FATIGUE LIFE PREDECTION OF THE AA2024-T351 ALUMINUM ALLOY

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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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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REDUCING LOW CYCLE FATIGUE LIFE SCATTER OF ADDITIVE MANUFACTURED ALSI10MG USING LASER SHOCK PEENING

Additive manufactured (AM) alloys are still prone to critical manufacturing flaws, such as gaseous bubble entrapment. These defects can lead to early crack initiation reducing fatigue life and increasing scatter, especially when near surface. This research investigated the effect of femtosecond laser shock peening (FLSP) on the fatigue life of AM AlSi10Mg. Due to the low penetration of the FLSP, fatigue life remained consistent between treated and untreated specimens. Of equal importance though, the scatter was found to be reduced in the FLSP treated samples. From the high resolution DIC results, the average strain per grain in the untreated specimens showed a higher increase of strain from initial loading to final fracture as compared to the FLSP samples. Implementing the use of FLSP onto AM materials could lead to more consistent fatigue life despite the presence of porosity, leading to a path of easier certification and improved confidence in their behavior.
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SIGNIFICANCE OF INTRA-BUILD DESIGN VARIABLES ON THE FRACTURE TOUGHNESS PROPERTIES OF ELECTRON BEAM MELTED TI6AL4V

Structurally reliable materials are essential for adopting additive manufacturing (AM) metals in safety-critical applications. Limited data on the damage-tolerance properties of metal AM materials exists, hindering the acceptance of AM metals in fracture-critical applications. A design of experiments (DOE) is used in this study to investigate the role of build space and part design parameters on the fracture toughness properties of Electron Beam Melted (EBM) Ti6Al4V. ASTM E399 tests were performed on over 100 compact tension (CT) samples in the as-built and machined conditions to obtain fracture toughness properties and evaluate the influence of part size and location within 80% of the build space. Results were comparable to wrought annealed titanium, with less than 10% variation in overall fracture toughness. Specimen location within the build envelope contributed to the observed variation, with an increase in properties with build height and specimens located in the center of the build envelope. The location-dependent properties result from changes in microstructure and porosity throughout the build space. While the experimental EBM Ti6Al4V fracture toughness properties are promising for future applications, it is crucial to consider the variation in properties due to build space location and design parameters when designing for consistency.
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COMPUTATIONAL MODELING FOR IDENTIFYING VOIDS IN ADDITIVELY MANUFACTURED AL-SI10-MG

Additive manufacturing (AM) is a quicker and more cost-effective technique to produce complex parts that can perform similar to or better than conventionally manufactured parts. However, due to the dissimilar microstructure compared to conventional parts, there is a lack of understanding in the physical and mechanical response of AM alloys under different loading conditions and strain rates, and thus the suitability of using AM parts is uncertain. Notably, the presence of voids in AM metal alloys is more prevalent. By developing a computational model that can represent plasticity and track fracture initiation at the void sites in AM alloys such as Al-Si10-Mg, the failure response can be predicted. Therefore, the objective of this research is to use in-situ micro-computed tensile testing to identify individual voids or networks of voids that are likely to cause fracture initiation in an AM Al-Si10-Mg alloy.
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FACTORS GOVERNING THE FATIGUE PERFORMANCE OF AM TI-6AL-4V COMPONENTS

Before an additively manufactured component can be safely used in a load bearing application, its mechanical performance must be qualified. Traditional qualification approaches, involving the fabrication and testing of many identical components, negate one of the greatest benefits of additive manufacturing, i.e. the ability to quickly and cheaply fabricate one-off components. Thus, qualification methods that rely less on mechanical testing and more on predictive modeling are of value. This is most true for high cycle fatigue performance, where mechanical testing requires significant resources and produces stochastic results.

High cycle fatigue failure is difficult to predict because it can depend nonlinearly on many parameters, e.g. part geometry, residual stresses, surface characteristics, material defect characteristics, grain and dislocation structures, mechanical and environmental loading characteristics and their history. This has motivated a succession of fatigue models with ever increasing mechanistic fidelity, with some now diving down to the atomic scale. This raises the question of: what level of mechanistic detail is required to sufficiently predict the performance of AM Ti-6Al-4V components? In this talk, I will give my perspective on this question, building from a decade of AM Ti-6Al-4V fatigue modeling and experimentation across scales.
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