: State of the art and challenges.

Undoubtedly, a better understanding and the further development of approaches for damage tolerant component design of AM parts are among the most significant challenges currently facing the use of these new technologies. This article presents a thorough overview of the workshop discussions. It aims to provide a review of the parameters affecting the damage tolerance of parts produced by additive manufacturing (shortly, AM parts) with special emphasis on the process parameters intrinsic to the AM technologies, the resulting defects and the residual stresses. Based on these aspects, basic concepts are reviewed and critically discussed specifically for AM materials: Criteria for damage tolerant component design;Criteria for the determination of fatigue and fracture properties;Strategies for the determination of the fatigue life in dependence of different manufacturing conditions;Methods for the quantitative characterization of microstructure and defects;Methods for the determination of residual stresses;Effect of the defects and the residual stresses on the fatigue life and behaviour. We see that many of the classic concepts need to be expanded in order to fit with the particular microstructure (grain size and shape, crystal texture) and defect distribution (spatial arrangement, size, shape, amount) present in AM (in particular laser powder bed fusion). For instance, 3D characterization of defects becomes essential, since the defect shapes in AM are diverse and impact the fatigue life in a different way than in the case of conventionally produced components. Such new concepts have immediate consequence on the way one should tackle the determination of the fatigue life of AM parts; for instance, since a classification of defects and a quantification of the tolerable shapes and sizes is still missing, a new strategy must be defined, whereby theoretical calculations (e.g. FEM) allow determining the maximum tolerable defect size, and non-destructive testing (NDT) techniques are required to detect whether such defects are indeed present in the component. Such examples show how component design, damage and failure criteria, and characterization (and/or NDT) become for AM parts fully interlinked. We conclude that the homogenization of these fields represents the current challenge for the engineer and the materials scientist.

Location, location &size: defects close to surfaces dominate fatigue crack initiation.

Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. Those defects are known to significantly alter the fatigue life of components. Yet very little is known, quantitatively, on the dangerosity of internal casting defects compared to surface ones. In this study, fatigue specimens containing controlled internal defects (shrinkage pores) are used to foster internal cracking. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones. These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards.

On the use of flat-fields for tomographic reconstruction.

Seeking for quantitative tomographic images, it is of utmost importance to limit reconstruction artifacts. Detector imperfections, inhomogeneity of the incident beam, as classically observed in synchrotron beamlines, and their variations in time are a major cause of reconstruction bias such as `ring artifacts'. The present study aims at proposing a faithful estimate of the incident beam local intensity for each acquired projection during a scan, without revisiting the process of data acquisition itself. Actual flat-fields (acquired without specimen in the beam) and sinogram borders (when the specimen is present), which are not masked during the scan, are exploited to construct a suited instantaneous detector-wide flat-field. The proposed treatment is fast and simple. Its performance is assessed on a real scan acquired at ESRF ID19 beamline. Different criteria are used including residuals, i.e. difference between projections of reconstruction and actual projections. All confirm the benefit of the proposed procedure.

X-ray micro-tomography of a coronary stent deployed in a model artery.

Coronary stents are small scaffolds routinely implanted during angioplasty procedures to re-open coronary arteries which have become narrowed by an atherosclerotic plaque. Despite the advent of drug-eluting stents to reduce restenosis (re-narrowing) after the procedure, complications still arise and mechanical factors may be partly the cause. Stents are being used in increasingly complex cases, so new forms of pre-clinical testing may be helpful in evaluating stent designs and deployment techniques. With this in mind, an in vitro experiment was conducted to evaluate the use of X-ray micro-tomography to image stents at various stages of deployment. A stent was deployed in an artificial artery while using a synchrotron X-ray source to obtain the tomography scans. A volume element (voxel) size of 5.3 microm was achieved, with a vertical field of view of 4.5mm. The imaging of the stent and artificial artery materials was better than expected, despite some attenuation artifacts and the high monochromatic beam energy used (25 keV). Experimental problems are discussed, together with recommendations for improving the technique. It is intended that this technique will be of interest to engineers and clinicians as a pre-clinical test.