The purpose of this styudy is to investigate the cyclic behaviour of the AA2024-T351 aluminum alloy widely used in the aircraft industry. This alloy shows a relatively low ductility at room temperature and is generally heat treated in various conditions to suit particular applications. Monotonic and cyclic tests have been conducted in order to characterize the fatigue behaviour and determine the fatigue life of aluminum alloy. Cyclic tests in the Low Cycle Fatigue (LCF) regime were performed under fully reversed total strain amplitudes ranging between 0.6% and 1.2%. The elastoplastic behaviour was analysed through the stress-strain hysteresis loops leading to evaluate kinematic and isotropic hardenings. The AA2024-T351 was also shown to be prone to cyclic strain hardening. Besides, symmetric High Cycle Fatigue (HCF) tests were also performed and the Stress-Number of cycles (S-N) curve until 107 cycles was plotted. A fatigue limit of about 150 MPa was found. Based on all LCF and HCF tests, the fatigue life could be represented in a strain approach by the Manson-Coffin-Basquin law. Moreover, observations of the fracture surfaces were carried out using a Scanning Electron Microscope (SEM) in order to detect the crack initiation and follow the propagation for the two fatigue regimes.
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Themes: Fatigue and Fracture of Additively Manufactured Materials
MECHANICAL RESISTANCE ASSESMENT OF 316L STAINLESS STEEL ADDITIVELY-REPAIRED STRUCTURES
To quickly characterize the static and cyclic mechanical strength of a structure repaired by additive manufacturing, a specific specimen is developed and then repaired using two processes (laser direct energy deposition and cold spray) with adjustable parameters. The fundamental role of the microstructure in the vicinity of the repaired area in the initiation and propagation of cracks under cyclic loading is highlighted and discussed
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FRACTURE TOUGHNESS OF A DUPLEX STAINLESS STEEL BUILT BY DIRECTED ENERGY DEPOSITION : EFFECT OF THE DEPOSITION DIRECTION
Additive manufacturing of duplex stainless steels (DSS) has recently seen some research interest. In particular, the use of directed energy deposition (DED) is still new and the fabricated materials remain to be fully characterized. In addition, materials produced by additive manufacturing can present anisotropic fracture properties. This study aims to characterize the fracture toughness of a DSS manufactured by DED, taking into account the orientation with regard to the printing strategy.
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ACCELERATED DESIGN AND INTEGRITY ASSESSMENT OF ADDITIVELY MANUFACTURED METALLIC STENTS USING MACHINE-LEARNING MODELS
In this work, we investigate the potential of laser powder bed fusion (L-PBF) to meet the stringent requirements imposed on metallic stents for the treatment of aortic dissection. Here, we use microstructure-based modeling to describe the mechanical properties of L-PBF 316L stainless steel. The derived structure-property relationships then serve as a database for training machine learning (ML) models, such as convolutional networks (CNN) and graphical neural networks (GNN). Based on the established modeling framework, we are able to predict the deformation and fracture behavior of 316L stents and identify the improved stent design in an efficient manner.
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EFFECTS OF DEFECT, LOADING MODE AND MICROSTRUCTURE ON LPBF 316L FATIGUE BEHAVIOR
The present study aims to investigate the high cycle fatigue (HCF) performance of steel 316L fabricated by the laser powder bed fusion (LPBF) process. Bending and torsional fatigue test specimens built horizontally (0°), inclined (45°), and vertically (90°) have been prepared and tested in the as-built and polished states. The presence of multiple lack-of-fusion defects at the surface or subsurface is detrimental to the endurance under cyclic loading. A more pronounced defect sensitivity in bending compared to torsion is found. Microstructural features are seen to compete with inherent defects to affect fatigue performance in the condition that the effective defect sizes are close to the critical fatigue crack size.
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CORRECTING FOR RESIDUAL STRESS EFFECTS ON FATIGUE CRACK GROWTH RATES OF ADDITIVELY MANUFACTURED TYPE 304L STAINLESS STEEL
Additively manufactured (AM) metal builds contain residual stress that can influence measured fatigue crack growth rates (FCGRs), which may then bias the interpretation of the performance of AM materials. In the present work, the on-line crack compliance (OLCC) method was used to determine the residual stress intensity factor, Kres, while simultaneously collecting fatigue crack growth rate data in edge crack compact (C(T)) specimens of both AM and wrought materials. Measured near-threshold FCGR data in AM 304L C(T) specimens appear elevated in comparison with data from wrought specimens over a range of applied ∆K. By quantitatively accounting for residual stress, the results for materials processed by the different methods are brought into good agreement, demonstrating the importance of accounting for residual stress when interpreting fatigue crack growth data in AM materials.
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SHORT CRACK GROWTH BEHAVIOR OF IN718 UNDER HIGH TEMPERATURE CONDITIONS IN CONSIDERATION OF PLASTICITY INDUCED CRACK CLOSURE
The behavior of short fatigue cracks under high temperature conditions is of significant importance for lifetime predictions and defect assessment of components in aircraft and gas turbine applications. The cyclic R(esistance) curve provides a possibility to describe phenomenologically the crack growth behavior below the long crack threshold. Within this paper experimentally determined cyclic R-curves of the nickel based alloy IN718 for varying load ratios (Rσ = -1, 0 and 0.5) at 650 °C are compared to each other. Furthermore, an approach supporting a mechanism-based understanding of the cyclic R-curves, taking plasticity induced crack closure (PICC) into account, is developed. Finite element analysis is used to estimate the amount of PICC in the short crack regime. For a systematic investigation of the impact of different ductility properties on short crack behavior, results from an additively and two conventionally (cast and wrought) manufactured material variants of IN718 are part of the investigations.
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RESISTANCE TO FRACTURE AND FATIGUE IN ADDITIVELY MANUFACTURED ALLOYS [Keynote]
Ti-6Al-4V fabricated by the laser powder bed-fusion (LPBF) process consists of metastable α’ microstructure and columnar prior β grain (PBGs) mesostructures. These micro-and mesostructures adversely affect fracture toughness (KIc) in as-built conditions. After an optimized post-processing heat-treatment, the KIc of LPBF Ti-6Al-4V improves by ⁓104%; however, the anisotropy in KIc persists due to preferential crack growth along the columnar PBGs. In another LPBF fabricated β Ti-alloy, Ti41Nb, the crack tortuosity from the mesostructures formed by compositional segregation improves the KIc by ⁓80%. These results demonstrate extrinsic toughening in AM alloys. While such toughening from mesostructures enhances AM alloys’ reliability, the processing-induced defects present in them, i.e., porosity, significantly reduce their high cycle fatigue (HCF) resistance. Therefore, in the second part of the present study, the HCF life of 316L and 17-4 PH steels produced by the binder jet printing process was investigated. The hot isostatic pressing (HIP) was employed on these steels to improve their HCF life. The HCF life of HIPed 17-4 PH steel is comparable to their conventionally manufactured counterparts; however, in 316L, HIP fails to improve fatigue life. Based on these findings, the microstructural origin for fracture and fatigue resistance in AM alloys are discussed.
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MICROSTRUCTURALLY INFORMED HIGH-VELOCITY IMPACT EXPERIMENTATION ON ADDITIVELY-MANUFACTURED METALLIC MATERIALS
This work presents a flexible experimental setup to study dynamic fragmentation of additively-manufactured metallic materials using two different configurations: (i) rapid axial penetration of thin-walled tubes, and (ii) rapid radial expansion of rings. In the first approach, the experiment consists of a light-gas gun that fires a conical nosed cylindrical projectile that impacts axially on a thin-walled cylindrical tube fabricated by 3D printing. The diameter of the cylindrical part of the projectile is approximately twice greater than the inner diameter of the cylindrical target, which is expanded as the projectile moves forward, eventually breaking into fragments. In the second approach, using a similar technique, a ring is inserted over a high-ductility tube, which expands after penetration by the conical projectile, pushing the metallic ring radially outwards, ultimately breaking into multiple fragments. The experiments have been performed for impact velocities ranging from 180 m/s to 390 m/s. A salient feature of this work is that we have characterized by X-ray tomography the porous microstructure of selected specimens before and after testing. Moreover, two high-speed cameras have been used to film the experiments and thus to obtain time-resolved information on the mechanics of formation and propagation of fractures.
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VERY HIGH-CYCLE FATIGUE BEHAVIOR OF ADDITIVELY MANUFACTURED TI-6AL-4V USING ULTRASONIC FATIGUE MACHINE AND SELF-HEATING TESTING.
Accelerated characterization of high-cycle fatigue properties is necessary in order to enable the optimization of parameters of additive manufacturing processes such as LPBF (Laser Powder Bed Fusion). Therefore, two accelerated characterization methods are applied and compared on Ti-6Al-4V samples produced using the LPBF process. The first method uses an ultrasonic fatigue machine and the second one determines the fatigue limit using self-heating testing. To study the interactions between the material and the accelerated testing methods, fatigue tests are carried out on different grades of Ti-6Al-4V-LPBF differing by their microstructure or their porosity. Three grades have the same microstructure but different porosity levels and three grades have different microstructures with the same porosity. Both properties showed a strong impact on VHCF strength and affected the mechanisms at fatigue crack initiation.
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