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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INFLUENCE OF THE CONTOUR PARAMETER IN MICROSTRUCTURE DUALITY AND FRACTURE INITIATION IN NON-COMBUSTIBLE MAGNESIUM ALLOYS FABRICATED BY LASER POWDER BED FUSION
Non-combustible Mg alloy components fabricated by laser powder bed fusion in as-built conditions have an average ultimate tensile strength (UTS) of 320 MPa, a significantly larger value than its casting counterparts, which present an average UTS of 200 MPa. In addition, it was determined that stable crack extension always starts at the outer surface due to the coarsened microstructure regions present in the area. Therefore, this paper will use fracture mechanics to predict the UTS value by determining the size of the coarsened microstructure region and considering it as a surface crack with the √area parameter. Then, by using a fixed fracture toughness value, the UTS will be predicted. Furthermore, a processing parameter known as contour, which is used for remelting the outer surface of the specimen, can also smoothen the microstructure and potentially increase the UTS value. Results showed that the √area of the surface crack responsible for fracture was 730 μm for the no-contour specimen and 630 μm for a contour specimen. Subsequently, using Murakami’s theory, the predicted UTS is 320 MPa and 345 MPa respectively. Finally, tensile testing was performed to confirm the prediction, showing similar results with an average deviation of 2.9%.
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FAILURE CHARACTERIZATION IN 17-4PH STAINLESS STEEL ACROSS MULTIPLE MANUFACTURING METHODS
Accurate models of additively manufactured (AM) materials require extensive mechanical testing for proper calibration and verification/validation. The process-structure-property relationships in 17-4PH stainless steel from multiple manufacturing modes were examined via mechanical testing across several strain rates and post-mortem characterizations of the fracture surfaces and microstructure. Under all manufacturing modes and testing conditions, optical and scanning electron microscopy showed ductile failure characteristics. Higher porosity concentration (determined by density measurement) resulted in lower ultimate strength in cast samples; the pores often acted as crack initiation points. Strain-rate dependence and failure modes were also affected by process-dependent anisotropy in the microstructure, which was quantified through electron backscatter diffraction (EBSD) imaging. This data will be used to inform models of failure in the 17-4PH for multiple manufacturing forms.
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EVALUATION OF STRENGTH CHARACTERISTICS FOR NON-COMBUSTIBLE MAGNESIUM ALLOY PRODUCTS FABRICATED BY LASER POWDER BED FUSION UNDER AS-BUILT CONDITION
It is difficult to evaluate fracture toughness according to ASTM standards for non-combustible magnesium alloy fabricated by Laser Powder Bed Fusion (LPBF) in as-built conditions. The reason is its microstructure duality between inner and outer surfaces. The microstructure duality can be eliminated by heat treatment. However, heat treatment reduces the strength of the material by around 11%. Therefore, heat treatment was not performed. In addition, the greatest advantage of LPBF is maximized when it can be used immediately without post-processing. Therefore in this study, the as-built condition was targeted. In the case of non-combustible Mg products, the mechanical properties of the inner and outer microstructures have a non-negligible difference. The difference is expected to affect the fracture behavior, so it is important to consider the difference in microstructure in strength evaluation. Therefore, this paper explains why ASTM standards are difficult to apply to non-combustible magnesium products fabricated by LPBF in as-built conditions with their microstructure differences. Furthermore, the alternative methods for measuring the fracture toughness of metals fabricated by the LPBF in as-built conditions with these characteristics are introduced and discussed.
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MODELING OF MIXED-MODE CRACK GROWTH BEHAVIOR IN LB-PBF TI-6AL-4V USING A CRITICAL PLANE FRAMEWORK
Many service loading conditions are multiaxial, and small cracks have been shown in many situations to grow in mode II or mixed-mode due to the orientation of defects and microstructural effects, particularly in additively manufactured metals. This paper uses fracture mechanics with a critical plane framework to predict crack growth rates using only mode I constants from the literature.
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